The original location of the White Pass & Yukon. Ry. reached the highest point of White's Pass, 18 miles out from Skagway, Alaska, by a switch-back up the canon at the summit; and on this line the road was operated until this winter, when trains were run over the new steel arch bridge across the canon, commencing early in January. The heavy snows at this point made it desirable to do away with the switchback, and the consequent expense of rotary snow plows and shoveling gangs, so that a new location was made, crossing the gorge by means of a bridge, consisting of viaduct approaches at each end, and a steel arch structure of special design, 400 ft. in length, over the gorge itself.
The only information in the possession of the railway company at the time the decision to put in a bridge at this point was determined upon was a center-line profile. No information of the side slopes was to be had except that they were probably not far from level. Nor was any information obtainable of the possible amount of water to be provided for in the gorge. It was known that any foundation work would necessarily be expensive on account of the very high price of labor. It was also desirable that the total
amount of foundation and falsework involved should be reduced to the least possible, owing to the short summer season available for prosecuting it successfully, and the great center-span height. A consideration of these conditions resulted in the adoption of the general design of the structure as built, which turned out to be of less weight than the tower trestle design first considered by the railroad company.
This Steel arch, as shown in Fig. 1, consists of two side spans of 80 ft. each, and a central span of 240 ft. The side spans were designed to act as shore, or anchorage arms, during erection, and were anchored down to the concrete shore piers by anchor rods passing through adjusting wedges. The central span was closed by erecting each half as a cantilever to the center, and connecting the structure with a central hinge pin; the entire structure being likewise of pin-connected design. After erection the shore arms were wedged up at the shore ends, so that the side spans acted as simple spans and the central span as a true arch, deflecting much less under live load than it would if allowed to remain as a cantilever with a loose center connection, and throwing the pier reactions
nearly normal to the slope of the bank instead of vertical.
The central span supports are concrete abutments carried on solid rock; the end piers are also located in the solid rock of the mountain side, all loose material being first removed, and the foundation pits blasted out into steps, on which was founded the concrete footing courses of the piers and abutments. The concrete is all made with imported Portland cement, in the proportion of one, two and five for the heart work, with a facing worked in, of a one, two and three mixture. Fig. 2 shows details of the skewback masonry for the arch feet. Above the foundation pits timber forms were used, into which concrete was placed and tamped; all anchor bolts being set in the concrete work as it
The superstructure was begun, as shown in Fig. 3, by starting from the end of the approach viaducts, assembling it panel by panel with an over-reach traveler of special design, made of materials which would afterwards be of use for other purposes by the railway company; the timbers all being in long lengths, while all of the guys were of
wire rope. On the traveler was a four-spool bridge erecting engine, which also acted as a counter-weight for holding down the traveler.
The bridge was designed to carry a load in addition to the dead weight of two 96-ton consolidation engines, followed by a train load of 2,700 lbs. per lin ft. The metal work is of medium steel, manufactured in accordance with Theodore Cooper's Specifications for Railway Bridges. The dead load was assumed at 2,000 lbs. per linear
foot. Fig. 1 shows the strain diagrams for the various parts of the structure, and Tables I., II. and III. show the nature and amounts of the various strains and the sections adopted to withstand them.
The erection of the bridge was carried on through very stormy weather in the early winter. For many days at a stretch the work could only be carried on for a few hours a day, so that progress was necessarily slow. When one-half of the structure had been erected, it was necessary to take down the traveler, and move it around by the
switchbaok to the other end to continue the work.
When the center of the span was reached, only 20 minutes was occupied in making the last connection, so carefully had the measurements been made in locating the masonry.
Upon the completion of the riveting the bridge was tested by running across it,two heavy engines and a heavy rotary snow plow, aggregating in all 270 tons. The maximum deflection under this load, which was somewhat heavier than in the case of the loading for which the bridge was calculated, was 3/4-in. at the quarter point of the
center span, while the deflection at the center amounted to 1/2-in. Fig. 4 shows the completed structure.
The anticipated results in being able to keep the road open through the winter were fully realized. In spite of the heavy snows occurring immediately after the completion of the work, it was found very much easier to keep the tracks clear across
The bridge was designed by the Atlantic, Gulf & Pacific Co., of Park Row Bldg, New York city, Geo. W. Catt, President and Chief Engineer; S. Wood, Designing Engineer, and was constructed under the supervision of Mr. Alfred Williams, Resident Engineer for the White Pass & Yukon Ry. Co., of which Mr. E. C. Hawkins, of Seattle,
Washington, is General Manager and Chief Engineer. The contractor for the structure was the Puget Sound Bridge & Dredging Co., C. E. Fowler, President and Chief Engineer. We are indebted to him and to Mr. H. S. Wood, the author of the design and calculations, for the information from which this article has been prepared.