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bridge, structure built over water or any obstacle or depression to allow the passage of pedestrians or vehicles. See also viaduct.
In ancient times and among primitive peoples a log was thrown across a stream, or two vines or woven fibrous ropes (the upper for a handhold and the lower for a footwalk) were thrown across, to serve as a bridge. Later, arched structures of stone or brick were used; traces of these, built from 4000 to 2000 B.C., have been found in the E Mediterranean region. The Romans built long, arched spans, many of which are still standing. Bridges built during the Middle Ages usually rested on crude stone arches with heavy piers (intermediate supports) that were a great obstruction to river traffic, and their roadways were often lined with small shops.
The best known early American design is the New England covered bridge, since wood was abundant and cheap, and did not demand trained masons. Colonial American bridge builders were willing to run the risk of rot or fire in exchange for such savings in time and manpower. Beginning with Abraham Darby's bridge at Coalbrookdale in 1779, most bridges began to be built of cast and wrought iron. Robert Stephenson, an English engineer, designed and built a bridge of this type across Menai Strait in North Wales (1850). Another is Victoria Bridge across the St. Lawrence at Montreal. The disadvantage of cast iron for bridges is its low tensile strength.
Modern Bridge Designs
There are six basic modern bridge forms: the beam, the truss, the arch, the cantilever, the cable-stayed, and the suspension. A beam bridge is made of long timber, metal, or concrete beams anchored at each end. If the beams are arranged in a lattice, such as a triangle, so that each shares only a portion of the weight on any part of the structure, the result is a truss bridge. An arch bridge has a bowed shape causing the vertical force of the weight it carries to produce a horizontal outward force at its ends. It may be constructed of steel, concrete, or masonry. A cantilever bridge is formed by self-supporting arms anchored at and projecting toward one another from the ends; they meet in the middle of the span where they are connected together or support a third member. In a cable-stayed bridge, the roadway is supported by cables attached directly to the supporting tower or towers. This differs from a suspension bridge, where the roadway is suspended from vertical cables that are in turn attached to two or more main cables. These main cables hang from two towers and have their ends anchored in bedrock or concrete.
The modern era of bridge building began with the development of the Bessemer process for converting cast iron into steel. It became possible to design framed structures with greater ease and flexibility. Single-piece, rolled steel beams can support spans of 50 to 100 ft (15–30 m), depending on the load. Larger, built-up beams are made for longer spans; a steel box-beam bridge with an 850-ft (260-m) span crosses the Rhine at Cologne.
Truss, Arch, and Cantilever Bridges
Cable-Stayed, Suspension, and Combination Bridges
The cable-stayed bridge is the most modern type, coming into prominence during the 1950s. The longest is the Russky Bridge, Vladivostok, Russia, which has a main span of 3,622 ft (1,104 m); the Sutong Bridge, Suzhou–Nantong, Jiangsu, China, with a main span of 3,570 ft (1,088 m), and the Stonecutters Bridge, Hong Kong, with a main span of 3,340 ft (1,018 m) are other long cable-stayed bridges. The longest cable-stayed bridge in the United States is the Arthur Ravenel, Jr., Bridge in Charleston, S.C., which has a span of 1,546 ft (471 m).
The suspension bridge is used for the longest spans. The earliest suspension bridges built in America were those constructed by the American builder James Finley. The design of suspension bridges advanced when J. A. Roebling, a German-born engineer who emigrated to the United States, developed the use of wire cables and stiffening trusses. His first completed suspension bridge spanned the Niagara River in 1854. He also designed the Brooklyn Bridge across the East River (completed 1883), which was the world's longest suspension bridge at the time of its construction, having a main span of 1,595.5 ft (487 m).
Today the longest spans in the world are suspended. The longest main spans are the Akashi Kaikyo Bridge, Hyogo, Japan, 6,529 ft (1,990 m); the Xihoumen Bridge, Zhoushan Archipelago, China, 5,414 ft (1,650 m); the Store Bælt Bridge, Denmark, 5,328 ft (1,624 m); the south span of the Runyang Bridge, Jiangsu, China, 4,888 ft (1,490 m); and the Humber River Bridge, Hull, England, 4,626 ft (1410 m). The longest suspension bridges in the United States are the Verrazzano-Narrows Bridge, New York City, 4,260 ft (1,298 m); the Golden Gate Bridge, San Francisco, 4,200 ft (1,280 m); the Mackinac Straits Bridge, Mich., 3,800 ft (1,158 m); George Washington Bridge, New York City, 3,500 ft (1,067 m); and the two Tacoma Narrows Bridges, Tacoma, Wash., 2,800 ft (853 m) each.
Combination spans are often used to bridge even longer stretches of water. The San Francisco–Oakland Bay Bridge, noted for its three long spans, of which two are traditional suspension spans and the third a self-anchored single-tower suspension, has a total length of 8.25 mi (13.2 km). The Hong Kong–Zhuhai–Macau Bridge's main section, which links Hong Kong with Macau and Zhuhai, China, across the Pearl River estuary, consists of a 14.2 mi (22.9 km) bridge and 4.2 mi (6.7 km) tunnel; the Chesapeake Bay Bridge-Tunnel has two 1-mi (1.6-km) tunnels along its 17.6-mi (28.2-km) length; and the 8-mi (12.9-km) Confederation Bridge, linking Prince Edward Island to the Canadian mainland, consists of three bridges. The longest cross-sea bridges are the Hangzhou Bay Bridge, 22.4 mi (36 km) long, which crosses the bay between Zhapu and Cixi, Zhejiang prov., China; the Donghai Bridge, 20.2 mi (32.5 km), which connects Shanghai, China, with the deepwater port on Yangshan Island in Hangzhou Bay; and the Jiaozhou Bay Bridge, 16.6 mi (26.7 km) long, which connects Qingdao with Huangdao, Shangdong prov., China; these bridges combine causeway with cable-stayed spans. The longest combination spans in the United States are the twin Lake Ponchartrain Causeways near New Orleans, Louisiana, whose parallel roadways stretch nearly 24 mi (38 km). Some viaducts carrying high-speed rail lines over land are even longer; the longest is the Danyang-Kunshan Grand Bridge, a 102.4-mi (164.8-km) viaduct in Jiangsu prov., China.
See D. Plowden, The Spans of North America (1984); H. Petroski, Engineers of Dreams: Great Bridge Builders and the Spanning of America (1995); J. Dupré and F. O. Geary, Bridges: A History of the World's Most Famous and Important Spans (1996); S. A. Ostrow, Bridges (1997); F. Gottemoeller, Bridgescape: The Art of Designing Bridges (1998); K. Willard, Bridges: Designing the Future (1999). See also bibliographies for articles on individual bridges.
a bridge with a movable span to permit the passage of ships. Drawbridges are usually built across rivers traveled by large ships when conditions make it technically and economically inadvisable to construct a bridge on high piers and with long approaches. The movable span of a drawbridge can be of the vertical-lift, swing, bascule, balance-beam, or rolling-lift type; the choice of type depends on local conditions. Construction of a drawbridge span requires either massive piers or towers to house the mechanisms and engines required for moving the span. Electric and hydraulic drives are the most common, and drawbridges are often equipped with backup drives from internal-combustion engines. The movable span generally has light-duty structural members, with trusses or beams of steel or lightweight alloys.