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A covered bridge is a timber-truss bridge with a roof and siding which, in most covered bridges, create an almost complete enclosure.〔(【引用サイトリンク】title=Covered bridge )〕 The purpose of the covering is to protect the wooden structural members from the weather. Uncovered wooden bridges have a lifespan of only 10 to 15 years because of the effects of rain and sun.〔(【引用サイトリンク】title=West Montrose Covered Bridge - The Kissing Bridge (built in 1881) )〕 Bridges having covers for reasons other than protecting wood trusses, such as for protecting pedestrians and keeping horses from shying away from water, are also sometimes called covered bridges. ==History and development== Early timber covered bridges consisted of horizontal beams laid on top of piles driven into the riverbed. The problem is that the length between spans is limited by the maximum length of each beam. The development of the timber truss allowed bridges to span greater distances than those with beam-only structures or arch structures, whether of stone, masonry, or timber.〔(【引用サイトリンク】title=Covered Bridge Manual )〕 Early European truss bridges used king post and queen post configurations. Some early German bridges included diagonal panel bracing in trusses with parallel top and bottom chords.〔 To solve the problem of deterioration of the wood upon exposure to weather, various forms of covering came to be employed.〔(【引用サイトリンク】title=History/Design )〕 At least two covered bridges make the claim of being the first built in the United States. Town records for Swanzey, New Hampshire, indicate their Carleton Bridge was built in 1789, but this remains unverified. Philadelphia, however, claims a bridge built in the early 1800s on 30th Street and over the Schuylkill River was the first, noting that investors wanted it covered to extend its life. Beginning around 1820, new designs were developed, such as the Burr, Lattice, and Brown trusses. In the mid-1800s, the development of cheaper wrought iron and cast iron led to metal rather than timber trusses, except in those areas of plentiful large timber.〔 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「Covered bridge」の詳細全文を読む スポンサード リンク
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