Modern Bridge Design
There are six basic modern bridge forms, the beam, the truss, the arch, the cantilever, the cable stayed and the suspension. A beam or "girder" bridge is the simplest and most inexpensive kind of bridge to construct. It is made of long timber, metal or concrete beams anchored at each end. In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers. The beam must be strong so that it does not bend under its own weight and the added weight of crossing traffic. When a load pushes down on the beam, the beam's top edge is pushed together (compression) while the bottom edge is stretched (tension). If the beams are arranged in a lattice, such as a triangle, so that each component shares only a portion of the weight on any part of the structure, the results is a truss bridge.
Arch bridges are one of the oldest types of bridges; they have great natural strength. Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch to the supports at each end. These supports, called the abutments, carry a load and keep the ends of the bridge from spreading out. Bowed shapes causes the vertical force of the weight it carries to produce a horizontal outward force at its ends. Arched bridges can be constructed of steel, concrete, masonry or wood.
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.
Cable-Stayed bridges may look similar to suspension bridges, both have roadways that hang from cables and both have towers. The two bridges, however, support the load of the roadway in very different ways. The difference lies in how the cables are connected to the tower. In suspension bridges, the cables ride freely across the towers, transmitting the load to the tower and the anchorages at either end. In cable-stayed bridges, the cables are attached to the towers, which bear the load. The cables can be attached to the roadway in a variety of ways. In a radical pattern, cables extend from several points on the road to a single point at the top of the tower. In a parallel pattern, cables are attached at different heights along the tower, running parallel to each other.
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 50 100 feet (15-30) meters, depending on the load. Larger, built-up beams are made for longer spans; a steel box-beam bridge with an 850-ft {260} meters span crosses the Rhine at Cologne.
Truss, Arch, and Cantilever Bridges
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 OR, can extend to 1,232 feet (376 meters). If the truss is shaped into an arch, even longer bridges are possible; such as the Bayonne Bridge between New York and New Jersey, the Sidney Harbor Bridge in Australia and the New River Bridge in West Virginia are the longest steel arch bridges at 1,675 feet (510 m), 1,670 feet (509 m) and 1,700 feet (518 m), respectively. Concrete arch bridges tend to be somewhat smaller, the largest being the Krk Bridge in Croatia and the Gladsville Bridge across the Parramatta River at Sidney, Australia, at 1,280 feet (390 m) and 1,000 feet (305 m), respectively. The longest concrete arch bridge in the United States is the Natchez Trace Parkway Bridge in Franklin TN, at 582 feet (177 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 feet (521 m) each. They remained the longest in existence until 1917, when the bridge over the St. Lawrence River in Quebec was built; it achieved a 1,800 feet (549m) span. The longest cantilever bridge in the United States is the Commodore John Berry Bridge in Chester, PA, which has a 1,644 feet (501 m) span.
Cable-Stayed, Suspension, and Combination Bridges
The cable-stayed bridge is the most modern type, coming into prominence during the 1950's. The longest is the Tatara Bridge in Ehime, Japan. It has a 2,920 feet (890 m) span. The Ponte de Normandie in Le Havre, France spans 2,808 feet (856 m). The Yangtze Bridge in Nanjing, China spans 2,060 feet (638 m); the Yangtze Bridge in Wuhan, China spans 2,028 feet (618 m). The longest cable-stayed bridge in the United States is the Dame Point Bridge in Jacksonville, FL with a span of 1.300 feet (396 m).
The suspension bridge is used for the longer 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 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 in New York City (completed in 1883). When completed, it was the world's longest suspension bridge at the time of its construction, having a main span of 1,595 feet (487 m)
Today the longest spans in the world are suspended. The longest main spans are:
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• The Akashi Kaikyo Bridge, Hyogo. Japan, 6,529 feet (1,900m).
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• The Izmit Bay Bridge, Marmara Sea Turkey, 5472 feet (1668 m).
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• The Store Baelt Bridge, Denmark, 5,328 feet (1624 m).
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• The Humber River Bridge, Hull, England, 4,626 feet (1,410 m).
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• The Tsing Ma Bridgem Hong, Hong, 4,518 feet (1,377 m).
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• The Verrazano-Narrows Bridge in New York City, 4,260 feet (1,298 m)
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• The Golden Gate Bridge, San Francisco, 4200 feet (1,280 m).
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• The Hoga Kusten (High Coast) Bridge, Vasternorriand, Sweden, 3,969 feet (1,210 m).
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• The Mackinac Straits Bridge, MI, 3,800 feet (1,158 m).
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• The Minami Bisan-Seto Bridge, Japan, 3,668 feet (1,118 m).
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• The Second Bosporous Bridge, Istanbul, Turkey, 3,576 feet (1,090 m).
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• First Bosporous Bridge, Istanbul, Turkey. 3,524 feet (1,074 m).
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• The George Washington Bridge, New York City, 3500 feet, (1,067 m)
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 suspension spans and the third a cantilever, has a total length of 8.25 miles (13.4 km). The Chesapeake Bay Bridge-Tunnel has 2 - mile (1.6 km) tunnels along its 17.6- miles (28.2 km) length. The 8-mile (12.9 km) Confederation Bridge linking Prince Edward Island to the Canadian mainland consists of three bridges. The longest combination spans are the twin Lake Ponchartrain Causeways near New Orleans, LA. Their parallel roadways stretch nearly 24 miles (38 km).
Movable Bridges
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 bridges are the lifting, bascule, and swing bridges. The lift bridge consists of a rigid frame carrying the road and rest abatement, over each of which rises a steel-frame tower. The center span, which in existing bridges is as long as 585 feet (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 in 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 of hoisting machinery, bascule bridge spans are limited to about 250 feet, (27 m). The swing bridge is usually mounted on a pier in midstream and swung parallel to the stream to allow water to pass.
Military Bridges
In wartime, where the means of crossing a stream or river is lacking or the enemy has destroyed a bridge, 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.2