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 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.
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.
In total, more than 12,000 covered bridges have been built in the United States, about 3,500 of them in Ohio. As of 2018, fewer than 1,000 authentic covered bridges are left in the United States.
The first known covered bridge constructed in the United States was the Permanent Bridge, completed in 1805 to span the Schuylkill River in Philadelphia. The structure endured beyond the estimate of 40 years offered by its architect, only being taken down in 1850 to make way for a new bridge more conducive to carrying railroad tracks.
About 1,500 covered bridges were built from 1820 and 1900, and most were built from 1825 and 1875. The longest ever built was over the Susquehanna River at 5,960 feet (1,820 m). Built in 1814, it was washed away in the freshets of 1832.
The oldest covered bridges in America date back to the 1820s: the 1825 Hyde Hall and Hassenplug bridges in New York and Pennsylvania, and the 1829 Haverhill-Bath bridge in New Hampshire and Roberts bridge in Ohio.
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.
- The 1992 novel, The Bridges of Madison County, featured the Cedar Covered Bridge, which has since been burnt by arson in 2002, replaced with a replica, and burnt by arson again in 2017.
- The Edgar Allan Poe story "Never Bet the Devil Your Head"
- Plot points in the 1988 comedy films Beetlejuice and Funny Farm refer to them.
- The Diehls Covered Bridge in Pennsylvania is featured in the opening scenes of the 1980s anthology horror television series Tales from the Darkside, created by George A. Romero.
- The Need for Speed included a track with covered bridges, where they were a prominent landmark and a jump at high speeds.
Baumgardener's Covered Bridge, showing the truss protected by the covering
The 1,282-foot (391 m) Hartland Bridge in New Brunswick is the longest covered bridge in the world.
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- "Programmation routière 2018-2020 - Plus de 157 M$ pour améliorer la sécurité et la qualité de vie des usagers de la route en Outaouais" (in French). Transports Québec. 5 March 2018. Retrieved 8 January 2019.
- "Covered Bridges". Government of New Brunswick. Retrieved 11 July 2019.
- "Covered Bridges". The Warren E. Roberts Museum of Early Indiana Life. Indiana University Bloomington. 2007. Retrieved 8 January 2019.
|Wikimedia Commons has media related to Covered bridges.|
- Covered Bridge Map, an interactive map showing locations of covered bridges in the United States and Canada