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.

The truss can span even greater distances and carry heavy loads; it is therefore commonly used for railroad bridges. A large truss span like that over the Columbia River at Astoria, Oreg., can extend to 1,232 ft (376 m); the Ikitsuki Bridge, connecting Ikitsuki and Hirado islands in SW Japan, has a continuous-truss span of 1,312 ft (400 m), the longest in the world. If the truss is shaped into an arch, even longer bridges are possible; the Chaotianmen Bridge in Chongqing, China, the Lupu Bridge, Shanghai, China, the New River Bridge in West Virginia, the Bayonne Bridge between New York and New Jersey, and the Sydney Harbor Bridge in Australia have the longest steel arch spans, at 1,811 ft (552 m), 1,804 ft (550 m), 1,700 ft (518 m), 1,675 ft (510 m), and 1,670 ft (509 m), respectively. Concrete arch bridges tend to be somewhat smaller, the largest being the Wanxian Bridge in China and the Krk Bridge in Croatia at 1,378 ft (420 m) and 1,280 ft (390 m), respectively. The longest concrete arch bridge in the United States is the Natchez Trace Parkway Bridge in Franklin, Tenn., at 582 ft (177 m), although the concrete and steel O'Callaghan-Tillman Memorial Bridge, Nev.-Ariz., near Hoover Dam, has the largest concrete arch, at 1,079 ft (329 m). The cantilever, however, is more common for spans of such lengths. The cantilevered Forth Bridge (1890) in Scotland was the first major structure built entirely of steel, the material that made possible its two record-setting spans of 1,710 ft (521 m) each. They remained the longest in existence until 1917, when the St. Lawrence River at Quebec Bridge was built; it has an 1,800-ft (549-m) span. The longest cantilever bridge in the United States is the Commodore John Barry Bridge in Chester, Penn., which has an 1,644 ft (501 m) span.

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.

Movable bridges are generally constructed over waterways where it is impossible or prohibitive to build a fixed bridge high enough for water traffic to pass under it. The most common types of movable bridge are the lifting, bascule, and swing bridges. The lifting bridge, or lift bridge, consists of a rigid frame carrying the road and resting abutments, over each of which rises a steel-frame tower. The center span, which in existing bridges is as long as 585 ft (178 m), is hoisted vertically. The bascule bridge follows the principle of the ancient drawbridge. It may be in one span or in two halves meeting at the center. It consists of a rigid structure mounted at the abutment on a horizontal shaft, about which it swings in a vertical arc. The lower center span of the famous Tower Bridge in London is of the double-leaf bascule type. Because of the need for large counterweights and the stress on hoisting machinery, bascule bridge spans are limited to about 250 ft (75 m). The swing bridge is usually mounted on a pier in midstream and swung parallel to the stream to allow water passage.

In wartime, where the means of crossing a stream or river is lacking or a bridge has been destroyed by the enemy, the military bridge plays a vital role. Standard types of military bridges include the trestle, built on the spot by the engineering corps from any available material, and the floating bridge made with portable pontoons.

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