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Types of Bridges
Suspension bridges are the longest-spanning bridges in the world. Prior to the record set by the Golden Gate Bridge in 1937, 4,200 ft (1,280 m), the longest-spanning bridge in the world was the George Washington Bridge, with a span of 3,500 ft (1,067 m). Today, the longest-spanning bridge in the world is the Akashi Kaikyo Bridge in Japan, with a span of 6,352 ft (1,991 m).
Arches were often used in ancient times to form bridges. Arches carry load primarily in compression (resisting pushing forces.) They can be made out of masonry (stones or bricks) because those materials are strong in compression. Each stone or brick pushes on its neighbor. Masonry is also durable, and most locations have a natural supply of stone that can be cut into shapes or clay that can be hardened with fire into bricks. The Romans built many arch bridges, such as the multi-level Pont du Gard that can be visited in France. That bridge served as an aqueduct to carry water over a river valley. Water ran in a trough at the top level of the bridge.
Cable-stayed bridges have straight cables extending from towers to hold up the roadway deck. As those cables (stays) pull up on the deck, they also pull toward the mast, and thus the roadway deck structure must be strong enough to resist a large compressive force. Crossing the Millau Viaduct in France, cars drive 1,132 ft (343 m) above the valley below.
The straight members that make up a truss are struts or "sticks" of steel in modern bridges. The struts form triangles, which are inherently rigid. Truss bridges carry weight effi¬ciently because each of their straight members is either pulled (in tension) or pushed (in compression). The tension or compression flows straight through the length of each member, without causing it to bend or flex. When the Firth of Forth Railroad Bridge was completed in Scotland in 1890, it was the longest-spanning bridge in the world (1,710 ft, 521 m) and one of the first to be made of steel rather than iron.
Beam bridges were invented when people first placed logs over streams and walked across. Most short-span bridges crossing roads or highways are supported by steel or reinforced concrete beams. If the beam only rests on supports at both ends, it tends to sag as it bends or flexes. This stretches the material (makes it feel tension forces) along the length of the beam's lower edge. Concrete is good at resisting compression (pushing) but is weak in tension. A concrete beam resting on supports at both ends, as shown here, needs steel reinforcing bars or steel cables embedded along the beam's length near its lower edge to resist tension. |