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 life span 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, are also sometimes called covered bridges.
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.
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.
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 Browntrusses.
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.
Examples of covered bridges
Buchfart - Alte Holzbrücke 1613
There are about 1600 covered bridges in the world.
China: covered bridges are called lángqiáo (廊桥), or "wind and rain bridges" in Guizhou, traditionally built by the Dong. There are also covered bridges in Fujian.Taishun County, in southern Zhejiang province near the border of Fujian, has more than 900 covered bridges, many of them hundreds of years old, as well as a covered bridge museum. There are also a number in nearby Qingyuan County, as well as in Shouning County, in northern Fujian province. The Xijin Bridge in Zhejiang is one of the largest.