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|carries=2 Vehicular lanes, 2 sidewalks |crosses=Allegheny River |locale=Pittsburgh, Pennsylvania |maint=Allegheny County |design=Self-anchored suspension bridge |lat=40.444269 |long=-80.000982 }} The Three Sisters are three very similar self-anchored suspension bridges spanning the Allegheny River in downtown Pittsburgh, Pennsylvania at 6th, 7th, and 9th streets, generally running north/south. The bridges have been given formal names to honor important Pittsburgh residents: * Roberto Clemente (Sixth Street Bridge) * Andy Warhol (Seventh Street Bridge) * Rachel Carson (Ninth Street Bridge) Designed by the Allegheny County Department of Public Works, they were all built in a four-year period, from 1924 to 1928, by the American Bridge Company, replacing earlier bridges of various designs at the same sites. Their construction was mandated by the War Department, citing navigable river clearance concerns. They are constructed of steel, and use steel eyebars in lieu of cables. The Three Sisters are historically significant because they are the only trio of nearly identical bridges, as well as the first self-anchored suspension spans, built in the United States. They are among the only surviving examples of large eyebar chain suspension bridges in America, and furthermore, unusual for having been erected using cantilever methods. The bridges’ design was viewed as a creative response to the political, commercial, and aesthetic concerns of Pittsburgh in the 1920s. The bridges were designed under the auspices of the Allegheny Department of Public Works, by T. J. Wilkerson, consulting engineer; Vernon R. Covell, chief engineer; A. D. Nutter, design engineer; and Stanley L. Roush, architect. The American Bridge Company built the superstructure; while the Foundation Company built the substructure. All three bridges are owned by Allegheny County. ==History== Like New York City residents, Pittsburghers are quite enamoured of suspension bridges in general and some of the country's earliest and finest examples can be found in the southwestern Pennsylvania area. From 1926 through 1931, four suspension bridges were constructed across the Allegheny and Monongahela Rivers. The fourth bridge (not covered in this article) is a wire cable bridge carrying South Tenth Street across the Monongahela. The "Three Sisters Bridges" represent an adaptive engineering design response to political and technical concerns. County engineers successfully maneuvered around federally mandated clearances, aesthetic and financial considerations raised by local agencies and the lack of adequate anchorage points along the river banks. They were the first self-anchored suspension bridges built in the United States. The design’s deck- stiffening girder provided compressive support while lowering visual barriers between Pittsburgh and the historically distinct North side (formerly Allegheny City), annexed in 1907 in a contentious fight. It may or may not be a coincidence that American Bridge Company the builder and steel supplier for these three bridges, was headquartered nearby, as was the Foundation Company which had the pier/abutment masonry contract. Local election campaigns during the period highlighted the intent to use local suppliers and labour. 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「Three Sisters (Pittsburgh)」の詳細全文を読む スポンサード リンク
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