Covered bridge

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Covered Bridge
Larrys Creek Covered Bridge.JPG
The Cogan House Covered Bridge, Pennsylvania
Ancestor Truss bridge, others
Descendant None
Carries Pedestrians, livestock, vehicles
Span range Short
Material Typically wood beams with iron fittings and iron rods in tension
Movable No
Design effort Low
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.[1] 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.[2]

History

Europe

The oldest surviving truss bridge in the world is the Kapellbrücke in Switzerland. Modern-style timber truss bridges were pioneered in Switzerland in the mid-1700s.[3]

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.[4]

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.[2]

In total more than 12,000 covered bridges have been built in the United States, about 3,500 of which in Ohio.[2]

In the mid-1800s, the development of cheaper wrought iron and cast iron led to metal rather than timber trusses.[5] Metal structures did not need protection from the elements and 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.[5]

Canada

In 1900 Quebec had an estimated 1000 covered bridges.[6] Relative to the rest of North America, Quebec was late in building covered bridges, with the busiest decade for construction being the 1930s.[7] 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. Five hundred of these were built in the first half of the 1900s.[7] The last bridge was built by the Ministry of Colonisation in 1958 in Lebel-sur-Quévillon.[7]

In 1900 New Brunswick had about 400 covered bridges. Today there are 58.[8]

Between 1969 and 2015, the number of surviving covered bridges in Canada declined from about 400 to under 200.[9]

Design

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.[5]

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.[5]

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.[3] In 1847 American engineer Squire Whipple published the first correct analysis of the way a load is carried through the truss,[10] which enabled him to design stronger bridges with fewer materials.

Covered bridges today

The covered bridge in West Montrose, Ontario, October 2018

There are about 1600 covered bridges in the world.[11]

The relatively small number of surviving bridges is due to deliberate replacement, neglect, and the high cost of restoration.[12] They tend to be in isolated places which makes them subject to vandalism and arson.[13] The oldest covered bridges in America date back to the 1820s:[5]

As of 2018 there are fewer than 1,000 authentic covered bridges in the United States.[14]

New Brunswick, Canada, has 58 covered bridges, including the world's longest, the Hartland Bridge.[15]

There are 82 covered bridges in Quebec. Transports Québec considers the Félix-Gabriel-Marchand Bridge, the province's longest covered bridge, to be an important tourist attraction.[16]

In fiction

As well as being practical, covered bridges were popular venues for a variety of social activities[2] and are an enduring cultural icon,[17] for example:

Gallery

References

  1. ^ "Covered bridge". Encyclopædia Britannica. Retrieved 8 October 2012.
  2. ^ a b c d "Ohio's Vanishing Covered Bridges - Back in Time - General Highway History - Highway History - Federal Highway Administration". www.fhwa.dot.gov. Retrieved 2019-01-08.
  3. ^ a b "Bridge - Timber truss bridges". Encyclopedia Britannica. Retrieved 2019-01-09.
  4. ^ Griggs, Frank Jr. "The Permanent Bridge". Structure Magazine. Retrieved 9 January 2019.
  5. ^ a b c d e "Covered Bridge Manual". U.S. Department of Transportation. Retrieved 9 October 2012.
  6. ^ "Ponts couverts". Transports Quebec. Archived from the original on 24 February 2012. Retrieved 26 September 2012.
  7. ^ a b c Lefrançois, Jean. "Les ponts couverts au Québec, héritage précieux" (PDF). Retrieved 8 January 2019.
  8. ^ Government of New Brunswick, Canada (2011-10-07). "New Brunswick's Covered Bridges - Transportation and Infrastructure". www2.gnb.ca. Retrieved 2019-01-13.
  9. ^ Walker, Nick. "Throwback Thursday: Covered bridges". Canadian Geographic. Retrieved 8 January 2019.
  10. ^ Allen, Richard Sanders (2004). Covered Bridges of the Northeast. Courier Corporation. p. 84. ISBN 9780486436623.
  11. ^ "World Guide to Covered Bridges". National Center for Wood Transportation Structures. Retrieved 2019-01-08.
  12. ^ 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.
  13. ^ Phares, Brent. "Covered Bridge Security Manual" (PDF). Government Publishing Office. Retrieved 9 January 2019.
  14. ^ "Guidelines to Restoring Structural Integrity of Covered Bridge Members" (PDF). US Department of Agriculture. 15 January 2018. p. 110.
  15. ^ Government of New Brunswick, Canada (2011-10-07). "New Brunswick's Covered Bridges - Transportation and Infrastructure". www2.gnb.ca. Retrieved 2019-01-08.
  16. ^ "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.
  17. ^ "Virtual Museum - Covered Bridges". www.iub.edu. Retrieved 2019-01-08.

External links

  • Covered Bridge Map, an interactive map showing locations of covered bridges in the United States and Canada
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