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Les cartes, planches, tableaux, etc., peuvent Atre filmAs A des taux de rAduction diff Arents. Lorsque le document est trop grand pour Atre reproduit en un seul clichA, 11 est filmA A partir de I'angle supArieur gauche, de gauche A droite, et de haut en bas, en prenant le nombre d'images nAcessaire. Les diagrammev sulvants illustrent la mAthode. ata ilure, A : 2X 1 2 3 f ^' * 2 3 4 5 6 (All rights reserved,) ADVANCI PnOOf-fSiihJed to reinawn). This Proof ia sent to you for dtaouMlon onfy, and on the exproM understanding that It Is not to be used for any other purpose whatsoever— ('^jte. Marie Bridge Co., to build u bridge across the St. Marie River at the " Soo," and a party was organized to make the preliminary surveys, from the results of which it was decided to construct th*; bridge at the upper end of the St. Marie's Rapids, where th't smooth water of the Lake commences to break away over the rooky bed of the river. The bottom is formed of smooth, level sand stone of the Potsdam formation, perfectly bare of gravel or debrit, with the excep- tion of Pier No. 1, where a considerable deposit was formed. In the winter of 1886-7 the contract for the construction was awarded to B. G. Reid, who the year before completed the St. Lawrence Bridge. Early in May the contractor and the engineering party arrived at "the Soo," and the work of triangulation commenced, and here it may not be out of place to go a little into the general detail of accurately deter- mining the position of the piers. The first operation was the measurement of a base line and th** calcu- lations of angles. The most f oientific and practical method of setting out piers is by mc:ins of angles from a measured base, direct meas- urement by a long wire has been advocated by some engineers, but the experience of the author on this method has not been satisfactory. It may appear a very simple matter to measure 700 or 800 feet on a wire, and then transfer this distimce between two points supported only at the ends, but the result of such is often unreliable, as was expe- rienced by the engineers at the Forth Bridge. For distances of 400 or 500 feet, with great earo, accurate results can be obtained, as the sequel will show. In setting out by angles, the instruments used'will first be discussed ; these arc ihrce in number, — a transit, .steel tape and picket. The transit should not be less than 8" di'imetcr, reading 'olO seconds. The one used at the St. Lawrence bridge and "the Soo was of this description, made by Hummcrsly of London, and furnished with a very powerful glass and long sensitive level underneath the telcticopc. There was no vertici'l arc, but simply the ordinary cir.nip screw, on this inii the erntro ot the upper end and tliepuinb bob swinging io tlio Cork belnw, thus ensuring the centre to be both exactly vcrtiuni and on the nail. The picket was then scoured by four wires fastened to the upper end and two stakes driven into the ground ; ihr^o wires could he iidjusicd for length by several screws, such as arc used to tighten the ordinary buck-saw frame. The pickets were painted white, nnd a Hack band placed at a little distance behind to make 'hem more distinct. Now, in selecting a base, it is prefcriible to have one on either side of the river ; one hnlf hifted, and the operation had to be repeated. When this angle was satiafacturily recorded, the vernier was set at 10", the hairs set on the first picket and the upper plate moved round to the second picket, the angle read, then moved round to 10" and checked to see if the hairs were on the first picket, and so on, in- oreasingby 10° each time, then the average was taken. By this means great accuracy was attained, any small discrepancy of a second or two was distributed between the three angles in proportion to their siie. In sitting out the piers the same precautions were taken. In reading the verniers, a sheet of white paper was placed close to the plate, and the reading taken through a small hole cut in the paper. This paper threw n shade on the vernier, and the graduations could be seen with great distinctness. Close beside each of these shanties, a signal was erected, which could be worked by the man at the transit, without taking his eye from the instruments as the caisson approached its pro- per position. A man on board held up a picket, which was immediately signalled into its exact position by the transitman, and by this means the men who guided the caisson were enabled to see which way it had to go without any confusion or mistake. When the caisson was placed by the transits, its positioc was verified with a steel wire of given length, usually the length of a span. This wire was about ^" diameter, the exact length marked on it by n loop soldered on at each end ; one loop of the wire was brought exactly over the starting point by means of a long adjustable screw arrangement, the other was carried in the boat to the caisson and strained to 200 lbs. and the exact distance marked. Very correct measurement can be made in this way, but it is necessary constantly to check the wire under strain with the standard tape, and also when not in use to roll it on :i reel made specially so as to prevent the wire getting bent. At the St. Lawrence Bridge the 408 ft. cantilever spans were measured in this way, and when the iron work was connected, the measurementfi agreed exactly on one of the 408 feet spans, and on th" other there was a discrepancy of only J inch. In setting out piers from a base line, the question will arise as to the length of base which will give the most accurate results, assuming that the transit reading may be liable to an error of (a seconds) when (a) may be I to 5. It may be shewn mathematically that this length should be equal to the length from the base to the pier measured along the base, or that the angle below the base line and a line from the transit to the pier should equal that between this line and the bridge lino. This would shew that theoretically the best results arc obtained by having a separate point on the base for settin" off each pier. The author believes that this was carried out at the International Bridge at Buffalo. The next operation was to level across the river; this was done by placing three of the best levels on each side of the river, and rending on a large rod graduated to A of a foot held up on the opposite shore; when the three readings were taken the rods were held up at as nearly as possible equal distances, but on the stime shore. Thus the errors of curvature and refraction were neutralised by these equal sights. The error in levelling by this method should not exceed loo in 3,000 feet, when the levels were transferred across two stands for the large transit wire set up just above the bridge line one on either side of the rivor, and opposite each a target placed, whose centre was exactly on the same level as the cross hairs of the transit when se'^i on the opposite stand ; thus by setting up the instrument and directing its cross hairs on this target, a correct level was ensured, the only source of error was in certain states of the Weather, the diffraction was so great as to cause an error of A of a foot in the line of levels ; but slight errors in levels, except in the ease of continuous girders, are of little consequence, much less than errors of distanci', as the appearance of a bridge not sitting fair on the centre of the piers has a very bad effect. During the time occupied in triangulation, the contractor was employed building a temporary wharf for landing stone and other 3 materia) and excavating fonndations for nbutmentK. Mmt of the plant used in tho confitruction of tlic 8t. Ldrronce Bridge wan shipped to " Tlie Hoo," and ooinpriwd two tugs, 22 boowh, three of which wen- furnidhud with Hteniii d(>n'icl(!i, two puinpH, hIx large Rnohorn, two olnni ithell dredges, iind other but lesH iniportiint iiiipleimmts timd on suoh worlc. The depth of the water on the site of the bridge varied from 2 to 12 feet. Although the extreme rise of the river, except after a westerly gale, does not exceed 2^ feet, it is nt any time liable to IJ feet rise in loss than nil hour, due to some wave motion of the Greiit Lakes. The swiftest current is wliere the water is deepest towards the Canadian side, and attains a velocity of 9 miles per hour. The shallowness of the water rendering it impossible to bring the tugs within loss than 300 feet of the piers, the scows and caissons had to be lowered down bj ropes. The plan of foundation is similiirto the St. Lawrence Bridge, namely, caissons with water-tight sides of 12" x 12" timber, without any bottom, the sides scribed to fit the rock. The caissons were so designed as to allow a space of 4' G" between the timber nnd the sides of the piers. A curtain of canvass was nailed round the inside of the caisson to check any current while depositing the concrete. The concrete was formed of sand and Portland cement in equal (|uan< tity, to which wos added as much broken stone as the mortar thus made would take. This concrete was deposited by the same boxes as were used at the St, Lawrence bridge, and dcscriboil in a paper by the author.* The depth of the concrete was never less than half the depth of the water with a minimum depth of two feet. The second day after the concrete was deposited the caisson was pumped dry, the bottom levelled, and ma- sonry commenced. In building with timber caissons and concrete bottoms, the depth of the latter should not bo less thun J the depth of the water, with a minimum depth of two feet. Less concrete than this is apt to cnuse troublesome leaks, and the concrete box should hold two cubic yards. The masonry of the small bridge on the Canadian side was commenced Jane 9th ; and the north abutment of the main bridge on June 2nd, but no masonry wus laid in the river piers until August 9tli ; as much time was lost in negotiating with the United Stntes Government as to duty on plant and material, etc. The last of the masonry of the Main Bridge was laid October 7th. The work proceeded rapidly and smoothly without serious mishaps or accidents of any kind, tin experience gained on the St. Lawrence Bridge by the men employed proving of great service. No. 1 Pier was the most tedious, as a large (juaiitity of gravel huii to be dredged off by means of a large rake attached to a long handlo, operated from scows anchored over tho foundation, nny rcinalniiii; gravel being cleared off after the caisson had been placcii. Tho most troublesome and costly foundation was the centre pier of the Swing Bridge, This had to be carried down 22 feet below water level through seamy rock, in anticipation of the enlargement of the oanal at some future date. The pier was circular, with a diameter of ,S0 feet, tho excavation being several feet wider ; leaving only about 20 feet of rock between the pier and the canal, through which the water poured like a rivor, requiring the constant use of two large rotatory wrecking punip.« until tlic masonry readied tlio water level. The stone used in the swing bridge and the bridge at ('anal Street was Ohio sandstone, the reuiainder being white limestone from the Owen Sound Quarries. The general plan of piers and abutments are similar to those of the St. Lawrence Bridge, the piers of the Main Bridge all being 10 feet wide on top. The following is a brief outline of the bridge : Commencing on the Canadian side, the approach to the bridge is of timber trestle 1500 feet in length, with two spans of iron crossing a small branch of the river, then the main bridge consisting of 10 spans of 242 feet each. Pin con- nect!' ii through girders, then 400 foot of bank which reaches the north abutment of the .swing bridge over the St. Marie canal. The swing has a totid length of soil feet, crossing the canal at an angle of about 60", then another bank of 200 lieet, and a plate girder over Canal Street reaches the southerly limit of the Bridge Co'. s property. * Vol. 1., Part I., page 39, Transactions Can. Soc. G. E. 4