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}} The Susitna River 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 is on the National Register of Historic Places. ==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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Susitna River Bridge」の詳細全文を読む スポンサード リンク
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