Covered bridge
Covered bridge
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Covered bridge

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Covered bridge

A covered bridge is a timber-truss bridge with a roof, decking, 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 typically have a lifespan of only 20 years because of the effects of rain and sun, but a covered bridge can last over 100 years. In the United States, only about 1 in 10 survived the 20th century. The relatively small number of surviving bridges is due to deliberate replacement, neglect, and the high cost of restoration.

Surviving covered bridges often attract touristic attention due to their rarity, quaint appearance, and bucolic settings. Many are considered historic and have been the subject of historic preservation campaigns.

Typically, covered bridges are structures with longitudinal timber-trusses which form the bridge's backbone. Some were built as railway bridges, using very heavy timbers and doubled up lattice work.

In Canada and the U.S., numerous timber covered bridges were built in the late 1700s to the late 1800s, reminiscent of earlier designs in Germany and Switzerland. They tend to be in isolated places, making them vulnerable to vandalism and arson.

The oldest surviving truss bridge in the world is the Kapellbrücke in Switzerland, first built in the 1300s. Modern-style timber truss bridges were pioneered in Switzerland in the mid-1700s. Germany has 70 surviving historic wooden covered bridges.

Most bridges were built to cross streams, and the majority had just a single span. Virtually all contained a single lane. A few two-lane bridges were built, having a third, central truss.

Many different truss designs were used. One of the most popular designs was the Burr Truss, patented in 1817, which used an arch to bear the load, while the trusses kept the bridge rigid. Other designs included the King, Queen, Lattice, and Howe trusses.

Early trusses were designed with only a rough understanding of the engineering dynamics at work. In 1847, American engineer Squire Whipple published the first correct analysis of the precise ways that a load is carried through the components of a truss, which enabled him to design stronger bridges with fewer materials.

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