Covered bridges
The Cogan House Covered Bridge, U.S.A.
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| Ancestor | Truss bridge, others |
|---|---|
| Related | Tubular bridge, Skyway, Jetway |
| Descendant | None |
| Carries | Pedestrians, livestock, vehicles |
| Span range | Short |
| Material | Typically wood beams with iron fittings and iron rods in tension |
| Movable | No |
| Design effort | Low |
| Falsework required | Determined by enclosed bridge structure, site conditions, and degree of prefabrication |
A covered bridge is a timber-truss bridge with a roof and siding which, in most covered bridges, create an almost complete enclosure. 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.
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.
Early timber covered bridges consisted of horizontal beams laid on top of piles driven into the riverbed below. However, this construction method meant that the length between bridge spans was limited by the maximum length of each beam. The development of the timber truss circumvented that limitation and allowed bridges to span greater distances than those with beam-only structures or arch structures, whether of stone, masonry, or timber.
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.
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.
There are about 1600 covered bridges in the world.
The 1,282-foot (391 m) Hartland Bridge in New Brunswick is the longest covered bridge in the world. In 1900 Quebec, New Brunswick, and Ontario had an estimated 1000, 400, and five covered bridges respectively. By the 1990s there were 98 in Quebec, 62 in New Brunswick, and one in Ontario, the West Montrose Covered Bridge.
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Wikipedia