The Susitna River Bridge, also known as the Gold Creek Bridge, was the longest bridge span on the United States Government Railway in Alaska. The steel 504-ft. through-truss channel spans the Susitna River. It was completed in February, 1921. The crossing of the Susitna includes two 121-ft. combination Howe truss flanking spans and two 70-ft. combination pony Howe truss end spans, with 392 ft. of trestle at the southern approach and 28 ft. at the northern approach, making a total length of 1,322 ft. It is located 150 miles north of Anchorage, and 264 miles north of the terminus at Seward. The bridge was listed on the National Register of Historic Places in 1977.
Study and design
In a study of the river under summer and winter conditions as a preliminary to the design of the bridge, high-water marks and ice scars on trees indicated the elevation reached by the ice while in motion. The spring break-up occurs usually before the ice has become soft, and in passing downstream massive pieces jam against the islands and bars and against the shore at points where the river takes a sharp turn, the water then rising until its pressure breaks the dams. It appeared evident that such rushes of accumulated water and ice would carry out any timber pier and that a concrete pier would be exposed not only to the tremendous pressure of the ice jam but also to undermining by the scour that would result from the creation of an ice dam against the pier. A comparative estimate for the structure over the main channel to consist of two 250-ft. steel spans on three concrete piers (with one pier near the center of the channel) or a single 504-ft. steel span on two concrete shore piers, slightly favored the long span, which therefore was adopted.
The combination of steel trusses and timber trusses in the same bridge is considered to be justified by the high price of bridge steel and concrete work at the time of construction, it being cheaper to build all approach spans of timber and to make provision for possible renewal with steel and concrete when the life of the timber has reached its limit. For this reason the main piers were designed to support 200-ft. steel flanking spans at some future time, each to replace the present 121-ft. and 70-ft. spans.
The Cooper E60 loading was adopted for the 504-ft. span, E50 for the timber and E44 for the trestling, which has 14-ft. spans. In the design of the steel span the 1910 specifications of the American Railway Engineering Association were followed mainly. Wind loading was an exception to this; the bottom lateral system was designed for 40 lb. per sq.ft. on the area of train and floor in elevation, and on li times the area of truss members of one truss not covered by train and floor, while the top laterals were designed for 500 lb. per lin.ft. of span.
Pier foundations and construction
Test holes 60 to 75 ft. deep to determine the character of foundation for the concrete piers disclosed compact sand, gravel and large boulders in layers of varying thicknesses, suitable for carrying pier loading without foundation piles.
Pier construction started in May, 1920, when the head of track was 22 miles south of the bridge site. Supplies were freighted by sleds on the ice until the river broke and then by boats and wagons. Transportation difficulties were practically overcome when the track reached the river in September, 1920. At that time excavation for the south pier was complete and that for the north pier had been begun. A temporary trestle 100 ft. downstream from the bridge site permitted tracklaying to proceed and facilitated erection of the bridge without interference to traffic and to other construction work north of the river. Piles for the trestle bents, piers and abutments had been driven and the falsework for the steel span had been started from the north end before the track reached the site.
An open caisson was used for the north pier excavation, and a cofferdam of Wakefield triple-lap sheet piling for the south pier. Excavation was started in the wet with orangepeel buckets operated by two stiff-leg derricks until hard material was struck; then the holes were pumped out and work was continued by laborers.