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.
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.
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.
In 1900, Quebec had an estimated 1000 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.
Between 1969 and 2015, the number of surviving covered bridges in Canada declined from about 400 to under 200.
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.
Covered bridges today
About 1600 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 which makes them subject to vandalism and arson. The oldest covered bridges in America date back to the 1820s:
- 1825: Hyde Hall and Hassenplug bridges in New York and Pennsylvania
- 1829: Haverhill-Bath in New Hampshire and Roberts bridges in Ohio
As of 2018, fewer than 1,000 authentic covered bridges are left in the United States.
- 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.
- Diehls Covered Bridge in Pennsylvania is featured in the opening scenes of The 1980's Anthology Horror Television Series Tales from the Darkside that ran that was created by George Romero.
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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- "Ponts couverts". Transports Quebec. Archived from the original on 24 February 2012. Retrieved 26 September 2012.
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- Government of New Brunswick, Canada (2011-10-07). "New Brunswick's Covered Bridges - Transportation and Infrastructure". www2.gnb.ca. Retrieved 2019-01-13.
- Walker, Nick. "Throwback Thursday: Covered bridges". Canadian Geographic. Retrieved 8 January 2019.
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- Ross, Robert. "Use of Laser Scanning Technology to Obtain As-Built Records of Historic Covered Bridges" (PDF). permanent.access.gpo.gov. Retrieved 9 January 2019.
- Phares, Brent. "Covered Bridge Security Manual" (PDF). Government Publishing Office. Retrieved 9 January 2019.
- "Guidelines to Restoring Structural Integrity of Covered Bridge Members" (PDF). US Department of Agriculture. 15 January 2018. p. 110.
- Government of New Brunswick, Canada (2011-10-07). "New Brunswick's Covered Bridges - Transportation and Infrastructure". www2.gnb.ca. Retrieved 2019-01-08.
- "Programmation routière 2018-2020 - Plus de 157 M$ pour améliorer la sécurité et la qualité de vie des usagers de la route en Outaouais". www.transports.gouv.qc.ca (in French). Retrieved 2019-01-08.
- "Virtual Museum - Covered Bridges". www.iub.edu. Retrieved 2019-01-08.
|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