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A log bridge is a timber bridge that uses logs that fall naturally or are intentionally felled or placed across streams. The first man-made bridges with significant span were probably intentionally felled trees. The use of emplaced logs is now sometimes used in temporary bridges used for logging roads, where a forest tract is to be harvested and the road then abandoned. Such log bridges have a severely limited lifetime〔National Parks Conference 1915, p. 59. "The chief objection to a log bridge ... is the shortness of its life."〕 due to soil contact and subsequent rot and wood-eating insect infestation. Longer lasting log bridges may be constructed by using treated logs and/or by providing well drained footings〔National Parks Conference 1915, p. 59. "If we can design the abutment so that no moisture can collect under the logs the life of the bridge is materially increased."〕 of stone or concrete combined with regular maintenance to prevent soil infiltration. This care in construction can be seen in the French bridge illustrated below, which has well locked dry set stone abutments and a footpath leveled with boards. ==Various log bridge designs== File:Log bridge - naturally fallen or man felled.jpg|Log bridge over a river in Papua New Guinea File:Zaire Log Bridge.jpg|Log bridge in Zaire, made of multiple parallel logs File:Crossing Nisqually River near Cougar Rock 01.jpg|Log bridge over the Nisqually River, United States, made of one large log with handrail File:Schwarz - Schwarztöbeli IMG 6621.JPG|Log bridge in Switzerland with flattened top and handrail File:Suchá Belá, Slovenský ráj, 2012, 004.JPG|Log bridge in Slovakia with additional boards on top File:Dřevěný most přes potok v Západních Tatrách.jpg|Log bridge in Slovakia with additional logs laid crosswise (a beam bridge) File:Vallorcine footpath bridge 2003-12-13.jpg|Log bridge in France with dry set stone abutments and a footpath leveled with boards File:Tibetan log bridge.JPG|Complex Tibetan log bridge made of multiple logs (a cantilever bridge) 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「Log bridge」の詳細全文を読む スポンサード リンク
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