The Cogan House Covered Bridge, Pennsylvania
|Ancestor||Truss bridge, others|
|Carries||Pedestrians, livestock, vehicles|
|Material||Typically wood beams with iron fittings and iron rods in tension|
|Falsework required||Determined by enclosed bridge structure, site conditions, and degree of prefabrication|
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 could last over 100 years.
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.
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 without an understanding of the engineering dynamics at work. In 1847, American engineer Squire Whipple published the first correct analysis of the way a load is carried through the truss, which enabled him to design stronger bridges with fewer materials.
The longest covered bridge ever built was constructed in Pennsylvania between Lancaster County (Columbia) and York County (Wrightsville). This bridge was over a mile in length and was completed in 1814.
About 1,600 covered bridges remain in the world. The relatively small number of surviving bridges is due to deliberate replacement, neglect, and the high cost of restoration. They tend to be in isolated places, making them subject to vandalism and arson.
About 14,000 covered bridges have been built in the United States, mostly in the years 1825 to 1875. Today, surviving bridges are numbered at fewer than 1,000 by the United States Department of Transportation, and at 814 by The National Society for the Preservation of Covered Bridges as of 2009.
The first known covered bridge constructed in the United States was the Permanent Bridge, completed in 1805 to span the Schuylkill River in Philadelphia. However, most other early examples of covered bridges don't appear until the 1820s. Extant bridges from that decade include New York's Hyde Hall Bridge and Pennsylvania's Hassenplug Bridge, both built in 1825, and the Haverhill–Bath Covered Bridge and the Roberts Covered Bridge, in New Hampshire and Ohio respectively, both built in 1829. The longest, historical covered bridges remaining in the United States are the Cornish–Windsor Bridge, spanning the Connecticut River between New Hampshire and Vermont, and Medora Bridge, spanning the East Fork of the White River in Indiana. Both lay some claim to the superlative depending upon how the length is measured.
In the mid-1800s, the development of cheaper wrought iron and cast iron led to metal rather than timber trusses. Metal structures did not need protection from the elements, so no longer needed to be covered. The bridges also became obsolete because most were single-lane, had low width and height clearances, and could not support the heavy loads of modern traffic.
Between 1969 and 2015, the number of surviving covered bridges in Canada declined from about 400 to under 200.
In 1900, Quebec had an estimated 1,000 covered bridges. Relative to the rest of North America, Quebec was late in building covered bridges, with the busiest decade for construction being the 1930s. Initially, the designs were varied, but around 1905, the design was standardised to the Town québécois, a variant on the lattice truss patented by Ithiel Town in 1820. About 500 of these were built in the first half of the 1900s. The last bridge was built by the Ministry of Colonisation in 1958 in Lebel-sur-Quévillon. There are now 82 covered bridges in Quebec, Transports Québec including the Félix-Gabriel-Marchand Bridge, the province's longest covered bridge.
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