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 can last over 100 years.[2] In the United States, only about 1 in 10 survived the 20th century.[3] The relatively small number of surviving bridges is due to deliberate replacement, neglect, and the high cost of restoration.[4]
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.
European and North American truss bridges
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]
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.[6][7] They tend to be in isolated places, making them vulnerable to vandalism and arson.[8]
Europe
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.[9]
Germany has 70 surviving historic wooden covered bridges.[10]
The
Ponte Coperto in
Pavia is a stone and brick arch bridge over the
Ticino River in Pavia, Italy. The previous bridge, dating from 1354 (itself a replacement for a
Roman construction), was heavily damaged by
Allied action in 1945. A debate on whether to fix or replace the bridge ended when the bridge partially collapsed in 1947, requiring new construction, which began in 1949.
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 with only a rough understanding of the engineering dynamics at work.[9] 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,[12] which enabled him to design stronger bridges with fewer materials.
The longest covered bridge ever built was constructed in 1814 in
Lancaster County, Pennsylvania, and spanned over a mile in length, but was destroyed by ice and flooding in 1832.[16] 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.[17][18]
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, so they 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]
As of 2004[update], there were about 750 left,[19] mostly in eastern and northern states. The 2021
World Guide to Covered Bridges lists 840 covered bridges in the U.S., although it states that only 670 of those were standing when the 1959 edition was published.[20] The tallest (35 feet high), built in 1892, is the
Felton Covered Bridge, just north of
Santa Cruz, California.[18]
Canada
Between 1969 and 2015, the number of surviving covered bridges in Canada declined from about 400 to under 200.[21]
In 1900,
Quebec had an estimated 1,000 covered bridges.[22] Relative to the rest of North America, Quebec was late in building covered bridges, with the busiest decade for construction being the 1930s.[23] 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. The designer is unknown. About 500 of these were built in the first half of the 1900s.[23] They were often built by local settlers using local materials, according to standard plans.[19] The last agricultural colony was founded in 1948, and the last bridge was built by the Ministry of Colonisation in 1958 in
Lebel-sur-Quévillon.[23] There are now 82 covered bridges in Quebec,
Transports Québec including the
Félix-Gabriel-Marchand Bridge, the province's longest covered bridge.[24] In Quebec covered bridges were sometimes known as pont rouges (red bridges) because of their typical colour.[19]: 11, 87
Like Quebec, New Brunswick continued to build covered bridges into the 1950s, peaking at about 400 covered bridges. These mostly used the Howe, Town, and Burr trusses. Today, there are
58 covered bridges in New Brunswick, including the world's longest, the
Hartland Bridge.[25]
Roofed, rather than covered bridges, have existed for centuries in southern Europe and Asia. In these cases, the cover is to protect the users of the bridge rather than the structure. Examples include:
Some stone arch bridges are covered to protect pedestrians or as a decoration—as with the Italian
Ponte Coperto and
Rialto Bridge, and the Chùa Cầu (the Japanese Bridge; see Gallery below) in Vietnam.
In China lángqiáo (
廊桥) covered bridges are found in
Guangxi, including the
Chengyang Bridge and the
Xijin Bridge. These are pier and girder bridges, not truss bridges. The
Chùa Cầu bridge is found in
Vietnam.
In addition to being practical, covered bridges were popular venues for a variety of social activities[2] and are an enduring cultural icon;[28] for example:
The
Joe Hill novel NOS4A2 features a character who uses a covered bridge called the "Shorter Way" as a portal to travel instantly across vast distances.
^
abcConwill, Joseph D. (2004). Covered bridges across North America. St. Paul, Minn.: MBI Pub. Co.
ISBN0-7603-1822-0.
^Caswell, William S. (31 October 2021). World Guide to Covered Bridges (2021 ed.). Concord, New Hampshire: National Society for the Preservation of Covered Bridges. pp. xii.
ISBN978-0-578-30263-8.
Covered Bridge Map, an interactive map showing locations of covered bridges in the United States and Canada. However, the data is only current as of the archive date.