POLE AND TOWER LINES McGraw-Hill BookCompany Electrical World The Engineering and Mining Journal Engineering Record Engineering News Railway Age G azette American Machinist Signal HnginE PRESS YORK PA PREFACE The rapid growth of the electric light and power industry within the last decade has caused an enormous increase in both the number and length of transmission and distribution lines. From a comparatively unimportant detail of little interest even to the owner the power lines have developed into quite pretentious systems whose relation to other industries and to the public must be taken into consideration. The occasional transmission systems designed for 11,000 volts and consisting of wood pole lines with spans of 120 ft. have developed into 150,000- volt lines on steel towers with spans from 500 to 800 ft. long, while the light wooden poles supporting a few street lighting circuits have been superseded in some cases by very heavy trunk lines of many cables. As is to be expected in a growing industry, no set of standards has been universally adopted. Moreover, it is impossible to apply any one specification or standard of construction universally, unless such a standard has some elasticity and is interpreted and enforced intelligently. In any attempt at standardization, either for one operating company or in a national specification, it is necessary to consider carefully the cost and the operating problems involved in the adoption of any general mechanical requirements. An apparently harm- less requirement may become a very serious matter if applied to all future construction. The number, size and voltage of wires and the type of insula- tion to be used belong to the field of electrical engineering, while the subsequent determination of the best method of carry- ing the conductors across country is purely a question of civil engineering. The terms "electrical" and "civil" are used in their narrow sense, but the division is important as denoting the proper apportioning or departmentizing of the work. In this age of specialists there is no more reason for electrical engineers to determine structural details, than there is for structural engineers to decide on the proper type of insulator. If laymen or workers in other branches of the engineering profession have hesitated to assume authority in electrical matters, the same cannot be said of the average electrical expert in the field of pure v 331010 vi PREFACE construction. This, no doubt, is due to the fact that those engaged in coordinate branches of the profession have done little or nothing to solve the problems properly assignable to construc- tion engineers. That this is no imaginary charge is substantiated by the history of transmission line construction. Until recent years the rule-of-thumb practices of telephone companies worthy results of the test of time as many of them are were followed blindly by those in charge of electrical transmission. As a result there are many improperly constructed lines and erroneous ideas prevail regarding the facts and principles involved in their design. This condition should no longer be allowed to exist. It is not the purpose of the writer to deal with purely electrical problems, such as the relation of voltage and size of the wires to the electrical characteristics of a line, or with such very specialized matters as the design of insulators. The problem is rather to develop a clearer preception of the application of the laws of mechanics to the case in hand. The writer wishes to acknowledge, with thanks, the assistance rendered by Mr. W. L. Cadwallader in preparing the tables and computations, and by those in charge of various properties in furnishing illustrations and data relating thereto. R. D. COOMBS. NEW YORK, N. Y. 1916. CONTENTS PAGE PREFACE v CHAPTER I TYPES OF CONSTRUCTION 1 Carrying Capacity of Supports Life of Materials Clearances Tree Trimming Right-of-way Factor of Safety Spans Sup- ports Location Plan. CHAPTER II LOADING 29 Sleet Wind^-Broken Wires. CHAPTER III WIRES AND CABLES 43 Copper Copper Covered Aluminum Steel Telephone Wire Catenary. CHAPTER IV DESIGN 60 Factors of Safety, Etc. Transverse Loads Corner Loads Broken-wire Loads Column Formulas Strength of Wooden Poles. CHAPTER V WOODEN POLES 76 Decay and Defects Seasoning Preservatives Pressure Treat- ment Open-tank Process Brush Treatment Framing of Poles Design of Wood Poles A-frames and H-frames. CHAPTER VI STEEL POLES AND TOWERS 103 Rivets and Bolts Lacing Angle Lacing Tower Connections Latticed Poles Curb-line Poles Triangular Poles Wide-base Towers Flexible Frames. CHAPTER VII SPECIAL STRUCTURES 141 Transposition Outdoor Sub-stations Switching Stations, Etc. High Towers Aerial Cable. vii viii CONTENTS CHAPTER VIII PAGE CONCRETE POLES . . . . 152 General Considerations Tests Sections Strengths. CHAPTER IX FOUNDATIONS 165 General Wood Pole Settings Bog Shoes Barrel Foundations Concrete Settings Tower Foundations "A "-frame Foundations Rock Foundations Concrete Cement Proportions Aggre- gates Water- Mixing and Placing Forms Workmanship Reinforcement Waterproofing, Salt Water, Alkali. Etc. CHAPTER X PROTECTIVE COATINGS ; ....... 180 Paint and Painting Galvanizing Standard Specifications for Spelter Standard Specifications for Galvanizing. CHAPTER XI LINE MATERIAL 189 Tie Wires Loops Splices Pin Insulators Pins Crossarms Crossarm Braces Lag Screws or Lag Bolts Guys and Guying Guy Anchors. CHAPTER XII ERECTION AND COSTS 217 Erection Costs. CHAPTER XIII PROTECTION 232 Ground Wires Neighboring Lines Cradles Clamping Devices CHAPTER XIV SPECIFICATIONS 248 Joint Report Specifications for Crossings General Specifications for Lines Galvanizing (see Chapter X). INDEX . . 269 POLE AND TOWER LINES CHAPTER I GENERAL CONSTRUCTION The choice of a type of construction for transmission lines, on private rights-of-way, may be said to depend on the cost of con- struction versus the cost of maintenance and of interruptions to service. This is also true, though to a less extent, of certain classes of railroad power lines. Where the lines of a transmission company are on public property, or cross other lines, or when high- voltage lines are used by railroads for electric-traction, the cost of failures of construction will be found to exceed the addi- tional cost of good construction. A decision as to the exact type of line, character of supports, and total wire capacity which will provide the highest ultimate economy, as well as excellence of service is perhaps impossible. A designer can only use good judgment in estimating future developments, and engineering skill on the immediate construc- tion. The number of years that any particular line, perhaps any or all lines, will need to remain in the original location cannot be definitely foretold. If the line remains in place for a long period of years, will it have any considerable scrap value at the end of that period? If removed, or considerably altered, in a shorter period, the value remaining in the material is scarcely less diffi- cult to determine. Apart from the economic needs of the owner 10 or 20 years hence, there is the question of the effect upon the existence of the line, of federal or municipal regulation at some earlier date. In addition to these considerations, there must also be taken into account the probable future wire-carrying capacity and voltage, the life of the materials, and finally the relative ultimate economy of one or another type of construction as judged by the above conditions. In order to obtain a better understanding of the various fac- tors which affect, from a construction standpoint, a decision as to the type of line, the following details may be noted : 1 POLE AND TOWER LINES Carrying Capacity of Supports. Experience with the lower voltages, at least, has shown that the capacity of the supports to carry additional wires has usu- ally been underestimated. The extremely heavy distribution lines often seen on city streets are not, however, an example of a proper ultimate wire capacity, but rather of an unwise over- loading. Exact maximum limits cannot be set, but the occasional failure of such overloaded lines FIG. 1. Overloaded wooden poles. FIG. 2. One-circuit wooden poles, 33,000 volts. causes an expenditure which should be a weighty argument for the alternative of more and lighter lines. It is true that two light lines will cost more than one heavy line in so far as the actual GENERAL CONSTRUCTION 3 construction cost is concerned, but the increased maintenance of the latter and its greater liability to extensive or prolonged inter- ruption to service, if difficult to foreteh 1 in terms of money, are not less costly on that account. Many otherwise competent engineers seem to ignore the very real cost in time, money, and reputation " saved" by a reduction in the initial construction account. The ordinary wood-pole distribution lines are subject to more alteration than transmission lines, and are affected by conditions not generally applicable to transmission lines. In transmission- line construction, the question is whether to provide for one or two, two or four, or four or more circuits upon a single line of supports, and in the higher voltages whether to carry one or two circuits. The financial ability or disability of the owner may settle the question of the number of circuits without reference to any further considerations whatever. The best may be the cheapest, but it may also be the impossible. Under such circumstances, the engineer can build only as well as is practicable with the funds available. Lines built under such restrictions are usually in less populous territory, and the penalties of accident are less severe than in the neighborhood of cities. The natural and entirely proper practice will be to provide relatively low supports, long spans, light structures, and very little future capacity. It may be noted, however, that proper side clearances, spacing, insula- tion, and guying, add but little to the cost compared to the gain in the efficiency of the line. In what may be called the better grades of construction, the number of circuits upon a single support depends very largely upon the location of the line and its present or future voltage. There is a quite important difference between lines upon private rights-of-way and those upon highways or private property. It is not unreasonable to assume that an operating company may utilize any combination of heavily loaded or duplicated lines it chooses upon its own property, though that property will eventually have to be fenced and patrolled. On the other hand, the occupation of highways, etc., by overloaded or perhaps any lines, may become subject to criticism. Again the holding of separated parallel, or looped, lines has real advantages from the commercial and service standpoint. Dividing the total capacity for wires between two or more pole 4 POLE AND TOWER LINES lines cannot be justified as a measure of reducing the cost. Such duplicate lines will always cost more than one heavy line, but the subdivision may be justified on other grounds. In case one line is given a different route, the maximum security from simul- taneous interruption to service is obtained and additional busi- FIG. 3. Two-circuit wooden poles, 33,000 volts. ness may be secured, but the cost will be more than that of two lines on one right-of-way, and nearly double that of one line of the total capacity. The number of circuits that may be carried by a single support depends on the electrical characteristics, the continuity of service GENERAL CONSTRUCTION 5 desired and to some extent on thfc length of span and the charac- ter of the supports. Thus the same weight of conductor divided between two circuits affords considerable protection against interruptions to service, with little added cost except for higher poles, larger or more numerous arms, more insulators, etc. A single circuit, however, can be arranged with more space between conductors and may be used more economically in long spans. Life of Materials. In general it may be assumed that a line should have the longest life consistent with a reasonable first cost, and that after selecting a type of construction and factors of FIG. 4. Two one-circuit lines, 55,000 volts. safety proper for the location under consideration, the matter of restrictive regulation must be left to the future. In connection with the probable life of materials the statement is frequently made that the life of steel is indefinite. This is literally correct though not exactly as intended. The life of properly protected steel is indefinitely long, while that of steel exposed to the weather and unprotected is indefinitely short. Many galvanized wind mills which are 20 years old are still in good condition, and some painted steel structures are much older. On the other hand, there are both galvanized and painted struc- tures whose probable life will be less than 20 years. The probable 6 POLE AND TOWER LINES average life of unprotected timber poles is about 10 years, while that of poles which have been properly treated with preserva- tives is from 16 to 20 years, depending on the treatment. As reinforced-concrete poles are a more recent development there has not been the same opportunity to obtain fair averages, but FIG. 5. Two one-circuit steel towers, 60,000 volts. from observing installations 10 years old it appears that their average life is comparable with that of steel. Clearances. Until recent years the only general clearance specified by engineers, or required by law, was that power wires should be maintained at a height of 25 ft. above highways or I i GENERAL CONSTRUCTION above the top of rail over railroad tracks. In some cases a clear height of 22 ft. was permitted. Such clearances, like most of the early requirements in transmission construction, were founded on and often a direct copy of existing telephone and telegraph practice. The overhead clearance necessary to permit the trolley pole on a large modern car to swing into its upright position, in case it FIG. 6. Two-circuit steel towers, 110,000 volts. accidently leaves the trolley wire, would perhaps indicate the minimum limit. While overhead systems for alternating-current operation of railroads or interurban lines are, as yet, but few in number the largest and best known installation being that on the New York Division of the New York, New Haven & Hartford Railfoad such construction should receive consideration in establishing a standard overhead clearance above railroad 8 POLE AND TOWER LINES tracks. In such cases the railroad company would probably desire to have their transmission circuits, if on a separate pole line, carried above all crossing lines, except possibly those of very high voltage systems. However, railroad trolley-contact wires cannot be elevated above other crossing wires. The mini- mum height of such overhead contact wires is approximately 23 ft. at the center of a span and 28 ft. at the supports. Linemen working on foreign wires, whether on joint-pole lines or not, should be protected from contact with power wires by providing a reasonable space between the two lines or sets of wires. In this connection mention may be made of one reason for requiring telephone and telegraph wires to be placed below power wires either at crossings or when located on joint poles, i.e., to prevent the harmless wires dropping into contact with the power wires, either during the process of stringing wire or due to the more frequent mechanical failure of the smaller telephone wires. It should be remembered that the stresses on poles increase directly with their height and, in fact, more rapidly than in a direct ratio when there is no adjoining protection from the wind. Omitting considerations of accidental contact and malicious in- jury, the lower line is usually the safer line. Assuming that the power or lighting circuit is, as it should be, the higher circuit, or in the case of several lines of different voltages that they are arranged in order with the highest voltage circuit uppermost, there should remain below the power wires a zone for harmless wires. This arrangement is especially necessary over highways or railroads where such inferior lines exist or may be reasonably expected in the near future. The proper separation of conductors to prevent their swinging into contact has never been definitely determined. It has been argued that long spans swing synchronously, and that experience has shown that they may be safely spaced much closer together than the distance required to provide for the maximum displace- ment. On the other hand, short spans with relatively greater separation have been brought into contact by what appears to be a purely electrical cause. It is doubtful whether engineers and executives realize the extent to which undesirable construction is installed, due to inadequate efforts to obtain the necessary concessions on the part of outside interests. Isolated trees, of perhaps no particular GENERAL CONSTRUCTION 9 value, have compelled the use of high poles or the unnecessary grading up of pole lines. Without wishing to appear an advocate of some of the common methods of "tree trimming," the writer believes that one large scraggly tree, more or less decayed, re- maining along a curb line where the other trees are of recent FIG. 7. Steel pole line, 11,000 volts, with space for lower voltages. regular growth, should not be allowed to interfere with the proper location of all the wires in that street. Everywhere throughout the country, there are towns in which the telephone and electric service lines occupy all sorts of zones, gradually getting higher and higher, until the latest line is driven to pole heights extremely difficult to obtain. It will also be found that many telephone lines have overbuilt the present 10 POLE AND TOWER LINES power wires and occupy the zone which should be used by future power wires of higher voltage. Telephone or telegraph wires should be placed underneath power wires, as it is impracticable to give the former a sufficient factor of safety to prevent mechan- ical failure. There is no established relationship between the span and sag, and the separation between conductors. Two general methods have been advanced for determining the separation be- tween conductors: (1) depending on sag and span, and (2) on sag only. The first is incorporated in the Pennsylvania Railroad wire-crossing specifications which prescribe a separation of 1 in. .5 5 fi 4 01 234 5 6 78 9 10 11 12 13 14 15 16 17 18 19 20 Sag in Feet FIG. 7 A Conductor separations. for each 20 ft. of span plus 1 in. for each foot of sag or fraction thereof. The second method, proposed by the writer in a paper before the National Electric Light Association convention in 1913, is shown in Fig. la. In both methods mentioned, the minimum separation may not be less than that given for shorter span lengths in paragraph 3 of the specifications on page 259. The use of suspension insula- tors involves an additional separation of one and one quarter times the length of the string of suspension insulators. Tree Trimming. In order to provide and maintain adequate separation between conductors and adjoining timber, a rather indefinite amount of tree cutting and trimming must be done. The actual amount of such work will vary between wide limits GENERAL CONSTRUCTION 11 for different lines even in the same locality. For lines located on streets the problem is first to select the most accessible and least- shaded street, and second, to adjust the height of the poles and the amount and character of trimming so as to obtain the maxi- mum protection with the minimum of offense. In cross- country lines, however, particularly those on private right-of-way, the problem is somewhat different. In this case, there is usually some freedom of movement through which the line may be so located as to avoid too close proximity while retaining the benefit of distant shelter. This latter feature seems to have been gen- erally disregarded, and yet, provided sufficient separation is main- tained to prevent falling contacts, the presence of timber land to windward is in the writer's opinion a considerable asset in the strength of ordinary lines. Cutting down trees, while generally deplorable, cannot always be avoided, so its justification must necessarily depend on the quality of the interfering tree and the importance and position of the line. Some trees will so outrank the ordinary power line, both in their real and in their popular value, that a change in route may have to be considered. Other trees are past their prime and have merely a sentimental value to a limited number of persons. In some cases permission for indiscriminate cutting will be freely given. Either in cutting or in trimming, a broad- minded liberal policy on the part of the power company, coupled with considerable tact, will ultimately justify itself. Unpruned trees with long scraggly limbs, instead of being injured, will generally be improved by proper trimming. Dead or dying branches are of no benefit to trees, whereas they are a serious menace to the power company; therefore they should be removed in the immediate neighborhood of the line. The methods in vogue in trimming trees are greatly in need of improvement. Aside from the serious loss in popularity and prestige, it is nothing short of criminal waste to unnecessarily injure grown timber. This country once possessed enormous forests, yet the present timber lands are only a pitiful remnant and cultivation is almost unknown. In pole-timber land it is no exaggeration to claim that every tree unnecessarily cut down or killed adds its mite to the future maintenance expenditures of the local power company. By exercising some care it is possible to trim so that killed trees or non-permanent clearances should be rare. The season 12 POLE AND TOWER LINES of the year in which trimming is done has a marked influence on the successful healing of the cuts. In general it is best to trim any tree during the dormant season. In removing large limbs they should be first undercut to pre- vent slivering and then sawed through close to the trunk. The stump should then be cut off flush with the trunk, leaving a smooth surface, which may be painted when it has dried. If this is not done, the stump will decay and the rot will spread to the trunk. Small limbs may be cut immediately beyond any forks, or close to the trunk; upright limbs should be cut on a slant and the surface should be finished smooth and then painted. When an entire tree is to be cut down, the stump should be short and be given a smooth ridge or roof similar to that on a pole top. Second-growth trees will then usually sprout from the stumps and be available for lumber in the future. In protecting or patching rotten cavities, the dead wood should be cut away to form a pocket with undercut edges as in dental work, and the cavity should then be painted and left as an open cavity or be filled with cement mortar or asphalt. On private right-of-way all trees, brushwood, sage brush, etc., should be cut down and cleared away. The amount of cutting on neighboring property will vary greatly, but should in all cases include the trimming of nearby dead branches and such trees as by their unusual position create a particular hazard. The attachment of guys, without tree blocks or shields, will frequently kill a part or all of a tree, lessen its value as a stub, and also render it a menace to the line. Trees grow by the addition of wood fiber to the outside of existing limbs, and the new growth is fed by a descending liquid in a thin layer, called the cambium, immediately inside the bark. If, therefore, the continuity of the cambium layer is broken the new wood is starved just below the break. If the cambium of all or a large proportion of the circumference is cut through or is prevented from growing with the tree, the entire tree starves. Nearly all trees are dormant in winter and may then be cut without excessive bleeding and without subjecting the wounds to attack by insects. Right-of-way. The simplest form of right-of-way is that of pole or tower rights, whether obtained along streets by franchise from the municipality or by lease or purchase from private owners. Although this form is by far the most common, it seems GENERAL CONSTRUCTION 13 probable that there will be a marked increase in the number of private rights-of-way for both pole and tower lines. Private rights-of-way, while naturally more expensive in original cost, permit the use of the most economieal types of construction and provide insurance against restrictive regulation or an excessive cost for increased facilities. The abnormal expenditures in- volved in hurried construction and the excessive payments often required to complete a right-of-way a species of blackmail are perhaps not fully realized. Such expenses would be greatly reduced in the case of a private right-of-way, particularly for sub- sequent lines installed thereon. It might be argued that ex- cessive payments would be demanded for a continuous right-of- way, as is usually the case in railroad construction. While such unit prices are unquestionably excessive for the land as such, they may not be excessive for an electric right-of-way at least this has been the case with railroads. Heretofore, private rights- of-way have been purchased chiefly where land was very cheap and when an important line on wide-base towers was to be con- structed. It is probable, however, that equally effective reasons may be advanced for private right-of-way in more settled com- munities, on which to build a series of pole lines. There seems to be no general standard or set of rules by which the width of a right-of-way may be determined. The two factors which appear to have had the greatest influence are the height of adjoining timber and the probable ultimate number of pole or tower lines. In addition to these, *there are several other conditions which should affect the width: the total desired security of^the lines; the character of the country traversed; and the character of the construction. Where the nature of the ground permits, some consideration should be given to patrol or transportation facilities between the lines of supports. Apart from any question of cultivation or possible railway facilities, the remaining conditions are all in- volved in the general one of security from interruptions. Inter- ruptions may originate either within or without the limits of the right-of-way. Those from within are generally due to mechan- ical or electrical failure and are best minimized by separating the lines. Those from without which usually exceed the former include falling trees, limbs, straw or other objects blown by the wind, fires, malicious mischief, etc. They are minimized by moving the supports in from the side lines. The two types of 14 POLE AND TOWER LINES interruption are therefore prevented by opposite action, but since the latter set is more important it is advisable to give them more weight in the location of the poles or towers. It has sometimes been stated that the distance from the side lines should equal or exceed the height of the tallest neighboring trees. Literally applied this rule would require a variable width, and in some localities excessive widths. While some degree of consideration may be properly given to the average height of timber, it must not be forgotten that the effective range of wind-blown branches is too great to permit absolute protection. It is advisable to cut down or trim the taller trees and to remove dead branches, since storms would presumably blow these onto the line before the smaller and live timber was affected. The character of the construction will affect the separation of the supports and their distance from the property lines because the wind-blown sag of the wires must be given proper clearances, either from each other or from the side lines. Long-span con- struction will, therefore, require greater side clearances than short-span construction. Steel poles or narrow-base towers per- mit the closest spacing of the lines, both on account of their narrow spread at the ground and because they are usually em- ployed with shorter spans and smaller sags. If the supports are staggered, i.e., the poles in one line opposite the center of spans of the adjoining line, less clearance is needed to prevent swinging contacts. The general security desired will affect the width of the right-of-way and the location of the lines thereon. One line in the middle of a wide right-of-way has the maximum possible security. In wild, treeless country two lines near the edges of the property are more immune against interruptions than with a smaller separation. One tall and one low line are more im- mune than two tall lines because the lower line can rarely affect the taller. For all other conditions, the lines should be located so as to permit the greatest freedom of future construction, a reasonable separation between lines, and a maximum clearance from the side lines. In order to facilitate the study of right-of-way clearances, three types of installation are shown. Fig. 8 represents a two- circuit steel pole or narrow-base tower located in the middle of a private right-of-way; Fig. 9 shows two one-circuit poles, while Fig. 10 shows two two-circuit wide-base towers. It is assumed that suspension insulators have been employed in each case. GENERAL CONSTRUCTION 15 FIG. 8. Right-of-way one two-circuit pole line. FIG. 9. Right-of-way two one-circuit pole lines. FIG. 10. Right-of-way two two-circuit tower lines. 16 POLE AND TOWER LINES To modify the diagrams for pin insulators it would only be neces- sary to adopt a slightly smaller value for the wind-blown deflec- tion B and for the tower clearance C. The windward-deflected sag BI in Figs. 9 and 10 is shown as one-half that of the leeward- deflected sag B. This assumption is, of course, arbitrary and would vary for different sizes of wires, spans and sags, In Table 1 are given the probable ranges of the various clear- ances, the summation of which determines the width of the right- of-way. Also shown therein is the probable range of the width W with three assumed minimum side clearances A, of 6 ft., 15 ft., and 25 ft. The presence of tall trees touching the right-of-way line is un- desirable, but it cannot always be avoided. The diagrams repre- sent the limiting condition. TABLE 1. APPROXIMATE RANGE OF RIGHT-OF-WAY CLEARANCES (IN FEET) One double-cir- cuit pole line Two single-cir- cuit pole lines Two double-cir- cuit tower lines Span 300 to 500 300 to 500 500 to 800 H B Bi 20 3 to 12 20 3 to 12 1 to 6 20 10 to 25 6 to 12 C Pi 3 to 6 3 to 5 3 to 5 4 to 8 3 to 6 If A = 6 ft., W If A = 15 ft., W If A = 25 ft., W 25 to 50 40 to 65 60 to 85 35 to 80 55 to 95 75 to 120 65 to 135 85 to 155 105 to 175 Factor of Safety. As in all construction work, by far the most important determination is the mechanical factor of safety. Indeed, the factor of safety will not only narrow the selection of the details of construction, but will practically determine the general type of line to be used. In past practice, however, the factors of safety have frequently been the last values to be defi- nitely determined, whereas they should be the basis for computa- tion. In other words, the method in general use is a cut-and-try method, in which several designs having in reality different fac- tors are first worked out, and the selection too often left to one individual's judgment or to the suggestion of a salesman. It is extremely doubtful whether "competitive" designs received by GENERAL CONSTRUCTION 17 most purchasers have been really comparable, in so far as their true mechanical factors of safety were concerned. In addition to the vagaries of competitive bidding, it may be claimed with considerable justice that the designs ordinarily made by a pur- chaser are not truly comparative. For an accurate survey of the conditions influencing the selection of the proper factors of safety for the various members involved, it is essential that considera- tion be given to the desired length of service of the line, as. well as to the characteristics of the members and materials involved in the construction. Included in the term "length of service" are many indefinite quantities which must be determined by judg- ment. Changes in the capacity of the line, a possible increase in voltage, or the entire elimination of the line from an operating standpoint may perhapfs be assumed with some degree of ac- curacy, but the probable life of the materials of construction and the possibility of restrictive public regulation are extremely diffi- cult to determine, either for a given line or for future develop- ments as a whole. The factor of safety, or as it is sometimes termed, "factor of ignorance," is a much-abused and generally misunderstood ex- pression. In reality it is a combination of the allowances for error, and consists of the summation of the individual allowances or elements of the factor, the term safety being somewhat mis- leading. The amount of the total factor depends, or should depend, on the accuracy with which the conditions of service and the characteristics of the members and material can be fore- told. If the possible variation of all the individual elements except one is known, then the allowances for the known elements entirely eliminate them in any further consideration of "safety," and a further increase in the total factor causes a disproportionate increase in the one unknown. For example, the factor of safety for. wires may be sub-divided into the following elements : (a) Increased loading. (6) Uncertain strength of the material. (c) Injuries during erection. (d) Errors in erection (improper sag). (e) Deterioration in the material. While the theoretical values of these elements should vary for each installation, the actual general case may be stated as ap- proximately : 18 POLE AND TOWER LINES a = 0.30 6 = 0.20 c = 0.10 d = 0.30 e =0.10 Total =1.00 Breaking strength = 1 . 00 Total factor of safety = 2 . 00 In other words, an analysis of the commonly used factor of 2.0 shows that it permits the loading to be underestimated 30 per cent., and the actual breaking strength of the wire 20 per cent., and allows a decrease of 10 per cent, from small injuries, an increase in stress of 30 per cent., due to improper stringing, and an ultimate deterioration of 10 per cent., before the span is theoretically at the point of failure. Omitting for the moment any consideration of the elastic limit, and the fact that stresses in excess thereof will necessitate pulling up slack (with or without other undesirable results), it is evident that any further increase in the factor of safety will greatly in- crease the allowance for the most uncertain element. On the other hand, if the strength of the wires assumed in the design cor- responds closely with the material as purchased, and the wires are strung with care and with sags having a close approximation to those in the design, it is apparent that the spans will safely withstand a very considerable increase in the assumed loading. A little consideration of the probability of exceeding the above elements in a line designed and erected with reasonable care may explain the excellent record of the wires in existing lines. It is more than probable that in many instances inaccurate wire stringing has entirely changed the actual strength of the wires in relation to external loads, and as indicated by the allowance of 0.30 in the above analysis, this is the most uncertain condition in the average installation. The above analysis is, in the writer's opinion, a fairly accurate statement of the average actual condition, but does not give the correct values for the elements of the factor of safety of wire spans designed and constructed under competent supervision. Spans. Theoretically, since the cost of the material between supports, i.e., the wire, is constant, except for the slight increase GENERAL CONSTRUCTION 19 in length due to the sag, the supports should be spaced far apart. With long spans the number of insulators is reduced, together with the probability of interruptions originating at the supports. However, other considerations usually prevent the adoption of the theoretically economic span length. The conductors must be spaced so as to provide sufficient clearance between adjoining wires and between the wires and the pole. Therefore with an increase in span length, with its consequent increase in sag, it becomes necessary to spread the wires further apart, thus length- ening the crossarms and increasing their cost. With compara- tively few wires in the line, it is possible to arrange them so that long spans can be used without excessively long or heavy cross- arms, but on heavy lines carrying many wires, this is not practi- cable without unduly increasing the height of the poles. There is quite a difference between the meaning of " aver age span" and " standard span," the former being the final result and the latter the original design which provides a theoretical clearance above the ground in flat country. Therefore, unless the towers can occupy hill tops, or an extra clearance is allowed in the design, it frequently happens that intervening elevations, or the loss of clearance on hillsides, materially decrease the actual span length. If lines are located on highways long spans are not always practicable as the length of crossarms may have to be restricted, or the wooden poles available may be incapable of withstanding the load due to long-span construction. The matter is further complicated by the mechanical limita- tions of standard, or stock, crossarms, pins, and insulators. On steep hills the spans must be decreased, or the supports lengthened, to maintain the overhead clearance. The size of the conductors has more effect on the proper or possible length of span than any other condition, since the large sags required for small wires in long spans would necessitate excessive wire spacings and pole heights. No exact economic span length has been determined either for one type of support or for one section of country. In fact, it is extremely probable that for any particular line there will be two designs of nearly the same estimated cost, and that the possible error in estimating the field work will far exceed any difference between the estimates of material. In wood-pole construction, the use of long spans with high 20 POLE AND TOWER LINES poles is subject to a serious error in estimating the probable re- placement cost. In view of the recent prices and scarcity of long poles, it may be possible that such lines cannot be rebuilt in timber at any reasonable cost. The wood-pole transmission line is an entirely proper type of construction in many cases, and it is also true that for one or two circuits the spans could often be lengthened with advantage, but such lines should be protected from decay and a high replacement cost used in estimating. While the economic design in traversing hilly country is un- doubtedly to cross ravines and small valleys by means of long spans, it is possible that this practice may be injudicious unless ample clearance is provided between the wires. There is little exact knowledge of the dependence to be placed on the parallelism of swinging wires, particularly if their horizontal spacing is only 5 to 10 ft. and the sag 15 to 30 ft. Besides the accidental contact of wires in the same horizontal plane, there have been instances of the lower wires being lifted by the wind into contact with those above. In case it will be necessary to pay rent for pole-rights on foreign property, it may prove economical to use long-span construction as the rent saved might more than pay interest on the increased cost of the supports. In ordinary country, the economic span is probably between 400 and 500 ft. for narrow-base supports, and between 600 and 800 ft. for wide-base towers. Supports. The poles or towers used up to the present time have been of wood, steel and reinforced concrete, and they have been used in the order given, both as to numbers and priority of installation. Wood poles, still the most common form of support particularly for low- voltage lines, have several objectionable features in that they deteriorate rather rapidly, do not resist fire and their cost is increasing. Under certain conditions, however, wood poles are still economically sound construction even for high-voltage lines, although the time is not far distant when they will no longer be employed for first-class installations. In changing from wood to metal, however, we may profitably pause to consider some of the characteristics of the structure which has rendered possible our progress in line construction. In theory as well as in fact the wooden pole is a precedent for the metal structure, and a too violent divergence from some of its good features may result in structures not relatively so excel- GENERAL CONSTRUCTION 21 lent as the wood they replace. A well-selected timber pole is very nearly of the ideal outline, due to the fact that the stresses imposed upon it in its original life were almost identical in nature with those encountered in pole-line service. It should not be forgotten that a wood pole has equal strength in all directions, both with and across the line, and a comparatively large strength in torsion. These qualities tend to minimize the effect of acci- FIG. 11. Bending test, FIG. 12. Tower of the rigid or windmill Coombs' concrete pole. type. dental loads or loads other than those assumed in design. Again, wood poles have considerable elasticity but not complete flexi- bility, a characteristic which enables them to deflect enough to equalize most unbalanced loadings while opposing a very con- siderable restraining force against the spread of failures along the line. This semi-flexible quality of wood poles, which is also obtainable in steel or reinforced concrete, is probably of much greater advantage than is generally realized. Another advantage of wood poles is that they are not easily injured in handling, and may be installed by men of ordinary intelligence and training. 22 POLE AND TOWER LINES Moreover, there are no long thin sections which may be bent and rendered useless and no flimsy connections in the make-up of a wooden pole. That the above good qualities have been largely instrumental in securing the excellent record of wooden poles in line work cannot be doubted by the analyst, and their lesson is well worth attention. The more permanent types of support may be divided into the rigid wide-base steel tower, the semi-flexible pole (either of steel or reinforced concrete), and the flexible steel pole or frame. Apart from their relative cost for any given line, there is to be considered the ultimate adaptability of each type. This adapta- bility will involve the questions of protective coating, rights-of- way, freedom from serious interruptions to service, and finally, and to the writer's mind of considerable importance, the relative prominence given the installation. In the progress of a rapidly growing industry there is always a tendency to apply methods of work to sections of the country to which they are less adapted than the locality of their previous successful use. The adoption of the so-called wind-mill tower may not be judicious in the densely populated districts of the East where climatic conditions are severe. In such regions, there are two considerations other than cost, i.e., the undue prominence of the line and the great importance of failures. The wind-mill tower is rather conspicuous, but it provides a type of support with which failures are practically confined to one span. Flexible- frame supports are not so noticeable, but they have little or no strength in the direction of the line and will therefore presumably be susceptible to a more severe type of failure. The semi-flexible pole or tower, occupying a position about midway between the two types of structures mentioned, has at least a theoretical ad- vantage over either. While some flexibility is useful in a narrow- base structure, to permit "pull back" by adjoining span wires, the amount of deflection need not be excessive. In actual service heretofore this movement cannot have been very great, for the reason that the commonly used attachments have not sufficient strength to transmit greatly unbalanced wire tensions. The desideratum is perhaps a certain elasticity rather than extreme flexibility. In fact, moderate bending or semi-flexibility is ob- tainable even in reinforced concrete. In sparsely "settled country or where the right-of-way is for any reason not accessible or not subject to cultivation, the spread GENERAL CONSTRUCTION 23 of tower bases is unimportant. If more than one high-voltage line is to be placed upon a private right-of-way, the separation of the lines will usually depend upon factors other than the spread of the bases. When land is valuable wide-base towers may be impracticable. For instance, there will probably be many power lines placed upon interurban railway rights-of-way. Such de- velopment is natural and necessary, but railroad rights-of-way do not provide space for wide-base construction. FIG. 13. Flexible A-frame. FIG. 14. Semi-flexible pole.' Wide-base towers and semi-flexible poles should, when properly designed, provide the maximum security against interruptions to service caused by insulator or wire failure. The greater strengths attainable in such structures allow the use of longer spans, with a consequent reduction in the number of insulators and of the probability of insulator failure. In case of wire failure, whether due primarily to insulator failure or not, the spread of 24 POLE AND TOWER LINES such failure along the line is arrested before it has influenced more than a span or two. The remaining factor of adaptability, i.e., the relative prominence of the various structures in the landscape, may prove of considerable subsequent importance. The writer does not mean to imply that a transmission line should be made FIG. 15. River-crossing tower. decorative, but rather that it be made inconspicuous. Even in regard to decorative effect, it is not absolutely necessary that it be an ungainly blot upon the landscape. Some attention to pleasing outlines is not amiss, for it is a well-known fact that GENERAL CONSTRUCTION 25 a gracefully designed structure is usually economical. In ad- dition to the question of appearance, if lines situated in settled communities are to remain undisturbed for any considerable period of time, they will have to be either unobjectionable in performance or invisible. The design of steel or reinforced -concrete poles and towers is fortunately becoming less hampered by demands for excessive cheapness, and the regulations current in other structural work are no longer entirely disregarded. The wisdom of this should be apparent when it is considered that a few hundreds or thou- sands of dollars " saved" on the line construction may jeopardize FIG. 16. River-crossing towers. the efficiency of an investment of millions of dollars. It is true that thus far existing construction has given fairly satisfactory service, but it is equally true that the more extended use of faulty designs would eventually bring disrepute upon the industry, and through failures invite the enforcement of severe regulations by various authorities. In any type of support the importance of eliminating long unsupported members and of providing a firm rigid base is now becoming more generally recognized. Location Plan. After the general location of a line has been determined from a study of maps and inspection of the ground, the prompt completion of the location plan is essential. The 26 POLE AND TOWER LINES rapidity of the compilation of this plan will depend on whether the line is to occupy, either entirely or in part, a strip of private right-of-way, highways, foreign rights-of-way, or pole-rights. In most cases, quite accurate preliminary data as to the plan view can be obtained from the right-of-way plans of properties such as steam or electric railroads, canals, highways, etc. When a private right-of-way has not been entirely secured, some changes in alignment may be expected, so the location plan is to that extent preliminary. Except for the desirability of having a correct permanent record, there is no particular need of determin- ing by accurate survey the exact distances between widely separated points. After the plan has been brought to a semi-final stage, the profile should be drawn upon the same sheet. In doing so, considerable future annoyance may be avoided by drawing a true profile and breaking the view at the corners, so that corresponding points will occupy their correct relative position in plan and profile, and both the center line and datum line will remain parallel to the bottom of the drawing. In some cases the distances in the profile have been measured along the inclined surface of the ground and then plotted horizontally, which results in a false profile and compels constant reference to plan and profile to identify a given point. It does not make a particle of difference in the excellence of a given section when completed whether the distance between the corners is 4000 ft. or 4050 ft. It is important, however, that the distance from small steep hills, etc., to one end of the section be accurately measured, so that the poles may be properly located to give the required clearance over the obstructions. The tentative, or paper, location of the supports can now be made on the drawing and scrutinized in the field by walking over the line. It is assumed, of course, that in the preliminary loca- tion reasonable care was taken to avoid natural or artificial obstructions including side hills, swamps, flood lands, or undue interference with other lines, and the use of private property. Almost invariably minor changes will have to be made to fit the paper location to the ground, -and local surveys can be made to plot cross-profiles at side hills, crossings, encroachments, etc. Some supports will have fixed locations, i.e., at the corners in the line, etc., so that the location of supports is reduced to distributing the poles or towers between the fixed points a series of short GENERAL CONSTRUCTION 27 locations. In a line having many changes of direction and eleva- tion, the previously assumed standard span length may seldom be used. The problem is then one of ascertaining the economic or desirable span length, not for a line in general, but for a given series of short sections having fixed ends and various intervening hills or other obstructions. 400' 17 250' FIG. 17. 300' In drawing the plan and profile, it will usually be found con- venient to use a horizontal scale of 200 ft. to the inch, and a vertical scale of 20 ft. to the inch. All obstructions, highways, crossings, etc., should be located in plan, and shown to scale in the profile. A sag and clearance templet should be made of tracing cloth, celluloid or even of thick paper, though the last is less con- venient as it is not transparent. Such a templet is shown in Fig. 17. The curve of sag is formed by plotting the maximum 28 POLE AND TOWER LINES sags of the given wire for various span lengths and usually for the condition of high temperature and no ice or wind load. Parallel to the sag curve, and at the distance of the overhead clearance below it, is the clearance curve. The templet should be extended to include span lengths well beyond the maximum span anticipated, in order that it may be used on hillsides. The method of applying the templet is shown in Fig. 17, in which it should be noted that the base of the templet has been kept horizontal and the templet itself shifted until the sag curve coincided with the points of conductor attachment on two supports, without causing the clearance curve to intersect the ground line. In case the standard height support in the assumed location will not permit this, the span must be decreased or the support be lengthened. CHAPTER II LOADING No detail of line construction has been the subject of such inaccurate assumptions and misstatement of facts as the con- ditions of loading which actual existing supports should or would withstand. Medium-voltage lines, located in regions known to be subject to heavy sleet, have been described and apparently designed on the assumption that no sleet load would occur, and that 15 Ib. wind pressure on the wires and 30 Ib. on the supports were proper assumptions. It has been stated that, in some instances, provision has been made for the supports to safely withstand broken-wire loadings which have varied from one wire to one-half or two-thirds the number of wires. Of this entire set of assumptions, that of one broken wire combined with a proper wind loading is often a more accurate statement of the actual strength of the existing structures. Fifteen pounds pressure on a No. bare wire is only 0.47 Ib. per linear foot, while 8.0 Ib. on J^-in. thickness of sleet on the same wire is 0.91 Ib. per linear foot. The pressure of 30 Ib. per square foot, on the tower corresponds to a wind velocity of 112 miles per hour, which is so excessive as to provide a little extra strength in so far as that con- dition is concerned. The maximum tension in the wires would probably be about 2400 Ib. per wire, and the usual pin insulators will not safely withstand such stresses. Furthermore, very few ties or clamps will dead-end a wire under such tension. Again, the crossarms used in certain of the lines under discussion would neither carry such unbalanced loads nor prevent a torsion which at less load would permit an insulator to incline and the wire to pull free. It is, therefore, evident that the assumed broken-wire stress could not be transmitted to the support. The writer believes that no consideration need be given accidental loads caused by falling objects such as trees, etc., and that a single ice and wind load will apply very satisfactorily in nearly every part of this country. It is true that in certain localities either a smaller or a larger loading may be justifiable, 29 30 POLE AND TOWER LINES and that some installations may warrant greater security than others, but these are questions for engineering judgment and should not influence general construction. FIG. 18. Snow-ice loads. Sleet. As it is hardly practicable to attempt the consideration of accidental loads which can be caused by falling objects, the only external loads to be considered are the ice and wind loads FIG. 19. on the wires and their supports. Severe loads of this nature are rare, and those producing very excessive stresses may be regarded as being in the category with tornadoes and similar LOADING 31 visitations which are beyond the limits of design. It has been shown by the records of the telephone companies, and is now more generally understood, that sleet loads may be encountered throughout nearly the entire United States, with the possible exception of certain restricted localities in the South and West. The maximum amount of sleet undoubtedly varies, but the effective variation of the combined wind and ice load is much less than is generally believed. Further, and neglecting the oc- casional extremely heavy deposits, it seems probable that a maximum thickness of 1 in. may be encountered. Experience has shown, contrary to the earlier assumption of many engineers, that sleet deposits will occur on wires carrying voltages up to 60,000 and possibly much higher. The heavier deposits are often of snow- ice and of less weight than clear ice, be- sides being more subject to removal by the sun and wind. Ice deposits, on the other hand, often remain intact even under a bright winter sun and a rising wind. It is extremely improbable that every span in any given line will ever be sub- jected to the simultaneous action of the maximum sleet and wind loads. The maximum sleet load, provided for in the design, is in itself a rare occurrence for any given span, perhaps happening once in ten years. Moreover, it is assumed that the sleet remains in place throughout the span and that the wind rises to a velocity which in itself should occur but two or three times each year during the winter months. It has some- times been specified that the thickness of sleet sho'uld be a factor of the diameter of the wire an assumption which is not borne out by the facts. Indeed the effect of the sleet load is far greater upon small wires, since their area and strength is much smaller, while the wind and sleet loading is only a little less than for larger wires. The following were reported, in 1915, as being the maximum loadings to which nine tower lines had actually been subjected in service, without injury. It should be noted that while the FIG. 20. 32 POLE AND TOWER LINES sleet load was usually measured, the wind load was not known and not reported. No. 1 no sleet, 2 in. thickness of snow. No. 2 2 in. thickness of snow. No. 3. . Y in. thickness of sleet and 2*4 in. of soft snow. No. 4 % in. thickness of sleet. No. 5 % in. thickness of sleet wind 35 miles. No. 6 1 in. thickness of sleet. No. 7 2 in. thickness of sleet. No. 8 2 cables parted during construction. No. 9 1 cable parted during construction. The most reasonable assumption for general use in regions where sleet is known to occur would seem to be a thickness of % in. all around the wires, combined with a wind load which will be discussed in the following section. Wind. It has been stated by some writers that it is necessary to know the probable direction of the wind and whether the wind and ice loads may occur together before the line may be designed. In ordinary broken country and with the usual changes in direction of a line, the former information, even if obtainable, could hardly be of great service. For example, it would be very bad practice to assume that the wind would blow in but one direction, and to use a structure incapable of resisting pressure from the opposite direction. The second condition mentioned contains a fallacious assump- tion in that it might be inferred that high winds and sleet will not occur together. For the vast majority of transmission lines in this country the sleet load, if there is ever sleet in the section in question, will probably occur during months in which high winds also occur. Again, it has frequently been claimed that high wind loads occurring during warm weather exert a greater effect upon the wires and their supports than the combined sleet and wind loads of the winter months. Disregarding cyclonic storms, which in many instances are beyond the limits of reasonable design, the writer believes the above claim to be entirely false and dangerously misleading. According to reports from the United States Weather Bureau, the maximum recorded wind pressures in many localities have occurred during the winter months. The combination of sleet and a moderately low wind velocity is greater in effect than the highest warm-weather wind pressures. LOADING 33 The Joint Report Specifications for crossings require a figured loading of % in. thickness of ice and 8 Ib. per square foot wind pressure on the ice-covered diameter of the wires. This loading was considered by the framers of the specifications as being generally reasonable, and with the designated factor of safety, FIG. 21. Conductors deflected by high wind. etc., to provide the proper construction for a crossing. It is not denied that thickness of ice greater than 0.5 in. or pressures of wind greater than 8 Ib. may occur, but it is improbable that they will occur simultaneously over large areas or so frequently as to make it desirable to impose a greater loading on all future 34 POLE AND TOWER LINES crossings. All of the spans in a given line would never be subjected to the maximum figured load, nor would a number of adjacent short spans, or even one very long span, be likely to receive the maximum load over every lineal foot. It is true that telephone lines fail every winter, and perhaps that some old or incorrectly built low-voltage lines occasionally fall, but the writer, has yet to learn of the failure of a single wire strung, even ap- proximately, to the Joint Report requirements. Further it should 16 12 10 sX 200 500 300 400 Span in Feet FIG. 22. Comparative normal sags of No. 1 wire for various loadings. be remembered that the loading and factors of safety in question and these must be considered in conjunction were recom- mended for crossings and for crossings only. As yet they have not been recommended by any authoritative body for general intermediate line construction. Some specifications have provided for a load of 0.25 in. of ice and 8 Ib. per square foot wind pressure with a stress limit of 0.9 of the elastic limit of the wire, while at least one crossing specification contains the severe requirement of 0.5 in. of ice and 20 Ib. per square foot wind pressure with a stress limit of LOADING 35 0.4 of the ultimate strength of the wire. In order to indicate more clearly the relative effect of various loadings and factors of safety, the approximate curves in Fig. 22 have been prepared to show the normal sag (at 60F. unloaded) of a No. 1, B. & S. gage, hard-drawn stranded copper wire under the following condi- tions : (1) ... . 5 in. ice + 20 . Ib. wind (120 miles per hour") max. stress, . 4 ultimate = 1580 Ib. (2) ... 1 . in. ice -j- 2.8 Ib. wind ( 40 miles per ho.ur) max. stress, . 5 ultimate = 1980 Ib. (3) ... . 5 in. ice -j- 8.0 Ib. wind ( 70 miles per hour) max. stress, . 5 ultimate = 1980 Ib. (4) ... . 5 in. ice + 8.0 Ib. wind ( 70 miles per hour) max. stress, . 6 ultimate = 2370 Ib. (5) . . .0.25 in. ice + 8.0 Ib. wind ( 70 miles per hour) max. stress, 0.9 elastic =2110 Ib. The records of the United States Weather Bureau omitting tornadoes, cyclones, and violent gales occurring in some par- ticularly exposed localities show a maximum indicated velocity of 100 miles per hour. The records at Bidston Observatory, Liverpool, England, covering the period from 1884 to 1888, give an actual velocity of 78 miles per hour as a maximum of 10 severe storms. 1 Table 2 shows the maximum velocities observed at a number of stations by the United States Weather Bureau. TABLE 2. MAXIMUM WIND VELOCITIES Observatory Period Maximum velocity indicated Observatory Period Maximum velocity indicated Chicago 111 1871-1906 90 Savannah Ga 1894-1903 76 Buffalo, N Y 1871-1907 90 Philadelphia, Pa 1872-1907 75 Galveston, Tex New York, N. Y Eastport, Me 1894-1903 1871-1907 1873-1907 84 80 78 Bismarck, N. Dak Boston, Mass Salt Lake City, Utah 1894-1903 1873-1907 1894-1903 72 72 60 Table 3 shows the three highest indicated velocities recorded each year by the United States Weather Bureau in its New York City station, during the period from 1884 to 1906 inclusive. This station was moved in March, 1895, from the Manhattan Life Insurance Building to the location at 100 Broadway; the latter is evidently in a more exposed position, as shown by the abrupt rise in, velocities after 1895. The maximum velocity of 80 miles per hour occurred during a sleet storm. 1 Extract from Overhead Construction for High-tension Electric Traction or Transmission, by R. D. Coombs, Transactions of American Society of Civil Engineer, Vol. LX. 36 POLE AND TOWER LINES TABLE 3. RECORD OF HIGHEST WIND VELOCITIES IN NEW YORK CITY Year Date Maximum velocity Date Maximum velocity Date Maximum velocity 1884 Oct. 18 44 Feb. 20 40 Dec. 9 40 5 Jan. 17 50 Dec. 7 50 Mar. 10 48 6 Feb. 26 64 Mar. 2 54 Jan. 9 44 7 Dec. 29 50 Nov. 16 48 Feb. 12 46 8 Jan. 26 60 Mar. 5 ' 52 . Mar. 13 50 9 Jan. 17 50 Feb. 1 48 Dec. 26 48 1890 Jan. 22 55 Dec. 17 48 Feb. 5 45 1 Dec. 30 53 Mar. 14 45 Jan. 11 44 2 Jan. 26 49 Mar. 11 40 Jan. 5 40 3 Aug. 29 54 Jan. 1 48 Oct. 13 48 4 Apr. 11 48 Oct. 10 48 Jan. 12 43 5 Dec. 27 73 Mar. 28 64 Aug. 4 62 6 Mar. 4 72 Feb. 7 65 Sept. 30 5fr 7 Jan. 18 60 Feb. 6 60 Oct. 17 60 8 Dec. 4 78 Sept. 7 72 Nov. 11 65 9 Mar. 20 80 Jan. 25 66 Feb. 27 64 1900 Oct. 16 76 Nov. 21 76 Jan. 26 76 1 Nov. 26 72 Jan. 19 72 Feb. 5 70 2 Mar. 19 74 Jan. 1 74 Feb. 2 74 3 July 2 72 Feb. 5 72 Sept. 17 65 4 Apr. 16 73 Sept. 15 68 Mar. 3 65 5 Dec. 10 64 Feb. 7 61 Apr. 10 56 6 Mar. 10 64 Jan. 6 61 ! Feb. 28 59 Table 4 is a record, by months, of the number of different 12-hour periods during which a maximum velocity of 60 miles, or more, was observed at the New York City station from 1895 to 1906 inclusive. Inasmuch as a maximum occurring late in one period and another early in the following period are both entered, a few of the entries represent the effects of the same storm. For the vicinity of New York City, Tables 3 and 4 indicate that: the maximum velocities occur during the winter months, when sleet may be on the wires; indicated velocities of more than 80 miles per hour will rarely, if ever, occur during the life of a given structure; and indicated velocities of 65 to 75 miles per hour may be expected several times each year, though much less frequently in conjunction with sleet. A rather complete study and tabulation of the U. S. Weather Bureau records of 43 observatories, located in 32 States, and LOADING 37 TABLE 4. NUMBER OF 12-nouR PERIODS IN WHICH WIND VELOCITIES OF 60 MILES PER HOUR OR HIGHER WERE OBSERVED IN NEW YORK CITY Month Indicated velocities, in miles per hour Totals 60 61 62 63 64 65 66 67 68 70 72 73 74 76 78 80 Jan 3 7 3 3 2 2 1 1 1 2 1 2 6 2 1 2 1 4 2 1 1 1 1 18 31 17 3 3 3 4 3 6 3 13 18 Feb 2 2 1 3 1 1 3 1 1 1 1 Mar Aur 1 May 1 1 1 1 1 1 July 9 1 1 1 1 1 Sent 1 2 1 2 3 2 1 1 1 1 Oct 1 4 6 1 1 2 1 , 1 1 1 1 1 1 1 1 1 Nov Dec Totals 26 8 8 9 18 10 9 2 5 4 10 3 4 4 1 1 122 covering periods of observation from 5 to 43 years results in the following summary 1 : Total number of sleet storms 487 Number with wind velocity over 40 miles per hour. .... 31 Number with wind velocity over 50 miles per hour 12 Number with wind velocity over 60 miles per hour 5 Sleet formation, ^ in. or less, during 90 storms Sleet formation, ^ in. to ^ in. during 62 storms Sleet formation, % in. to 1 in. during 42 storms Sleet formation, over 1 in. during 17 storms Total recorded =211 Temperature fell below F., after sleet deposit .... 3 storms Maximum recorded wind velocity and maximum sleet deposit occurring in same storm 19 storms Since the publication of Sir Isaac Newton's law for the pressures exerted by moving fluids which, for wind pressures, may be reduced to the form P = F 2 370 in which P = pressure, in pounds per square foot, and V = velocity, in miles per hour many investigators have experi- mented, with a view to the determination of values for the 1 Handbook on Overhead Line Construction, National Elec. Light Assoc. 38 POLE AND TOWER LINES constant, K. For normal pressures against thin flat surfaces, most of the results indicate values between and P = 0.0035F 2 P = 0.0049 7 2 (1) (2) These formulas, modified to apply to cylindrical surfaces, become P = 0.0021 7 2 (3) and P = 0.0029 V 2 (4) The Berlin-Zossen high-speed tests, in which wind pressures against trains were measured, gave the formula, P = 0.0027 F 2 and, using a rounded "nose" on the forward end, P = 0.0025 V 2 In Table 5 are given the equivalent actual velocities corre- sponding to those indicated by anemometer readings, and the pressures per square foot produced on flat and cylindrical surfaces. TABLE 5. WIND PRESSURES AND VELOCITIES CORRESPONDING TO ANEMOM- ETER READINGS Indicated velocities, mi. per hr. Actual velocities, mi. per hr. Pressure per sq. ft. on cylinders, P = 0.0025F2 Pressure per sq. ft. on flat surfaces, P = 0.004272 30 25.7 1.7 2.8 40 33.3 2.8 4.6 50 40.8 4.2 7.0 60 48.0 5.8 9.7 70 55.2 7.6 12.8 80 62.2 9.7 16.2 90 69.2 12.0 20.1 100 76.2 14.6 23.3 110 83.2 17.3 29.1 120 90.2 20.3 34.2 P = pressure, in pounds per square foot. V = velocity (actual), in miles per hour. Assuming an indicated velocity of 70 miles per hour, or an actual velocity of 55.2 miles per hour, the above equation for LOADING 39 obtaining pressures against flat surfaces becomes P = 12.8 Ib. per square foot of projected area, while for pressures against cylindrical surfaces P = 7.6 Ib. per square foot of projected area. On long spans, the maximum pressure at one point may be considerably in excess of the equivalent uniform pressure along the wire, while very short spans may be exposed to the maxi- mum pressure throughout their length. In view of the rare, if not improbable, occurrence of indicated velocities greater than 80 miles per hour, and the further improbability of such winds accompanying sleet storms, or of the sleet remaining in place, the following pressures seem to be reasonable for general use: P = 13.0 Ib. per square foot of projected area for flat surfaces; P = 8.01b. per square foot of projected area of wires covered with 0.5-in. deposit of ice. In applying wind loads to the supports, wooden, concrete or cylindrical metal poles should be considered as flat surfaces. By so doing the excess loading will compensate for the increased surface at the top of the poles due to arms, braces, insulators, etc. Latticed steel poles or steel towers should be treated as having flat surfaces equal to the exposed area of the members on the windward side, increased by 50 per cent, to allow for pressure on the leeward side of the poles and by 100 per cent, for wide-base towers. On the other hand some decreases in pressure would be justi- fied on the lower part of poles or towers, except when set on hill-tops. Broken Wires. In determining the proper wire loads and factors of safety to be used, it is important to bear in mind the effect of any further requirement such as a provision for dead- ending or carrying broken wires, inasmuch as the effect of the latter requirement is to impose from 5 to 40 times the former load- ing upon the insulator connections and the supporting structures. Dead-ending and corner-turning are different only in degree, and designing for a broken-wire load is more or less equivalent to designing all structures as corner structures. Fig. 23 is a graphical representation of the relative effect of what is termed balanced transverse loading and a broken-wire condition. The ordinates are the ratios of loads caused by one broken wire to the loads caused by balanced spans. In other words, the ordinates show how many times more severe a broken- 40 POLE AND TOWER LINES wire condition is than the load of the same wire unbroken under identical ice and wind loads. For instance, a No. 1 hard-drawn stranded copper wire in spans of 200 ft. would, for a broken- wire condition, impose 16 times the stress upon its support that it would unbroken. 40 38 36 . 34 32 30 "g 26 CI | 24 3 22 1 20 h 13 g 16 'S 14 'S 12 2 d 10 8 6 4 2 o, FIG. \ I \ \ \ \ \ \ $1 \ \ 0*$^ >od \ % > \ <*> % \ X ^ \^ ',, b ^X ^ x^ \ ^ \ \ x ^^ ^^ \ <^o ^"^ ^r ^ lt<1 "^ . Sf ^ ~~-~. .. 15 100 150 200 2SO Span in Feet 23. Relative effect of balanced and broken wire loads. In determining a mechanical factor of safety for any material it is customary, whether so stated or not, to assume certain por- tions of the factor as safeguarding each of the possible elements of danger, such as errors of design, workmanship, excess loads and deterioration of material. When using wire cables a rel- atively low factor may be assumed, since a wire catenary, both in material and as a structural member, is more uniform in sec- tion, strength and elasticity, and less influenced by eccentric LOADING 41 loads or errors of workmanship, than any other engineer- ing structure. Therefore, failure in the wires may be con- sidered as resulting usually from electrical causes such as arcs in the span or at the insulators. Assuming the provision of adequate clearance and proper spacing of wires in the span, the majority of wire troubles should occur at the insulators. Further, and in view of the small number of failures per in- sulator in the existing installations, it would seem that the in- creasing tendency to improve the insulation should have some effect in lowering the number of broken wires per support. In consideration of the above it is, in the writer's opinion at least, utterly indefensible to assume a severe broken-wire con- dition in designing all poles and towers. Particularly is this true if future construction must actually accord with specifications. There can be no engineering justification for a specification which premises a large proportion of wires broken under full load, when the devices fastening the conductors to the sup- ports would not withstand any considerable part of such a load. Again, it is probably a fact that many of the structures in exist- ing lines are not as strong as the preliminary test structure. This may be due to a variety of causes, such as local injury or in- cipient bends in light sections, lack of rigidity in the founda- tions, weakness in torsion, and last but not least, the usual difference between test specimens and the least perfect field product. In working backward from the results of practical expe- rience over large areas, the tendency is to overestimate both the actual loading and the strength of the structures. In other words, many existing lines, particularly the heavy wooden pole lines, remain in service without failing not because they have a strength equivalent to some recent requirements, but simply because they have never been subjected to such loads. There- fore if a severe mechanical requirement is placed in a standard specification it must be assumed that designers will eventually be driven to literal compliance therewith, and the net result will not be equivalent to the designs of the transition period upon which the requirement is supposed to be founded. The assumed load on the wires should equal as nearly as possible the maximum load that may be expected on some indeterminate number of spans during some indeterminate interval of time. One or more spans in a given line may con- 42 POLE AND TOWER LINES ceivably, and properly, receive a greater load during their life- time. Such excess loads may, or may not, be harmful, depend- ing on the factor of safety. A large factor of safety will un- doubtedly continue to protect inaccurate assumptions of load- ing, but the use of unreasonable loads and impossible stresses does not establish wise engineering standards. Bridges and buildings are not designed to withstand tornadoes, nor need power wires be absolutely immune from failure. It is, however, becoming more and more important to provide continuous service and to establish a standard which will satisfy all conflicting interests without unduly burdening a great industry. The difficulty in specifying a sliding scale of broken wires is that in reality only one or two wires may be reasonably ex- pected to break, whether there is one circuit upon the structure or one dozen circuits. If there are only three wires on a pole, the load requirement of one broken wire is relatively greater than the requirement of several broken wires on a structure of many wires on account of the pullback of adjoining spans. Again, if broken wires are to be considered, the wire con- nections must be designed to withstand a broken-wire loading, otherwise the broken-wire load could not be transmitted to the support. To allow pullback in a consistently designed line is correct both theoretically and practically, but its accurate computation is quite difficult, and the inclusion of such a condition in a general specification is probably inadvisable. It appears, there- fore, that a broken-wire load should be applied to the arms and wire connections, but that its application to the supports may depend in a measure upon the character of the supports. CHAPTER III WIRES AND CABLES The qualities desired in electric-service wires, in so far as the construction is concerned, are mechanical strength, tenacity, and ability to resist corrosion or other deterioration. In ordinary practice, the breaking strength required for a wire of a given span will depend entirely on the sag, because increasing the sag will reduce the wire tension approximately in proportion to the sag. Practical considerations, however, indicate a rather in- definite minimum, below which it is undesirable to go. Any surface injury, such as local pitting by arcs and nicks caused by careless handling, or any weakness in the material due to errors in manufacture, will have relatively greater effect on a small wire than on a large one. Moreover, such faults are more serious in solid wires than in stranded cables, and in hard-drawn wire than in soft-drawn wire. In stranded cables, an injury to a single strand affects only a fractional part of the entire sec- tion; in hard-drawn wire the surface, or skin material, has ap- proximately twice the unit strength of the interior mass, so that an injury will have a relatively greater effect on hard-drawn wire. Fortunately, however, the process of wire drawing in- sures a large amount of work per unit of mass, so that the finished product is a very homogeneous and trustworthy material. This quality, combined with the reduction in stress resulting from any increase in sag caused by stretching, explains the com- parative immunity from mechanical failures. Copper. The good qualities of copper wire are a matter of common knowledge, and as stated previously, its manufacture and method of use combine to make it almost unique as a ma- terial of construction. It is fairly immune from corrosive action, as ordinarily used in transmission-line work, although it is not absolutely indestructible. The principal sources of injury to copper are due to its softness and low melting point. The former renders it liable to nicks or broken strands in stringing and clamping, and the latter to burning by arcs. 43 44 POLE AND TOWER LINES Unless the voltage is such that insulated or weatherproof wire affords some real protection, there is no logical structural reason for using it. Otherwise, it merely serves as an additional load and offers a greater diameter for sleet deposits, besides deteriorat- ing far in advance of the rest of the construction and frequently hanging in unsightly streamers. Since the sheen of freshly strung copper is greatly lessened after exposure, it becomes problematical whether the attention of the casual observer would in reality be attracted more by the copper or by the size and spacing of insulators which cannot be disguised. Copper Covered. A comparatively recent development in transmission-line wires is the use of a steel wire covered with copper. This product is produced by drawing out an ingot of steel which has been previously encased in a copper covering. The thickness of the shell of copper may be varied within wide limits, the usual commercial 'proportions being an amount of copper that, combined with the lower conductivity of the steel core, produces a wire having either 30 or 40 per cent, of the conductivity of a copper wire of the combined gage. Thirty per cent, copper-covered wire is about 5 per cent, stronger than 40 per cent, wire of the same gage. The thickness of the shell of copper is quite small, depending in part on the size of the wire. Such wire should, therefore, be handled at least as carefully as copper wire. Since the thickness of the copper decreases with the size of the wires in the cable, it is preferable, at least for overhead ground wires, to use the 40 per cent, grade or else to use cables of few strands. The steel from which the wire is drawn is a high-carbon steel having an ultimate strength of about 90,000 Ib. per square inch, and a correspondingly high elastic limit. During the process of copper coating and wire drawing, there is an annealing effect followed by hardening, the net result being to produce a wire having an ultimate strength not greatly below that of the origi- nal ingot material. In general, and with the grade of steel com- monly used by manufacturing companies, the ultimate strength of copper-covered wire is from 20 per cent, to 40 per cent, greater than that of the corresponding sizes of hard-drawn copper. The principal uses of this material in transmission, work are for overhead ground wires, telephone wires, and the power wires of the lighter and lower capacity lines, where little future growth of business may be expected. WIRES AND CABLES 45 Aluminum. Aluminum wire, as now used for power-line pur- poses, is usually employed in the form of stranded cables, and when so used is no longer subject to some of the troubles incident to the earlier installations. As a material, it is quite different from copper, although used for similar purposes. Therefore, in making price comparisons, it is necessary to consider not only the price per mile per unit of electrical rating, but also the changes in the general construction of the line. The conductivity of aluminum is about 60 per cent., based on the Matthiesen standard for copper, making aluminum cables about 1.5 times the area and 1.25 times the diameter of copper cables having equal conductivity. As the specific weight of aluminum is about 0.33 that of copper, the weight of aluminum cable will be about 0.5 times that of copper cable having the same conductivity. The strength of aluminum is about 0.8 that of soft copper and 0.4 the strength of hard copper. The net result of these differences is best shown by a concrete example. Thus, a No. 00 aluminum cable has about the same conductivity as a No. 1 stranded hard-drawn copper cable, and their other characteristics for a 400-ft. span are as follows: Xo. 00 aluminum No. 1 copper Breaking strength, pounds 2500 3600 Elastic limit, pounds 1460 2180 Wind pressure on ice-covered diam., Ib. per foot.. . Weight of ice-covered cable, Ib. per foot Resultant load, Ib. per foot.. 0.943 0.691 1 168 0.885 0.770 1 173 Maximum wire tension (factor 2 0), pounds 1250 1800 Transverse load, per wire, pounds 377 354 Normal sag 20 ft. 10ft. Maximum sag. . 21 ft. 12ft. Since the coefficient of expansion of aluminum is considerably higher than that of copper, aluminum cables are more affected by temperature changes. Relatively greater sags will therefore occur in hot weather, while at low temperatures there is a greater increase in tension due to the contraction of the material with its resultant decrease in sag. In addition, the lighter weight of aluminum renders it more liable to local displacement by wind pressure. It is necessary, as a result of these differences in the 46 POLE AND TOWER LINES material, to provide greater pin separation and overhead clearance for aluminum conductors. From a construction viewpoint, however, the lighter weight of aluminum does not possess any particular merit, except possibly greater ease of handling the reels and pulling out wire in stringing. The saving in dead load on the supports is negligible and is more than offset by the greater sag and separation required. Further- more, the increased diameter imposes a greater wind load. The choice between aluminum and copper will depend on the relative cost of the two materials and their accompanying con- struction, together with the allowances which can be made for the scrap values of the two installations. Steel. Steel cable can be obtained of almost any desired unit breaking strength, the commercial grades ranging from the low grade steel of guy wire, which has an ultimate strength of 60,000 Ib. per square inch, to steels of 200,000 Ib. or more. For transmission-line purposes steel cable is used chiefly for overhead ground wires or as power wires for very long spans. The occasional use of steel messengers for telephone or insulated cables does not involve any considerable quantity of such material employed on typical transmission lines. Steel cables for line work should always be galvanized, and should be larger than is actually required for strength. The galvanizing of cable is by no means as permanent a protec- tion as the hot-dip, unwiped process applied to structural steel; therefore some allowance should be made for future corrosion. Despite the temptation to use small-gage cables made of the higher grade steels, on account of their greater strength, it is generally preferable to adhere to medium grades such as the Siemens-Martin. Guy-strand steel cable is the lowest com- mercial grade and its quality is relatively much lower than that of any of the higher grades. Large diameter cables, particularly of high-grade steel, are rather difficult to handle as they are very stiff. It should be noted that all cables of great strength require special clamping attachments for dead ending, the ordinary quota of clips and clamps being inadequate to transmit the tension. Telephone Wire. Supporting a telephone circuit on long-span transmission-line structures introduces a difficulty, in that the small wires which are sufficient for telephone service do not have the mechanical strength to carry safely in long spans. WIRES AND CABLES 47 It is practicably impossible to string any ordinary telephone wires so that they will be reasonably secure on long-span lines. There have arisen, therefore, two general methods of procedure; one is to use larger and stronger wires; and the other to con- template failure in the telephone circuit as a necessary evil. Solid steel wire No. 6 BWG, sometimes called river crossing wire, has an ultimate strength about equal to that of No. 00 stranded hard-drawn copper and can, therefore, be strung equally well in long-span lines. Catenary. If two ends of an imaginary wire having perfect flexibility and uniformity of material but no ductility were supported at two points in the same horizontal plane the wire would take the shape of a curve known as the catenary. For all practical purposes it may be assumed that actual wires and cables possess the same characteristics. The curve of the wires between the supports is therefore known, if the span and sag are known. Inasmuch, however, as the equation of the catenary is rather complicated, while that of the parabola, which closely resembles it, is simple, the latter is usually employed instead; thus, y* = ax "a" being a constant found by substituting the known value of x for the point on the curve at the support, i.e., S 2 a = j-j in which S is the length of the span and d is the sag. Assume a uniform load on each lineal foot of the span, and imagine half of the span removed and the wire held in place by the tension T at the middle of the span. Then considering the moments about the remaining support we have the weight of the half span multiplied by its lever arm which is one-quarter of the span, equals the balancing force T multiplied by its lever arm d. Since the total weight W = the weight per foot, X the half span TI7 WS orTF = T we have fxf- Therefore 48 POLE AND TOWER LINES and 55 ' Sd or, the tension in the wire equals the weight per foot times the span squared divided by eight times the sag. It is necessary, however, to take into consideration the effect of a change in length of the wire due to temperature and loading, and a simple arrangement of formulae in which this is done is given below. The following mathematical treatment is not new, 1 but the writer has found the arrangement convenient: 5 = span, in feet. d = sag, in feet. W = load per lineal foot in plane of wire. A = area of wire, in square inches. E = modulus of elasticity. 6 coefficient of expansion. t = change of temperature, in degrees. e = elongation or change of length, within elastic limit. Lo = length, in feet, of imaginary wire (W = 0) at normal temperature. L oc = length, in feet, of imaginary wire, cold (tF. below normal temperature). L oh = length, in feet, of imaginary wire, hot (tF. above normal temperature). Index to Subscripts. No subscript = normal conditions. c = cold: F. below normal + dead load. t - = cold : ice load + dead load. ew = cold: wind load + dead load. iw = cold: ice + wind + dead load. h = hot: tY. above normal + dead load. WM is the resultant of the vertical dead + ice loads and the horizontal wind load. W cw is the resultant of the vertical dead load and the hori- zontal wind load. 1 Overhead Construction for High-tension Electric Traction or Trans- mission, by R. D. Coombs, Trans. Am. Soc. C. E., Vol. LX (1908). WIRES AND CABLES 49 Stresses. Substitute normal values in Eqs. 1, 2, 3, and 4. Assume values of T h , T iw , T c , T iy or T ew , such that Eqs. 5 and 6 will give identical values of d h , d iw , etc. The tension that will give the same sag by Eqs. 5 and 6 (independently) is the tension resulting from that sag and the given loading. ' (2) Lo = L - e (4) (ZF. above normal, with dead load.) L oh = L (l -f Bt h ) e h = L h = L oh + e h d h = 0.612\/,S~(Z A - S) (5) , W h XS* dh= ~WT (t~F. below normal, with dead + ice + wind loads.) LOC = L (\ Ot c ) iw = ^Tj Liw == L oc ~\~ iw d iw = O.Ql2Vs(L iw - S) (5) , W iw X S 2 diw= ~ST~~ (tF. below normal, with dead load.) L X T L oc = L (l dt c ) e c = C g L c = LOC + e c d c = 0.612V / ^(L C - S) (5) W c X S* dc = ~wr~ ( ' 6) (tF. below normal, with dead + ice loads.) Lv T. Li i -i r\i \ oc ^ * T T I <,c = L a (l et e ) di d di i = 0.612s(Li - S) (5) Wi X S ' 50 POLE AND TOWER LINES (tF. below normal, with dead + wind loads.) X J- cw L oc = L (l etc) e cw = EA L or I c/cto d cw = 0,612V S(L CU , - S) W cw X S 2 dr.,:, = 8T e (5) (6) In Tables 6 to 13 are given the physical properties and the wind and ice loads for various wire gages.* TABLE 6. PROPERTIES OF WIRE MATERIAL Ultimate strength, Ib. per square inch Elastic limit, Ib. per square inch 1 Modulus of elasticity, E Coefficient of expan- sion, per F. Copper, solid soft-drawn Copper, solid med.-drawn 32 to 34,000 40 to 50,000 16,000 22 to 27,000 14,000,000 15,000,000 0.0000096 . 0000096 Copper, solid hard-drawn 50 to 60,000 30 to 35,000 16,000,000 . 0000096 Copper, strand soft-drawn Copper, strand med.-drawn Copper, strand hard-drawn Copper clad, solid, hard-drawn.. . Copper clad, strand hard-drawn. . Aluminum, strand 30,000 45,000 55,000 60 to 90,000 70 to 90,000 23 to 24,000 75 000 15,000 25,000 33,000 35 to 53,000 41 to 53,000 14,000 8,000,000 10,000,000 12,000,000 21,000,000 18,000,000 9,000,000 25 000 000 0.0000096 0.0000096 . 0000096 . 0000067 0.0000067 0.0000128 0000066 150 000 25 000 000 0000066 180 000 25 000 000 0000066 Steel solid ex-high-strength 187,000 29,000 000 0000066 It has been urged by some that using the 0.5-in. ice and 8-lb. wind load with the parabolic formula for computing the stress in the wire does not give results which accord with experience, since actual spans erected with less than the specified sags have not failed in service. On the other hand, it is sometimes claimed that allowing maximum stresses near the elastic limit is danger- ous. The facts of the matter are: First, the true catenary formula is scientifically and mathe- matically correct within the elastic limit. Second, the para- bolic formula, ordinarily used for simplicity, gives results which in the vast majority of cases are closer to the exact values than the actual wire stringing will be to the specified stringing. Third, the material of a wire catenary is more uniform in section, * Tables from R. D. Coombs & Co. design standards. 1 The elastic limit used is in reality the yield point, or point of appreciable extension, as this value seems more applicable to wire stringing than that obtained by accurate laboratory tests. WIRES AND CABLES 51 strength, and other characteristics than that of any other engi- neering structure; therefore the error of design is correspondingly less. Fourth, the stretch of a ductile material, such as copper, permits the sag to increase and the stress to decrease and, within limits, does not perceptibly decrease the cross-section at any 1 42 Span in Feet 00 200 300 400 500 600 7C K) 800 900 I / 7 I 42 40 38 I 7 38 3C 34 32 30 28 26 ?24 .622 18 i i / 1 1 i I / / 34 32 30 28 26 22 a 18 16 14 12 10 8 6 4 2 i I 1 1 / i 1 1 / ' i / / 1 i / / 1 1 i / y \\ * 1 i < i * i $ // 7 / 7 / / / / 1 / / / 14 12 10 8 6 4 2 1 / 1 / i '/ / 1 / / / / / / y / / / / / ' / / , / / / / 7 / ' / /^ / . / ^ / ^ y ^ ^ ^ 30 200 300 400 500 600 700 800 900 Span in Feet FIG. 24. Sags and tensions of No. 1 H. D. copper. point. Therefore, a loaded span stretches enough to relieve the stress and does not fail unless the load is very excessive. Fifth, the specified maximum loading is an emergency loading and is not a general or frequent occurrence on any span or line. The facts of the matter are that the spans in service 52 POLE AND TOWER LINES have either not been subjected to loads in excess of those re- quired by the catenary formula to develop their elastic limit, or the wire has stretched and the sag has increased. Pos- sibly there are two other reasons for seemingly overstressed lines giving satisfactory service. One is that the poles have bent 100 34 200 Span in Feet 400 500 700 800 "v A /y 900 144 100 200 300 400 500 600 700 800 900 Span in Feet FIG. 25. Sags and tensions of No. H. D. copper. or the wires slipped through the ties, thus temporarily increasing the sag. Second, the wires may have become tempered or hardened by tension, possibly by atmospheric changes or other action, and by becoming harder have been able to sustain a greater load. WIRES AND CABLES 53 100 200 300 c22 18 16 14 12 Span in Feet 400 500 600 700 800 900 */ -v/ Lit 100 200 300 FIG. 26. Sags 400 500 600 700 800 Span in Feet and tensions of No. 00 H. D. copper. 54 POLE AND TOWER LINES 188! !8 iOSCOCOO COC5GOHHO b- rH 00 1C JrHiMiOt^ rHCOrHrHO OOb-INiCC OSHHOOIN t^rHOCiCIN 00 1C CO O C a$tDQO* od- CO O2 CO CO O OOCOiC^CO iCCo'rHOiVr JQCOW'eOei' (N'rHrH'rHrH' "S 15 ,5*8 901 -u + - - qi-o'8 -q t C8.S Oo rH O O O>O t^ O O C^ CO lOCCCOi I0t>0 (Ni-Hi-H l CO COOOOiOi-H ooooo oo O rH CO rH 00 iCCOOOOirH l- CO 1C *' *' 1>CO -^ COl> >O CO GO O tH (N CONOCO OOO CO IN > STt< O3 1-1 t ^(M^H (N O C ^ - CD COlO >C 1C CONNMt-i .-i ^H ^H rH O OOOOO OO 1C05TJHCOO CO 1C *' CO CO >C OO CO O3CO IM Tf O2 CO 00 CO iO CO (NO CD(N CO (N t (N t^ 1C t-i O5t>. CO 1C- rHrH^-trH OOOOO OOOOO COt^t^ COt^ OOOOO Tt rH OOOOO C CO COCO (N (N i-( I-H OOOOO COrHrHCOOO t^GCOOGOOO " CD OOOO r-l >C O 1-1 fOSrHCOCO OOOOO O5 00 *C O CO C rH O CO CO O 00 CO >C N-O 00 * OS rH 1C 00 (NIC iC--* CO COIN OOOO OOOOO OOOOO O< <>CCOI>00 000000>CTj< TJ< GO (N CO 00 CO ( iC >C >C>C O5 CO * >C OOOOO OOI>t>COiC OOOOO OOO300CDCO OOOO5O5rH OOOOO O OOOCO^OO COI I CO IN IN i-H rH rH >OO( I t^ Tf< < OOOOO OO cot^^irioo i>co O3 CO 00 M< rH O3 (NOOOOlrH co"iCr)Tco"'co"' 00 t^ CO 1C * OOOOO l-~O * >C T}( O5 00 rH C O5 CO OOOCOOO O CO rH Ttl COCOINrHIM COO COO OO CO 1C HH CO CN IN C005COCOO oo OOOOO CO CO (N CO O OOOOO 00 OOCOfNOlN CO-* OCOH rH OOOo'd OOOOO O Q' Q' O O O O 000 O O *C ooooo *C O ^C O *C t^ iC <*< ^ CO ! a; ^ ' G = 1C 53 ub N. ado' PQ O WIRES AND CABLES 55 a .2 r + - aoi - + -bs 301 'Ut-C'O ' ill -O 2 saqoui ' Ofl 0OCOO O05XO3 O Tf O CO r-i Ol t iO COI>f~CO "5-HO5IM Ot^CO-* OCX-HCi ^OSWSC^ O5t>-CO'<* 00 N CO i-HCOO'*' COO O l^ C^ >OCOCOC I-H co M o (NOOOO t^COCOO U3-* I-H I-H d d d d d d d d r- ^ c rt n - -c c ~ rf O O N >O O O IN O b- COC-*CO (NO T}< CO o co oo COOO-*.-! Ot^iO-* CON gt^l"- CO Oi-H O W t co-j< oo x ^H o rf O 10 cooo*3< oocoooco co- COQOi-iCO CJt^O^ 1-1 ( 00 CO tO * CO IN 0 O >0 O 10 O tC- O i * 00 O IO'^'CO'IN" NrHi-HiH > 1C 10 D o OOiOO iC^CO 2221 8888 S o' o' d d d dod p p_ ^* ~ i-i 5 >"o CO 5 io Tf (N o'o'do oodo oo 1 3 p ' 10 o -* i 'S'S'S 56 POLE AND TOWER LINES ii O O O O O O OOOO OOOO O O O O O OOOO OOOO I s - t^ Oi T~I CD O^^t^-O OOt^C^ & 3 ^ of I-H" o" od *o" CO t>T ^"^00^ TjJ'oo^Hic' CO t- ** ^ ? OlOOCO^ 00t^05 1C rH 00 "^ i-H I>- ^i-HOit^- ^CTfCCC^ co co" of - IN 00 *}< "3 C^I-^X(N lOCOOOO <> 00 i-H CC 1C l*~ T-tCOCDrH OINOS'O t^ IT! CO lO * 00 CQIMi-HrH OOO5O5 M < + 2 5 a 6'Jb H>> i-tQ -4-J o "B c o N +J fi ' - T 05 ^H O 05 i-H 10 OSCOCOIN lOrHr-lrH O 00 >O I-H 00 ^ ^Ht>.^^H OOCDT}<(N C^ i 1 i i I-H O O OO5O5O3 OOOOOOGC 1 O w Ho Q06 G I **" D 1 < -a +.i 'O C 12 Q *" 6 Vertical +J | O 10 O >0 CO Tj< i-i U5 -H J>. (NOOC^Tf 00 O CO LO t^ O5 00*00200 O5"OOOIOT}< ,_, rt rH r-l O O OOOO OOOO I O * 00 -O^CO * * CO 00 N O iO O O >O >O OiO>OO OOOO O 1-1 C<1 00 <* iO OOi-HOO5 iOlNO5t>- 05* 00* t-* CD* 10* rjT 00* 00* (N* ^H* T-H" r4" H T}< 00 00 1-1 O5 ^ i-tOJOOO OOCOOO (N O ifl O -OOO C Tt (N d -oo dodo 000'09 g-s O_"3 OO 00 00 o -S ss 6 d 3*3 .8 II . . . 'S'3'S Jl g till ^ Bq E = 16,000,000 copper ' c* TjTcsfoo" .<*OiO^< .CO-i-l-l ; ; ; ; ; *> | 1 pBOT a IN ^ O - >0 <}< OSOOOO odd tf 9 a : : : : : lineal fc orizont 331 ui-c-d + -qi-0'8 tcoo t*.*w t~t~t~ ooo 1 m : : : : h^l 301 ui-e^-o + Pa<3 O iO O : Soo So ' ooo J V 30t Ut-C-Q + P3Q . ll Jj cqot^co :S dooo M o 3 S-S eg 5 > 8.s ooooo O O CC W^f ooot^t^ t aa-Cg W i-l Ot<-n-i-i C 2 a a IB ooo X(Mt^ (N_O 10 - i j '-H >cq -3=3 o^ ooo o RSS S ^ 6* S-51 s aJ **: ^ -^< oo 10 n issll o' o o o o 6 .5 00 O O> O5D-*^O Q ooooo i o odd d >>' ^' CSffiCQ - r r. N ft ft oo i! ' J .0 - d-S 1' J+ + 'S'S'S ^^^ ii ii n III < o 58 POLE AND TOWER LINES 1 C 7 I II I Copper Wire Normal Sags, (60 F. no Ice or Wind) Factor of Safety, at Max Load =2.0 Max. Load=^'lce+8.0 Lb. Wind 0F. Span in Feet FIG. 27. Normal sags, 1 copper wires and cables. 'Overhead Line Construction Committee (N.E.L.A.). WIRES AND CABLES 59 . \. 00 *000 0000 Stranded Aluminum Wi Normal Sags,(60F. no Ice or Factor of Safety, at Mar. Loa Max. Load-J^'lce+8.0 Lb. Wl re Wind) d-2.0 jid,OF. / t / / / / / / / / , / / y / / / / / / / / / / / / / / / / / / > / / ' / / / / / ^ / / / / / / ' / / / ' , / / / '/ / / / ' / / / / /\ ' / // / / sS / s ^^ ^ iH m ^ s ^^ri \ Span in Feet FIG. 28. Normal sags, 1 aluminum cable TABLE 13. PROPERTIES OF WIRE MATERIAL (From 1911 Report of Overhead Line Construction Committee of National Electric Light Association) Ultimate strength per sq. in. Elastic limit Modulus elasticity, E Coefficient of expansion Copper, solid, soft-drawn 32-34,000 28,000 12,000,000 0.0000096 Copper, solid, hard-drawn 50-55-57-60,000 30-32-34-35,000 16,000,000 0.0000096 Copper, stranded, soft-drawn . . 34,000 28,000 12,000,000 0.0000096 Copper, stranded, hard-drawn. 60,000 35,000 16,000,000 0.0000096 Aluminum, stranded 23-24,000 14,000 9,000,000 0.0000128 Steel, stranded, Siemens-Martin 75,000 29,000,000 . 0000064 Steel, stranded, high-tension. . . 125,000 29,000,000 0.0000064 Steel, stranded, ex-high-tension 187,000 29,000,000 . 0000064 Overhead Line Construction Committee (N.E.L.A.) CHAPTER IV DESIGN Since it is impracticable to include herein a sufficient explana- tion of the laws of mechanics or the theory and practice of structural design, to enable the inexperienced to acquire even a reasonable facility in their use, no detailed exposition thereof has been attempted. The computation of stresses, and the determination of sections for such structures as steel towers, requires a working knowledge of subjects already covered by various text-books. However, there are a number of general conditions in which transmission-line work does not follow the 'V* / fr fe ~ FIG. 29. Relative strength of telephone and power lines. accepted standards and methods of other structural design, and a discussion of such matters should be of value to otherwise competent designers whose experience has been obtained in a different field. Factors of Safety, Etc. If a given line is to be designed in a logical manner and with a minimum of "cut and try" methods, the first step is the assumption of the various loads and factors of safety. These assumptions will enable the designer to men- 60 DESIGN 61 tally predetermine, to some extent, the general nature of the supports, or at least to narrow the field of choice. In a broader sense, it will also limit the choice to one kind of material for the supports, since a wooden-pole line cannot have a total factor of safety equal to that possible in steel construction. The first and easiest factor of safety to assume is that for the wires. It is the easiest to assume, since once chosen it can be maintained without much effect upon the type of support. Again, a reasonable factor in the wires will have only a beneficient effect upon all the remaining construction. Further, there is a more general consensus of opinion regarding this assumption than on any other element affecting transmission-line construction. The literal expression of various wire loads and factors may appear quite dissimilar, but the ultimate result when the wires are strung presents a fair average. Unbalanced loads and factors of safety have been very common, and they are greatly to be condemned both as a misstatement of fact and as providing an excuse for future errors of design. Sleet may be encountered, during the probable life of a well-built transmission line, in a great many sections throughout America, even in the South. Moreover, it may be desirable to provide a sag corresponding to the standard sleet load, even in non-sleet regions, because such sags decrease both the normal and the maximum loads on the line and, in general, produce a line which is well able to distribute and equalize excess stresses. In general the "J^-in. ice plus 8.0-lb. wind" loading is logical in origin, as shown by the writer in 1908; 1 further it is more universally accepted than any other. A factor of safety in wires of 2.0 is approximately equivalent to a working stress of 0.9 of the elastic limit, and so is not too conservative when errors of stringing and possible reduction of strength at splices are considered. An exception may be made to the above loads and factor in the case of very long spans, in which the load may be consistently reduced about 25 per cent. Thus in designing one line with 800-ft. spans, the writer believes that his use of a 6.0-lb. wind pressure was logical. An additional exception to the above factor, if not to the load, is the short-span distribution lines in city streets. Such lines are usually well sheltered, designed for low voltages, heavy and numerous wires, and better guyed 1 Proceedings American Society of Civil Engineers, 1908. 62 POLE AND TOWER LINES when the lateral restraint of wires and the guys at street corners are considered. As proven by actual experience with many thousands of miles of such lines, they may consistently be given a factor of one (1) with the above maximum loading. It is true that many transmission lines have been built with tighter stringing than recommended above only a few having had more conservative values but as a general statement, slack lines are safer. If the number of wires will permit liberal separa- tion of the conductors, a little increase in the sag will often help out in the design of the supports. It seems probable that for every span in which slack stringing (and improper separation) have caused accidental contacts there have been three cases in which tight stringing has broken pins, insulators or supports. After making the foregoing assumptions there still remain to be determined certain loads and factors for the supporting structures. The specified wire load applies also to the supports, but in addition there is the broken- wire load to be considered, as well as the factor, or factors, for the poles or towers. Broken-wire loads are dis- cussed in more detail elsewhere (pages 39 to 42), the writer's recommendation being that for transmission lines the effect of one broken wire should be combined with the above-mentioned ice and wind loads, while for short-span city distribution lines broken-wire loads may be neglected. While the requirements just mentioned would literally apply to insulators and pins on all supports, it is impracticable and un- necessary to so interpret the broken- wire load. It will, there- fore, apply only to insulators and pins at corners, dead ends, crossings and special points. On intermediate tangent poles single-pin insulators and tie wires may properly be used, although they may not always have the required broken-wire strength, and the broken-wire effect on the pole might be considered as the equivalent result of one broken wire or of several un- balanced wires. Owing perhaps to unfamiliarity with the structural questions involved in the construction of transmission lines, engineers have, until recently, specified the test loads which sample towers or poles must withstand. Dependence on a large, and more or less certain, factor of safety to cover uncertain design, however, should have no permanent place in line construction. On the other hand, the theory of this practice, i.e., that of working to known ultimate strengths, has much to commend it. Moreover, DESIGN 63 tests of sample towers have been of considerable use in adding to our somewhat meager store of data on the ultimate strength of columns. Unfortunately for the entire success of this procedure, a test load is very rarely an accurate representation of the maximum which may be obtained in practice, nor is the condition of the test structure similar to that of many of the structures when installed. Test loads are almost always applied regularly and slowly, and in many cases uneccentrically. A test structure most assuredly will have at least a fairly good foundation, and be composed of members free from incipient bends or other defects caused by mishandling. It would also be very well bolted together and plumbed with greater accuracy than the average line structure. In general it may be said that an expert structural assembler should be able to obtain test loads quite noticeably in excess of the presumptive average strength of the finally erected structures. It appears, therefore, that the period of usefulness for this practice is past, and that competent designers should be able to produce structures which will have actual strengths much nearer their predetermined strengths than the actual loads will be to the assumed loads. If statically indeterminate frames are used, such as poles with incomplete web systems, design tests are necessary, but the transfer of stresses and the construction of efficient details are now so well understood that actual tests of determinate structures are in a measure a confession of ignorance. The failure of steel poles and towers has almost invariably been caused by the buckling of main compression members, and this may or may not be superinduced by inefficient bracing. Owing to the possible application of the load from the opposite side of a structure, line supports must have the same main compression section at each corner regardless of the tension stress. The com- pression stress per square inch in the main legs is, therefore, the first and most important determination. A secondary condi- tion which should be borne in mind during the calculations is that the section selected must be of a size suitable for the connec- tion of the desired bracing. Admitting that there is a difference in the kind of service or, in all events, in the number of applications of the load, between transmission and building work, there is a very marked variation in the attitude of engineers toward bolted connections in the two types of construction. In building work single bolts are dis- 64 POLE AND TOWER LINES couraged or, if used, low strength values are allowed. In towers, however, all joints are bolted, and then usually with one-bolt connections in which no reduction of value is assumed. The theoretical value of a one-flange connection should be reduced as the full strength of the connected member is not available ; this has not been so assumed in tower work. Again, the bearing of a bolt is assumed as being on a surface as thick as the member; in fact the bearing will be only on a line. The strength of a one-flange connection is approximately but 80 per cent, of the strength of the angle; therefore, since many tower joints are one-flange connections, a somewhat conservative unit stress should be assumed in the design of bracing. Further, if a member has one flange "blocked off," i.e., cut entirely away for clearance, or if the flanges are mashed together or flattened out, the strength of the member at that point is no longer the strength of an angle but of a flat. In addition, there is con- siderable likelihood that such blacksmith work may result in burning or cracking the material at the point in question. As a long slender member is not well adapted to withstand compression, it has been customary in other work to limit the relation of the length to the radius of gyration. In transmission- line construction, however, very much higher values of this ratio have been used than are generally permitted. It is probably not necessary to adhere to the low limits of building construction, but it is equally probable that too much latitude has been taken in some cases heretofore. The more recent designs of transmission line supports do not employ any castings in the main structure, although cast-steel or malleable-iron castings are perhaps properly applicable to wire connections. Similar reasoning should prohibit the use of castings for hoops or bands in reinforced-concrete poles. Transverse Loads. Before entering upon any detailed dis- cussion of design, it is necessary to consider briefly the forces acting upon a pole line and the character of service required of its component parts. As already stated, the function of the pole is that of a cantilever beam rather than of a column. The external forces are due to dead, ice, and wind loads, which, with the exception of the pressure on the pole, must be transmitted to the pole by the wires. The weight of the wires and their coating of sleet, together with the weight of crossarms, insulators, and the pole itself, DESIGN 65 is a vertical load which the pole carries as a column. The pressure of the wind on the wires, whose diameter is in- creased by the sleet, and upon the pole structure, is assumed as acting horizontally and at a right. angle with the line, and, 14- E -s --Si- Pole A FIG. 30. Transverse loading. Pole A Pi = Transverse load at ground wire = wind load per ft. of wire a (~^-\ n ) Pa = Transverse load at top power wire = wind load per ft. of wire b(- -\ -^ . Pt = Transverse load at lower power wires = wind load per ft. of wire &(o +~ p f- j- H- jo ^ ^ -^ "" X y X ^ ^ / / / / A / / / / ' / / / / / / / / / 10 20 30 40 50 60 70 80 90 100110120 130 140150 160 170180 Degrees FIG. 32. Corner pole loading. 1 Following the common practice in low-voltage construction, the statement is frequently made that at corners the neighboring spans should be shortened to minimize the stress on the corner structure. In the absence of further elaboration, and in view of the usual lack of expert advice during erection, it would appear that the above statement is thought to be self-sufficient. 1 R. D. Coombs & Co. Design Standards. DESIGN 67 This is decidedly not the case, since the stress to which the corner support is subjected is not reduced materially by shorter spans, unless advantage is taken of the short spans to increase the sag in those spans. This is due to the fact that the greater part of the load upon the corner support is from the tension in the wires, and unless this tension is reduced by slack stringing, there is no particular advantage in short spans. x With short-span low- voltage construction, an increase in sag of a few inches, made by the line foreman to "ease up the corner," will be inconspicuous and very efficient, provided that the tying-in of the wires is effective for some distance each side of the corner. Long-span high- voltage construction, however, requires a material change in sag and there is a redistribution of stresses to be pro- vided for, unless entire dependence is to be placed on the bending of supports and on the slipping of wires at the supports. Slack corner spans may have a maximum wire tension from 500 to 1000 Ib. less than the standard stringing; so if this reduc- tion is to remain effective the unbalanced tension must either be held by and at the adjoining supports or be carried back and distributed over a number of supports. It is needless to say that the standard stringing curve, in case one is provided, is not applicable to such construction. Further, it is useless to attempt to distribute unbalanced wire tension by means of a slip-shod single pigtail tie wire. Broken-wire Loads. In case the sags in adjoining spans are not adjusted so as to balance the tension in the wires each side of a pole, there will be an unbalanced pull in the direction of the line, which must be considered in conjunction with the vertical and horizontal forces first mentioned. Unbalanced tension may also be produced by unequal ice and wind loads in adjoining spans. Further, if it is assumed that all, or part, of the wires may be broken, then the poles must withstand a longitudinal force equal to the tension in the wires in the unbroken span. On the other hand, it can be shown by a rather complicated mathematical demonstration that, owing to certain properties of the catenary curve, a slight bending in a number of poles will balance the tensions in adjoining spans. This is due to the fact that the tension in a wire is greatly decreased if the span length is shortened while the length of wire per span remains unchanged. Vice versa, increasing the span length while the length of wire per span remains unchanged increases the tension. 68 POLE AND TOWER LINES If it is assumed that all the wires in one span are broken, then the first pole is subject to the unbalanced tension of all the wires in the unbroken span and bends away from the break. This shortens the next span length, decreases the tension in that span, and allows the second pole to be bent away from the break. Successive bending occurs in decreasing amounts, until a point of equilibrium is reached at which the wire tension next to the break is considerably less than the original tension. If it is assumed that less than the entire number of wires are broken, then the bending of the first pole increases the span length of the remaining wires and by increasing their tension causes them to exert a greater pull toward the break and thus decreases the unbalanced pull on the pole. However, the ordinary attachments for fastening line wires to the insulators do not always have sufficient strength to develop the strength of the wire and, therefore > a broken wire would pull through into the adjoining spans before exerting its maximum ten- sion on the poles. For this reason, and because equilibrium by bending may result in over-stressing the poles, wires, pins or insulators, it is not always possible to take advantage of this method of design. In assuming the possibility of broken wires, it becomes neces- sary to assume which wires may break as well as their number. If the wires farthest from the pole are broken, the effect on the crossarms is much greater than in the case of wires near the pole. If all the broken wires are on one side of the pole, the torsional effect on the pole must be considered. Column Formulas. Inasmuch as the strength of the main- leg members of a pole or tower, as well as most of the bracing, is predicated upon their strength as compression members, the most important requirement of a specification next to the broken-wire condition is the formula for compression members, known as the column formula. Unfortunately, the many column formulas in existence are always expressed in terms of "safe working unit stresses," which renders them almost valueless to the inexpert transmission-line designer, unless their factor' of safety is known. This is due to the fact that in transmission line construction it is the ultimate or breaking strength which must be determined in order that a specified factor of safety may be applied thereto. The writer is aware that the last statement may be criticised DESIGN 69 as a defense of a dangerous practice, in that no protection is afforded by the working stress for incompetence on the part of the designer. On this point the writer wishes to emphasize his belief that, provided a portion of the factor of safety is present to offset minor errors of design, the unit stress should not be ex- pected to afford such protection. As a matter of fact, when bid- ding under a specification requiring a test in which a sample pole or tower "must withstand" certain loads, the competing de- signers are compelled to work as close to the probable ultimate or buckling strength as seems to them advisable. Therefore, it would serve to eliminate the personal equation of the designer, together with the false security arising from test towers, if there were available accurate rules by which to compute the ultimate strength of statically determinate structures. In general there are such rules, and aside from the difficulties or inaccuracies of computing eccentric or torsional stresses, the chief uncertainty is in the column formula. The engineering profession has long awaited a complete series of ultimate com- pressive tests and the derivation therefrom of a set of generally accepted column formulas. It is to be regretted that the hundreds of tower tests which have been made to date have not resulted in a more accurate and more general addition to our knowledge of the subject. In pole and tower design, the compression members are simple in type, usually single angles with relatively large ratios of the unsupported length to the radius of gyration, i.e., "Failure" occurs when such members buckle, as the structure becomes dis- torted and useless, although it may not fall to the ground. It is readily apparent that any incipient bends in such columns will very markedly affect the theoretical compressive strength. In addition it is quite possible to select sections such as 4 in. X 4 in. X J-i-in. angles, for example, whose theoretical strength by the column formula exceeds their actual strength. This is due to the fact that in such large thin sections failure may start by the local buckling of the legs of the angle. Columns have been divided into classes according to the nature of their end connections, whether "fixed," "pin-ended," or "round- ended" and free to move. A pole set in an adequate concrete foundation probably approaches the condition of one fixed and one free end, if the whole pole is under consideration. The 70 POLE AND TOWER LINES columns formed by tower members in general might be con- sidered as stronger than flat-ended and weaker than fixed-ended columns. A discussion or compilation of the various column formulas and their derivation is beyond the scope of this book, but in Fig. 31 are shown several formulas expressed in terms of ultimate 42000 4 40000 \ \ OOAAA ^> \ T" ,3 2 \ V \ \ \ \ Y \\ \ \ , V \ \ \ N 1 \ \\ \ \ \ s\ \ 28000 V % \ \ \ v s \ to ~ 24000 \v \ \ M 2200 N 4^ s\ \ ^ -S *3 1ROOQ *-^ X \ 5 IfiOOO \ \ \ \ \ \ \ ^ 2 \ ^ \ \ \ \ ^ 2s o \ \ 8000 x o ^ \ 3 \ x 6000 \ ^v. ^x >^ 1 4000 ""-- ^. -. 2000 n 20 40 Ratio of FIG. 33. Column formulas. strength. It will be noted that the chief differences are at the ends of the curves, i.e., either for very small or very large values of l/r. It should be noted that the ultimate-strength curves are based on the assumption of good design, workmanship and material, the use of medium steel, and freedom from incipient injury. They DESIGN 71 represent approximate average values and must be used with caution by the inexpert. They are chiefly useful in showing the derivation of the allowable working values, and to predetermine breaking strengths. Ultimate-strength Curves: 40000 (1) R. D. Coombs & Co.: - 1 + 1 16,000 r (2) Joint Report Crossing Specifications : 54,000 180 - (3) American Bridge Co. (4) American Railway Engineering Association : Working-stress Curve: (5) R. D. Coombs & Co. (with a factor of safety of 2.0) : or (with a factor of safety of 2.5) : 57,000 - 300^ 39,000 - 150^ 39,000 max. 48,000 - 210^ 42,000 max. 20,000 1 + 1 16,000 r 2 16,000 1 + 1 16,000 r 2 Formulas 2, 3 and 4 shown in Fig. 31 were not issued in that form by their authors, but in terms of allowable unit stresses, as follows : 2. 18,000 - 60- 19,000 - 100 - 13,000 - 50 ^ 4. 13,000 max. 16,000 - 70 - 14,000 max. The diagrams shown were, therefore, obtained by increasing these allowable units to their apparent ultimate values. The difficulty in comparing the usual allowable-unit formulas arises from their variable and uncertain factors of safety. Such formu- las were rarely intended for use with the large ratios of l/r com- 72 POLE AND TOWER LINES 24.UUU 23,000 22,000 21,000 20,000 19.000 18,000 Z4.UW 23,000 22,000 21,000 20,000 19,000 18,000 17,000 -g a 16,0002 15,000 1 14,000 S 13.000 i 12,000 1 11,000-3 10,000 9,000 | 8,000 | 7,000 6,000 5,000 4,000 3,000 2,000 1,000 ffl Graphical Comparison of Various Formulae for Allowable Working Stresses in Steel Columns "--. x. X \ \ \ S - ^: 1 15,000 cr 00 14 nnn X ~~'- s x>; \ \ *^ ^ ^ <^- ^ .000 .pto oo> -V. fnit Stresses per ! I ! 1 1 1W V Oapl ^ 5 >^ $" : v^ V s ^ ^5 ^ ^ ^ ^txj ^12 boo-u pto c <^ 1 ^ ^\\ js ^< ^-, ^x X, "^ ~^- ^ [^ ^ X \: X tl> x r ^- -. U)1U,UW s 8 9.000 | 8.000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 ^~ ^^ ^ ^ ^v ^ ^ X ^ 3 , s ^x ^ ^i ^ ^ <^ ^^? ^ X 2 V **> -..^ \^ ^~ ^ ^ ^ ^ X; *., ~~ - N SS 2 1 1 \ *- \ \ s \ ""^ 4? ^ ; <^ X; \ x \ \ ^ ss s ^ ^ S \ s \ X n ^X ^ \ S a 10> K x f ^ x ^ ! 23 N s 1 \ ^ B 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 2 Ratio -^r FlG. 34. - 17,LM>( OaVinrno fTiv ^i-ionifi nnf 'na^ 10. 15,200- 58 y 11.15,000-57- r I i *_ 2. 20,300 - 70 Chicago Bridge & Iron Wks. r (Tank Towers) L+ 36,OOOr2 4. 17,100 57 Carnegie, Jones &Laughlin, r Buffalo-Minneapolis 5. 16,000-55 - Bethlehem Steel Co. r t o F^nn 12. 12,500 42 13. 11,300-35 15,000 11 Z2 1 13,500r2 17,000 i I ' 2 Laughlin, Phoenix Bridge L ' 36,000r2 Co. 12,000 7. . 2 Boston 1+ 36,000r2 1 Il,000r2 16,250 1 18,000r2 9. 16,000-60- J. A.L. Waddell ("De Pon- r tibus") 16. lz + ll,000r2 New York City, Washing- ton Nat'l. B'd. Fire Un- derwriters Passaic Steel Co. C. E. Fowler ("Steel Roofs & B'ldgs.") H. E. Horton Chesapeake & Ohio Ry., Norfolk & Western Ry., Phila. & Reading Ry., Penna. R. R., N. Y. C. (for H'way Bridges) Virginia Br. & Iron Co., Baltimore & Ohio Ry., Chesapeake & Ohio Ry., Long Island R. R., Deep- water Ry., Phila. & Reading Ry. Philadelphia DESIGN 73 mon in tower work, nor were the conditions of loading with which they were used as unusual as those encountered in transmission- line construction. In other words, tower members are less likely to approach their theoretically perfect condition and, therefore, should have a high factor of safety, while the loading for which they are usually figured may never occur, and on that account they should have a low factor. TABLE 14. ULTIMATE STRENGTHS OF TIMBER IN BENDING N.E.L.A. Overhead Line Construction Committee, 1911 A.R.E.A. Wooden Bridges and Trestles Committee, 1909 . Lb. per square inch Lb. per square inch Port Orford cedar 6900 Long-leaf yellow pine 6600 6500 Douglas fir Short-leaf yellow pine 6000 5700 6100 5600 White oak 5700 5700 Chestnut 5100 Washington cedar Idaho cedar . . 5100 5100 Redwood 5100 5000 Bald cypress (heart wood) Red cedar 4800 4200 4800 4200 Eastern white cedar 3600 Juniper 3300 Catalpa 3000 Spruce 4800 Western hemlock 5800 It should be noted that the extension of the ultimate-strength curve, 1, to values of l/r of 300 is not intended as a recommendation of such values, but rather to illustrate the decrease of strength. Further, even the reduced values shown will not always be ob- tained in practice owing to errors of design, workmanship, and injuries from handling. 17. 19,000-100 (to 120") Amer. Bridge r \ r Co. 13,000- 50 (120^200) r \r ) 13,000 Maximum 18. 2 '?P H.B. Seaman 8,000r2 19. 16,000-70 Am. Ry., Eng. & M. W. r Assn.,N. Y. C. R. R. Bos. & Me. Ry., Cana- dian Pacific Ry., Grand Trunk Ry., Maine Central Ry., Mo., Kansas & Tex. Ry., Nash., Chatt. & St. L. Ry., N. Y., N. H. & H. Ry. St. Louis & San Fran. Ry., St. Louis & Southwestern Ry., Wabash Ry., M. S. Ketchum ("Steel Mill Bldgs."); C. C.Sch- neider ("Structural Design of Bldgs.") 20. 18,000- 90 Southern Ry. 21. 15,000- 75 L. D. Rights 22. 12,000 \1- (i200 j. R. Worcester 23. Rule. J. R. Worcester 74 POLE AND TOWER LINES The logical procedure would seem to be to assume only the loads which could reasonably be expected, i.e., the ice and wind loads with very moderate broken- wire conditions, to prohibit excessive values of l/r, as well as the very thin sections, and to use a fairly low factor of safety. The American Railway Engineering Association values are for squared timbers used in railroad construction, assumed to be of good commercial quality, and neither very green nor seasoned. It should be remembered that these values are the breaking strengths of fairly good lumber, and a more conservative factor of safety must be assumed than would be necessary in more uniform material. Further, pole timber is not squared and is more likely to contain crooked grain, knots and rot. It would be more consistent to use lower ultimate strengths for poles than the N.E.L.A. values, and a smaller factor of safety than 6.0, which is that used therewith. Strength of Wooden Poles. WEAKEST POINT IN WOODEN POLES (a) Neglecting Wind on Pole y = distance of weakest section below load. d = diameter of pole at load. t = increase in diameter per inch of length. P = resultant load. All dimensions in inches. c = I c ds _ dy ~ 0.098 2 (d + ty)* (d + ty) - 3ty = A y ~ 2t Let di = diameter of pole at distance y below the load. Then t == d L --d y From above equation, _ d_ _ dy y o* a/ - d) DESIGN 75 , - d)y = dy That is, the diameter of the pole at the weakest section is one and one-half times that at the load. When the diameter at the ground line is less than one and one-half times the diameter at the load, the weakest section is at the ground line. WEAKEST POINT IN WOODEN POLES (6) Including Wind on Pole y = distance below load P. d = diameter of pole at load. t = increase in diameter per inch of length. a = distance of load P below top of pole. w = wind load on pole per inch of length. M Py w(y + a) 2 '' I ~ 0.098(d + tyY 2X0.098(d + ty)* 2P d . 4P 2 2Pd , SPa , d 2 2da w = ~. r "& *i 9 w t \ w 2 wt w or "S~T' v\^ CHAPTER V WOODEN POLES The total number of wooden poles in use in the United States is probably 40,000,000, while the yearly additions approximate 4,000,000. Of these the majority are used by telephone, tele- graph, and railroad companies. The greater part of the 4,000,000 new poles are less than 40 ft. long, and such poles FIG. 35. Wooden pole lines, 60,000 volts. are rarely used for transmission lines. The lengths of the poles used for transmission lines are increasing, while those for other purposes are decreasing. In the East the timber generally used is chestnut, while cedar is more common in the West. For distribution lines not only the length of the poles is increasing, 76 WOODEN POLES 77 but also the number^ installed per year. The cost of poles in- creases very rapidly with increases in length. Approximately 90 per cent, of the timber poles used are either chestnut or cedar, the former being about 18 per cent, and the latter 72 per cent. Chestnut, which is second in use to cedar, is durable and is stronger than cedar, and its taper is not excessive. On the other hand, it is heavier and harder, it shrinks and checks more easily, and is not so straight or free from knots. Cypress is durable, but its size and taper often make it unsuit- able for pole purposes. Pine is not durable and is heavy, but it grows in suitable sizes. Douglas fir, spruce and red- wood are durable and make ex- cellent poles when of suitable size. The consumption of the FIG. 36. Ring shakes in chestnut. FIG. 37. Cat-faces in chestnut. former for pole purposes is increasing. Redwood, on account of its size, is used almost exclusively in the form of sawed poles. The useful life of a timber pole, in contact with the soil, de- pends in part on the chemical action of the earth's ingredients, the attack by fungi, and on the ability of the timber to resist insects. Disintegration will, therefore, advance more rapidly in some soils than in others, but in general the use of good native timber 78 POLE AND TOWER LINES for local use will be found advisable. Decay at the ground line weakens the body of a pole until this critical section is so emaciated that it will no longer sustain its load. In the dry season this decayed portion is much in the nature of dry tinder, so if the pole is located on a grassy right-of-way, grass fires may char away still more of the critical section. Decay and Defects. The decay of wood is generally due to the activities of certain low forms of plant life known as fungi, punk, toadstools, etc. Bacteria are also known to cause decay, but their action is not well understood. These plants have their origin in minute spores borne from place to place by the wind. FIG. 38. Butt-rot in chestnut. Those that lodge in a suitable situation for growth, which may be on either living or dead timber, germinate, and provided the conditions are favorable, at once attack the wood. The plants grow with great rapidity, sending out numerous threads which penetrate the wood and attack the contents of the wood cells and finally the cell-walls. The most favorable conditions for the growth of fungi and other organisms of decay are an abundant food supply, heat, moisture, and air, the amount of each depending on the kind of organism. A certain amount of moisture must be present or de- cay cannot set in. Air is also essential, and thus may be explained the lasting qualities of wood when kept perfectly dry, and the perfect state of preservation of wood which has been under water for long periods, moisture being lacking in the first case and air in the second. Again, if the wood is protected by a germicide or WOODEN POLES 79 antiseptic, it will not decay At the butt of the pole, though moisture is present, air is excluded, while above the ground the pole is generally dry. Decay begins where moisture and air are both present, the former perhaps being drawn by capillary attrac- tion from the ground. Since the decay of timber is due to the attacks of wood-destroy- ing fungi, and since the most important condition of the growth of these fungi is water, anything which lessens the amount of water in wood aids in its preservation. Cold, or extreme heat, will prevent the growth of fungi, although the necessary degree of the latter is beyond the limits FIG. 39. Ant-eaten butt. of natural temperatures. The character of the soil may have a marked effect on the decay of timber, owing to the ability of certain soils, like heavy' clay, to hold water or to discourage in- sect life. The decay of poles before their installation in the line may be of several kinds and together with the several kinds of cracks or other injuries constitute what are known as "defects." The former include butt rot, heart rot, ring rot, and rotten knots; and the latter, seasoning checks, wind shakes or ring shakes, cat faces, and loose knots. Since in designing the strength of a pole is computed as that of a solid homogeneous cylindrical section, it is evident that pro- nounced defects may materially affect the actual strength. Therefore, either pole specifications and inspection must be 80 POLE AND TOWER LINES strict enough to eliminate poles whose actual strengths are not reasonably close to their assumed values, or else a large factor of safety must be employed to take care of irregularities. Some detailed instructions to inspectors would seem very desirable, since exactly the same defect may have an entirely different significance in two locations on the pole. Thus a rotten heart, very common in cedar, may be in the center of the cross-section and, therefore, of the least effect, or it may be well toward one side. A wood pole is not greatly affected by hollow- ing out a small portion at the center of the cross-section, but the strength is decreased by any loss of area near the circum- ference or any reduction of diameter. The strength is pro- portional to the cube of the diameter, and the relative strengths for different conditions would be: 15-in. sound diameter = 100% 15-in. pole, 5-in. rotten heart at center = 99% 15-in. pole, 5-in. rotten heart 2^ in. off center = 89% 12-in. sound diameter (13^ in. decay) = 51% Seasoning. Under present methods much timber is rendered unfit for use by improper seasoning. When exposed to the sun and wind the water will evaporate more rapidly from the outer than from the inner parts of a log and more rapidly from the ends than from the sides. The evaporation of water from timber is largely through the ends. The evaporation from the other surfaces takes place very slowly out-of-doors, with greater rapidity in a kiln. The rate of evaporation differs with the kind of timber and its shape. Air-drying out-of-doors takes from two months to a year, the time depending on the kind of timber and the climate. As the water evaporates the wood shrinks, and when the shrinkage is not fairly uniform the wood cracks. When wet wood is piled in the sun, evaporation may occur with such unevenness that the timber splits and cracks so badly as to become absolutely useless. Such uneven drying can be largely prevented by careful piling. When solid piles are placed side by side and many together, the air cannot cir- culate freely between the timbers. Open-crib piles, however, will allow free air circulation even when closely spaced. For this reason green timber should be piled in as open piles as possible, as soon as it is cut, and kept so until it is air dry. No timber should be treated until it is air dry. Seasoning is ordinarily understood to mean drying, but it WOODEN POLES 81 really involves other changes besides the evaporation of water. It is very probable that these consist in changes in the substances in the wood fiber, and possibly also in the tannins, resins and other incrusting substances. One of the first steps in preparing naturally short-lived timber for preservative treatment is to season it properly. More benefit will result from taking care of the short-lived timbers than from treatment of those with longer life. The bark should be peeled from poles before seasoning, and particularly from those that are to be treated, as the inner bark offers considerable resistance to impregnating fluids and if not removed will peel, leaving the untreated wood exposed to the attack of fungi. Bark will also retard and almost pre- vent seasoning. Care should be taken in handling and felling trees, as those which are split in felling or are otherwise roughly handled may afterward undergo serious checking. Whether poles are to receive preservative treatment or not, there can be no doubt that it invariably pays to season them properly before putting them into service. Under ordinary conditions, the life of a well-seasoned untreated pole should be at least 30 per cent, greater than that of an untreated green pole. Poles should be cut from sound timber, which may or may not be live timber. Poles cut in the late winter or spring have immediately before them the best period for seasoning, but late f alf and winter offer the best conditions for cutting, and facilitate the cultivation of new sprouts from the winter-cut stumps. Preservatives. The creosotes are employed most generally for protecting timber against decay, and they are apparently the best type of preservative. The terms creosote and tar are rather general expressions, and not very definitely inter- preted by the ordinary purchaser. Briefly, coal-tar is produced by the distillation of coal and is obtained from two distinct and different sources, i.e., that from coke ovens and that from gas works. Crude coal-tar is subjected to different refining proc- esses, and yields various commercial derivatives, such as the different grades of creosote, carbolics, naphthas, etc. The protective effect of all preservatives is due to their exclusion of water and to their antiseptic or poisonous effect on the fungi which cause decay. In order to protect timber permanently, it is necessary that the preservative be maintained either as an impervious coating on the surface or as an impregnation through- 82 POLE AND TOWER LINES out at least the outer layers of the wood. A thin permanent surface coating is impracticable; therefore, to obtain successful results the preservative should be injected into the timber. The deeper the penetration and the more insoluble and non- evaporative the injected material, the more successful will be the treatment. It is possible to inject a light solution to greater depths than a heavy solution, but it may be that the lighter solution, which may contain more volatile matter, will evaporate or wash out more readily than a heavier solution. It is cus- tomary, however, to regard the greatest penetration as the most desirable, and to grade the treatment by the weight of preservative injected into a given volume of timber. Such units of measurement, however, are only comparable if the qualities of the injected fluids are similar. In addition to the three regular grades of creosote oil, i.e., the A.R.E.A. Nos. 1, 2 and 3, the specifications of which are recognized standards, there are in use various combinations of creosote and coal-tar as well as a number of proprietary creosotes, or creosote-tar combinations. Since creosote is obtained from tar, and is a part of crude tar, the distinction between creosote, as the word is ordinarily used, and crude tar is very indefinite. Undoubtedly a proper impregnation with A.R.E.A. No. 1 creosote is the most economical treatment in ultimate result. Aside from this grade of material, however, there is considerable difference of opinion as to the relative merits of using greater quantities of the Nos. 2 or 3 grades of straight creosote, or of using creosote-coal-tar combinations, or the proprietary solutions. Since the electric companies are frequently also operators of gas plants, it is probable that there is a good commercial argu- ment in favor of their developing the combination creosote-coal- tar treatment, even though theoretically the highest grade treatment is ultimately economical for absolutely permanent construction. In other words, the writer is of the opinion that, since such companies use large quantities of timber for in- stallations the existence of which is not permanent, they would be justified in obtaining a less effective preservation at a lower cost. Further, if the development of their own resources would encourage their more general use of preservatives, the ultimate result would be advantageous, whether or not the kind of treat- ment for any particular case were absolutely the best possible. Gas-house tar and coke-oven tar are practically alike chem- WOODEN POLES 83 ically but differ greatly in the percentage of free carbon, the former having a comparatively high percentage and the latter usually a low percentage. High free-carbon tar, whether gas house or coke oven, should not be used as an addition to creosote oil. If, therefore, the combination material is to be used, only a low-carbon tar should be allowed in the creosote-coal-tar combination preservative. Pressure Treatment. Turning now from a discussion of the materials to be used in protecting poles from decay we find that several processes of treatment are in common use; the high- pressure, the open-tank, and the brush treatment, these being stated in the order of their effectiveness. When treated by the high-pressure method, the timber is placed in metal cylinders and subjected to a steaming or heating process followed by a vacuum. After the vacuum has been maintained from one to two hours, the tank is completely filled with creosote oil and pressure is applied and maintained until the specified amount of creosote has been forced into the timber. Open-tank Process. The open-tank process consists in treat- ing the butts of the poles only. Seasoned timber is immersed in a tank of hot preservative and kept there for a period of from one to three hours. The timber is then suddenly transferred to a bath at atmospheric temperature and kept there from one to three hours longer. By this process the air and moisture in the cells of the wood are first expanded and some driven off, while in the second bath the air and moisture contract drawing the pre- servative liquid into the timber. Brush Treatment. By the brush method, dry seasoned timber is given two or more coats of hot preservative applied with three- or four-knot rubber-set or wire-bound roofing brushes. The creosote should be heated to about 200F. and kept at that tem- perature while being applied. A liberal quantity of liquid should be used and it should be well brushed into all crevices in the timber. Before applying preservative, the poles must be stripped of bark, inner skin, or dirt, and in fact should be scrubbed clean. Sufficient time should elapse between the application of the different coats for the preceding one to be absorbed ; not less than one-day intervals are generally satisfactory. Poles should not be used for two or three days after treatment. In general it is more economical, both in labor and in the efficiency of the operation, to treat poles while they are in temporary storage, although some 84 POLE AND TOWER LINES benefit is unquestionably derived from even a cold application at the site. Whenever brush treatment is employed the entire butt should be coated up to about 2 ft. above the ground line. In addition, all crossarm gains, roofs and bolt holes should be painted with preservative. SPECIFICATIONS FOR WOOD POLES The purchaser shall have the right to make such inspection of the poles as may be desired. The inspector representing the purchaser shall have the power to reject any pole which is defective in any respect. Inspection, however, shall not relieve the contractor from the re- sponsibility of furnishing proper poles. Any imperfect poles which may be discovered before their final accept- ance shall be replaced immediately upon the order of the purchaser, even though the defects may have been overlooked by the inspector. Poles shall be subject to inspection by the purchaser, either in the woods where the trees are felled or at any point of shipment or delivery. Any poles failing to meet the requirements of these specifications may be rejected. Seasoned poles shall have preference over green poles, provided they have not been held for seasoning long enough to have developed any of the timber defects hereinafter referred to. All poles shall be reasonably straight, well proportioned from butt to top, shall have both ends squared, the bark peeled and all knots and limbs closely trimmed. DEAD POLES. No dead poles and no poles having dead streaks covering more than one-quarter of their surface shall be accepted under these specifications. Poles having dead streaks covering less than one- quarter of their surface shall have a circumference greater than otherwise required. The increase in the circumference shall be sufficient to afford a cross-sectional area of sound wood equivalent to that of sound poles of the same class. TWISTED, CHECKED OR CRACKED POLES. No cracked poles, no poles containing large seasoning checks, and no poles having more than one complete twist for twenty (20) ft. in length shall be accepted under these specifications. CROOKED POLES. No poles having a short crook or bend, a crook or bend in two planes, or a reverse crook or bend, shall be accepted under these specifications. The amount of sweep measured between the six (6) ft. mark and the top of the pole shall not exceed one (1) in. for every six (6) ft. of length. MISCELLANEOUS DEFECTS. No poles containing sap rot, evidence of internal rot as disclosed by careful examination of black knots, hollow knots, woodpecker's holes, or plugged holes, and no poles showing WOODEN POLES 85 evidences of having been eaten by ants, worms or grubs shall be accepted except that poles containing worm or grub marks below the six (6) ft. mark may be accepted. CAT FACES. No poles having "cat faces," unless the latter are small and perfectly sound and the poles have an increased diameter at the "cat face," and no poles having "cat faces" near the six (6) ft. mark or within ten (10) ft. of their tops shah 1 be accepted. WIND SHAKES. No poles shall have cup shakes (checks in the form of rings) containing heart or star shakes which enclose more than ten (10) per cent, of the area of the butt. BUTT ROT. No poles shall have butt rot covering more than ten (10) per cent, pf the total area of the butt. If butt rot is present it must be located close to the center in order that the pole may be accepted. KNOTS. Large knots, if sound and trimmed close, shall not be con- sidered a defect. No poles shall contain loose, hollow or rotten knots. DEFECTIVE TOPS. Poles having tops of the required dimensions shall not be accepted unless the tops are sound. Poles having tops one (1) in. or more in excess of the required circumference may contain one (1) pipe rot not more than one-half (0.5) in. in diameter. Poles with double tops or double hearts shall be free from rot where the two parts or hearts join. DEFECTIVE BUTTS. No poles containing ring rot (rot in the form of a complete or partial ring) shall be accepted under 'these specifications. Poles having hollow hearts may be accepted under the conditions shown in the following table : TABLE 15 Average diameter Add to butt requirement (circumference) of rot 25- and 30-ft. poles 35-, 40- and 45-ft. poles 50-, 55-, 60- and 65-ft. poles 2 in. Nothing Nothing Nothing 3 in. lin. Nothing Nothing 4 in. 2 in. Nothing Nothing 5 in. 3 in. lin. Nothing Gin. 4 in. 2 in. 1 in. 7 in. Reject 4 in. 2 in. 8 in. Reject 6 in. 3 in. 9 in. Reject Reject 4 in. 10 in. Reject Reject 5 in. 11 in. Reject Reject 7 in. 12 in. Reject Reject 9 in. 13 in. Reject Reject Reject Scattered rot, unless it is near the outside of the pole, will be considered being the same as heart rot of equal area. 86 POLE AND TOWER LINES DIMENSIONS. The dimensions of the poles shall be not less than the values given in the following tables, the "top" measurement being the circumference at the top of the pole and the "butt" measurement the circumference six (6) ft. from the butt. The dimensions specified for the six (6) ft. mark shall be required in all cases, but the top circumferences may differ from those shown in the following tables by not more than one-half (0.5) in. No pole shall be more than six (6) in. longer or three (3) in. shorter than the length for which it is accepted. If any pole is more than six (6) in. longer than is required it shall be cut back. TABLE 16. CHESTNUT Circumferences of poles in inches Cl asses Length of poles (ft.) A B C Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) 25 20 30 30 24 40 22 36 20 33 35 24 . 43 22 40 20 36 40 24 45 22 43 20 40 45 24 48 22 47 20 43 50 24 51 22 50 20 46 55 60 22 22 54 57 22 22 53 56 20 49 65 22 60 22 59 70 22 63 22 62 75 22 66 22 65 80 85 22 22 70 73 22 22 69 72 90 22 76 22 75 WOODEN POLES 87 TABLE 17. EASTERN WHITE CEDAR Circumferences of poles in inches Cl asses Length of poles (ft.) A B C Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) 25 22 32 18% 30 30 24 40 22 36 18% 23 35 24 43 22 38 18% 36 40 24 47 22 43 18% 40 45 24 50 22 47 18% 43 50 24 53 22 50 18% 46 55 24 56 22 53 18% 49 60 24 59 22 56 TABLE 18. WESTERN WHITE CEDAR, RED CEDAR, WESTERN CEDAR, IDAHO CEDAR Circumferences of poles in inches Length of poles (ft.) Classes A B C Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) 20 28 30 25 28 22 26 22 28 32 25 30 22 27 25 28 34 25 31 22 28 30 28 37 25 34 22 30 35 28 40 25 36 22 32 40 45 28 28 43 45 25 25 38 40 22 22 34 36 50 28 47 25 42 22 38 55 28 49 25 44 22 40 1 60 28 52 25 46 22 41 65 28 54 25 48 22 43 SAWED REDWOOD POLES The material desired under these specifications consists of poles of redwood (Sequois Sempervirens) sawed to shape as hereinafter set forth. QUALITY OF TIMBER AND WORKMANSHIP. All poles shall be of sound No. 1 Common Redwood; and shall be reasonably straight and well sawn. 88 POLE AND TOWER LINES TABLE 19. SAWED REDWOOD Dimensions in inches Classes (ft.) A B Top (in.) Butt (in.) Top (in.) Butt (in.) 24 6X6 6X6 4X6 4X6 25 7 X7 10 X 10 6X6 9X9 30 7X7 11 X 11 6X6 10 X 10 35 7X7 12 X 12 6X6 11 X 11 40 7X7 13 X 13 6X6 12 X 12 45 7X7 14 X 14 6X6 13 X 13 50 7X7 15^ X 15> 6X6 14 X 14 The sectional dimensions of the sawn poles shall not be more than one-quarter (>) in. under, or three-quarters (%} in. over, the dimen- sions specified in the above table. No poles shall be more than three (3) in. longer or shorter than the lengths required in the above table. SAP WOOD. No poles shall have sap wood covering more than four (4) per cent, of the area of all the surfaces. No pole shall have sapwood for a distance of more than eight (8) ft. from the top. No sapwood shall be deeper than one (1) in. at any point. KNOTS. In 4" X 6" poles sound knots with a diameter smaller than one (1) in. may be present in any number. No 4"X 6" pole shall be accepted which contains in each five (5) superficial ft. more than one sound knot having a diameter of one (1) in. or more, or which con- tains any knots with a diameter greater than one and one-half (1>^) in. All other sizes of poles covered by these specifications may contain any number of sound knots with a diameter smaller than one and one- half (13^) in. No pole shall be accepted which contains in each five (5) superficial ft. more than one sound knot having a diameter of one and one-half (1>^) in. or more, or which contains any knots of a diameter greater than two and one-half (2^) in. NOTE. Where diameters are specified in connection with knots a knot shall be rated on the basis of its average diameter. SPECIFICATIONS FOR CREOSOTED YELLOW-PINE POLES These specifications shall apply to Classes A, B and C poles of sputhern yellow pine treated with dead oil of coal tar. QUALITY OF POLES. All poles shall be sound southern yellow pine (longleaf , shortleaf, or loblolly yellow pine) squared at the buft, reason- ably straight, well proportioned from butt to top, peeled and 'with WOODEN POLES 89 knots trimmed close. All poles shall be free from large or decayed knots. All poles shall be cut from live timber. It is desired that all poles be well air-seasoned before treatment and such poles shall be treated in accordance with the requirements for treating seasoned timber contained in the " Specifications for Creosoting Timber" hereinafter referred to. The poles shall not be held for season- ing, however, up to the point where local experience shows that sapwood decay would begin. Unseasoned poles shall be treated in accordance with the requirements for treating unseasoned timber contained in the above-mentioned specifications. All poles shall be sufficiently free from adhering inner bark before treating to permit the penetration of the oil. If the inner bark is not satisfactorily removed when the pole is peeled, the pole shall be either shaved or allowed to season until the inner bark cracks and tends to peel from the pole. DIMENSIONS. The dimensions of the poles shall be not less than those given in the following table. TABLE 20. CREOSOTED YELLOW PINE Circumference of pole in inches Classes Length of pole (ft.) A B C Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) Top (in.) 6 ft. from butt (in.) 25 22 33 20 30 18 28>i 30 22 35 20 32 18 30H 35 22 38 20 34 18 32 40 22 40 20 36 18 34 45 22 42^ 20 38 18 36 50 22 44^ 20 40 18 38 55 22 47 20 42^ 18 40 60 22 49 20 44^ 18 42 65 22 51 20 47 18 44 70 22 53 20 49 18 46 75 22 55 20 51 80 22 57 Framing of Poles. Before the poles are treated with creosote they shall be framed, unless otherwise ordered, in the following manner and as shown in drawing No. ( ). The top of each pole shall be roofed at an angle of ninety (90) degrees. All Class A poles shall have eight (8) gains, all Class B poles shall have four (4) gains and all Class C poles shall have two (2) gains. The gains shall be located on the side of the pole with the greatest 90 POLE AND TOWER LINES curvature, and on the convex side of the curve. The faces of all gains shall be parallel. Each gain shall be four and one-quarter (434) in. wide and one-half (Y^) in. deep and twenty-four (24) in. center to center. The center of the top gain shall be ten (10) in. from the apex of the gaWe. A twenty- one thirty-second ( 2 > 2 ) in. hole shall be bored through the pole at the center of each gain perpendicular to the plane of the gain. INSPECTION. The quantity of dead oil of coal tar forced into the poles shall be determined by tank measurements and by observing the depth of penetration of the oil into the pole. If the poles have more than one and one-half (1^) in. of sap wood, the depth of penetration shall be not less than one and one-half (1^) in. If the sap wood is less than one and one-half (1^) in. thick, the dead oil of coal tar shall pene- trate through the sap wood into the heartwood. The depth of penetration shall be determined by boring the pole with a one (1) in. auger. The right is reserved to bore two holes at random about the circumference for this purpose, one hole to be five (5) ft. from the butt and one hole ten (10) ft. from the top. After inspection each test hole shall be filled first with hot dead oil of coal tar and then with a close-fitting creosoted wooden plug. The rejection of any pole because of insufficient penetration shall not preclude its being retreated and again offered for inspection. Design of Wood Poles. If in Fig. 40, which represents a standard Class A chestnut pole, we assume for the present that all the bending loads are represented by one load, P of 1200 lb., 35 ft. from the ground, the bending moments and unit stresses at sections X2, XI, and at the ground line, will be as follows: At X2, M = 18.25 ft. X 12 in. X 1200 lb. = 262,800 in.-lb. S = 0.0982 X 12 3 = 169.69 F = M/S = 1550 lb. per square inch At XI, M = 24.5 X 12 X 1200 = 352,800 in.-lb. S = 0.0982 X 13.12 3 = 220.75 F = M/S = 1600 lb. per square inch At ground, M = 35 X 12 X 1200 = 504,000 in.-lb. S = 0.0982 X 15 3 = 331.42 F = M/S = 1530 lb. per square inch It should be noted therefore, that the point of greatest stress is not necessarily at the ground line but may be at some section above WOODEN POLES 91 the ground. If the pole under consideration were disproportion- ally heavy at the butt, any computations made at the ground line might be quite erroneous, although the difference in the example given is negligible. This condition results from the fact that the unit stress "at any point depends on the distance from the load and on the diameter of the pole at that point. Provided there are no serious defects in a pole which may make some particular point unusually weak it will, in theory, break at the point where the diameter is 1.5 times the diameter at the point where the load is applied. Therefore, poles may or may not fail at the A > J "o 1 ^ a ; ~ - 3 1 x. 3.1 r 1200 FIG. 40. ground line depending on the taper. Further, if the butt diameter exceeds the above critical diameter the pole may experience some decay at the butt without becoming any weaker. Referring to Fig. 41, if the following wires are to be carried, with a minimum clearance of 30 ft., and a maximum stress in the wires of 0.9 of the elastic limit, we have, if a 200-ft. span is assumed, One ^g- m - Siemens-Martin galvanized stranded steel. Three No. 1 hard-drawn stranded copper, 33,000 volts. Span 200 ft. Normal sag = 1 ft. 3 in. Normal tension = 1020 Ib. Maximum sag = 2ft. in. Maximum tension = 1960 Ib. Elastic limit, No. 1 cable = 2180 Ib. Wind pressure on wires: %-in. ground wire = 0.917 Ib. X 200 ft. = 183 Ib. No. 1 power wire = 0.885 Ib. X 200 ft. = 177 Ib. 92 POLE' AND TOWER LINES The bending moment at the ground, for transverse loading, straight-line poles and no broken wires, is, Ground wire, 183 Ib. X 37 ft. = 6,770 ft.-lb. Power wires, 177 Ib. X 34.5 ft. = 6,100 ft.-lb. Power wires, 177 Ib. X 2 X 32 ft. = 11,330 ft.-lb. 07 C2 Wind on pole, 0.9 sq. ft. X 13 Ib. X -g- ft. = 8,225 ft.-lb. Bending moment = 32,425 ft.-lb. f^__l - T - 1 r~t. -i- - so ^ f- .. Cleara :-15 L FIG. 41. 44-ft. pole. The shear on the pole is, Ground wire, 183 Ib. Three power wires, 531 Ib. Wind on pole, 438 Ib. and Total shear, 1152 Ib. 32,425 1152 28.1 WOODEN POLES 93 Or the load is equivalent to a single load, P = 11521b., 28.1 ft. above the ground. If the weakest section of the pole is assumed as being at the ground line, which is usually not correct, the unit stress under the first condition of loading is, M SI M - c 32,425 ft.-lb. X 12 in. ~T~ c I 1 15 3 - = X diam. 3 (approx.) = = 337.5 C -LU J.U S = 1150 Ib. per square inch, bending stress. To obtain the maximum unit bending stress in the pole: Center of gravity of wire loads below top of pole: 183 Ib. X 6 in. = 1,098 in.-lb. 177 Ib. X 36 in. = 6,372 in.-lb. 354 Ib. X 66 in. = 23,364 in.-lb. 714 Ib. 30,834 in.-lb. 30,834 in.-lb. -f- 714 Ib. = 43 in. below top of pole. The location of the point of maximum stress below the center of gravity of wire loads can be found from the formula, page 75 : 2P d In this case P = wind on wires = 714 Ib. w = wind per inch of pole = 1 Ib. di = diam. of pole at top = 7.4 in. dz = diam. of pole 6 ft. in. above butt = 15 in. dz diam. of pole at point of maximum stress. t = increase in diameter per inch of length 15 - 7.4 _!_ ~ 38ft. X 12 ~ 60 d = diam. of pole at load P = 8 in. a = dist. of load P below top of pole = 43 in. 94 POLE AND TOWER LINES Substituting these in the above formula: y = 304 in. = 25 ft. 4 in. The maximum stress occurs, therefore, 25 ft. 4 in. below the center of gravity of the wire loads, or 28 ft. 11 in. below the top of the pole. d 3 = d + ty = 8 in. + ^ X 304 = 13 in. Maximum stress in pole M P a 2 '' _ 0.098d 3 3 c 714 X 304 -f * (304 + 43) 2 Zi ~ 0.098 X 13 3 277 260 = 01 g o = 1290 Ib. per square inch. ' ZlO.O As the breaking strength for a chestnut pole is 5100 Ib. per square inch, the factor of safety is T^T: = 4. The stress at the ground line equals 714 X (450 - 43) + 0.098 X (14.9) 3 391 848 - = 1200 Ib. per square inch. The bending moment at the ground for a pole at a 5 corner, is found as follows: Maximum wire tension = 1960 Ib. Component due to corner (Fig. 32) = 0.10 tension or 1960 Ib. X 0.10 = 195 Ib. per wire. Wind on wires and pole (same as before) = 32,425 ft.-lb. 195 Ib. X 37 ft. = 7,215 ft.-lb. 195 Ib. X 34.5 ft. = 6,725 ft.-lb. 195 Ib. X 2 X 32 ft. = 12,480 ft.-lb. Total bending moment = 58,845 ft.-lb. WOODEN POLES 1152 Ib. wind oh wires and pole 780 = 195 X 4 wires, corner loading 95 1932 Shear and 58,845 1932 = 30.4 ft. FIG. 42. Corners on single pin insulators. Therefore P = 1932 Ib., equiv. load 30.4 ft. above ground 8 = > sq ' In Fig. 42 is shown a one-circuit wood-pole line, in which the wires turn rather sharp corners on single-pin insulators, a prac- tice which is objectionable. The illustration shows considerable right-of-way clearing, but it appears that the poles will pre- sumably not be subjected to very severe wind loads on account of 96 POLE AND TOWER LINES the shelter afforded by adjoining timber. It is also evident that a very wide clearing would be ne'cessary to entirely protect the line from adjoining trees. FIG. 43. Design for wooden A-frame. A-frames and H-frames. Timber A-frames composed of two poles spliced together at the top and with their butts separated transverse to the line are useful chiefly where large timber is ex- WOODEN POLES 97 pensive, as such construction permits the use of slender poles, one of which would not have sufficient strength. These frames have not been used to any considerable extent, however, in this country. In the direction of the line, the strength is twice that of the single poles, while it is considerably greater in a transverse direction, the amount depending largely on the bracing provided FIG. 44. One-circuit H-frame. to prevent buckling of each pole. Except at corners, these frames are relatively too strong across the line as compared with their strength in the direction of the line. The H-frame, on the other hand, while having less theoretical strength across the line, is a useful type of construction, par- ticularly for heavy lines in bad ground. Its width at the top permits a larger number of wires per crossarm, while utilizing the strength of the arms as simple beams instead of cantilevers. In Fig. 44 is shown a one-circuit H-frame, consisting of two light timber poles. This is typical of the characteristic usefulness 7 98 POLE AND. TOWER LINES of an H-frame in that two slender poles can be used to provide adequate strength. On account of their strength H-frames FIG. 45. H-frame crossing, metal grounding arms. may also be employed to support heavy lines, although they are more frequently used for heavy telephone and telegraph trunk lines than for transmission lines (Fig. 86). WOODEN POLES 99 Q.2 o '? |*~ 1 i .5 aj > > 8 '-5 2 S S I a O o 00 T3 r O CO CO 100 POLE AND TOWER LINES w a a PH 5. ^ o ^ x* ^ 00 C j| 8 i j T3 -2 ol IM "3 c ^ iL c ~ IN (N 0) aS-5 S Q}*^ l-H aS- ^ 09 2 00 ^1* OO 00 000 reatment 1 pq 2-Brush 1 "3 Creating) now o3 I 2-Brush Fter erec- tion 55 l 1 P IH- pq ' O 03 pq 13 T3 T3 T)T3 a *C OJ a a ^3 g 1 T3 03^ SH o 03 O U a o O O 1-1 a a a 6c3 pq*" g s OQ ' 1 > . * o % CO CO co c^ H O 3 E* W WOODEN POLES 101 2 T3"* 1 * .s ao T3 3 a o ** o I O 00* i 00 I 1 00 1 00 1 o 00 **^ o o o o o vf wx X fc 1 :: S 1 3 a I 30 '; oj >> ^ rt CM i 5 S * if ii tj, CS Q.^3 V 1 1 1 1 1 1 1 3 1 o a i pi X PQ PQ PQ ffl PQ PQ CNI II i a o Material 1 o O | O d a d 3 o "3 55 1 i o t-jU 13 o 00 H ao o 00 ooo .8 1 ''*" JP o ^ CO 3 1 ~~^ o d i 6 o sj i c S CO 1 " f treatment Butt Creosote ii7 111 1 t_ PQ 1 Open tank 1 PQ Open tank | i -1 1 1 1 55 & O "3 'oS'w d . o O 1 1 a 6 a o O Galv.Sl H V O3 1 00 000 3 00 IN o . I |1 ^cc 11- 0> c O2 'w o 1 02 1 02 63 3 S| -3 O * |o * 02 O 3 O O IV e to q 02 x lff\ J - "3 d a . 0) i d d TJ 09 ^.^ .0 . a^j O a o 1 03 03^^ 33 If e i 02 6 3 02 & O o O 'I si 4 ^0 ^ i| *<> 00 CO IN CO It CO COO coco COCO * o Q f 312 o 1 si , 1 8SS OO 8 02 2 12 o TjHIN^HCO 5\ 00 8 o OIN o (N NCO ,4 3 80 00 8 3 Ml o O O O o o (N 1* - 2 22 CO^H e OiCO COi-H 1-1 2 2 si ^ 6 1 d PH a o 8 6 O a d O W O OH' ^ d O *. o J W w d 8 OH S ^ OH ^i ^ 02 H M CHAPTER VI STEEL POLES AND TOWERS OPEN-HEARTH STEEL Manufacturers' Standard Specifications. Issue of Feb. 6, 1914. (Abstracts') All tests and inspections shall be made at the place of manufacture prior to shipment. The tensile strength, limit of elasticity and ductility shall be deter- mined from a standard test piece cut from the finished material. The elongation shall be measured on an original length of 8 in. Rivet rounds and small bars shall be tested full size as rolled. Two tests pieces shall be taken from each melt or blow of finished 'material one for tension and one for bending but in case either test develops flaws, or the tensile test piece breaks outside of the middle third of its gaged length, it may be discarded and another test piece substituted therefor. Material which is to be used without annealing or further treatment shall be tested in the condition in which it comes from the rolls. When material is to be annealed or otherwise treated before use, the specimen representing such material shall be similarly treated before testing. Finished bars shall be free from injurious seams, flaws or cracks, and have a workmanlike finish. MAXIMUM PHOSPHORUS. 0.10 per cent. RIVET STEEL. Ultimate strength, 48,000 to 58,000 Ib. per square inch. Elastic limit, not less than one-half the ultimate strength. 1 400 000 Percentage of elongation, - Bending test, 180 flat ultimate strength on itself, without fracture on outside of bent portion. RAILWAY BRIDGE GRADE. Ultimate strength, 55,000 to 65,000 Ib. per square inch. Elastic limit, not less than one-half the ultimate strength. Percentage of elongation, Bending test, ultimate strength 180 to a diameter equal to thickness of piece tested, without fracture on outside of bent portion. MEDIUM STEEL. Ultimate strength, 60,000 to 70,000 Ib. per square inch. Elastic limit, not less than one-half the ultimate strength. 103 104 POLE AND TOWER LINES Percentage of elongation, 1,400,000 Bending test, 180 to a ultimate strength' diameter equal to thickness of piece tested, without fracture on out- side of bent portion. For material less than { 6 in. and more than y in. in thickness, the folio wing -modifications shall be made in the requirements for elongation: For each decrease of ^ 6 in. in thickness below % 6 in., a deduction of 2% per cent, shall be made from the specified elongation. In rounds'of % in. or less in diameter, the elongation shall be measured in a length equal to eight times the diameter of section tested. The variation in cross-section or weight of more than 23^ per cent, from that specified will be sufficient cause for rejection, except in the case of sheared plates, which will be covered by certain permissible variations. Rivets and Bolts. A well-made hot-driven rivet is superior to even a turned bolt because the excess length of the rivet has been upset into contact with the sides and irregularities of the hole, and because in cooling the contraction of the rivet presses the riveted pieces together and develops a friction which increases- the strength of the joint. The use of turned or machined bolts in drilled or accurately reamed holes would be of prohibitive cost for pole or tower work, while drawn-wire bolts (which are truly circular), in holes enlarged with a conical reamer, are not worth the additional expense. Since the number of bolts per joint is small, rarely over two and frequently but one, it is probable that the friction between the connected pieces is negligible. For transmission line construction the strength of a rivet or bolt is either its shearing or its bearing value. The former is the shearing strength per square inch of the rivet or bolt material multiplied by the area of the cross-section, and assuming, as will usually be the case, that 48, 000-58, 000-lb. material is used, we have the following shearing values: TABLE 21. SHEARING VALUES OF RIVETS AND BOLTS Working values (in Ib.) Diameter rivet or bolt (in inches) Ultimate shear (in pounds) (36,000 Ib. per square inch) Shop rivets (15,000 Ib. per square inch) Field rivets or bolts (1 2,000 Ib. per square inch) H 7,000 2,900 2,400 M 11,000 4,600 3,700 y 16,000 6,600 5,300 Vs 21,500 9,000 7,200 STEEL POLES AND TOWERS 105 The bearing value of a rivet or bolt is that of the effective area of metal pressed together during the transfer of stress, and, therefore, equals the product of the diameter of the rivet or bolt, the thickness of the thinner riveted piece, and the unit bearing value of the material. TABLE 22. ULTIMATE BEARING Ultimate bearing (72,000 Ib. per square inch) Diameter rivet or Thickness of thinner connected piece (in inches') H Me N Me M Me H 4,500 6,700 9,000 11,300 13,500 15,700 H 5,600 8,400 11,200 14,000 16,800 19,600 X 6,700 10,100 13,500 16,800 20,200 23,600 % 7,900 11,800 15,800 19,700 23,600 27,600 TABLE 23. BEARING VALUES OF RIVETS AND BOLTS' Working values Diameter rivet or Shop rivets (30,000 Ib. per square inch) Field rivets or bolts (24,000 Ib. per square inch) inches) Thickness of thinner connected piece (in inches) tt Me M Me H H. || H Me ^ Me| H Me H 1,800 2,800 3,700 4,700 5,600 6,500 1,500 2,200 3,000 3,700 4,500 5,300 N 2,300 3,500 4,700 5,900 7,000 8,200 1,800 2,800 3,700 4,700 5,600 6,500 H 2,800 4,200 5,600 7,000 8,400 9,800 12,200 3,300 4,500 5,600 6,700 7,900 % 3,200 4,900 6,500 8,200 9,800 11,500 2,600 3,900 5,300 6,600 7,900 9,200 In bridge and building construction, it is customary to specify that the distance between rivet holes shall be not less than three times the diameter of the rivet, and the distance from the center of the hole to the end or edge of the piece shall be not less than one and one-half times the diameter of the rivet. In practice, however, particularly when using small thin sections, these minimum distances are often reduced. In transmission line structures, the stresses to be transferred from the bracing to the main members are usually small and there is little danger of the material failing between or outside the holes, provided an excess- 1 In the working values for M-m- material, a more conservative factor of safety has been used, since the theoretical ultimate values are not always obtained in practice. For instance, it has been found by test that ^-in. material may crumple, allowing the bolt to pull through the hole at a stress less than the theoretical ultimate bearing value. 106 POLE AND TOWER LINES ively close spacing is prohibited. The distance from a hole to a rolled edge may be made slightly smaller than that to a sheared edge or end, since the material of the former is free from any injury due to the shearing process. In Fig. 46 are shown the minimum spacing, edge, and end distances, for each size of rivet or bolt, below which further reduction is inadvisable. Where clearance will permit, the end distances shown should be increased about in. to in. From ( a ) Minimum Spacing of Rivets & Bolts ( b ) " End Distance " ( C ) " Edge Distance FIG. 46. Minimum spacing, edge, and end distances. the edge distances given and assuming the usual thickness of material and sizes of nut, the minimum section of angle to be used for each diameter of bolt is found to be : TABLE 24. MINIMUM ANGLE SECTIONS Diameter rivet or bolt (in inches) Minimum angle Yz IK in. L 5 /8 1% in. L % 234 m. L % 2Y 2 in. L Lacing. The function of lacing is to stiffen the connected members by reducing the unsupported length of the compression section and also to transmit shearing stresses. If the shear is relatively large, the limiting condition may be the number of STEEL POLES AND TOWERS 107 rivets connecting the lacing to the main section, otherwise it will be the stiffness of the lacing itself. That is, the lacing is a compression member whose strength depends on its ratio of stiffness, or l/r. Since the minimum radius of gyration of a flat or bar is much smaller than that of an angle, the unsupported length of the former must be less. Again, flat lacing is more sub- ject to accidental injury than angle lacing because a slight bend in the direction of the thickness may easily occur and make the theoretical compressive strength negligible. D FIG. 47. Single flat lacing. FIG. 48. Double flat lacing. The value of r of a lacing bar is approximately 0.3 of its thickness; therefore, increased stiffness can be obtained only by additional thickness or by reducing the length. When double lacing is used, some reduction in effective length may be assumed because of the connection at the intersec- tion. With flat lacing, however, it is not correct to assume that the effective length is the distance from the end hole to the intersection. The inclination or angle a of the lacing affects both the length of the bars and their stress. The compressive stress in lacing is: shear S X sec. a or 108 POLE AND TOWER LINES C = 1.155 S for a = 30 C = 1.414 S for a = 45 C = 2.000 S for a = 60 Therefore, both th length of the bar and the stress will be in- creased 73 per cent, by increasing the angle a from 30 to 60. The reduced strength and increased stress may cause either the thickness of the bar or the strength of the rivet to become the limiting condition. In addition, it is unwise to use values of a. much in excess of 30 for single bars, 45 for double bars, and 45 for angles, as the stiffening effect of a light member connected by one rivet is very small with excessive inclinations. Angle Lacing. Owing to the fact that the radius of gyration of an angle is larger than that of a flat, the for- mer section allows a considerable in- crease in the width of the main mem- bers with less material in the lacing. The angle section may depend on the size of the bolt needed to transmit the stress in the lacing or, if the latter is turned in, on the permissible end and edge distances. In Table 25 will be found the values of r for various sections, and the length corresponding to different ratios of l/r. The maximum permissible value of l/r will depend to some extent on the character of the service 'expected of the bracing, as well as on its position in the structure. Bracing for secondary members, which are not liable to accidental injury or torsion, may be allowed larger ratios than main compression members, which from their position may be subject to both injury and torsion. Again, in selecting angle sections and the ratio l/r for any member, some consideration should be given to its position and protective coating, as the effect of these may make it advisable to employ a thicker angle. Thus, a bracing angle with the outstanding flange turned in and upward, and the angle itself in a vertical or in- clined plane, is less subject to injury either from accident or corrosion than a similar angle reversed or in a horizontal plane. FIG. 49. Single angle lacing. STEEL POLES AND TOWERS TABLE 25. ANGLES 109 Section Area Weight Least r Length in inches corresponding to various values of //r 60 80 100 120 150 180 220 l^XlHXfia 0.48 1.6 0.26 x y\ 0.63 2.1 0.26 1WX1V4XH 0.36 1.2 0.30 18 27 30 36 45 54 66 XMo 0.53 1.8 0.29 17 23 29 35 44 52 64 xu 0.69 2.3 0.29 17 23 29 35 44 52 64 154X1J4XH 0.36 1.2 0.27 XMo 0.53 1.8 0.27 XH 0.69 2.3 0.27 1?4X1%XH 0.42 1.4 0.35 21 28 35 42 52 63 77 XMo 0.63 2.1 0.34 20 27 34 41 51 61 75 XH 0.82 2.8 0.34 20 27 34 41 51 61 75 2 XlHXMe 0.57 2.0 0.27 17 23 29 35 44 52 64 XH 0.75 2-. 6 0.27 17 23 29 35 44 52 64 2 X2 X*io 0.72 2.4 0.39 23 31 39 47 58 70 86 XH 0.94 3.2 0.39 23 31 39 47 58 70 86 2MX2J4XM* 0.81 2.8 0.44 26 35 44 53 66 79 97 XH 1.07 3.6 0.44 26 35 44 53 66 79 97 2V$Xl^X?i6 0.72 2.4 0.33 20 26 33 40 49 59 73 XH 0.94 3.2 0-32 19 26 32 38 48 58 70 2^X2 X^e 0.81 2.8 0.43 26 34 43 52 64 77 95 XJ4 1.07 3.6 0.42 25 34 42 50 63 76 92 2HX2J4XM6 0.91 3.1 0.49 29 39 49 59 73 88 108 XW .19 4.1 0.49 29 39 49 59 73 88 108 XSid .47 5.0 0.49 29 39 49 59 73 88 108 2HX2HX91* .00 3.4 0.54 32 43 54 65 81 97 119 XH .32 4.5 0.54 32 43 54 65 81 97 119 x^ .63 5.6 0.54 32 43 54 65 81 97 119 3 X2 X3io .91 3.1 0.44 26 35 44 53 66 79 97 XN .19 4.1 0.43 26 34 43 52 64 77 95 Xtf* .47 5.0 0.43 26 34 43 52 64 77 95 3 X2^X^6 .00 3.4 0.53 32 42 53 64 79 95 117 X!4 .32 4.5 0.53 32 42 53 64 79 95 117 X51o .63 5.6 0.53 32 42 53 64 79 95 117 3 X3 XMe .09 3.7 0.59 35 47 59 71 88 106 130 XH .44 4.9 0.59 35 47 59 71 88 106 130 XMe .78 6.1 0.59 35 47 59 71 88 106 130 3KX2HXH .44 4.9 0.54 32 43 54 65 81 97 119 XMo .78 6.1 0.54 32 43 54 65 81 97 119 3^X3 XW .56 5.4 0.63 38 50 63 76 94 113 139 XM .94 6.6 0.63 38 50 63 76 94 113 139 3HX3HXM 2.09 7.2 0.69 41 55 69 83 103 124 152 XH 2.49 8.5 0.68 41 54 68 82 102 122 150 XMo 2.88 9.8 0.68 41 54 68 82 102 122 150 XMi 3.25 11.1 0.68 41 54 68 82 102 122 150 4 X4' Xff 2.41 8.2 0.79 47 63 79 95 118 142 174 XX 2.86 9.8 0.79 47 63 79 95 118 142 174 XMe 3.31 11.3 0.78 47 62 78 94 117 140 172 X^ 3.75 12.8 0.78 47 62 78 94 117 140 172 5 X5 X9i 3.61 12.3 0.99 59 79 99 119 148 178 218 XJii 4.19 14.3 0.98 59 78 98 118 147 176 216 X*4 4.75 16.2 0.98 59 78 98 . 118 147 176 216 6 X6 XH 4.36 14.9 1.19 71 95 119 143 178 214 262 XMfl 5.06 17.2 1.19 71 95 119 143 178 214 262 X^ 5.75 19.6 1.18 71 94 118 142 177 212 260 XHe 6.44 21.9 1.18 71 94 118 142 177 212 260 110 POLE AND TOWER LINES Tower Connections. As it is usually advantageous, from a manufacturing standpoint, to maintain like punching on both flanges of the main leg angles, the panel points are often at the same elevation on all four sides of the tower and the holes are opposite each other. This necessitates clipping the out- standing flange of one brace at each panel point, to clear the inside brace on the adjoining face and also to provide space for the insertion of the inside connection bolt (Fig. 50). An alternative method is to stagger the main panel points a few inches and thus obtain the necessary clearances without clipping. To do this, and also maintain like bracing angles on all faces of the tower, it is neces- sary to make the tower out of square, so that the increased width of two opposite faces will compensate for the greater length of the diagonal bracing in its lowered position. FIG. 50. Bracing connection, FIG. 51. Bracing connection. The outstanding flanges of horizontal or inclined angles should always be turned up, as in this position they drain and dry quickly and do not collect dirt or hold water. For similar reasons it is inadvisable to use any closed pockets, or semi-closed pockets, anywhere in the structure, as they are certain to become clogged STEEL POLES AND TOWERS 111 with refuse and filled with water. Since moisture is a necessary condition of all decay and corrosion, rapid and thorough drainage are essential to a good design whether the material be timber or steel. FIG. 52. Suspension insulator FIG. 53. Strain insulator connections, connections. FIG. 54. FIG. 55. Single 3^-in. bolt connections should be prohibited in the main bracing system of wide-base towers, except possibly for the connection of secondary members such as sub-panel struts, whose sole function is to reduce the unsupported length of other 112 POLE AND TOWER LINES members. Such struts have frequently been given odd inclina- tions as compared with the main diagonal system, with a result far from pleasing in appearance. It is generally true that the diagonal bracing on tangent towers with well-inclined main legs has to withstand only relatively low stresses. In the frantic endeavor to reduce costs, this fact has led to the use of widely separated bracing, which is incapable of properly supporting the most important section the main leg members. In other cases, in an effort to obtain the greatest possible theoretical strength, the bracing has been closely spaced, but cut down in section to such members as \Y in. X IJi in. X -Hj-in. angles with large values of FIG. 56. The connection of disc-type insulators to steel poles or towers is usually made by providing a U-bolt or plate into which a hook is inserted. In other cases, the connection hole is made in the crossarm angles as shown in Fig. 54. There is no particular merit in one form of connection rather than in another, provided the thickness of the crossarm material is sufficient to transmit the stress to the arms. The thickness of the material outside the hook hole must be about % in. on account of the spread of the hook and the inside edges of the hole should STEEL POLES AND TOWERS 113 preferably be rounded, otherwise the hook will bear upon two points only. Latticed Poles. Square, latticed, structural-steel poles may be of any width from that of true narrow-base poles used along curb lines to the wide poles which are in reality towers. There is no fixed dividing line between a pole and a tower, unless it be that of strength and rigidity, or possibly the use of widths which preclude shop riveting and shipment assembled. By far the FIG. 57. Steel crossing pole. greater number of the structural-steel poles used are square in cross-section, one angle at each corner, and assembled and riveted before shipment. In the case of long poles, it will fre- quently be found advantageous to ship in two sections, and bolt them together in the field. There is no reasonable objection to the use of such field bolts, provided a splice is used of sufficient strength and length. The splice angle can be made an interior splice, with the root of the angle ground to fit the fillet of the main legs, and thus be comparatively unobtrusive in the final appearance of the pole. Several types of poles are in use, the most common being 114 POLE AND TOWER LINES those with a regular bevel or those with parallel legs. Parabolic slopes have been used and they present a very graceful appear- ance under favorable conditions, although the rapid increase in width for longer poles may result in an inconvenient spread at the ground line. The parabolic slope has its true function in the application to very high towers of uniform height. In some cases, the top portion of regularly sloped poles has been made with parallel sides in order to maintain like punch- FIG. 58. Railroad crossing pole, 13,000 volt wires. ing, length and pin spacing of the crossarms. This does not always give a good appearance, however, on account of the marked bend at the lowest arm. The foundation or anchorage of latticed poles may be made in three general ways, i.e. : by simply burying the lower portion, by providing separate ground-stub angles as in a rigid tower, or by a base plate or plates attached to anchor bolts. Further, STEEL POLES AND TOWERS 115 the material entering the ground may be either painted or galvanized, although the former should be encased in concrete. Base plates and anchor bolts are sometimes more expensive in material and workmanship than either of the other designs, but allow concrete foundations to be built in advance with the least probability of error in setting the connections to the super- structure. FIG. 59. Arrester house and guyed terminal pole. As shown in Fig. 59, some quite elaborate bases have been used, the general purpose of these being to more firmly fix the base of the column and to provide an excess of material against cor- rosion. The real benefit derived from such construction, however, is open to question, and the gain does not seem com- mensurate with the cost. Steel poles can readily be protected against injury from accidental collisions, overflow, high water, 116 POLE AND TOWER LINES FIG. 60. Two-circuit steel poles. FIG. 61. Steel poles with wooden arms, 11,000 volts. STEEL POLES AND TOWERS 117 etc., by the simple expedient of encasing the lower portion in concrete. Either a section of the latticed pole itself may be encased or long stub angles, embedded in high foundations, may be attached to the superstructure in the usual mariner (Fig. 60). Figs. 63 and 64 are views of the north and east sides respectively of a square latticed structural steel pole tested to FIG. 62. destruction. The views are unusually good in that they show very clearly the typical compression failure by buckling. It will also be apparent that angle lacing is very effective in preventing buckling in the plane of the lacing, but that it has only a little restraining influence at right angles to its plane. It is further apparent that with latticed steel poles failure does not necessarily cause the pole to fall or break off. Instead the 118 POLE AND TOWER LINES pole buckles near the base allowing the top to deflect, thus de- creasing the wire tension but without causing the wires to fall to the ground. The concrete foundations shown are relatively large for the poles they support, but were considered desirable in view of the location in a river bank. The design of square latticed poles, in general, may be resolved into a determination of the stresses at the ground line or rather in the first panel above ground. This statement is based on FIG. 63. FIG. % 64. Latticed steel pole, test. the assumption that, owing to the tops being relatively wider than in wood poles, the upper portion of the pole has an excess width as compared with the lowest panel. It is further predi- cated on there being no attempt made to seriously reduce the sections of the material in the upper half. In the case of para- bolic slopes, stress determinations must be made at various heights since the widths presumably follow, more or less closely, the changes in bending moment, so the weakest section may be anywhere. Owing to the greater rigidity of pole frames, STEEL POLES AND TOWERS 119 the breaking strength per unit of area in a pole will exceed that in a wide-base tower. Again, since the main legs have little inclination, the web system is compelled to carry the shearing stresses, which in a tower are partly carried by the main legs. For these reasons, the web or lattice is more often limited by the strength required than is the bracing of a wide-base tower. Shearing stresses must, therefore, be computed, and the lattice and its connection to the main legs designed accordingly. Single flat lacing should not be used, except for small stresses and in narrow widths, since, as stated before, its strength is low and it is easily injured. Double flat lac- ing is applicable to greater stresses and widths, but it is often not as economical as angle lacing. In any case the strength of the pole depends on the unit strength of the weakest unsupported length, which is usually the lowest panel but may be the entire pole if the width is small and the height great. That is, the l/r of the entire cross- section of the pole may be greater than that of any single panel. The character and spacing of the lattice will determine to a large extent the amount of support afforded by it to the main leg angles at the panel points. When both faces of a pole are connected with lacing at the same level, the unsupported length of the main leg is the distance between panel points. If, however, the lacing is staggered, so that the support is in one direction only at each panel point, the unsupported length of the main leg is somewhere between a half and a whole panel length. FIG. 65. 120 POLE AND TOWER LINES DESIGN OF STEEL POLE One %-in. Siemens-Martin galvanized stranded-steel ground wire. Three No. 1 hard-drawn stranded copper, 33,000 volts. Span = 400 ft. Nor. sag = 9 ft. in. Nor. tension (GOT.) = 570 Ib. Max. sag = 10 ft. 6 in. Max. tension (0F., ^-in. ice, 8 Ib. wind) = 1960 Ib. Ground Wire - 70 v_ Clearance } T k2'2 ? 36 J c.g. FIG. 66. Design of steel pole. Elastic limit, No. 1 wire = 2180 Ib. Wind pressure on wires (^-in. ice, 8 Ib. per square foot wind) : %-iri. ground wire = 0.917 X 400 = 367 Ib. No. 1 conductor = 0.885 X 400 = 354 Ib. Wind on pole = 13 Ib. per square foot X 1^ times exposed area of windward side = 20 Ib. per lineal foot. STEEL POLES 'AND TOWERS 121 With the above transverse loading and no broken wires, the compressive stress in the leg in the lowest full panel above the foundation is ob- tained by taking moments of the forces about the panel point 2 ft. 1 in. above the foundation. Ground wire 367 Ib. X 42.4 = 15,560 ft.-lb. Power wires 354 Ib. X 39.9 = 14,120 ft.-lb. Power wires 354 Ib. X 2 X 37.4 = 26,480 ft.-lb. 42 4 2 Wind on pole = 20 Ib. X = 17,980 ft.-lb. Total bending moment = 74,140 ft.-lb. Since the lever arm of the resisting forces = 1.9 ft. : 74,140 ft.-lb. -r- (1.9ft. X 2 legs) = 19,500 Ib. Vertical load steel = 1700 Ib. Vertical load wires and insulators = 1500 3200 Ib. -=- 4 legs = 800 Ib. Total compressive stress in each leg = 20,300 Ib. Since 1 L 3 X 3 X M = 1.44 sq. in., the maximum unit stress in each leg = 20,300 -5- 1.44 = 14,100 Ib. per square inch. If the side face of the pole is the same as the view shown, the leg is restrained from buckling in one direction by the intersect- ing diagonals, so the maximum l/r will be either the full panel length divided by the radius of gyration parallel to the leg, or the half panel length divided by the least radius of gyration, 1 i.e., I 44 in. I 22 in. The ultimate strength of the angle based on the greater value of l/r and the curve in Fig. 33 is 35,000 Ib. per square foot; there- 35 000 fore the factor of safety is ' = 2.5. Similarly the tensile stress in the other leg can be obtained by taking moments about the lowest panel point and subtracting the vertical load stress, which is always compression. For tensile stresses the area of the angle should be reduced by the area of one rivet hole. Since the ultimate strength in tension is about 60,000 Ib. per square inch, the compressive stress is generally the governing factor. Stress in Diagonals. The horizontal shear at the lowest crossarm is: x See page 119, the assumption of the half panel length is conservative, 122 POLE AND TOWER LINES Ground wire = 367 Ib. XI = 367 Conductors = 354 Ib. X 3 = 1062 Wind on pole = 20 Ib. X 6 = 120 Total = 1549 Ib. This will be carried by the web systems of two faces, making a shear of 775 Ib. per face. With inclined legs, the stress in the diagonals will decrease from the lowest arm to the ground. Since the taper of the legs in this case is small, the stress in the diagonal just below the arm can be said, with only a small error, to equal the shear multiplied by the secant of the slope. Assuming a 45 slope for the diagonals, the stress will be 775 Ib. X 1.414 = 1100 Ib. Stress in Crossarms. The crossarms should be designed for either the maximum ice and wind loads on both spans or for the maximum ice and wind loads on one span combined with a longitudinal load due to the break- ing of the wire in the other span. CONDITION No. 1 Vertical load (*^-in. ice on wires) 0.770 Ib. per foot X 400ft = 308 Ib. Insulator and pin = 25 Ib. 333 Ib. 333 Ib. X 35 in. . . = 11,680 in.-lb. 3 I 2 Weight of arm = 15 Ib. per foot X ^ X 12 = 860 in.-lb. Horizontal load (8 Ib. per square foot wind on wire - 0.885 Ib. per foot X 400 ft.) = 354 Ib. X 12 in = 4,250 in.-lb. Total bending moment = 16,770 in.-lb. 2 Is 3> in. X 3 in. X Ke in. - = 2 X 0.95 = 1.90. c Max. unit stress in crossarm = 16,770 -*- 1.90 = 8800 Ib. per square inch. CONDITION No. 2 Vertical load (3^-in. ice on wires) 0.770 Ib. per foot X 200ft = 154 Ib. Insulator and pin = 25 Ib. 179 Ib. STEEL POLES AND TOWERS 179 lb. X35in. 3.1 2 Weight of arm = 15 lb. per foot X ~ 2 ~ X 12 Horizontal load (8 lb. per square foot wind on wire 0.885 lb. per foot X 200 ft.) = 177 lb. X 12 in.. 123 6270 in.-lb. 860 in.-lb. 2120 in.-lb. Total bending moment = 9250 in.-lb. 9250 -4- 1.90 = 4900 lb. per square inch. Longitudinal load: 1960 lb. X -J^RQ = 3720 lb. -=- 1.94 sq. in. = 1900 lb. per square inch. Maximum unit stress in crossarm = 6800 lb. per square inch. It should be noted that comparatively few insulators and pins can safely carry the longitudinal load assumed in Condition No. 2, even if the tie wires were able to transmit the load to the insulator. Generally, only strain poles or towers are able to FIG. 67. Curb-line poles. fully meet this condition, as they are provided with either strain insulators or double-pin insulators and more effective tie or clamping devices. Curb-line Poles. Where high-voltage lines are located along curbs, it is important that the construction be of a high degree of excellence, and that structures be used which combine strength, 124 POLE AND TOWER LINES a restricted width, and at least some esthetic qualities. A rela- tively high amount of insulation, with the consequent freedom from electrical failure, affords the greatest protection for the least expenditure. The width of poles must necessarily be restricted at the ground line, the maximum permissible width being from 24 in. to about 28 in. These dimensions are not fixed by any definite rule, but result from the precedent established by the use of large wooden poles. There are many places, however, where greater widths would not create any real ob- struction nor presumably any active criticism. There should, in fact, be less objection to a line of well-designed steel poles than to wood poles, since their appearance is superior and only about 'one-half as many poles are required. The surfaces of the poles which may possibly come into contact with pedestrians should be free from projecting edges; therefore, the latticing should be inside the main legs and its connections riveted rather than bolted. It will sometimes be found advantageous to prevent the climb- ing of poles by unauthorized persons. This can be done by clamp- ing wire netting against the lacing a short distance above the ground, or by filling the interior of the pole with concrete. The latter is not expensive, the forms being extremely simple, and it strengthens the pole both for general use- and as a hub guard. Triangular Poles. The three-legged poles used heretofore have generally been of a proprietary type employing U-shaped main legs fastened at intervals with horizontal cast spreaders, but a few have been built of structural angles. The material of the former is usually rerolled rail of greater unit strength but much harder and more brittle than structural steel. Owing to the shape and small flanges of U sections, as well as the hardness of the material composing them, it is not practicable to lattice the main legs by a true web system. In poles of this type the main legs are inclined more than is usual in square latticed steel poles, and the shearing stresses must be carried by the main leg sections. In any structure having a triangular cross-section, the strength is not the same in all directions; therefore, three-legged poles are not well adapted by their form to withstand heavy loads. When built of homogeneous material, which is difficult to in- sure in rerolled stock, such poles deflect considerably and will bend without actual fracture much more than square latticed STEEL POLES AND TOWERS 125 poles. Failure of three-legged poles should, therefore, be con- sidered as occurring when a permanent bend is produced, or when the fastenings become loosened. The logical service for poles of this design has been demon- strated by practice to be for supporting light lines in locations 65 Ft. High 110000 Volts 64 Ft. High 110000 Volts 43 Ft. High 102000 Volts 62.5 Ft. High 70000 Volts /\/\ 60 Ft. High 60000 Volts 60 Ft. High 50000 Volts 60 Ft. High 45000 Volts 60 Ft. High 44000 Volts FIG. 68. Types of towers involving difficult transportation, the poles being " knocked down," shipped in light-weight packages, and assembled in the field. Wide -base Towers. The forms of the frames which have been used in wide-base towers are of many types, as shown by Fig. 68a, 6, c, d, e, f, g and h. The majority of designs are 126 POLE AND TOWER LINES determinate frames, i.e., those in which the stresses may be computed directly and definitely. Some, however, are what are termed indeterminate frames, since all the stresses cannot be computed directly owing to the fact that there are several paths through which the loads may be carried to the ground. In such designs the actual distribution of stress will depend in part on the relative rigidity of the different paths. Although it is entirely possible to build indeterminate frames having any necessary strength, the practice involves a liability of error. In general, the designs show a direct transfer of the tension and compression elements of the bending moment through the main legs, and a more or less complicated stiffening system whose chief function, in some cases, is to provide local support for the main legs. The actual sections used for various members, even for some- what similar installations, present marked variations. In fact there is no known structural theory which would make some de- signs desirable from either the purchaser's or the manufacturer's standpoint. For instance, a certain installation has 1 L 4" X 4" X M" f r the main legs, the panel length being 13 ft. The corresponding l/r is therefore 202, which is excessive for a main compression member. In order to show more clearly the relative undesirability of the section in question, it may be compared with two other sizes of angle as follows: Area Length l/r Breaking strength per square inch Total breaking strength 1L 4in.X4in.X% in. 1L 5in.X5in.X%6 in. 1L 6in.X6in.XK6 in. 5.44 5.31 5.06 13ft. 13ft. 13ft. 202 159 131 12,000 Ib. 16,000 Ib. 20,000 Ib. 65,200 Ib. 85,000 Ib. 101,200 Ib. It is evident from the foregoing that an L 6" X 6" X KG"? having a much smaller value of l/r, would have been stiffer, stronger, lighter, and more readily fabricated than the section used. In fact, either the 5" X 5" or the 6" X 6" angle would have been a stronger and cheaper section. It may be observed further that the actual factor of safety of the construction in question, under the maximum load assumed in its design, is a minus quantity. This is due solely to the ex- cessive value 'of l/r or, in other words, inadequate bracing. STEEL POLES AND TOWERS 127 FIG. 69. FIG. 70. FIG. 71. TABLE 27. KEY TO TOWER SECTIONS. Figs. 69, 70, 71. Mark Section 1 ................ : ............. L \Y 2 X IY 2 X H 2 .............................. L1%X m X YB 3 .............................. L2 X \ 1 A X M 4 .............................. L2 X2 X K 5 ............ , 6 7 .............................. L2} X2 X 8 9 10 ......................... .'.... L3 X3 X 12 X 3^ X 13 .............................. L4 X 4 X 14 15 16 L4 X 4 X M X y 1Q L4 X 4 X K X H Chan. 4 in. 5.25 Ib 17 .............................. Chan: 5 in. 6.5 Ib. Flexible Frames. The steel A-frame trolley-wire support and transmission pole shown in Fig. 72 is typical of some of the lighter installations in Europe, where the A-frame was originated. This view is of additional interest in that the light brackets to which the trolley-wire messenger cables are attached would have 128 POLE AND TOWER LINES little or no strength to withstand a broken cable, thus tending to show that failures in wires are not very seriously considered by the European designers of such structures. The illustration also shows a pair of grounding arms under the transmission cir- cuits at the top of the pole. TABLE 26. RECORD OF SINGLE AND DOUBLE CIRCUIT WIDE BASE TOWERS. REPORTS (1915) PROM SIXTEEN LINES HAVING A TOTAL OP 7362 TOWERS IN SERVICE Number of towers Tower failures Remarks 244 None A crossarm twisted, due to burnt conductor. 378 None 1041 None 370 None 64 None 243 None Three conductors burnt, due to contacts when heavy sleet unbalanced sags during removal. 1079 None 593 None 184 None 33 None 851 None 110 None 324 None Four broken insulator connections. 913 None A number of crossarm hanger rods have failed. Several conductors burnt. 748 1 Due to guy failure. Also about 70 breaks in con- ductors. (Storm of Apr. 2, 1915 worst on record.) 187 1 Compression leg of tower carrying 1250-ft . span. Also a number of slight buckles in other towers. 7362 (Storm of Apr. 2, 1915.) When flexible frames were first used in this country, it was customary to insert a rigid or dead-ending tower at corners and at intervals of about five spans on tangents. In recent years, however, there has been a tendency to omit some of these stiffening structures on the theory that there was no real danger of the line falling longitudinally like a "house 'of cards." In view of a number of accidents that have occurred on such lines, it would seem desirable either to return to the former practice or to obtain the effect of stiffening structures by a more liberal use of guys. A general objection to the flexible pole or frame is not in- STEEL POLES AND TOWERS 129 tended, as they may have a proper usefulness in the construction of the lighter and less important lines. It is further probable that some criticisms of such construction would be more accu- rately directed to their shallow foundations, span lengths, details, and incorrect installation than to their use under favorable conditions. Too much emphasis has been placed upon the need of flexi- bility, and to spend any efforts in providing greater flexibility than is found in the usual forms of support is a move in the wrong direction. A structure 40 ft. or more in height with only the resistance to bending inherent in such members acting as FIG. 72. A-frame trolley poles. European installation. cantilever beams has naturally very much more flexibility than is required. To balance wire tensions only a slight movement of the pole top is required. Narrow-base A-frames have some- times erroneously been termed poles or semi-flexible structures. A pole or tower is an enclosing or box-girder structure with four planes of bracing, whereas the narrow-base A-frame or latticed channel has but one central plane of bracing and is a true flexible frame. Assuming that a reasonable amount of skill has been employed in selecting spans, heights and main sections, the next most important step in building an adequate A-frame line is to pro- 130 POLE AND TOWER LINES vide an overhead ground wire and substantial foundations. The ground wire, which should have considerable strength, may be given a little less sag than the conductors so it will serve as a continuous head guy, the value of which can hardly be overestimated. In fact, it is difficult to string the conductors unless there is a ground wire in place to steady the frames. The foundations are also of great impor- tance, since flexible frames are not well adapted to withstand eccentric loading. If the base of one main leg settles, or is erected at a different level than the other, the deviation of the top of the frame will be about seven times as much as the settlement, depending on the height and spread at the base. Moreover, as the failure of a frame will usually result from the buckling of the main chan- nels, anything which disturbs an equal distribution of stress be- tween the legs will promote failure. That considerable foundation stress may be developed is shown by the fact that in a number of -tests the bent rods used to attach the anchor members have been straightened out at the bend (Fig. 73). It seems probable, therefore, that more rigid attachments are needed. FIG. 73. FIG. 74. The conditions which produce buckling are not very clearly understood, or rather their limits are not definitely known. If the main channels are assumed to be made of absolutely identical material and the base of the foundation is firm and unyielding, some difference in the lateral supports at the ground line or in the rigidity of the bracing connections may allow sufficient STEEL POLES AND TOWERS 131 deflection to start buckling. As the failure is a compressive failure in a relatively long column, any measures which restrain the main legs from moving sideways at any point will be of effective service. A comparatively long stiff connection of the bracing with the main legs is useful as it stiffens the column locally. Such connections, therefore, should never be made with less than two rivets, and should preferably be not less than 6 in. long. Further, the diagonal braces should not have any slack, and if made of rods or other adjustable members, should be tightened as near equally as possible. One of the most important requirements in A-frame construc- tion is to provide guys at all corners above about 3, and to pro- FIG. 75. A-frame failure. vide strain towers at corners above 10. This is in addition to a more or less definite number of head guys on tangents. In wire stringing, it is almost impossible to pull out the wires unless an overhead ground wire has been previously clamped in posi- tion to steady the frames. It is further necessary to pull all three wires of a circuit at one time, using a dynamometer and an equalizing rig to balance the tension in the wires. If an attempt is made to string one wire at a time, the frames may be twisted by the unbalanced loading. It is entirely possible to design and construct A-frame lines which will be satisfactory in cost and operation, but this cannot be done without the exercise of care and skill both in the design and in the erection. 132 POLE AND TOWER LINES The present tendency, and the writer ventures to believe it a proper tendency, is toward the use of galvanized ground-stub angles, whether the superstructure is painted or galvanized and with either concrete or earth back filling. Galvanizing such members is a relatively inexpensive operation. If desired, the galvanized surface can be painted over at the ground line. No reduction of section on account of the protective coating should be permitted in the ground stubs. As flexible frames are painted as readily as narrow-base poles, the cost being in the neighborhood of $2 per structure, there is no objection as far as cost is concerned to the use of painted rather than galvanized superstructures. RECORD OP FLEXIBLE A-FRAMES. REPORTS (1915) FROM Six LINES HAVING A TOTAL OP 958 FRAMES IN SERVICE Number A . frame A-frames failures Remarks 158 None 262 None 62 None 100 None 290 3 86 56 Two cross arms twisted. Four broken insulator connections. Foundations pulled up. Storm of Dec., 1914. 958 STEEL POLES AND TOWERS 133 ui suoionpuoa J - ii-'^j "ZUOH j ^.1 aouuisap 'UBds -puejg 5 ON 51 a I CD.S I ^ "35 o Southern Power Co. t. S. rvic nn. Central Lt. & wr. Co. & jo S |S I & ! Re e. 1(3 W * 1O 134 POLE AND TOWER LINES 1 i o 8 PI CO O o ^. o 00 IM o 00 ^1 s X s s s * a 6 I Canada 1 1 a _a Canada 6 03 O oj s '1 43 o 0505 O5 O5 05 CO 05 8 o O5 a fc o a Ji o PQ q PQ j>1 1 >1 51 |1 5 51 1 SI > >-s 0-5 a-o O * 02 o o O o 02 o O o O * O O O xpO spO spO rts ^s o (D * NpO , * 1 a a 6 ^ a o O o u a a o O 3 a o O U a ^li 1 1 1 S 18 i 11 cr I CO S I g 8 a i 1 18 I O i = I I c cs s Is 02 ? P Penna. Water Power Co. I 1 K- a^ &sl i s^J |HO hern California son Co. Falls & l Power toria ransv o. Jl 1 8 18 136 POLE AND TOWER LINES o O .5 08 o >rt . o O 00 it X * Oo 60 fcfc Canada "3 O d 3 d 3 Canada c3 03 oj 03 PH i O Oi ,-HOO 0505 05 O5 1 05 O5 05 co 05 CO 05 A PH' PH g i-g H |l PH" oi 5 00 O I | O 05 05 | | o o 3 o S o 1 00 O.I o "0 O 'O t^0005 II ' > - > ... ... ... ... : ... '. . ... ' ' i s 5 s JS NOC - (NCO ** (N (M (N rH IS d . 3i 1 d d O d o O O ^ 6 & O & O O 4 s i oo oo 250,000 250,000 300,000 i - 8 08 s 300,000 8 1 IN 1 d 02 02 d 02 i 02 t 02 d 02 d 02 d 02, i 02 d 02 d 02 d 02 d . 1 d 02 1 1 |. 1 | 1 8 | | | ' | | | 1 o i? 8 8 8 8 8 O O O O i s 5 5 o CO (N Tf* so CO ithern Power Co. ithern Power Co. iwinigan Water Pwr. Co. D 1 s s 1 ffl 33 G a) CO 01 6 c3 ^ h fe 6 28 dro-Elec. Pwr. 3 in mission. 0) 5 : fi ll O O bama Power Co. a 1 fa bC O FH Q) S 03 1 M * OTJ E-i ^^ w w o^ 00 O5 O l-j ~ 1 sjojonpuoo no puij& 10 1-1 o 2 I ui '(llpj) }33JS jo" ssaujiaiqj, 22* 3S X aot^eooq Mexico JA "3 a CQ "oS O ! 1 d a CM 1 g ,,mq9 A CO a> CO 03 CO OS rH : CO 2 CO 2 'M } H : s m- W "^ A S 1 S 1 S o co o 3 e 1 W . TO li -i-C - I I CO CO CO sis ^ inemrtaiiv ._. ... ... ... ... ' O s P 5 )A\O[[B dOUttJtjap peaqjSAO 'Uij-\[ i s co 1 3 1 U.4N. o ; I 1 a 1 y 'uBds -put3}g 1 1 1 1 1 g 1 EH * I "08 1 0) O"S 11 1 s i i ^ g ~_5 P g 1 o H > Z -aj o o i 5- o "3^ 1 1 02 i 11 1. " s N00 eds * s B * -H c -" N 2 23 j2 55 140 POLE AND TOWER LINES I >j | 3 03 G .2 9 * HH HH HH G H q H 2 ^ CD M j^ CO S3 1 CO I CD . a^ 2 1 L CO CD 1 1*1 CO Q O 03 S J 1 H ! | "S G S 6 I "S O a c^ a a o | S CO ' g 05 ' A 00 O "2 r-gg t> 1 o 03 G S s a S a 02 c * f i 1 ) CC 00 I 1 I c 1 | f 5 .L o3 .2 S 6 H IH '> O ( D '-3 O s 0> 3 03 1 s c & I r Societa Italian trochimica. Societa Genera dell'Adamello. Societa Elettri di Ponente. Muncie Electric a 3 .2 G .2 2 - ! G s o CD 03 o 3 1 NewH * CO * > i H 5 O5 o J 00 OJS 05 ^ tc ^ 1 a w 2 o oo coco 2 I ^ C G . o3 a> I s ' ' 1 S -i-j g 10 i 02 "cS -s 4) EH ^ tl G O 13 1 1 ) in. high. Glass or earthenware jars for hardware samples. Vessels for washing samples. Tray for holding jars of stock solution. Jars, bottles and porcelain basket for stock solution. Cotton waste. Hydrometer cylinder three (3) in. diameter by fifteen (15) in. high. Thermometer with large Fahrenheit scale correct at 62 and 68. Hydrometer correct at 1.186 at 65F. CHAPTER XI LINE MATERIAL Tie Wires. The usual function of the wire "pigtails," used to attach the conductors to the insulators, is to prevent falling and creeping rather than to effect dead-ending. Even assuming the most efficient form of tie attachment, there is some unknown relation between the size and number of turns of the tie wire and the size of the conductor which will be dead-ended thereby. Further, the effectiveness of a tie wire is almost entirely depend- ent on workmanship and the attachment must be made without nicking the power wire. Stranded cable, particularly in. large sizes, can be tied more effectively than solid wire, owing to the grip of the tie between the strands. Tie wires should either be made of the same material as the con- ductors, or be coated with that material to prevent electro- lytic action. The wire itself must be "dead soft" or free from any tendency to spring loose after wrapping. Many kinds of ties have been devised, although they are all variations of two gen- eral types, i.e., those depending upon a severe constrictive action in one or two turns, and those in which friction is developed over the extended surface provided by a number of turns. The latter type is generally preferred, especially for soft copper and alu- minum, as it is less liable to injure the conductor. The efficiency of the tie has a very direct bearing on the con- ditions of loading which can logically be assumed for the poles or towers, although the existence of this relationship seems to have been generally ignored. Thus, it is probable that in very many installations, certain assumptions are made as to the stresses that may be transmitted by the wires to the supports, without any very direct information either as to the type of the tie to be used or its ability to transmit such loads. For example, if a given tie, or any tie, is incapable of developing a stress of 5000 lb., a No. 0000 copper cable cannot exert its maximum tension on the structure, because the tie would fail before transferring such a load. The example chosen is perhaps 189 190 POLE AND TOWER LINES unnecessarily severe, since a No. 0000 copper cable cannot be successfully dead-ended by any tie or pin insulator with which Tie Wire Fig-Tall or Holding-Down Tie. For Aluminum Conductor use Strands of 214 000 C.M. Aluminu Cable for Holding-Down lie. For Copper Conductor use No.10 N.B.8. Annealed Telephone (_ Wire or Annealed Strands of No. 00 Copper Cable. About 1% Ft. of Wire Necessary. FIG. 111. Ties. the writer is familiar. The principle, however, applies to all cases so that, except at corners, the poles cannot be subjected to LINE MATERIAL 191 greater wire loads than the ties used will transmit. The writer does not infer that supports in general have been made too strong, but that the reasons given for the strengths used are not logical and not in accordance with the facts. Loops. Loop cables, either as shown in Fig. 112 or having Crosby or other clips instead of a wrapped splice, have some- times been used to dead-end or securely attach the conductors to pin-type insulators. The loop is placed around the neck of the insulator and the end is clamped to the conductor with three- bolt clamps or several clips. If properly made, this attachment has a much greater strength than the average insulator or pin, but care must be exercised to avoid injuring the conductor. With soft-drawn cables particularly, any attempt to secure FIG. 112. Loop cable. strength by overtightening two clips will probably result in cut- ting the strands under the clips; therefore, three or more clips should be used in order to reduce the grip at any one point. The strength of loop cables is primarily a matter of workman- ship, either to secure an efficient splice or to so attach the loop to the power wire that the ultimate strength of the latter may be developed without injury to the strands. A few tests, incidental to others, gave the following results: Test No. 1. A loop cable of 250,000 c.m. soft copper directly attached to the testing machine failed in the loop splice at 7200 lb., after developing the entire breaking strength of the cable. Test No. 2. A similar loop cable connected to the conductor with Crosby clips began to slip at 2900 lb. and continued tightening of the clips cut the strands of the cable. Test No. 3. A loop cable of ^-in. steel failed at 9800 lb. Splices. The methods of splicing conductors, ground wires and telephone wires varies considerably, the older and less effi- cient forms of splice being still quite common on short-span work. For the higher voltage longer span lines, however, splices are now generally made by the use of splicing sleeves, or special connectors. 192 POLE AND TOWER LINES By such methods, the electrical conductivity may be maintained without a very great decrease in mechanical strength. Since hard or medium-hard conductor material and high-carbon steel ground wires are commonly used in true transmission-line con- struction, it is not advisable to do any soldering or sharp bending at splices. The former anneals the conductor and very consid- erably reduces its strength while the latter is always objection- able in wires subject to heavy loading. Sleeve splices on the other hand may be given any desired con- ductivity, combined with great mechanical strength. Some care is necessary in the selection of the material, length of the sleeve, and the number and character of the twists. Even small high-strength solid steel wires sometimes used for long-span telephone wires can now be successfully spliced by steel sleeves. FIG. 113. FIG. 114. Pin Insulators. By reference to the section on insulator pins, it will be noted that few if any pin insulators are suitable for turning sharp corners, particularly with large conductors. Even providing double arms will not give factors of safety commensu- rate with those presumably required on the supporting construc- tion. This weakness of the pin insulator is not, accurately speak- ing, chargeable to the porcelain but to the pin. To use stronger pins would require larger pin holes in the insulators and, there- LINE MATERIAL 193 fore, new designs with greater neck diameter. However much one might wish that designers had foreseen the present tendency toward rational construction, the present purchaser of small quantities of insulators is compelled to turn to a different type of construction, if any considerable degree of strength is desired. The stock, or standard insulators and pins shown in all manu- facturers' catalogs include a great variety of designs most of which could be dispensed with to the great advantage of both the manufacturer and purchaser. By handling fewer designs manufacturers would not be compelled to retain so many molds and so much stock, while purchasers would per- force have their choice limited to a smaller number of well- selected types. Particular reference is made to the wood top, porcelain base, wood base, or all-wood-top pins, and to all pins having bolts J,{Q in. or J in. in diameter. Mechanically, the excuse for such designs in their application to severe loading, defies analysis. The strength of an insulator or of a pin should be determined by a test made after the two are assembled and with the wire and its attachment in place. Tests on pins alone may involve a reduc- tion in the lever arm of the load, and in any case give no indication of the result of pin bending on the insulator. If there is con- siderable bending, which is exaggerated on wooden arms, the attachment of the wire may slip over the head of the insulator. In general it is more than possible that the values obtained by piece tests on rigid supports could not be even approximately duplicated in actual practice. That pin insulators have an en- tirely proper function may not be denied. Further, they are probably the most economical type for all of the more common voltages. Although used up to 80,000 volts, with the expressed satisfaction of the users, the writer is of the opinion, taking into consideration the mechanical as well as the electrical features, that their proper field of usefulness is below 60,000 volts. At heavy corners, however, the design should be changed to the disc type, in which far greater strength is obtainable. In addition it would seem entirely logical to assert that if pin-type insulators are used on tangents and the insulators in question have only a fraction of the strength of the wire, then the supports need not be built to re- sist a wire tension, which the insulators can never transmit to them. 13 194 POLE AND TOWER LINES There may be reasons for desiring a certain strength in the sup- ports, but certainly broken-wire loads at maximum tension on such construction cannot be one of them. Insulators of the suspension or disc type are more satisfactory from a mechanical viewpoint than pin insulators, since they trans- mit the wire tension directly to the crossarms without introducing the torsion due to the height of a pin insulator. When used either in the suspended or strain position, disc insulators require some additional height of pole or tower and an additional width of crossarm to provide clearance for the jumper or the suspended FIG. 115. Suspension insulators. FIG. 116. Through-pin insulator. string of insulators. When used in the strain position on medium- voltage lines, it is now thought desirable to add one disc to the number used in the suspended position. For the high- voltage lines, there is now a rather general objection to using the strain position at all as it is claimed that a large proportion of the trouble from lightning has occurred at strain connections. To avoid in- stalling insulators in the strain position and still maintain, at least in a measure, an auxiliary attachment, the arrangement shown in Fig. 162 has been used. LINE MATERIAL 195 The types of insulators shown in Figs. 1 15, 1 16 and 117 are much stronger mechanically than single-pin or double-pin insulators, that in Fig. 116 being particularly suited for turning corners with heavy wires. It should be noted that the through pin acts as a simple beam instead of as a cantilever, and transmits its load to double arms without torsion. Further, as this pin can be made a rolled- steel rod about 1% in. in diameter, it is not liable to bend and crack the porcelain. Unfortunately, however, the electrical strength of these insulators does not render them advisable for use much above 20,000 volts, although they are rated as 30,000- volt material. Their mechanical strength is usually about 12,000 FIG. 117. Heavy -strain insulator. Ib. A corner, particularly an important one, should be insulated approximately up to crossing requirements. Suspension insulators, in general, have mechanical strengths ranging from 9000 Ib. to 16,000 Ib. For extremely heavy stresses, strain insulators of the type de- signed for railway service and shown in Fig. 117 may be used. These insulators, while expensive, can be equalled in strength only by some paralleled system of the disc type, an arrangement both cumbersome and costly. The insulator shown has a mechanical strength of 20,000 Ib., while the strength of a larger size is 35,000 Ib. In transmission-line work these insulators have an occasional use in very long span construction. It has sometimes been specified that the mechanical strength of guy insulators should not be less than 1, 1J, or 2 times that of the guy in which they are placed. If this re- quirement were strictly enforced it would penalize the use of extra heavy guys which are desirable on account of corrosion, 196 POLE AND TOWER LINES since it would be difficult, perhaps impossible, to obtain insulators of the requisite strength. Interlocking insulators of the " goose- egg" type have great mechanical strength and retain a measure of the guy action after failure, although the guy would be slack. Such insulators, however, do not have the high electrical factor of safety of the disc type. The insulation of guys on poles carry- ing high-voltage wires is undesirable from a structural viewpoint, although it may at times be desirable as a matter of policy for the protection of pedestrians, particularly in the case of wooden poles. FIG. 118. Guy insulators. Pins. To a certain extent, the day of the wood pin is passing, though it is still used on systems operating below 11,000 volts, and apparently gives satisfactory service. Mechanically, locust pins are inferior to metal pins and, by causing the removal of a large section of the timber, materially decrease the strength of the crossarms. It is generally assumed that the common forms of straight-line insulator pins are " strong enough," even that locust pins will "do the work." It is true that almost any insulator and pin will support a straight unbroken line, but it is not therefore true that the same members will withstand the stresses which are in the wires under maximum loading. If the connection of the wires to the insulators will not transmit more than a fraction of the maximum stress in the wires, then the pin need withstand only the transverse load. As, however, it is often specified that the pins must dead-end the wires, it would seem necessary first to so attach the wires that they will remain attached under maximum tension and second to provide a pin and insulator which will remain intact under this stress. LINE MATERIAL 197 Wood pins are generally of yellow or black locust and should be straight grained and free from knots except that small sound knots, J in. to M m - diameter, may be permitted in locations where they will not materially impair the strength or durability of the pin. They should be free from wane or sap wood, and from checks or worm holes. The standard thread is four per inch and the threads should be cut cleanly and uniformly to provide a tight fit in the insulator. Unless well-seasoned timber is used, the pins will probably vary from the standard dimensions and the protective coatings will be less effective. Such coatings are paint, creosote, linseed oil, and perhaps more commonly paraffin. The holes in the crossarm should be so bored that the tapered FIG. 119. Types of insulator pins. shanks of the pins will fit tightly therein and the pin be per- pendicular. A six-penny nail may be driven into the shank from the middle of one side of the arm. Aside from the possibility of rupturing the porcelain in case the pin bends, it has been found by test that certain classes of pins deflect as a whole and allow the top of the insulator to be subjected to shear and tension. If such pins are carried by wood arms the angular movement may be quite large, due to the penetration of the pin base in the timber, with the result that the wire fastening 198 POLE AND TOWER LINES either slips over the head of the insulator or shears the top from the insulator. It is probable that the character of the pin, particularly in re- gard to its hold on the insulator, is of equal importance with the strength of the insulator itself. A cemented pin is somewhat ob- jectionable because it cannot be readily replaced and to overcome this difficulty various thimbles and separable-top pins have been devised. i. FIG. 120. Standard locust pin. The writer does not know of any pin having a wood thimble in which the strength of the complete pin is more than about one- half that of the bolt. This is due to the fact that the thimble is too thin and does not have sufficient bearing on the base. The weakest part of this type of pin is not in the porcelain insulator but in the design of the pin itself. LINE MATERIAL 199 Again referring to the cemented pin and assuming it rigidly fastened to a metal crossarrn, it has been found that some of the ordinary low-voltage porcelain insulators and metal pins cannot withstand long-span loading, or safely support the transverse bending caused by heavy cables on angle poles. On high-voltage lines the insulator pins should be of metal. There are, however, a number of 11,000 to 22,000- volt lines c. . (a) (6) (c) FIG. 121. Wooden pins. F C E \%" 10" 2 1% 13 3 1U 17 3 D equipped with wooden pins. The low mechanical strength of such pins and the possibility of their disintegrating or burning has raised the question of the limiting conditions under which wooden pins are permissible. In general, pins should extend well into the insulator to reduce the mechanical stress on the material of the insulator. On account of the improbability of frequent painting, metal pins should be galvanized, or otherwise protected against corrosion. 200 POLE AND TOWER LINES It should be noted that in case of a broken wire some of the long pins now in use would develop a very large torsional effect upon the crossarms. The calculated strengths of the insulator pins shown in Figs. 124, 125 and 126, are those at which the bolts should begin to bend, thereby allowing the insulators to tilt. In making the computations it was assumed that there was a complete, level contact between the pin base and crossarm and that the bolt was All Wood Top 'Bolt FIG. 122. Tension applied at neck of insulator ; average of three tests. 500 Ib. pin started to bend; 590 Ib. pin failed by splitting the wood thimble. not subjected to preliminary bending by any slack in the adjustment. If wood crossarms are used, allowing greater preliminary bend- ing due to the bolt compressing the fibers, the strength and ri- gidity of the construction would be further reduced. On the other hand, if bolt steel having a yield point of 32,000 Ib. were used instead of the steel of 28,000 Ib. assumed above, there would be an increase of about J i n the tabulated strengths. Wood-top or porcelain-base pins of this general type are all relatively weaker, owing to the fact that both timber and porce- LINE MATERIAL 201 lain have a much lower crushing resistance than cast-iron, malleable-iron, or cast-steel. The following conclusions seem to be justified: (a) That most of the standard designs of pins now in use are undesirable in that the metal parts are weaker than the porcelain; (6) that ordinary insulator pins are not at all suitable mechanically for corner con- struction or for dead-ending; (c) that much stronger pins can be designed for metal arms if a little additional thickness is allowed in the insulator neck and a larger bolt is employed. FIG. 123. Tension applied at neck of insulator; average of three tests. 770 Ib. pin started to bend; 920 Ib. pin failed by splitting or crushing the wood thimble. The type of pin shown in Fig. 127 was designed to avoid holes in wood crossarms. It would appear, however, that the addi- tional cost and material are not justified, provided comparison is made with properly designed pins. Tests have shown that under heavy loading the critical condi- tion is often not the strength of the porcelain or of the metal pin, but the ability of the arm to resist tilting. If the arm, as a whole, will rotate under torsion, or if the base of the pin cuts into the 202 POLE AND TOWER LINES timber, or twists on the arm, the consequent tilting of the pin may permit the wire or attachment to slip over the head of the insulator or to shear the top from the insulator. The following short series of tests indicate the foregoing tendencies: FIG. 124. Cemented-type all-metal pin. L W B Elastic limit of bolt 5K" 6" 7^r 2H" 2>r 3" H" KG" 7 A4" p P P 1 9^ v ^finn iv> 1325 lb. 540 lb. 520 lb. 5.25 X 1 9^ V ^fiOO Ih " 6.0 X = 44 X 2600 lb.- = / .0 Tension at neck of insulator, transverse to arm. It should be observed that the strength along the arm in the second set of tests is considerably in excess of that in the direction of the wires. In the tests in question, short yellow-pine arms LINE MATERIAL 203 were used and as standard length arms attached to a pole would have less rigidity against torsion, the effects of the tilting would have been aggravated in actual practice. FIG. 125. Cemented-type all-metal pin. L TF B Elastic limit of bolt 5K" 3" X P = H X 8450 lb. = 2300 lb. 6H" 3" H" P = i^ X 8450 lb. = 1950 lb. 9H" 4" K* P = ^ X 8450 lb. = 1780 lb. If we allow a factor of safety of 2.0 in the pin construction, and assume that the ultimate resistance to failure of some sort is about 2000 lb. for single arms and 4000 lb. for double arms, 204 POLE AND TOWER LINES FIG. 126. Separable-thimble type all-metal pin. L w * Elastic limit of bolt 8" 4" X" p = |^ X 5480 Ib. = 1990 Ib. H" 4" K" p 9 = H X 8450 Ib. = 1780 Ib. 13H 5" H" p = ~ X 8450 Ib. = 1690 Ib. lm 5" H" p = ^X84501b.= 1560 Ib. *Test No. 1. 2380 Ib., insulator uninjured, excessive pin tilting, bottom strap bent. Test No. 2. 2400 Ib., insulator failed, excessive pin tilting, bottom strap bent. Tension at neck of insulator, along axis of arm. Test No. 3. 2600 Ib., insulator uninjured, pin tilted cutting into wood arm. Test No. 4. 3300 Ib., insulator failed, pin tilted cutting into wood arm, bottom strap bent and split. Test No. 5. 4080 Ib., insulator failed, pin tilted cutting into wood arm, bottom strap bent and split. the maximum wire tensions which they would dead-end are 1000 Ib. and 2000 Ib., respectively. The maximum tension in * Tests of type shown in Fig. 127. LINE MATERIAL 205 conductors larger than No. 1 gage, however, will usually be in excess of 2000 Ib. Crossarms. The standard crossarm, particularly for the so- called low-voltage lines, is now, and will undoubtedly remain for some years, a wood arm. Even in view of the great increase in the price of timber, the wood arm is the cheapest and the most easily obtainable throughout the country as a whole. Assuming that the price of wood arms will continue to increase, it is still probable that, for some years to come, metal or other materials will not seriously compete with timber. ^ ^'JO"" JL FIG. 127. FIG. 128. It might be supposed that preservative treatment, which will un- doubtedly be extensively applied to poles, would prolong the use of wood arms. To some extent this may be true, but while the creosote treatment is rapidly growing in favor for poles, the same cannot be said of its application to arms. A preservative which would make arms less inflammable, would not drip on passersby, and would not injure the hands or clothing of workmen, would be more desirable for crossarms than creosote. The difficulty of standardization, of foretelling accurately the uses to which an arm will be put, makes metal arms undesirable 206 POLE AND TOWER LINES for small growing properties. The question of the relative benefits to be derived from the insulating qualities of a wood arm is a mooted one. Voltages, at least those below 13,000 volts, can on a dry day be successfully insulated by the wood arm alone. Under such conditions, if an insulator is shattered and allows the wire to fall upon a dry or comparatively dry wood arm, no interruption of service need result nor is there any injury to wire or arm. Some unknown additional humidity, or degree of damp- ness of the arm, will cause burning, with the possible falling of the arm which may or may not carry other wires on the burned- off portion. The advocates of the metal arm contend that it is economical in final cost and that the insulators should be ^4^12'^-12 / ^-12^--15V>^---15'-^-12' : -<-12'^7*-12V4^ "Ao Through Bolt Hole^-19 -->t< 19- -^ /%' Brace Bolt Hole H ! I*" 8 Pin Arm - - 11/16 Th 4 Pin Arm FIG. 129. Standard wood arms. designed to do all the insulating necessary, also that the wires falling on a metal arm shut down the service and compel proper maintenance. The average crossarm would not withstand dead ending under maximum stress, combined with the torsional effect due to the lever arm of a long pin, without allowing a distortion which would presumably permit the wire to become unfastened from the insulators. The more commonly used steel arms consist of single angles with the same general dimensions and punching as standard wood arms. When used on wood poles, crossarm braces are necessary. LINE MATERIAL 207 On structural steel poles to which crossarms have two points of connection, braces are rarely used, as it is simpler and nearly FIG. 130. Substantial crossarm construction for eccentric loading. FIG. 131. Wish-bone crossarms. as economical in material to increase the section of the crossarms themselves. Except with painted steel poles, and sometimes even in that case, metal arms are galvanized. 208 POLE AND TOWER LINES In addition to the standard angle arms, several types of pat- ented arms have been used to some extent, the more important being the so-called " wishbone" and the "bo-arrow" arms. In both of these, adjoining arms are brought together so that there may be two points of attachment to counteract rotation. The upper pole bolt must not be placed too near the pole top as the leverage exerted by an unbalanced pull on an arm may split the pole top. The use of special arms like these has been confined more or less to one section of the country, and it is perhaps true that familiarity will remove the sense of strangeness with which they are first seen. Crossarm Braces. The standard brace is a flat \Y X M"> 26 in. center to center of holes and 28 in. overall. There are also various modifications of the standard, such as changes in length and reduction of thickness to %2 m - or Me m - The term iron is still in common use, although the material is usually soft steel. In fact " wrought iron" or soft steel is fre- FIG. 132. Angle brace. quently specified, whereas a high-carbon steel would be more effective. The function of a brace is to support and prevent rotation of the crossarm, and it acts in either tension or compres- sion. As a tension member, its strength is excessive and its rigidity against buckling as a column is, therefore, the critical condition. Owing to the shape of the section, the rigidity is a function of the thickness and the strength of the material, and as the present rather ineffective thicknesses will presumably be retained, some added stiffness may be secured by the use of the stronger steels. The angle brace, in which two flat braces are replaced by LINE MATERIAL 209 a single angle about 1J" X 1M" X Me" nas a much greater strength, but almost twice the weight of material at a corres- ponding increase in cost. Braces have their greatest usefulness when the crossarms are unequally loaded. When but one of two circuits is installed or when all wires are placed on one side of the pole, the arms will frequently tilt, particularly on lines of medium voltages carrying heavy wires. FIG. 133. Crossarm with two pole bolts, no braces. FIG. 134. Wooden braces. In theory at least, it is somewhat undesirable to use lag-screw connections to the pole or to the arms but, in practice, the timber about the screws will be in fab* condition when replacement of the pole or a change of arms is necessary from other causes. It is probable, however, that longer, if not heavier, lag screws should be used than are always employed. Sometimes crossarm braces have been omitted and two through- 14 210 POLE AND TOWER LINES bolts used to connect the arm to the pole. This is not, however, as rigid or as strong as the more standard arrangement of one- pole bolt and an angle brace. When a short crossarm is used for the top wire of a single-circuit pole, as shown in Fig. 133, the ground-wire post may be made a very efficient brace. Wood braces are not good construction as it is difficult to ob- tain a strong permanent connection to the pole. The pole shown in Fig. 134 is well adapted to provide adequate longitudinal pin separation, something which is not readily obtainable in double arming with large insulators. Lag Screws or Lag Bolts. The fetter-drive or cone-point screws generally required in former years and still shown as standard, will probably be replaced by the gimlet-point type in future work. There is in fact no possible advantage in the former and the continuance of a double standard is quite objectionable from a manufacturing standpoint. FIG. 135. Cone-pointed lag bolt. FIG. 136. Gimlet-pointed lag bolt. In theory, lag screws are supposed to be screwed into place, either into a small bored hole, or after being started by hammer- ing. This should be done to enable the threads to pass through the timber with the minimum shearing and injury to the fibers of the wood. Hardware such as braces, bolts, lag screws, etc., should be galvanized by the hot-dip process, at least until such time as other methods will have clearly demonstrated an equal excellence. It has sometimes been advocated that bolts, etc., be electrically galvanized and not subjected to the standard test, the reason being fairly obvious. By the use of the rolled thread, in distinction to the cut thread, it is possible to use the hot-dip process without recutting the bolt threads and thereby removing the protective coating. While not commonly done, nuts may be made with extra loose threads and after galvanizing retain at least some measure of protection on the threads. It is probable, however, that unpro- tected threads on the nut and rolled threads on the bolt, both being hot-dipped, are superior to other methods. LINE MATERIAL 211 Guys and Guying. Guys or support braces are of three types: timber push poles, steel-cable guys and rod guys. The first are unsightly and their use is chiefly justified in places where guys are needed in two directions and can be allowed in but one. Provided a timber brace is properly set and well connected with the line pole, it is capable of resisting stresses in either direction and to some extent may act as longitudinal reinforcement. When used in such double service the setting must be adapted to resist either depression or uplift. Under exceptional conditions rod guys may be used as they have adjustable connections, form a rigid anchorage and may be made with an excess of material to provide for corrosion. In general, however, guys are of stranded galvanized-steel cable and when properly installed are a component part of the support. The writer is utterly unable to agree with the view, sometimes expressed, that guys are a makeshift attachment of doubtful service. In fact, it is his firm belief that only the good service of the power and guy wires is retaining many existing lines in position. It cannot be denied, however, that there are many guys of less than no usefulness owing to extreme slack, inadequate section, or excessive corrosion. A very slack guy is no guy at all, but a small, unsightly load on the structure. So-called iron wire about % in. in diameter is not an efficient guy, though it might serve for very light lines if the material were in reality wrought iron, and the galvanizing as efficient as would be desirable. Steel cable not less than ^{Q in., and preferably not less than % in., with a heavy coat of galvanizing is the desirable standard guy. Greater strength may be obtained by the use of larger diameters, or higher grades of steel, but the latter are stiffer and more difficult to handle in the field and therefore less popular. It is possible to obtain any desired grade of steel cable from the standard-guy-strand of about 60,000 Ib. per square inch to extra-high-strength steel with a strength of 180,000 Ib. per square inch. In general, however, the standard or the Siemens-Martin grades should be used, but with sufficient diameter to provide ample strength, bearing in mind the much more rapid corrosion of galvanized cable than of galvanized unwiped structures. The exact location of guys must depend on local conditions and their number on the character of the supports. With wood 212 POLE AND TOWER LINES poles and flexible frames, guys should be used more plentifully than with semi-flexible poles, and the latter in turn require more guys than semi-rigid towers, while the true rigid towers require" no guys. In general, wood poles and flexible frames should be side guyed at all corners, at tops of steep hills, and usually wherever a very long span occurs. They should be head guyed on steep hillsides, long spans, hill tops and at intervals on long tangents. There is no valid objection to the intelligent use of guys. Structures so designed that their light flimsy nature renders them overliable to buckling by guys should not be used in a transmis- sion line. Further, the majority of the existing pole lines derive much of their strength from guys. // a guy is not overtightened its presence must inevitably increase the strength of the structure to which it is attached. In guying it is necessary to adapt the number, size, position and tension of the guys to the service required. When practi- cable, guys should not be anchored too close to the structure they support. The angle of inclination, however, is not fixed. The insertion of a turnbuckle in a guy, particularly in a long guy, permits a more careful adjustment of tension than is practi- cable with wire clamps. Looping a guy around the entire tower is not good practice, except in unusu-al cases where there are no steel edges to be dis- torted and where the structure is adequately braced both verti- cally and horizontally at the point of guy connection. Guy con- nections should be made close to a panel point of the vertical bracing to prevent distortion of the main legs. When a single guy is used on the tension side of a tower it is generally desirable and frequently essential to attach it to both main legs on that side so that the pull will be exerted squarely on the tower, and not merely on one corner. Ordinarily, a guy should be attached as close as possible to the conductors whose pull it carries to the anchorage. Except in pole lines consisting of many wires where guys attached among the upper crossarms are absolutely neces- sary, the guys should not cross over conductors, but should be connected near the bottom crossarm. With steel structures guy insulators are not required by the standard specifications, their use being optional, but if used they form a weak point in the guy. LINE MATERIAL 213 T t = total tension, or pull on pole to be balanced by guy. T g = total tension in guy. = L g X sin a Assuming that L L g = 3 ft., we have: POLC FIG. 136a. TABLE 30. TENSION IN GUY DUE TO T t = 1000 LB. L g (ft.) (ft.) 20 25 30 35 40 45 50 5 4,750 5,710 6,680 7,680 8,660 9,670 10,650 10 2,580 3,020 3,480 3,950 4,430 4,920 5,410 15 ,920 2,170 2,460 2,760 3,060 3,370 3,690 20 ,620 ,790 ,980 2,190 2,410 2,620 2,850 25 ,470 ,580 ,720 ,870 2,030 2,200 2,370 30 ,380 ,460 ,550 ,670 1,800 1,920 2,060 35 ,320 ,380 ,450 ,530 1,630 1,740 1,840 40 ,290 ,320 ,380 ,440 1,520 1,610 1,700 45 1,260 ,280 ,320 ,380 1,440 1,500 1,590 50 1,240 ,260 ,290 ,320 1,380 1,430 1,490 While guys of special steel are sometimes used to obtain great strength it may be more advisable to use heavier guys of standard material, as the latter is more easily handled and its use will tend to encourage an allowance for corrosion. Guy Anchors. Patented guy anchors are of many kinds, generally variations of either the old patent for screw-piles, or of the unfolding type. The diameter of the disc or blade varies from about 6 in. in the smaller sizes to 12 in. in the largest. The holding powers claimed for such devices should be used with the reserva- tion of a factor of safety, as the character of the soil, either in general or at the time of test, ex- ercises a very great influence on all foundation values. The resistance of an anchor to uplift depends primarily on its depth and bearing area and on the weight and cohesion of the superim- posed soil. The depth and area of the anchor blade are lim- FIG. 137. 214 POLE AND TOWER LINES ited by the means available to install it, but the weight and cohesion of the soil will vary from place to place and from season to season. Depending on the Nature of the Rock FIG. 138. Wooden dead-man guy anchor. "Dead men" are more efficient than patent anchors, since they are always larger and can be made of any desired size. Large sound logs, about 10 in. in diameter and 6 ft. long are desirable for ordinary guying. Logs treated with creosote are still better, while the best type of anchor is a concrete-covered steel 1 In. Diameter x&. is in. or over in Length \^ beam or reinforccd-concrcte block. In any case the anchor rod or rods should be galvanized by the hot-dip process (N.E.L.A. Specifications) and preferably incased in concrete or in a concrete- filled pipe from the anchor to a point about 1 ft. above the ground line. The published values of the holding FIG. 139. Rock anchor. power of various sized anchors are ap- parently based on tests in clay and therefore should be re- duced about 25 per cent, for sandy soil. Further, it is very difficult to arrive at any acceptable standard values for the LINE MATERIAL 215 holding power of anchors, as would be evident from an analysis of the values heretofore published. For instance, -if the holding power of a 6-in. anchor buried vertically .5 ft. is 15,000 lb., the pressure on the top surface of the 6-in. disc whose area is 28.3 sq. in. would be 76,500 lb. per sq. ft., or 38 tons per sq. ft. If the holding power is due first to the weight of the cone whose sides have an inclination of 45 to the vertical (and such in- clinations have sometimes been limited to 30 in foundation work), and second to the cohesion of the earth on the periphery of the cone, the following values will result: Angle a assumed as 30: Volume of superimposed cone of earth 56 cu. ft. Weight of cone at 100 lb. per cubic foot = 5,600 lb. Cohesion, or shear, at 150 lb. per square foot = 9,400 lb. Published holding power Angle a assumed as 45: Volume of superimposed cone of earth Weight of cone at 100 lb. per cubic foot Cohesion or shear, negligible Published holding power 15,000 lb. = 151.5 cu. ft. = 15,150 lb. = Olb. = 15,000 lb. The pressure of 38 tons per square foot is greatly in excess of that permitted by any foundation specifications and the inclu- sion of cohesion is not specifically permitted by such specifica- tions. Therefore, it would seem either that the ordinary require- ments for foundations, which are generally assumed as having a 216 POLE AND TOWER LINES factor of safety of 5 or 6, are unnecessarily conservative for anchor installations in fairly good ground, or that the generally accepted holding powers of anchors are excessive. The writer believes the former to be the case, at least under favorable conditions, but the above analysis may serve to explain the wide discrepan- cies in anchor values. It must be admitted that the efficiency of any given type or size of anchor will depend on the soil in which it is placed. Therefore, since the character of the soil at various anchor locations is not usually known in advance, it is, perhaps, advisable to use disc or unfolding anchors only for light guys and to rely on the installation of " dead-men" to resist heavy stresses. Far greater holding power can be obtained by the use of a good dead-man than can possibly be provided by any of the patented anchors whose area is necessarily much less than that of any dead-man. Since the initial stress on the guy anchor will be approximately one-third of its maximum stress, care should be taken to disturb as little of the adjoining earth as possible during construction, in order that the anchor may have a high initial resistance without depending on the additional strength resulting from a future compacting of the soil. CHAPTER XII ERECTION AND COSTS Erection. In stringing wires it is, of course, of importance to cover as much ground daily as possible, but this should not be done at the expense of injury to such an important and expensive item of the construction as the wire. Copper wires, whether solid or stranded, cannot be dragged without injury over the ground or over the crossarms. If either of these methods is adopted, it will result in nicks in the solid wire, or broken strands in the cables. Such injuries may not be visible and, with good fortune, may never cause failure, but anyone who has seen soft stranded copper wire snarl into a veritable " rat's nest" when removed through snatch blocks, will not deny that injuries may result from improper stringing. It is also necessary to be constantly on the lookout to avoid kinks, twists, or broken strands, either in unreeling or in stringing. When broken strands or injurious kinks do occur, a new section of cable should be spliced into the line. It is almost impossible to remove the cable from a reel without forming kinks unless the reel rotates about its axis. The reel should, therefore, be sup- ported on a horizontal shaft arranged to turn freely, but not too fast, as all cable has a tendency to kink as soon as a little slack occurs. If kinks do form it is, of course, desirable to remove them if possible to do so without injury to the cable. By immediate attention and adherence to the proper methods of manipula- tion, it is possible to remove a kink without injury to the strands. To do this it is necessary, as shown in Figs. 1 141, 1, 2, 3 and 4, to straighten the wire by pushing the ends apart without altering the lay of the strands. If this is not done cor- rectly, some of the strands will be stretched and the spiral of the wires will be distorted at the point of bend, and thereafter the cable wih 1 fail by the parting of the individual strands at much less than the ordinary strength of the cable. In stringing wires they should be pulled out through snatch 1 Illustrations from Yellow Strand Broderick and Bascom Rope Co. 217 218 POLE AND TOWER LINES blocks with wooden sheave and frame and ball or special bear- ings and afterward lifted into place on the insulators or in the clamps. Any simple device which can be quickly attached to a m 9 \ FIG. 141. Straightening kinks. crossarm to hold the top groove of the snatch block at the eleva- tion of the clamp, will be found to be very useful. Periodic inspections should be made of the condition of the snatch blocks, to prevent injury to the wire. If a dynamometer is used to ad- ERECTION AND COSTS 219 FIG. 142. Stringing wire with derrick car. FIG. 143. Derrick wagon raising pole. 220 POLE AND TOWER LINES just the sags the wires can be transferred from the blocks to their final positions. Dynamometer stringing is particularly desirable in long-span construction, although it is uncommon in the more general classes of short-span work. It should be checked oc- casionally by measurement of the resulting sag. It is advisable to string all spans to balance at normal temperature and no wind or ice, even though this results in some unbalancing under load. Otherwise, the tensions might balance at maximum load, per- haps once in 10 years, and be unbalanced the rest of the time, with the consequent continuous loading of their supports. When a track is available paralleling the pole line, erection with a derrick car will give the greatest possible distances per day. In the absence of a track it is frequently practicable to use motor or horse-driven derrick cars for the erection of poles or for string- FIG. 144. Derrick wagon. 1 ing wires. The economy of such erection equipment is very great under favorable conditions, but it must be kept moving to attain its greatest usefulness. For this reason no direct com- parison can be made between the erection costs of two lines, if one is long and accessible, and the other is short and inaccessible. The wagon derrick, shown in Figs. 143 and 144, consists of a wide stout wagon base carrying short, double shear legs from which is hung a wood mast. The mast is hung from an axle and a universal joint, thus allowing the top to travel in an arc limited by the length of the groove which restrains the base. By this arrangement sufficient overhang is obtained to reach holes about 10 ft. from the wagon. The mast is back guyed by ropes attached to the top ring and snubbed around any convenient object, or bar driven in the ground beyond the wagon. Since 1 Designed and built by Wm. A. Ladue, Supt. Public Service Elec. Co., Hoboken, N. J. ERECTION AND COSTS 221 the height of the shear legs is not great and as the mast may be rotated in both directions, the rig with the mast in the horizontal and axial position can pass under bridges, trolley wires, etc. When but one of two ultimate circuits is to be immediately installed, as shown in Fig. 145, it is frequently placed on one side of the pole and on the highway or most accessible side. This practice is not commendable as it tends to tilt the arms, as is faintly visible in the illustration, and because erection of the second circuit is made much more difficult. When substantial angle braces are used such eccentric loading is probably unob- FIG. 145. Highway-side loading. jectionable under ordinary conditions, but it has been observed that all side-arm construction suffers more in severe storms than balanced arms. If, however, only standard flat braces are used, some arms are certain to become tilted. Placing the first wires on the highway side of a pole line reduces slightly, but only slightly, the original cost of erection. The second circuit, however, will have to be erected under much less favorable conditions than the first would have been if placed on the inside. It is better construction, when both circuits are to occupy the top arm, to place two wires on the inside and one on the outside. This will necessitate longer shut-downs on the first 222 POLE AND TOWER LINES circuit during the second stringing, but it will reduce the ultimate cost of that stringing and provide a stronger original line. Poles should be set vertically and in line, except that at corners and dead-ends they may be given a slight rake, though this is unusual for steel poles. In building foundations or setting poles, particularly in long-span lines, it is necessary to exercise some care to obtain a firm unyielding support for the pole. Since providing a layer of concrete in the bottom of the excavation is not always practicable, or might result in considerable expense, the bottom of the excavation should be compacted by tamping, and perhaps by adding and tamping a 6-in. layer of broken stone or gravel. The back filling should always be well tamped in thin layers, and to insure this it is frequently required that one shoveler be used to three tampers. If broken stone or gravel is removed from the FIG. 146. A frame erection with gin-pole. excavation, or readily obtainable nearby, a very efficient founda- tion may be obtained by back filling with a considerable propor- tion of rock, being careful to pack earth or sand in the spaces between the pieces. Since the back filling will settle and become more compact after it has been completed, and after rains, a por- tion, at least, of the excess excavation should be piled up around the base of the pole. Later an examination s'hould be made of the foundations and back filling added wherever it may have settled below the surface. On wood-pole lines it is customary to set adjacent poles with the crossarm gains facing opposite ends of the line. Guys should be installed before any wires are strung and should be inspected ERECTION AND COSTS 223 and adjusted if necessary after the stringing is completed, other- wise, the structures may receive an overloading, while without the guys. If some wood poles have unusually large tops, the regular crossarm bolts may be too short. In such cases it is better to obtain a few long bolts, than to injure and weaken the top of the pole by cutting it down to the shorter bolts. An occasional heavy pole, or one with a large top and regular taper, is a real asset to the structural strength of any wood-pole line, therefore such poles should not be weakened by excessive top cutting. Costs. In most contract work it is fairly accurate to assume that in general the work will approach the estimated average FIG. 147. Erection with house-derrick. and that under-estimates of some portions of the work will be balanced by exceptional records made in other portions where organization and familiarity are given a fair test. For this to be true, however, it is necessary that there be a fairly large volume of work in a few locations. Where work is scattered the expense is in moving, beginning, and stopping, not in the actual work itself. Further, there are more kinds of work in line construction than in most other classes of contract work, since each pole or tower loca- tion is a small job and in some way unlike the last. It is, there- fore, impossible to attain the speed of piece-work in a fixed loca- tion, as a very considerable portion of an employee's time is spent 224 POLE AND TOWER LINES in " thinking " about the next step, or in moving to a new position, or in getting a new tool. In a comparatively short installation, even so unconsidered an item as a specially rainy week will have a marked effect on the unit cost per structure. Rain or snow not only stops or delays progress except the progress of the " straight-time" pay-roll on the day in question, but also usually delays the work of the following day. Holes are filled with water or snow, equipment or material is buried, slides have occurred, walking and teaming is more difficult, and in general it is a poor day's rain which cannot count as two. FIG. 148. Hauling a small concrete pole. Work carried on between spring and late fall should cost at least 20 per cent, less for labor than that done during the remainder of the year. Instances are rare in which published accounts cover the matter of accident insurance. An owning company may and perhaps usually does, include such contingencies in overhead expense, but the cost is nevertheless directly applicable to the line erection. A contracting concern, on the other hand, usually tabulates the insurance as part of the cost estimate and as it is almost univer- sally paid as a direct percentage on the actual labor pay-rolls, it is a very real item. The amount paid for insurance varies with ERECTION AND COSTS 225 different classes of labor and in different states. Under certain recent legislative enactments, the liability of employers is now only partly protected by premiums as high as 15 per cent., so the casual omission of such items of expense is at least censurable. Again in comparing the costs of a previously established method of construction for any company, such as the ordinary wood-pole line, with for example, steel-pole lines, the omission of general expense items in the former is quite common. For instance, the company may maintain a small force of travelling inspectors and purchasing agents in order to obtain their quota of wood poles, and this expense with that of handling, storage, trimming, etc., is properly chargeable to the cost per pole delivered. In general, the work done at odd times on regular construction mate- FIG. 149. Raising a small concrete pole. rial will not be charged thereto as certainly as the unusual charges to new types of construction. Proper charges for plant and equipment, particularly for small tools, are noticeable by their absence in most cost estimates. It has been stated that the cost of steel poles or towers varies directly as the square of the height, a very evident error since their weight, under constant conditions of design, will be more nearly in direct proportion to the height, and the greater heights usually indicate a smaller number of structures per mile. As a matter of fact, it is surprising to note the relatively small differ- ence in estimated cost between different designs of equal or nearly equal general excellence. The conditions of manufacture, accessi- bility of the site and character of the ground usually influence the ultimate cost much more than is often realized. In addition, 16 226 POLE AND TOWER LINES it will be found by investigation that very few existing lines are directly comparable on account of differences in design. Published accounts of erection costs are usually misleading and frequently inaccurate. Unless the local conditions are similar and the methods of erection equally efficient there will be no equality between two sets of costs. Furthermore, a difference in the extent of the work and in the organization of the field forces may cause a relatively great difference in the cost of two lines of exactly similar construction. In making a comparative cost estimate of two different types of construction, as for example wood poles and steel poles, it is necessary either to make two complete and distinct tabulations, or to use care to include all credits and debits due to the differ- ences in construction. In either case an accurate estimate should include allowances for maintenance, renewals, and interest charges. In making estimates it should be remembered that the use of a long-span steel-pole line will effect a saving of about two-thirds of the cost of the insulators, pins, ties, pole rights, and founda- tions, and of the erection of the insulators, pins and ties. Having fewer insulators than the shorter span wood pole line, there will be less probability of insulator failure and, therefore, less inter- ruption to 'service. In addition, some credit is probably due long-span steel construction on the ground that the maintenance expense will be lower and that a high-grade line with few poles will cause less criticism. If shop-assembled steel poles with galvanized butts approxi- mately 24 in. square are employed, they can be set in the ground without concreting the holes, so the cost of the foundations per pole should not greatly exceed that for wood poles, and a saving of nearly two-thirds of the hole digging would result. Steel poles are distributed and erected the same as wood poles, therefore with the proper field equipment a mile of steel poles should be set at least as quickly as a mile of wood poles. Established costs of concrete-pole lines are practically non- existent, and even such pole costs as have been published are rarely applicable to ordinary transmission line work. The more recent, best built, and most important concrete-pole lines have been for telephone and telegraph, rather than power service. Some short " back-yard" poles for purely distribution service have been built at an approximate average cost of $10 per pole. True ERECTION AND COSTS 227 transmission line poles of adequate height and strength, unless made in quantities, would probably cost at present, about the same as structural steel poles. As previously stated, it is necessary to use considerable judg- ment in making cost estimates or in interpreting them. The following costs which have been compiled from time to time, as well as the author's estimates, cannot be considered universally applicable. Indeed they are only reasonably accurate for par- ticular cases. CALGARY WOOD-POLE LINE (Electrical World, Jan., 1912) One ground wire, 24-in. steel. Three conductors, No. aluminum, 55,000 volts. Two telephone wires. Pin insulators. Spans, 150 ft. Height, 40-ft. poles. Average cost per mile, $2000. * * * STEEL-POLE LINE One ground wire, Ke-i n - steel. Three conductors, No. 2 copper, 33,000 volts. Two telephone wires, No. 10 copper-clad. Span, 400 ft. Height of poles, 43 ft. Poles $690 Freight and cartage 25 Foundations 65 Guying 30 Erection . . 55 $865 Wires $685 Insulators, pins and ties 145 Erection.. 105 ; $935 Clearing, damages, etc 25 Right-of-way 195 Supervision 100 Miscellaneous. . 110 Total cost. 228 POLE AND TOWER LINES * * WIDE-BASE TOWER LINE This line was erected under adverse weather conditions, and in extremely rough, and rather inaccessible country. The soil was hard clay. One ground wire, No. 00 copper. Six conductors, No. 00 copper. (One circuit installed). Standard spans, 800 ft. Cost per tower Hauling $14 . 50 Setting footings 73.20 Assembling tower 24 . 80 Raising 22.00 $134.50 Hauling four-horse teams, driver and one to three helpers. a portion of line material distributed from rail- road. Setting Foreman four diggers six to eight templet, level and survey men. Assembling foreman and 5 to 20 men, depending on weather. Raising foreman, four-horse teams, shear legs, etc., and eight men (average four towers per day). Cost of common labor, $2.25 a day. 1 Average of 104 towers. * STEEL-POLE LINE (NARROW-BASE TOWERS) This line was erected in 1905, under circumstances that were not at all favorable to a low cost. The work was done in winter weather; the foundations were expensive both in design and in construction, and the line is crooked and difficult of access Twenty-four conductors, 250,000 circ. mils, soft-drawn stranded copper, 11,000 volts. Eight feeders, 500,000 circ. mils, soft-drawn stranded copper, 650 volts. Wood crossarms. Standard spans, 150 ft. Pole heights, 40, 45, 50, and 55 ft. Weight of standard 40-ft. pole 3000 Ib. Weight of standard 45-ft. pole 3300 Ib. Weight of standard 50-ft. pole 3800 Ib. Weight of standard 55-ft. pole 4000 Ib. Weight of special or angle poles . . . 5000 Ib. to 6000 Ib, ERECTION AND COSTS 229 APPROXIMATE AVERAGE COSTS Poles Cost per mile Steel poles $3,550 Wood crossarms 200 Erection 550 * Foundations 5,800 Guying 50 Painting poles and arms 200 Total $10,350 ESTIMATED COST. ONE-CIRCUIT LINE WITH STEEL POLES One ground wire. Three conductors, No. 1 copper, 22,000 volts. Two telephone wires. Standard spans, 450 ft Poles per mile, 12. Poles: Cost per mile Material at $50 per pole $600 Freight 35 Hauling at $2 . 50 per pole 30 Foundations (earth) 7 at $2 = $14 Foundations (braced) 3 at $3=$9 Foundations (concrete) _2_at $11 = $22 12 $45 45 Erection at $3 per pole 35 Guying 15 Painting 20 $780 $780 Wires and Line Material- One ground wire %-in. galvanized steel cable .... $60 Three conductors, No. 1 stranded copper 670 Two telephone, No. 6 BWG Siemens-Martin steel. . 80 $810 Ties, guys, splices, etc 50 33,000-volt insulators and pins 27 insulators on tangent poles 18 insulators on corner poles 45 insulators and pins at $0. 75 each = $35 45 telephone insulators and pins . . at $0 . 20 each = $10 Hauling , 10 Erecting 110 $1025 $1025 Clearing, trimming, etc 25 Right-of-way at $5 per pole 60 Supervision 100 Contingencies and miscellaneous 25 Total per mile of standard line $2015 Crossings and special structures $ 230 POLE AND TOWER LINES The following comparative estimates of the costs of a steel- pole line with long-span construction, and of a wood-pole line with short-span construction, both for the same location in access- ible rolling country, indicate an ultimate saving in favor of the steel line. ESTIMATED COST. ONE-CIRCUIT LINE WITH WOOD POLES One ground wire, ^-in. steel. Three conductors, No. 2 copper, 33,000 volts. Two telephone wires, No. 10 copper-clad. Standard spans, 120 ft. Poles per mile, 44. Pin-type insulators. Metal arms. Poles: Cost per mile Poles 35 ft. long, 7-in. tops at $5 each $220 Crossarms, galvanized 167 Telephone brackets 5 Pole steps and hardware at $0 . 75 per pole 33 Framing and trimming at $0 . 50 per pole 22 Creosoting butts at $0 . 20 per pole 9 $456 Hauling at $1 per Dole $ 44 Digging holes at $1 .20 each $ 53 Bog shoes or braces 6 Setting poles at $1 . 80 each 79 Miscellaneous 4 $142 $142 Guying 30 $672 $672 Wires and Line Material- One ground wire $ 54 Three conductors 544 Two telephone 50 $648 Ties 5 Soldering materials 5 33,000-volt insulators 66 Pins 49 Telephone insulators 5 Ground-wire connection 16 Stringing 3 miles, No. 2 copper 45 Stringing 2 miles, No. 10 copper-clad . . 20 Stringing 1 mile, %-in. steel 18 Miscellaneous 4 $881 $881 ERECTION AND COSTS 231 Clearing, trimming, etc 10 Miscellaneous materials and tools 15 Right-of-way at $5 per pole 220 Supervision, engineering and general expense .... 100 Contingencies and miscellaneous 25 Total per mile of standard line $1923 Crossings and special structures $ ESTIMATED COST. ONE-CIRCUIT LINE WITH STEEL POLES Wires same as before, except that a Ke-in. steel ground wire was assumed. Standard spans, 400 ft. Poles per mile, 13. Three-disc suspension-type insulators. Poles: Cost per mile Material at $53 per pole $689 Hauling at 2 . 25 per pole 29 Digging holes at $1 .50 each $19.50 Concrete at corners 40 . 00 Crushed stone .6.00 $65.50 $65 Erection at $2 .25 per pole 29 Guying 30 Painting at $1 .50 per pole 20 Miscellaneous -. . 8 $870 $870 Wires and Line Material: One ground wire $75 Three conductors 544 Two telephone wires 50 $669 Soldering materials, etc 5 Insulators and clamps 137 Telephone insulators 5 Stringing 3 miles, No. 2 copper 54 Stringing 2 miles, No. 10 copper-clad. 24 Stringing 1 mile, Ke-in. steel 20 Miscellaneous 6 $920 $920 Clearing, trimming, etc 10 Miscellaneous materials and tools 20 Right-of-way at $7 per pole 90 Supervision, engineering and general expense. .. 100 Contingencies and miscellaneous 25 Total per mile of standard line $2035 Crossings and special structures $ CHAPTER XIII PROTECTION Ground Wires. In the light of our present knowledge, "ground" or "sky" wires seem to be desirable on lines of 11,000 volts or more, but are not necessary on lines which are in more or less shel- tered locations. If, however, the ground wire is of less durable material than the conductors under it, or is improperly connected Ground Wire FIG. 150. Eccentric location of ground wire. to the supports, it becomes a menace rather than a safeguard. A poorly constructed ground wire will eventually cause interrup- tions in the service of the power wires below it. The relative merits of galvanized-steel, galvanized-iron, copper- covered and copper wire are not definitely known and the subject is worthy of much more careful consideration than it has thus far 232 PROTECTION 233 received. The second material mentioned, i.e., galvanized iron, is more or less of a misnomer, as there is said to be little or no real iron wire used for transmission purposes. A large portion of the so-called iron wire is in reality soft steel, which does not have the ability to resist corrosion like the old-fashioned wrought- iron wire. In the process of galvanizing wire cables the excess coating is wiped off, resulting in a thinner coat than is usually obtained on galvanized shapes which are merely allowed to drain. This explains in some measure the increased life of windmill towers over that of guy cables. It need not be inferred that galvanized ground wires are undesirable in all instances, as in certain locali- ties they will prove economical. In general, however, the probable life of galvanized wire in the lo- cality under consideration should be scrutinized with care before such material is placed immedi- ately over copper conductors. The choice between copper- covered and copper cable, is chiefly a matter of cost, if it is assumed that the relatively thin shell of the copper-covered cable will be effective in preventing corrosion. A smaller gage cop- per covered steel wire or cable will have the strength necessary to permit a sag equal to that of the larger copper cables. If a copper-covered ground wire is used, it should have a heavy coating of copper. A number of lines have been built with ground wires of the same section and material as the conductors, but the greater number have galvanized-steel cable. It is also probable that the majority of ground wires are of a smaller gage than the power wires. Good practice seems to indicate, however, that galvanized- FIG. 151. Two-circuit tower, two ground wires. 234 POLE AND TOWER LINES steel ground wires should be cables % in. or more in diameter and that copper-covered stranded-steel should be heavily coated. Furthermore, it is desirable to use cables having few strands in order to obtain a relatively thicker coating of copper. The ground wire connection differs from the power connections in that it should be treated as a dead-end connection at every pole or tower. A variation in sag due to the accidental, or intentional, slip of a conductor has less opportunity to cause trouble than a similar slip in the ground wire. In the case of the so-called flex- ible towers, by which is meant those having little theoretical strength in the direction of the line, a firmly attached ground wire is needed to serve as a partial guy to help minimize the extent of tower failure. The ground wire attachments should, therefore, be well tight- ened, regardless of the condition of the power-wire attachments. On some supporting struc- tures the ground wire is con- nected with a vertical earth wire leading to a ground plate beneath the support. Such connections should be arranged to preserve, as much as possi- ble, the original strength of the ground wire. The junction is necessarily at the point of maxi- mum mechanical stress in the ground wire; therefore soldered or bent connections are particularly undesirable. The proper location for a ground wire with respect to the con- ductors is at the apex of a 60 to 90 angle enclosing the latter wires. In recent practice the ground wire is usually placed a distance above the upper conductors equal to about one- half the horizontal space between them. On some high-voltage lines with two circuits in vertical spacing, two ground wires have been used, one over each set of conductors. This method un- doubtedly gives some increased protection, but its relative effec- tiveness is uncertain. On the other hand some designs for one-circuit poles have placed the ground wire in the position opposite the upper insula- FIG. 152. Crosby clip. PROTECTION 235 tor connection, so that it is in no sense over two of the power wires (Fig. 150). It seems, therefore, in considering the protection afforded by overhead ground wires and in judging the results obtained in actual installations, that some weight should be given to the rela- tive location of wires on the lines in question. The attachment of the ground wire to its supporting structure FIG. 153. Ground wire clamping cap. is a detail of great importance. If a long, smooth, well-rounded wire seat in the clamps is a wise provision for the attachment of the power wires, it is equally desirable for a ground wire which over- builds the power wires. It is a matter of common knowledge that a short rigid metallic connection with a small U- or hook-bolt biting into a wire has a tendency to cause wire failure. Therefore, such connections should not be used in the very worst possible place on a power line. FIG. 154. Ground wire clamping cap. Further, the use of ridges or teeth in the contact surface is decidedly poor practice in any connection to copper wire. Such projections merely serve as cutting edges to injure the softer copper material when the device is tightened. The indications are that better results are obtained with copper wires by the use of a long contact surface without change of direction, rather than by short clamps with pronounced waves. Since clamps are fre- quently galvanized, care should be taken to obtain a smooth sur- 236 POLE AND TOWER LINES face finish, both before and after galvanizing, on the portions which will come into contact with the wire. Sand spots or edges on the black material, or improper draining of the zinc coating, may cause the formation of sharp projections which will injure the wire. The ends of the wire grooves should be bell-mouthed with a gradual slope, in order that the wire may not be bent sharply, or even appreciably, at any point. In other words, there should be a reasonably long tangent contact surface ending in curved orifices, the sides of which should confine the wire, as nearly as possible, to the exact position where its curve of sag would cause it to lie under any conditions of loading. Neighboring Lines. Whether or not the location of a proposed transmission line is in a measure fixed by existing property rights, as may be the case with lines on electric railways, it is necessary, or at least very advisable, to confer with the owners of any exist- ing and adjacent wire lines to predetermine what measures, if any, may be required to prevent interference with the proper operation of such foreign lines. There are two general classes of neighboring lines; other trans- mission lines, and the so-called "no-voltage" lines such as the telephone and telegraph. In relation to both of these classes of lines, there are two classes of adjacency, crossings and parallelism. Crossings may vary from single-span, right-angle crossings to several-span, oblique or "skew" crossings. The term parallelism is ordinarily used to indicate two lines on separate structures, though it also applies to over building whether on the same or separate structures. Interferences may also be divided into two classes, inductive and contact. Interferences of the first class are probably con- fined to cases of parallelism while the latter may occur with either parallel or crossing lines. The theory of inductive interference is not yet thoroughly understood, so generally effective measures for its prevention have yet to be devised. It appears that induced currents may occur with rather widely separated lines, between which there can be no physical contact either direct or indirect. The matter of induction, therefore, should be considered as a separate sub- ject. In its relation to the physical characteristics of a trans- mission line, induction need, therefore, be considered only as a reason for the inclusion of transpositions. PROTECTION 237 Interferences by contact may be of many kinds and may occur wherever two lines are near each other. Reasonable security at crossings is not difficult to obtain, but the matter becomes" rather complicated where transmission lines and no-voltage lines are closely parallel. In general, it will be found advantageous to locate the proposed power line as far as practicable from the no-voltage line, prefer- ably on a different route. Otherwise, the two lines should be separated as widely as possible, and ordinarily occupy opposite sides of the highway or right-of-way. In some instances it will be necessary to move all, or parts, of an existing line in order to maintain a proper separation. Again it may be advisable to consolidate two existing lines to provide space for the power line. In view of the fact that most of the existing lines are of rela- tively remote origin, and together with the power line frequently subject to governmental supervision, it should be unnecessary to point out the propriety of an investigation as to the relative rights, contracts and responsibilities of the conflicting lines. Un- fortunately such investigations seem to have been the exception rather than the rule. In an attempt to reduce the possibilities of interference by con- tact, it immediately appears that physical separation is the most effective method. The amount, or distance, of separation is not a fixed quantity, nor is it essentially a function of any physical characteristic of either line. Each installation should be con- sidered as a special case since the local topographical conditions have almost as much bearing on the effective separation as 'the pole heights, spans, factors of safety and details of construction of the lines. For example, it is evident that there can be no physical contact between the supporting structures of two lines, if they are separated by a high embankment. Again, a low line cannot come into structural contact with a tall line if the poles of the short line are set opposite or near the mid-span of the other. In both of the above cases, however, there is a possibility that wind-blown wires of either class may afford contact with the parallel line. The relative positions of the two lines with regard to locations on side hills and exposure to storms will have a very direct bearing on the possibility of contact. In addition to the foregoing, consideration must be given to the presence of inflammable material near either line, 238 POLE AND TOWER LINES the probability of wind-blown branches, etc., and the details of construction of both lines. An investigation, in 1914, of the number of failures at cross- ings in the States of Idaho, Oregon, and Washington on a total of 1953 crossings, for periods from 2 to 8 years, showed a total of six failures, only one of which resulted in the damage to the company crossed. 1 Voltages Crossing years Failures Damage 5,000- 7,000 635 11,000-15,000 3,397 4 None 22,000-44,000 1,209 55,000-66,000 5,544 2 Phones burned ($25.00) 10,785 6 Cradles. A cradle or guard net is a wire basket in the more elaborate form, a wire tunnel formerly used to separate two power systems, or to protect a telephone, telegraph, or similar system from an electric light or power line. A cradle to be effective must practically inclose one system of wires, since there is no justification for the assumption that a broken wire will fall vertically and remain within the confines of a flat net of restricted area. That the use of cradles was a natural development in the prog- ress of transmission line construction, may be admitted; it is, however, an indisputable fact that in recent years they have fallen into great disfavor. Inasmuch as prevention is more desirable than cure, the latest practice in this country is to so install the power line that the conductors will rarely break and in some instances to further insure against a falling wire by the use of an auxiliary attachment designed to hold the wire in case of the failure of the insulator or of the wire at the insulator. Clamping Devices. The Joint Report specifications for cross- ings (Edition of 1911) state explicitly what the connection of the power wires to the supporting structures at crossings should ac- complish, but do not state the exa~ct means by which the result should be attained. The general theory of the clamping device is to require an effi- 1 Idaho Power and Light Mens Assoc. PROTECTION 239 cient insulator and a positive dead-ending attachment of the wire to the insulator through a device having sufficient mechanical strength to resist the tension of the wire in case the insulator fails ; that the device should have sufficient mass to resist burning, at least to some extent; and that the points of attachment to the power wires be at a sufficient distance from the insulator to mini- mize the danger from arcs. The attachment should be so de- FIG. 155. signed that it cannot fall free of the pin in case the insulator is shattered. It should not require delicate adjustment and should be firmly clamped to the wire without injuring it in any way. In order to prevent the burning of wood pins and crossarms by arcs from defective insulators, or fallen wires, by causing the circuit breakers to act, some specifications have required that ICT FIG. 156. FIG. 157. crossarms should be of metal, or be provided with metallic strips and that they should be grounded. In other instances, metal grounding arms have been placed below the wires so that a falling wire would come into contact with them, as in the H-frame cross- ing, Fig. 45, in which the auxiliary chain attachments have not yet been clamped to the power wires. On the other hand, the grounding of wood crossarms results in the loss of the insulating 240 POLE AND TOWER LINES value of the wood arm. In dry weather a wire falling on a wood arm would not necessarily burn either the arm or the wire. With insulated wire there is a possibility of injuring the wire in removing the insulation and this may outweigh the effect of the insulation in preventing a tight grip on the power wire itself. It is not clear to the writer how a device at the support can in- sure against accidents from breaks out in the span. Moreover, it would appear that the majority of failures occur at the insula- tors, so that the greatest practicable benefit would be obtained by improving, if any improvement is necessary, the construction at the insulators. It is claimed by some engineers that nothing is required except a first-class insulator and a tie wire, and that the cost of auxiliary devices would better be expended for higher grade insulators. Further, as the low-voltage lines are not subject to much electrical trouble, a slight increase in insu- FIG. 158. FIG. 159. lation might practically insure such lines against failure at the insulators. In some cases an auxiliary or second attachment of the power wire is used, but unfortunately this is not always as effective as it appears. For instance, if such an arrangement requires dead- end connection on a single-pin insulator, it is in itself undesirable and mechanically impracticable for heavy stresses. It has also been proposed that the wire be protected from arcs by the use of an arcing strip, cap, etc., but these are not effective if a shattered insulator allows the attachment and wire to fall. Again, it is not possible by the use of any device at the support to prevent a wire broken out in a span from coming into contact with a line beneath it. However, as the majority of failures occur at the insulators, it seems wise to neglect this possibility and concentrate attention on other features of the connections at the supports. In the writer's opinion too little value has been attached to the ability of a wire to hold, over a doubled span length, in case of pole failure. With ordinary short-span construction and reason- ably low voltages there is little reason for doubting that the wires PROTECTION 241 will be less liable to injury if the crossarms are ungrounded. Therefore, any grounding device should include some provision to prevent the actual separation of the wire into two spans, either of which may fall. The clamping device or dead-ending attachment, illustrated in Fig. 160, is typical of the erroneous idea that protection may be afforded foreign interests without due regard to the protection of the power line. In other words, the benefits from the method FIG. 160. used to eliminate danger from falling conductors are more than offset by the possibilities of accident introduced by the protective construction. As shown, the span adjoining the crossing is dead-ended on one pin type insulator and this insulator must carry a heavy me- chanical load, not merely when a break occurs, but at all times. Apart from the fact that this is one of the things most to be avoided in line material installation it introduces an unbalanced load on the support. In general, it compels the insulators, pins 16 242 POLE AND TOWER LINES and towers to withstand a continuous loading far in excess of their continuous loading under ordinary construction. The crossing span is supported from the strain insulator con- nection and the power cable is not continuous over the supports, the clamps having to serve both as a mechanical and an electrical connection. In actual construction it is probable that the power cable would be passed around a large stout thimble and possibly "served" at the end connection; otherwise the detail seems an undesirable one for copper cable. The constant mechanical stress to which the strain insulators are subjected is undesirable and at high voltages might be very objectionable, though it is true that such insulators have greater mechanical strength than those of the pin type. Clamp Porcelain Insulator, Metal Pin FIG. 161. Pin insulators with caps and saddle. Further, it seems at least possible that an electrical breakdown of the strain insulators might cause an arc which would melt the jumper cables immediately above them thus rendering the aux- iliary connection useless. The purpose of the grounding arm beneath the jumper is to ground the latter in case a conductor breaks outside of the clamps, i.e., out in the span. There does not appear to be any great assurance, however, that the ground would be effective before the long end of the wire falling free could come into contact with wires beneath it. The use of the rigid clamping cap on the pin insulators is not considered the best practice, the general tendency now being to allow the power cables to balance themselves about a smooth porcelain surface, and to have any auxiliary connections "ride" on the line. PROTECTION 243 If there is any basis of fact in the recent theory that lightning troubles are aggravated by bends in conductors, this construction would be open to such criticism. Nests of insulators supporting a cast saddle have been used to some extent for the conductors of unusually long spans, such as river crossings and in some instances for railroad crossings (Fig. 161). It is possible in this manner to provide sufficient pin strength to withstand heavy stresses and at the same time obtain a long contact surface to which the conductors can be clamped. The chief objection to this construction, apart from the cost, is the difficulty of removing and replacing shattered insulators. There is usually a considerable weight of cable carried by the ; j FIG. 162. Two strings of insulators, suspended position. saddle and also a vertical component of the wire tension, due to the fact that the wires often descend from the saddle to a lower adjoining pole. The clamping attachment of the saddle as well as all portions of the wire seat should be smooth and well rounded to avoid breakages due to the rigidity of the saddle and the vibra- tion of the wires. Another and perhaps a better method of supporting moderate- length spans is to use two strings of suspension type insulators hung as shown in Fig. 162. Two strings of insulators in the strain position have also been used and have generally given satisfactory service except at the higher voltages. The design shown in Fig. 162 has the merit of a lower mechan- ical stress in the insulators, and should be less conducive to 244 POLE AND TOWER LINES impact in case of wire failure while retaining, in a measure, the auxiliary or double connection. This method of attachment, however, requires considerable separation between crossarms to provide clearance for wires which descend sharply on one side of the tower. Several years' experience with a number of unusually high river crossing towers in different localities has led the writer to believe in the propriety of over-insulating the wire attachments on such towers. Considering six pairs of towers ranging in height from 96 ft. to 188 ft. separated from 10 to 100 miles and all in a region subject to rather severe electrical storms, there have been but two cases of insulator failure. Both insulators were of the single-disc type and were hung in the strain position. While the insulators were badly burnt and shattered, in neither case did they break apart or allow the wire to fall. Two crossings have pin insulators and the others have disc insulators in the strain position. The failures occurred on one of the two sets which did not have an overhead ground wire. One failure resulted directly from lightning and the other apparently from a heavy surge fol- lowing a head puncture, by lightning, of a pin insulator on an- other structure. All of the towers are grounded. No other trouble of any nature has been reported on any other wires, although the various structures carry from six to twenty-four 13,000-volt wires each. DISCUSSION OF JOINT REPORT SPECIFICATIONS FOR CROSSINGS (Edition of 1911-1914) The writer was a member of two committees signing the Joint Report and perhaps for the very reason that he has been compelled to occupy a double standpoint some explanatory discussion of various disputed requirements may not be out of place. The fact is frequently forgotten that this specification is a pioneer and in its very nature a compromise. There was previously no standard even approximately acceptable to conflicting interests, whereas with the Joint Report as a basis, an electric service company and any company whose lines are crossed are able to agree promptly on mutually satisfactory construction without the old interminable delay. Objections have been made that the specifications, when literally enforced, are oppressive in certain special cases. This is not a reasonable objection since it may be said with equal truth of any specification. PROTECTION 245 To consider the specifications by clauses we have: 1. SCOPE. The limitation of 5000 volts in relation to telephone lines, while no doubt desirable from a telepone standpoint, may be a hardship to power lines located in streets. If it is a fact that protective apparatus is not effective above 2300 volts, there seems little logic in limiting the power line to 5000 volts, if the probability of failure on a well-constructed 13,000- volt is not measurably greater than that of the ordinary 2300- volt line. The limitation is an old standard which conceivably should not now apply with equal force under improved methods of construction. The words " constructed over'* are perhaps unfortunate since they in- troduce the inference that the clause applies to joint poles and parallel lines. In reality the specification is as stated a crossing specification, a crossing being a single span or perhaps two or three spans. 8. CLEARANCE. Side clearances of (12 ft. in.) are not properly en- forceable in case the track occupies a city street within that distance of the curb, provided always that reasonable clearance may be obtained, or that trees, etc., already occupy the curb line. 10. While the 8 ft. in. clearance above other wires is reduced by paragraphs 16 and 33, to 6 ft. in. under certain conditions, it would be an improvement to specifically permit 6 ft. in. clearance for all con- struction in which the wires are securely fastened and when the poles are not subjected to much bending. Further, such clearances should be extended to cover the voltages above those embraced in the various clearances given in the specifications. 12. Dead-ending through disc insulators in the strain position should not be mandatory as more recent experience tends to prove this method undesirable in some instances. Provided other requirements are com- plied with, this clause should not be blindly enforced. 19. GUYS. While inferentially guys are permitted on any type of structure, they should be specifically permitted with proper regulation. 23. GROUNDING. As indicated heretofore, the writer, personally, is not in favor of a general enforcement of the grounded arm and in view of the general lack of agreement on the subject since developed by the specification, it would seem preferable to make the requirement voluntary. 34. LOADS. The sliding scale of broken wires, while apparently a reasonable compromise between none broken and all broken, should specifically include provision for designing with "pullback" from the unbroken wires. This is what actually occurs in fact on heavy lines and unless this interpretation is made, such lines are unduly punished. Further, it is illogical to design all multiple-pin arms for large broken cables without pullback. Pullback is not mentioned directly in the specification, though it may be inferred. 37. FACTORS OF SAFETY. The pin and insulator are a structural unit after erection and the porcelain does not really need a larger factor 246 POLE AND TOWER LINES than the pin, nor is its individual factor readily determinable. For heavy cables the factors given for pins and insulators are prohibitory if liter- ally enforced. The factor of 3.0 for structural steel is not exactly correct as a single statement and should be omitted since the matter is covered by the allowable unit stresses given in paragraph 69. Concrejbe poles if properly made are unnecessarily penalized if a factor of 4.0 is used. The factor of 2.0 for foundations should not be blindly enforced as the conditions of loading presuppose at least a semi-frozen soil and the general methods of computing foundations err on the side of safety. 45. CONDUCTORS. There seems to be little justice in using this paragraph to require an existing solid wire on ordinary construction to be replaced by a stranded cable. The clause is founded on the greater strength of large stranded cables for long-span construction and on the greater chance of injury to solid wires in erection. Therefore, short spans and careful erection, particularly with insulated wire, would be unjustly treated by a literal and general enforcement. 48. INSULATORS. The strength of the guy insulator should be twice that of the guy stress, not of the guy strength. This is evidently a confusion of intent since the latter would penalize extra strong guys used for protection against corrosion. Again, the interlocking feature is not generally applicable, indeed impracticable for high voltages and the requirement should not be mandatory. 54. CONCRETE. At the time the specifications were written, the then forthcoming Joint Committee Report on Concrete and Reinforced Concrete seemed the most general authority on the subject. There is now grave question, however, that it can be applied to crossings, since it covers a different field of work, i.e., bridges and buildings, and is only fairly enforceable in certain individual requirements, though education- ally of benefit. 59. STRUCTURAL STEEL. Reference to the preceding sections on structural design will show that the use of large ratios of l/r are not conducive to the best types of construction, nor safely permissible for general use. Some reduction in the figures given, such as to 150 and 200, would be no great hardship and of some real benefit to the general excellence of future work. 60. Similar reasoning indicates an increase in minimum thickness to % e in. for galvanized material. 64. FOUNDATIONS. Although accurate data on foundation design are wanting, the strict application of the clause works a hardship by in- ferentially omitting the unknown value of earth shear, arch action, etc. 73. TIMBER. Paragraphs 37 and 73 seem unnecessarily conservative, particularly for selected timber treated with preservative, or whose PROTECTION 247 deterioration will be closely watched. It may be granted that a fair and accurate requirement is almost impossible, but some increase of unit stress is certainly reasonable for lines in city streets. In conclusion and in spite of the foregoing interpretations many of which have been developed only by actual use, the writer again repeats his firm belief that the specifications, if used with an honest desire to correlate divergent interests, are a very efficient work and a great advance on all previous measures. CHAPTER XIV JOINT REPORT SPECIFICATIONS FOR OVERHEAD CROSSINGS OF ELECTRIC LIGHT AND POWER LINES (Edition of 1911-1914) GENERAL REQUIREMENTS 1. Scope. This specification shall apply to overhead electric light and power line crossings (except trolley contact wires), over railroad right-of-way, tracks, or lines of wires; and, further, these specifications shall apply to overhead electric light and power wires of over 5000 volts constant potential, crossing telephone, telegraph or other similar lines. It is not intended that these specifications shall apply to crossings over individual twisted pair drop wires, or other circuits of minor importance where equally effective protection may be secured more economically by other methods of construction. 2. Location. The poles, or towers, supporting the crossing span preferably shall be outside the railroad company's right-of-way. 3. Unusually long crossing spans shall be avoided wherever practicable and the difference in length of the crossing and adjoining spans generally shall be not more than 50 per cent, of the length of the crossing span. 4. The poles, or towers, shall be located as far as practicable from inflammable material or structures. 5. The poles, or towers, supporting the crossing span, and the adjoin- ing span on each side, preferably shall be in a straight line. 6. The wires, or cables, shall cross over telegraph, telephone and similar wires wherever practicable. 7. Cradles, or overhead bridges, shall not be used beneath the cross- ing wires or cables ; but in cases where the crossing wires or cables cross beneath the railroad wires, telephone, telegraph, or other similar wires, a protection of adequate strength and proper design between the two sets of crossing wires or cables may be required. 8. Unless physical conditions or municipal requirements prevent, the side clearance shall be not less than twelve (12) ft. from the nearest track rail, except that at sidings a clearance of not less than seven (7) ft. may be allowed. At loading sidings sufficient space shall be left for a driveway. 9. The clear headroom shall be not less than thirty (30) ft. above 248 OVERHEAD CROSSINGS 249 the top of rail under the most unfavorable condition of temperature and loading. For constant potential, direct-current circuits, not exceed- ing 750 volts, when paralleled by trolley contact wires, the clear head- room need not exceed twenty-five (25) ft. 10. The clearance of alternating-current circuits above any existing wires, under the most unfavorable condition of temperature and load- ing, shall be not less than eight (8) ft. wherever possible. For con- stant potential, direct-current circuits, not exceeding 750 volts, the minimum clearance above telegraph, telephone, and similar wires may be two (2) ft. with insulated wires and four (4) ft. with bare wires. 11. The separation of conductors carrying alternating current sup- ported by pin insulators, for spans not exceeding 150 ft., shall be not less than: Line voltage Separation Not exceeding 7,000 volts ; . 12 in. Exceeding 7,000, but not exceeding 14,000 20 in. Exceeding 14,000, but not exceeding 27,000 30 in. Exceeding 27,000, but not exceeding 35,000 36 in. Exceeding 35,000, but not exceeding 47,000 45 in. Exceeding 47,000, but not exceeding 70,000 60 in. For spans exceeding 150 ft. the pin spacing should be increased, depending upon the length of the span and the sag of the conductors. 1 With constant potential, direct-current circuits "not exceeding 750 volts, the minimum spacing shall be ten (10) in. 12. When supported by insulators of the disc or suspension type, the crossing span and the next adjoining spans shall be dead-ended at the poles, or towers, supporting the crossing span, so that at these poles, or towers, the insulators shall be used as strain insulators, or the height of the wire attachments shall be such that with the maxi- mum sag in the crossing span, occurring from failure of the construc- tion outside the crossing span, and taking into account the deflections in the strings of suspension insulators, the minimum clearances, as given in Paragraphs 9 and 10, shall be maintained. 13. The clearance in any direction between the conductors nearest the pole, or tower and the pole, or tower, shall be not less than: Line voltage Clearances Not exceeding 10,000 volts , . ' 9 in. Exceeding 10,000, but not exceeding 14,000 12 in. Exceeding 14,000, but not exceeding 27,000 15 in. Exceeding 27,000, but not exceeding 35,000 18 in. Exceeding 35,000, but not exceeding 47,000 21 in. Exceeding 47,000, but not exceeding 70,000 24 in. 1 NOTE. This requirement does not apply to wires of the same phase or polarity between which there is no difference of potential. 250 POLE AND TOWER LINES 14. Conductors. The normal mechanical tension in the conductors generally shall be the same in the crossing span and in the adjoining span on each side. 15. The conductors shall not be spliced in the crossing span nor in the adjoining span on either side. Taps to conductors in the crossing span are generally objectionable, and should not be made unless necessary. 16. The ties or devices for supporting the conductors at the poles, or towers, shall be such as to hold the wires, under maximum loading, to the supporting structures, in case of shattered insulators, or wires broken or burned at an insulator, without allowing an amount of slip which would materially reduce the clearance specified in Paragraphs 9 and 10. 17. Ground wires when installed as protection against lightning, shall be thoroughly grounded at each of the crossing supports. In case of their installation on steel supporting structures, they may be clamped thereto. In case they are installed on wooden structures, the ground wire shall be grounded at each of the structures with a solid copper wire, with as few bends as possible, and no sharp bends, and not less than No. 4 B. & S. gage or equivalent copper section. The ground wire itself, in the crossing span and the adjacent spans, may be of the same material as the conductors, or a steel strand not less than 5/16 in. in diameter may be used, double galvanized, and having a breaking strength of not less than 4500 Ib. and in general shall follow the minimum factors of safety as provided for the rest of the crossing construction. If crossarms are grounded, the same ground wire may be used for grounding the lightning protection wire as in grounding crossarm strips. 18. Where there is an upward stress at the point of conductor attach- ment, the attachment shall be of such type as to properly hold the conductor in place. 19. Guys. Wooden poles supporting the crossing span shall be side- guyed in both directions, if practicable, and be head-guyed away from the crossing span, and the next adjoining poles shall be head-guyed toward the crossing span. Braces may be used instead of guys. 20. Strain insulators shall be used in guys from wooden poles, except when the guys are through grounded to permanently damp earth. The insulators shall be placed not less than eight (8) ft. from the ground. Strain insulators shall not be used in guying steel poles or structures. 21. Clearing. The space around the poles, or towers, shall be kept free from inflammable material, underbrush and grass. 22. Signs. In the case of railroad crossings, if required by the rail- road company, warning signs of an approved design shall be placed on all poles and towers located on the railroad company's right-of-way. OVERHEAD CROSSINGS 251 23. Grounding. For voltages over 5000 volts, wooden crossarms, if used, shall be provided with a grounded metallic plate on top of the arm which shall be not less than % in. in thickness and which shall have a sectional area and conductivity not less than that of the line conductor. Metal pins shall be electrically connected to this ground. Metal poles and metal arms on wooden poles shall be grounded. 24. The electrical conductivity of the ground conductor shall be adjusted to the short-circuit current capacity of the system at the crossing and shall be not less than that of a No. 4 B. & S. gage copper wire. 25. Temperature. In the computation of stresses and clearances and in erection, provision shall be made for a variation in tempera- ture from - 20F. to + 120F. A suitable modification in the tem- perature requirements shall be made for regions in which the above limits would not fairly represent the extreme range of temperature. 26. Inspection. If required by contract, all material and workman- ship shall be subject to the inspection of the company crossed; pro- vided that reasonable notice of the intention to make shop inspection shall be given by such company. Defective material shall be rejected and shall be removed and replaced with suitable material. 27. On the completion of the work, all false work, plant and rubbish incident to the construction shall be removed promptly and the site left unobstructed and clean. 28. Drawings. If required, by contract, ( ) complete sets of general and detail drawings shall be fur- nished for approval before any construction is commenced. LOADS 29. The conductors shall be considered as uniformly loaded through- out their length, with a load equal to the resultant of the dead load plus the weight of a layer of ice Y 2 in. in thickness and a wind pressure of 8 Ib. per square foot on the ice-covered diameter, at a temperature of 0F. 30. The weight of ice shall be assumed as 57 Ib. per cubic foot (0.033 Ib. per cubic inch). 31. Insulators, pins and conductor attachments shall be designed to withstand the mechanical tension in the conductors under the maximum loadings with the designated factor of safety. 32. Sags should be such that the stress on the pin falls within the limits of paragraph 31, unless methods be employed to prevent an undue slip in case of pin failure. (See paragraphs 9, 10 and 16.) 33. The pole, or towers, shall be designed to withstand, with the designated factor of safety, the combined stresses from their own weight, the wind pressure on the pole, or tower and the above wire 252 POLE AND TOWER LINES loading on the crossing span and the next adjoining span on each side. The wind pressure on the poles, or towers, shall be assumed at 13 Ib. per square foot on the projected area of solid or closed struc- tures and one and one-half times the projected area of latticed structures. 34. The poles, or towers, shall also be designed to withstand the loads specified in paragraph 33, combined with the unbalanced tension of: 2 broken wires for poles, or towers, carrying 5 wires or less. 3 broken wires for poles, or towers, carrying 6 to 10 wires. 4 broken wires for poles, or towers, carrying 11 or more wires. 35. Crossarms shall be designed to withstand the loading specified in paragraph 33, combined with the unbalanced tension of one wire broken at the pin farthest from the pole. 36. The poles, or towers, may be permitted a reasonable deflection under the specified loading, provided that such deflection does not reduce the clearance specified in paragraph 10 more than twenty-five (25) per cent, or produce stresses in excess of those specified in para- graphs 69 to 73. FACTORS OF SAFETY 37. The ultimate unit stress divided by the allowable unit stress shall be not less than the following: Wires and cables 2 Pins 2 Insulators, conductor attachments, guys 3 Wooden poles and crossarms 6 Structural steel 3 Reinf orced-concrete poles and crossarms 4 Foundations 2 NOTE. The use of treated wooden poles and crossarms is recommended. The treatment of wooden poles and crossarms should be by thorough impregnation with preservative by either closed or open-tank process. For poles, except in the case of yellow pine the treatment need not extend higher than a point 2 ft. above the ground line. 38. Insulators. Insulators for line voltages of less than 9000 shall not flash over at four times the normal working voltage, under a pre- cipitation of water of y$ in. per minute, at an inclination of 45 to the axis of the insulator. 39. Each separate part of a built-up insulator, for line voltages over 9000, shall be subjected to the dry flash-over test of that part for five consecutive minutes. OVERHEAD CROSSINGS 253 40. Each assembled and cemented insulator shall be subjected to its dry flash-over test for five consecutive minutes. The dry flash-over test shall be not less than: Line voltage Test voltage Exceeding 9,000 but not exceeding 14,000. . . . 65,000 Exceeding 14,000 but not exceeding 27,000. . . . 100,000 Exceeding 27,000 but not exceeding 35,000. . . . 125,000 Exceeding 35,000 but not exceeding 47,000. . . . 150,000 Exceeding 47,000 but not exceeding 60,000. . . . 180,000 Exceeding 60,000 3 times line voltage Each insulator shall further be so designed that, with excessive potential, failure will first occur by flash-over and not by puncture. 41. Each assembled insulator shall be subjected to a wet flash-over test, under a precipitation of water of ^ in. per minute, at an inclina- tion of 45 to the axis of the insulator. The wet flash-over test shall be not less than: Line voltage Test voltage Exceeding 9,000 but not exceeding 14,000. . . . 40,000 Exceeding 14,000 but not exceeding 27,000. . . . 60,000 Exceeding 27,000 but not exceeding 35,000. . . . 80,000 Exceeding 35,000 but not exceeding 47,000. . . . 100,000 Exceeding 47,000 but not exceeding 60,000 120,000 Exceeding 60,000 twice the line voltage 42. Test voltages above 35,000 volts shall be determined by the A.I.E.E. Standard Spark-gap Method. 43. Test voltages below 35,000 yolts shall be determined by trans- former ratio. MATERIAL 44. Conductors. The conductors shall be of copper, aluminum, or other non-corrodible material, except that in exceptionally long spans, where the required mechanical strength cannot be obtained with the above materials, galvanized or copper-covered steel strand may be used. 45. For voltages not exceeding 750 volts, solid or stranded conductors may be used up to and including 0000 in size; above 0000 in size, stranded conductors shall be used. For voltages exceeding 750 volts and not exceeding 5000 volts, solid or stranded conductors may be used up to and including 00 in size; above 00 in size, conductors shall be stranded. For voltages exceeding 5000 volts, all conductors shall be stranded. Aluminum conductors for all voltages and sizes shall be stranded. 254 POLE AND TOWER LINES The minimum size of conductors shall be as follows: No. 6 B. & S. gage copper for voltages not exceeding 5000 volts. No. 4 B. & S. gage copper for voltages exceeding 5000 volts. No. 1 B. & S. gage aluminum for all voltages. 46. Insulators. Insulators shall be of porcelain for voltages exceed- ing 5000 volts. 47. For pin type insulators, there shall be a bearing contact between the pin and the insulator pin hole up to the level of the top of the tie wire groove, the purpose being that the pin should directly take the strain imposed upon the insulator. 48. Strain insulators for guys shall have an ultimate strength of not less than twice that of the guy in which placed. Strain insulators shall be so constructed that the guy wires holding the insulator in position will interlock in case of the failure of the insulator. For less than 5000 volts, strain insulators for guys shall not flash over at four times the maximum line voltage under a precipitation of water of one-fifth of an inch (3/g in.) per minute, at an inclination of 45 to the axis of the insulator. For voltages of more than 5000 volts, the strain insulator or series of strain insulators shall not fail at the line voltage under the above precipitation conditions. 49. Pins. For voltages of 5000 and over, insulator pins shall be of steel, wrought iron, malleable iron, or other approved metal or alloy, and shall be galvanized, or otherwise protected from corrosion. (See paragraph 47.) 50. Guys. Guys shall be galvanized or copper-covered stranded steel cable not less than % 6 in. in diameter, or galvanized rolled rods, neither to have an ultimate tensile strength of less than 4500 Ib. 51. Guys to the ground shall connect to a galvanized anchor rod, extending at least 1 ft. above the ground level. 52. The detail of the anchorage shall be definitely shown upon the plans. 53. Wooden Poles. Wooden poles shall be of selected timber, reasonably straight, peeled, free from defects which would decrease their strength or durability, not less than 8 in. in diameter at the top, and meeting the requirements as specified in paragraphs 19, 33, 34 and 37. 54. Concrete. All concrete and concrete material shall be in accord- ance with the requirements of the Report of the Joint Committee on Concrete and Reinforced Concrete. 1 STRUCTURAL STEEL 55. Structural steel shall be in accordance with the Manufacturers' Standard Specifications. 1 NoTE. This may be found in the February, 1913, Proceedings of the American Society of Civil Engineers, Vol. 59, No. 2, pp. 117-168. OVERHEAD CROSSINGS 255 56. The design and workmanship shall be strictly in accordance with first-class practice. 57. The form of the frame shall be such that the stresses may be computed with reasonable accuracy, or the strength shall be deter- mined by actual test. 58. The sections used shall permit inspection, cleaning and painting, and shall be free from pockets in which water or dirt can collect. 59. The length of a main compression member shall not exceed 180 times its least radius of gyration. The length of a secondary compres- sion member shall not exceed 220 times its least radius of gyration. 60. The minimum thickness of metal in galvanized structures shall be y in. for main members and ^ in. for secondary members. The minimum thickness of painted material shall be y in. PROTECTIVE COATINGS 61. All structural steel shall be thoroughly cleaned at the shop and be galvanized, or given one coat of approved paint. 62. Painted Materials. All contact surfaces shall be given one coat of paint before assembling. All painted structural steel shall be given two field coats of an ap- proved paint. The surface of the metal shall be thoroughly cleaned of all dirt, grease, scale, etc., before painting and no painting shall be done in freezing or rainy weather. 63. Galvanized Material. Galvanized material shall be in accord- ance with the Specification for Galvanizing Iron and Steel. Bolt holes in galvanized material shall be made before galvanizing. Sherardizing for small parts is permissible. FOUNDATIONS 64. The foundations for steel poles and towers shall be designed to prevent overturning. The weight of concrete shall be assumed as 140 Ib. per cubic foot. In good ground, the weight of "earth" (calculated at 30 from the vertical) shall be assumed as 100 Ib. per cubic foot. In swampy ground, special measures shall be taken to prevent uplift or depression. Concrete for foundation shall be well worked, very wet, and shall not be leaner than one part Portland cement, three parts clean, sharp sand, and six parts of broken stone, or one part Portland cement to six parts of good gravel, free from loam or clay. 65. The top of the concrete foundation, or casing, shall be not less than six (6) in. above the surface of the ground, nor less than one (1) ft. above high water, except that no foundation need be higher than 256 POLE AND TOWER LINES the base of the railroad company's rail, or the top of the traveled roadway. 66. When located in swampy ground, wooden crossing and next adjoining poles shall be set in barrels of broken stone or gravel, or in broken stone or timber footings. 67. When located in the sides of banks, or when subject to wash- outs, foundations shall be given additional depth, or be protected by cribbing or riprap. 68. All foundations and pole settings shall be tamped in six (6) in. layers, while backfilling. It is desirable in backfilling that the earth be suitably moistened. WORKING UNIT STRESSES Obtained by dividing the ultimate breaking strength by the factors of safety given in paragraph 37. 69. Structural Steel: Lb. per sq. in. Tension (net section) 18,000 Shear 14,000 Compression 18,000 - 60- 70. Rivets, Pins: Lb. per 8q . in Shear 10,000 Bearing 20,000 Bending 20,000 71. Bolts: Lb. persq.in. Shear 8,500 Bearing. 17,000 Bending 17,000 72. Wires and Cables: Lb. per sq. in Copper, hard-drawn, solid, B. & S. gage 0000, 000, 00 25,000 Copper, hard-drawn, solid, B. & S. gage 27,500 Copper, hard-drawn, solid, B. & S. gage No. 1 28,500 Copper, hard-drawn, solid, B. & S. gage Nos. 2, 4, 6. 30,000 Copper, soft-drawn, solid 17,000 Copper, hard-drawn, stranded 30,000 Aluminum, hard-drawn, stranded, B. & S. gage under 0000 12,000 Aluminum, hard-drawn, stranded, B. & S. gage No. 0000 and over 11,500 OVERHEAD CROSSINGS 257 73. Untreated Timber: Lb.persq.in. 1-^ Eastern white cedar 600 600 Chestnut 850 850 Washington cedar 850 850 Idaho cedar 850 850 Port Orford cedar 1150 1150 Long-leaf yellow pine 1000 1000 Short-leaf yellow pine 800 800 Douglas fir 900 900 White oak 950 950 Red cedar 700 700 Bald cypress (heartwood) 800 800 Redwood 650 650 Catalpa 500 500 Juniper 550 550 L = Length in inches. D = Least side, or diameter, in inches. 17 GENERAL SPECIFICATIONS FOR ELECTRIC LIGHT AND POWER LINES BY R. D. COOMBS CLEARANCES 1. Conductors. The clear headroom above a highway, under the most unfavorable condition of temperature and loading, shall be not less than 20ft. 2. The vertical overhead clearance from any telephone or similar wire on the power line, shall be not less than: Line voltage Not exceeding 6,600 volts Exceeding 6,600 but not exceeding 22,000 but not exceeding 45,000 but not exceeding 66,000 but not exceeding Exceeding Exceeding Exceeding Exceeding 22,000 45,000 66,000 88,000. 88,000 but not exceeding 110,000. Exceeding 110,000 Clearance 2ft. 4ft. 5ft. 6ft. 7ft. 8ft. 10 ft. .9 5 ,01 234 5 6 78 9 10 11 12 13 14 15 16 17 18 19 20 Sag in Feet FIG. 1. Conductor separations. 3. The separation of alternating-current conductors on pin-type insulators in a horizontal plane shall be in general not less than that required for the sag in question in Fig. 1 nor less than: 258 GENERAL SPECIFICATIONS 259 Line voltage Spacing Not exceeding 6,600 volts 12 in. Exceeding 6,600 but not exceeding 13,000 18 in. Exceeding 13,000 but not exceeding 22,000 24 in. Exceeding 22,000 but not exceeding 33,000 30 in. Exceeding 33,000 but not exceeding 45,000 40 in. Exceeding 45,000 but not exceeding 66,000 50 in. Exceeding 66,000 but not exceeding 88,000 60 in. 4. The separation of conductors supported by suspension type insulators, in a horizontal plane, shall be that required in paragraph 3, plus one and one-quarter times the length of the suspension string. 5. The clearance between a conductor and any part of the structure shall be not less than: Line voltage Clearance Not exceeding 6,600 volts 9 in. Exceeding 6,600 but not exceeding 13,000 12 in. Exceeding 13,000 but not exceeding 22,000 15 in. Exceeding 22,000 but not exceeding 33,000 ". . 18 in. Exceeding 33,000 but not exceeding 45,000 21 in. Exceeding 45,000 but not exceeding 66,000 24 in. Exceeding 66,000 but not exceeding 88,000 27 in. Exceeding 88,000 but not exceeding 110,000 29 in. Exceeding 110,000 30 in. NOTE. This requirement does not apply to the distance between the crossarms for an insulator of the through-phi type. 6. The side clearance between a conductor supported by suspension insulators and the supporting structure when the insulator string is deflected 45, shall be not less than that specified in paragraph 5. 7. Ground Wire. The longitudinal overhead ground wire or wires shall be in general not more than 45 from the vertical through the adjoining conductor, and with a separation of not less than that required by the table in paragraph 3. LOADS AND FACTORS OF SAFETY 8. Ice and Wind Loads. The conductors shall be considered as uni- formly loaded throughout their length, with a load equal to the resultant of the dead load plus the weight of a layer of ice % in. in thickness and a wind pressure of 8 Ib. per square foot on the ice-covered diameter, at a temperature of 0F. 9. The weight of ice shall be assumed as 57 Ib. per cubic foot (0.033 Ib. per cubic inch). 10. The wind pressure on poles or towers shall be assumed at 8 Ib. per square foot, on the projected area of solid or closed poles, and on one and one-half times the projected area of latticed poles, and on twice the projected area of wide base structures. 260 POLE AND TOWER LINES In regions in which there is no sleet, the ice load may be omitted and the wind pressure increased to 15 Ib. per square foot. 11. Conductors and Ground Wires. For spans exceeding 150 ft. and for lines not on streets, the conductors and overhead ground wires shall be designed to withstand the above ice and wind loads, without exceed- ing the elastic limit of the material. 12. For spans not exceeding 150 ft. and for all lines on streets, the above loading may be reduced 25 per cent. 13. Ground-wire Connections. The ground wire connections shall be designed to withstand the maximum stress in the ground wire, without exceeding the elastic limit of the material. 14. Supports. For spans exceeding 150 ft. and for lines not on streets the poles or towers shall be designed to withstand the combined stresses from their own weight, the wind pressure on the structure and the above wire loading on the adjoining spans combined with the effect of one broken conductor, with a factor of safety of 2.0. 15. For spans not exceeding 150 ft. and all lines on streets, the sup- porting structures shall be designed to withstand the above loading. NOTE. Guys may be used to obtain the strength required by para- graphs 14 and 15. 16. Insulators and Pins. Insulators and pins at corners or bends in the line shall be designed to withstand the transverse loads resulting from the above ice and wind loads on the conductors, combined with the horizontal component due to the tension in the wires and the angle in the line, with a factor of safety of one and one-half (1.5). NOTE Double arms may be used to obtain the requisite strength. 17. Suspension Type Insulators. Suspension type insulators and their connections shall be designed to withstand the maximum tension in the conductors, with a factor of safety of one and one-half (1.5) when used in the suspension position and 2.0 when used in the strain position. 18. Guy Insulators. Strain insulators for guys shall be designed to withstand the maximum stress in the guy, with a factor of safety of two (2). 19. Guys. Guys shall be designed to withstand their maximum stress with a factor of safety of two (2). 20. Guy Anchorages. Guy anchorages shall be designed to withstand the maximum stress in the guys with a factor of safety of one and one- half (1.5). 21. Foundations. The foundations of unguyed poles and towers shall be designed to resist overturning, with a factor of safety of two (2) . 22. Temperature. In the computation of stresses and clearances and in erection, provision shall be made for a variation in temperature from 20F. to 120F. A suitable modification in the temperature require- GENERAL SPECIFICATIONS- 261 ments may be made for regions in which the above limits would not fairly represent the extreme range of temperature. 23. Guys or Special Supports. Guys, or supporting structures of greater strength than required to withstand the preceding loads, shall be installed approximately as follows: Wooden poles Side guy all bends or corners. Head guy steep hills. Head guy unusually long spans. Head and side guy light lines every 1500 ft. Head and side guy heavy lines every 1000 ft. Flexible structures Side guy corners over 5. Head guy steep hills. Head guy unusually long spans. Head guy or special structure, every 2000 ft. Steel or concrete poles Side guy sharp corners. Head guy steep hills. Head guy unusually long spans. Head guy light lines every 3000 ft. Head guy heavy lines every 2000 ft. Rigid towers Special structure sharp corners. Special structure every mile. MATERIAL 24. Overhead Ground Wire. The material of ground wires shall be copper, copper-covered steel, galvanized iron, galvanized steel or an approved alloy; sizes over No. 4 B. & S. gage shall be stranded. NOTE. The use of galvanized steel ground wire is not recommended except in sizes % in. or more in diameter. 25. The attachment of the ground wire to the structure shall be by means of a smooth grip with well-rounded ends, and a contact length of not less than three inches (3 in.). 26. Conductors. Conductors shall be of copper, aluminum or other approved material. 27. Insulators. Insulators shall be of porcelain, glass or other ap- proved material. 28. Wooden Pins. Wooden pins shall be sound, straight grained yellow or black locust or other approved species, free from knots over Y% in. in diameter, except on the shoulder or lower half of the shank, and free from checks, sapwood and worm holes. 29. Metal Pins. For voltages over 13,000, insulator pins shall be of 262 POLE AND TOWER LINES steel, wrought iron, malleable iron, an approved metal or alloy or a combination of steel with wood, metal or porcelain. NOTE. Wood pins may be used for higher voltages in regions having favorable climatic conditions. 30. Wooden Crossarms. Wooden crossarms shall be of seasoned timber, reasonably straight grained, out of wind, free from large, loose or unsound knots, wane, large pitch pockets, pitch pockets which enter the pin or bolt holes, through shakes, shakes or checks over 3 in. long and rot or worm holes. 31. Wooden Poles. Wooden poles shall be of approved species of timber, peeled, with trimmed knots, reasonably straight, well propor- tioned from butt to tip, with squared ends, free from defects which would materially decrease their strength or durability and of not less than 7- in. minimum diameter at the top. 32. Guys. Guys shall be stranded, galvanized steel or copper covered cable, not less than ^ 6 in. in diameter. 33. Pole Steps. Pole steps shall be of forged or rolled iron or steel, in accordance with the Manufacturers' Standard Specification. 34. Steel. Structural steel work shall be of open-hearth steel, in accordance with the Manufacturers' Standard Specification. STRUCTURAL DESIGN 35. Frame. The form of the frame shall be such that the stresses may be computed with reasonable accuracy. 36. The sections used shall permit inspection, cleaning and painting and shall be free from pockets in which water or dirt can collect. 37. The length of a main compression member shall not exceed 125 times its least radius of gyration. The length of a secondary compres- sion member shall not exceed 180 times its least radius of gyration. 38. Minimum Sections and Connections. The minimum thickness of metal shall be one-quarter inch (y in.) for main members and three- sixteenth inch (% 6 in.) for secondary members. In wide-base structures the minimum angle shall be not less than l;Hj X 1^ X %6 m - an d the minimum main bracing connections shall be two bolts. 39. Rivets and Bolts. The minimum diameter of rivets and bolts shall be one-half inch (^ in.). 40. The diameter of a rivet or bolt hole shall not exceed the diameter of the rivet or bolt by more than one-sixteenth of an inch (^ 6 in.). 41. The distance center to center of rivet or bolt holes shall be not less than: Diameter of bolt or rivet Spacing K in. IH in. % in. 1% in. % in. 23^ in. in. 2^ in. GENERAL SPECIFICATIONS 263 42. End and Edge Distances. The distance from the center of a bolt or rivet hole to a rolled edge or to a sheared end shall be not less than: Diameter of bolt or rivet Edge distance End distance H in. % in. % in. % in. % in. % in. % in. 1 in. 1^ hi. K in. IH in. \Y in. 43. Rods. The minimum diameter of rod bracing shall be one-half inch (Yz in.). 44. Main diagonal rod bracing shall be provided with adjustable end connections having right and left threads. PROTECTIVE COATINGS 45. All structural steel shall be thoroughly cleaned at the shop and galvanized or given one coat of approved paint. NOTE. In view of the thin sections used in the class of work covered by these specifications, the cleaning and painting or galvanizing required herein will be rigidly enforced. The make and brand of paint, or the mixture to be used, for both shop and field coats, shall be given hi a written notification, a copy of which may be furnished the paint manufacturer. 46. Metal pins shall be galvanized or otherwise protected from corro- sion. 47. Hardware. All bolts, braces, lag screws, washers, etc., used on wooden or reinforced-concrete poles, shall be galvanized or sherardized in accordance with the Standard Specifications for galvanizing. 48. Guys shall be galvanized or copper-covered. 49. If required, wooden erossarms shall be treated with an approved preservative or given two coats of approved paint. 50. Wooden pole tops, crossarm gains and bolt holes shall be treated with paint or preservative. NOTE. The application at the ground line of at least a double-brush treatment with preservative is recommended. 51. Painted Material. All contact surfaces shall be given one coat of paint before assembling. 52. All painted structural steel shall be given one field coat of an approved paint. 53. The surface of the metal shall be thoroughly cleaned of all dirt, grease, scale, etc., before painting, and no painting shall be done in freezing or rainy weather. 54. Galvanized Material. Galvanized material shall be in accordance with the Standard Specification for galvanizing. 55. The spelter material shall be Prime Western for structural steel and Grade A for wire, or equal. 264 POLE AND TOWER LINES FOUNDATIONS 56. Tn swampy or otherwise uncertain ground, the line supports shall be provided with broken stone, gravel, concrete or timber footings, and when located in the sides of banks or subject to washouts, shall be given additional depth or protected by cribbing, riprap, etc. 57. When possible, the top of a concrete foundation or casing shall extend not less than six inches (6 in.) above the ground nor less than one foot (1 ft.) above high water. 58. The top of the foundation shall slope down toward the sides, and be built up in the corner of angles, etc., to provide efficient drainage. 59. The thickness of the concrete casing around the butt of a steel pole shall be not less than three inches (3 in.). 60. Wooden Pole Settings. Poles shall be set in the ground to depths not less than those specified in the following table: DEPTH OF SETTING Total length of pole (ft.) Straight lines (ft.) Curves, corners and points of extra strain (ft.) 30 5.0 6.0 35 5.5 6.0 40 6.0 6.5 45 6.5 7.0 50 6.5 7.0 55 7.0 7.5 60 7.0 7.5 65 7.5 8.0 70 7.5 8.0 *75 8.0 8.5 80 8.0 8.5 61. Steel and Concrete Pole Settings. The penetration below ground of a steel or concrete pole shall be not less than given in paragraph 60, except that concrete poles or steel poles of greater butt diameter than twenty inches (20 in.) incased in concrete, may have nine inches (9 in.) less penetration than otherwise provided. 62. Towers or Wide Base Structures. Wide base towers or structures not provided with a web system below ground, shall be secured in a foundation designed to resist lateral movement at the ground line, either by the use of sufficient superficial area, concrete or cribbing. 63. The penetration shall be not less than six feet (6 ft.). 64. The anchorage plate shall be designed to withstand the maximum stress in the main leg, with a factor of safety of two (2), and in general shall be not less than four hundred square inches (400 sq. in.) in area. GENERAL SPECIFICATIONS 265 65. Excavation. The bottom of the excavation shall be compacted and if required shall be covered with a rammed layer of broken stone and sand, or gravel, or be covered with concrete. CONCRETE 66. Cement The cement shall be Portland, and shall meet the requirements of the Standard Specifications. 67. Aggregates. Aggregates shall consist of sand, gravel, broken stone or other approved material, graded from fine to coarse, free from vege- table matter and soft particles and reasonably clean. 68. Water. Water used in mixing concrete shall be free from oil, acid and injurious amounts of alkalies or vegetable matter. 69. Proportions. For plain concrete or mass foundations, not less than one part cement to a total or nine (9) parts of fine and coarse aggregates, measured separately, shall be used. 70. For reinforced concrete not less than one (1) part of cement to a total of six (6) parts of fine and coarse aggregates, measured separately, shall be used. 71. Such relative amounts of fine and coarse aggregates shall be used as will produce a dense uniform concrete. 72. Mixing. The materials shall be well mixed, using sufficient water to form a mixture wet enough to flow in the forms and about the rein- forced or incased metal, but which will not permit the separation of the coarser aggregates from the mortar. 73. Workmanship. Proper precautions shall be taken to prevent the freezing of concrete. 74. Mortar or concrete which has partially set shall not be remixed and used. 75. Exposed surfaces of the concrete shall be rubbed smooth without plastering. 76. Exposed edges or corners shall be rounded or beveled. 77. Concrete Poles. Reinforced-concrete poles shall be made strictly in accordance with the best practice in workmanship, using approved aggregates and producing a concrete of great density. 78. No part of the reinforcement shall be less than one inch (1 in.) from the surface. 79. No cast metal shall be used in the skeleton reinforcement. 80. The reinforcing rods shall be capable of being bent cold 180 around a circle of four diameters. 81. The entire surface of the concrete shall be rubbed to a smooth finish without plastering. 82. Poles shall be straight, of the required dimensions, and provided with the necessary holes for crossarms, brace and guy bolts and sockets for pole steps. 266 POLE AND TOWER LINES TESTS 83. Insulators. Each insulator shall be so designed that, with excess- ive potential, failure will first occur by flash-over and not by puncture. 84. Previous to the electrical tests the separate parts of an insulator shall be subject to inspection for mechanical defects in material or workmanship. No part shall contain soft porcelain, crazing, serious deformations or cracks in the grooves or in the unglazed portions that would materially decrease the value of the insulator. 85. The assembled insulators shall withstand a voltage of 5000 volts less than specified in paragraphs 90 and 91, for five consecutive minutes, without injury, abnormal static strain, noise, or flash-over. 86. Each separate part of a built-up insulator, and each assembled and cemented insulator shall be subjected to an approved factory test. 87. The wet flash-over test shall be made under a precipitation of water of one-fifth of an inch per minute, at an angle of 45 to the axis of the insulator. 88. Test voltages above 35,000 volts shall be determined by the A.I.E.E. Standard Spark-gap Method. 89. Test voltages below 35,000 volts shall be determined by trans- former ratio. 90. Pin Insulators. The flash-over design test voltage shall be not less than: Line voltage Dry flash-over Wet flash-over Less than 11,000 twice line voltage 11,000 60,000 30,000 22,000 90,000 50,000 33,000 100,000 60,000 45,000 125,000 90,000 50,000 150,000 100,000 60-70,000 180,000 120,000 80,000 240,000 160,000 91. Suspension Type Insulators.- The flash-over design test voltage of the string of suspension units shall be not less than: Line voltage Dry flash-over Wet flash-over 11,000 80,000 50,000 22,000 160,000 90,000 33,000 160,000 90,000 45,000 220,000 130,000 66,000 270,000 175,000 88,000 310,000 220,000 110,000 340,000 265,000 125,000 460,000 300,000 140,000 470,000 335,000 GENERAL SPECIFICATIONS 267 92. When insulators of the suspension type are placed in the strain position, one additional insulator unit shall be used in series. 93. For line voltages not exceeding 9000 volts, strain insulators for guys shall have a wet flash-over of not less than four times the maximum line voltage. 94. Reinforced-concrete Poles. If required, the strength of concrete poles shall be determined by testing one pole from each lot of 95. The expense of testing the poles which withstand the specified conditions of loading shall be borne by the purchaser and the expense of testing those which do not meet the requirements shall be borne by the contractor. 96. Poles shall be tested by applying a horizontal pull at the center of gravity of the wires, and continually increasing the stress until it is equivalent to the specified loading. 97. Test poles shall be set in a firm foundation. 98. The test load shall be measured by a dynamometer, or scale. INDEX Aerial cable, 149 duct line, 150 A frames, record of, 132 steel, 22, 23, 127 to 132 wooden, 96, Aluminum, 45 Anchors, dead-man, 214 holding power, 215 patent, 213 Angles, properties of, 109 Auxiliary attachments, 240, 241 Bearing, rivets and bolts, 105 Bi-metallic wire, 44 Bog shoe, 166, 167 Bolts, 104, 105 Braces, crossarm, 208, 209 Bracing (see Lacing), 110 Broken wires (see Loading) Brush treatment, 81, 82, 83 Butt treatment, 83 C Cable, properties of, 54 to 57 straightening, 217, 218 Capacity, of line, 2 to 5 Catenary, 47 Cedar, 87 Chestnut, 77, 86 Clamps, Crosby, 234 crossing, 238 to 243 ground wire, 235 Clearances, 6, 27, 236, 245, 248, 249, 258, 259 Coal tar, 81, 82' Column formulae, 68 to 73 Concrete, aggregates, 174 cement, 173 Concrete, forms, 176 mixing, 175 proportions, 173 reinforcement, 177 specifications, 254, 265 waterproofing, 178 workmanship, 177 Concrete poles (see Poles) Copper, 43 Costs, general, 223 to 227 steel pole line, 227, 228, 231 steel tower line, 228 wood-pole line, 227, 230 Cradles, 238, 248 Creosote, 81, 82, 89 Crossarm braces (see Braces) Crossarms, metal, 206 wish-bone, 207 wood, 205, 206, 207, 209 Crossing clamps, 238 to 243 failures, 238 river, 145 to 148 specifications, 244, 248 D Dead man, 214 Decay and defects, 77 to 80, 84 Deflection (see Sag) Derrick wagon, 219, 220 Edge and end distances, bolts and rivets, 106 Erection, 217 to 231 costs, 227 to 231 derrick wagon, 219, 220 gin pole, 222 hauling, 224 raising, 219, 222, 223, 225 straightening cable, 218 stringing, 219 269 270 INDEX discussion of, 244 L Lacing, 106, 117 Factor of safety 16, 17, 18, 154, 193, Joint re P rt specification, 248 245, 252 Failures, A-frames, 130, 131, 132 crossings, 238 towers, 165 Flexibility, 21, 22, 129 Flexible frames (see A-frames) Foundations, barrel, 169 bog shoe, 166, 167 concrete, 169 rock, 172 rotten rock, 172 tower, 165, 171, 172 Frames (see A-frames, H-frames) G Galvanizing, 5, 182 to 188, 233 Gin pole, 222 Grounding arms, 98, 206, 239, 245 poles, 239, 251 Ground wire, 232 to 236, 244, 250 clamp, 235 Guys and guying, anchors, 213, 214 general, 211, 212, 245, 250, 254, 261 insulators, 254 tension, 213, 254 wire, 211, 262 Hardware (see Line Material), 263 H-frames, 97, 98 High towers, 145 to 148, 244 House derrick, 223 Induction, 236 Insulator connections, 111 Insulator pins (see Pins) Insulators, disc, 194 guy, 195, 196, 254 pin, 192 angles, 108 flats, 107 Lag screws, 210 Life, 1, 5 Line material, 189 to 216 Loading, broken wire, 39 to 42, 193, 245, 252, 260 corner, 65, 95, 260 sleet, 30, 37, 251, 259, 260 transverse, 65 wind, 32, 33, 38, 251, 259, 260 Location, 25 Loop cables, 191 Map (see Plan) M N Neighboring lines, 236 O Outdoor substation, 142, 143, 145 Paint, and painting, 180, 181, 182, 255, 263 Parallelism, 236 Pin insulators (see Insulators) Pins, metal, 197, 200 to 205, 254 tests, 200 to 204 wood, 196 to 199 Plan, 25, 26, 27 Poles, concrete, 152 to 164 costs, 156, 226 design, 160 to 164 erection, 224, 225 flexibility, 155, 157 life, 6, 154 tests, 157, 267 specifications, 252. 253, 254, 266 Poles, steel, curb line, 113 to 123 strain, 195 thro-pin, 194 Iron (see Steel) data on existing lines, 140 design, 120 latticed, 113 to 123 INDEX 271 Poles, steel, triangular, 124 tests, 118 Poles, wood, data on existing lines, lines, 99 to 102 design, 90 to 94 life, 6, 99 settings, 168, 264 specifications, 84 to 90 weakest point in, 74 Preservatives, 81 to 84 Pressure treatment (see Preserva- tives) Pressure, wind, 32 to 39 Profile, 25, 26, 27 R Redwood, 87 Right of way, 3, 12 to 16 Rivets, bearing, 105 shear, 104 sizes, 106 spacing, 106 River crossings (see High Towers) Rock anchor, 172 Sag, comparative, 34 computation, 47 curves, 51, 52, 53, 58, 59 plotting, 27 tables, 55 to 59 templet, 27 Scale, 27 Shear, 104 Signs, 250 Sleet (see Loading) Spans, 18, 19, 133 to 140 Specifications, cement, 265 concrete, 254, 265 concrete poles, 265 crossings, 248 galvanizing, 186 line construction, 258 spelter, 185 steel, 103, 254, 262 wood poles, 84 to 90 Spelter, 185 Splices, 191 Steel, specifications, 103, 246, 254, 262 wire, .46, 57 Stranding, 246, 253, 261 Stringing, 217 to 221 Substations, 142, 143 Supports (see Poles, Towers), 20 Survey, 26 Swinging contacts, 8, 10 Switching station, 143, 145 Synchronism, 8, 33 Tables, angle sections, 109 bearing values, rivets and bolts, 105 maximum sizes, rivets and bolts, 106 shearing values, rivets and bolts, 104 strength of timber, 73 Terminal frames, 141, 145 Tests and testing, concrete poles, 157, 159, 160, 267 galvanizing, 186 insulator pins, 200 to 205 steel poles, 118 Ties, 189, 190, 205 Timber, 73, 76 to 90, 246 Towers, anchors, 171 bracing, 106 to 110, 125, 127 costs (see Costs) data on existing lines, 128, 133 to 139 failures, 128, 165 Transmission line crossings (see Crossings) Transposition, 141 Tree trimming, 8 to 12 U Unit stress, 70, 72, 256 V Velocity, 35, 36, 37 272 INDEX W Wire, tables, 54 to 57 telephone, 46, 57 Weatherproof wire, 55 Wood (see Poles) Wind (see Loading) decay and defects, 77 to 80 Wire, aluminum, 45, 56 preservatives, 81 copper, 43, 54, 55 seasoning, 80, 81 copper-covered, 44 properties of, 50, 59 Y steel, 46, 57 straightening, 218 Yellow pine, 88 UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW Books* not returned on time are subject to a fine of 50c per volume after the third day overdue, increasing to $100 per volume after the sixth day.. Books not in demand may be renewed if application is made before expiration of loan period. === FEB 6 1917 APR 1 1921 JAN 1924 360221970 50m-7,'16 331010 UNIVERSITY OF CALIFORNIA LIBRARY