Reading 3

SUSPENSION BRIDGES

1                                  The suspension bridge is a very ancient device that has long been used for foot traffic in many regions of the world. These simple foot bridges are of the type called catenary, a term referring to the curve or sag that a hanging cable takes under its own weight when supported only at its ends. In catenary foot bridges, the footway follows the natural curve assumed by the hanging cables. By the fifteenth century, the Chinese started using the catenary concept to suspend a level roadway, often using iron chains instead of bamboo rope in bridge construction.

2                                  The main parts of the suspension bridge are the roadway, the cables, the towers, and the cable anchorages. The level roadway hangs from the cables, which bear the load of the roadway and the bridge traffic. The cables pass over the towers and connect to the cable anchorages. If the cables were to end at the tops of the towers, their own weight combined with the weight of the roadway and the traffic would bend the towers toward each other. To prevent this from happening, the cables extend beyond the towers and are attached to concrete anchorages that are embedded in solid rocks. The weight of the cables creates a strong vertical force that is transferred into the ground at the anchorages and down through the tower foundations.

3                                  Suspension bridges of great size did not appear in the West until the nineteenth century. The first major suspension bridge in Europe was built across the Menai Strait in Wales. Designed by Thomas Telford and completed in 1826, the Menai Bridge is a massive structure in which the suspension units are heavy iron bars and rods. Sixteen huge chain cables, each made of 935 iron bars, support the bridge’s 176–meter span. On either side of the strait, towers constructed of limestone support the cables and the roadway. The original wooden roadway was later replaced with a steel deck.

4                                  In North America, the great pioneer of the suspension bridge was John Roebling, who designed the Brooklyn Bridge in New York. Completed in 1883, the Brooklyn Bridge was the longest suspension bridge in the world at that time, with a span of 486 meters, steel cables, two enormous granite towers, and a total length of 1,825 meters. The Brooklyn Bridge is a hybrid of the suspension bridge and the cable–stayed bridge, one of the oldest examples of either type in the United States. A cable–stayed bridge has one or more towers, each with a number of cables connected to the bridge deck on both sides. The primary load–bearing structures are the towers, which transmit the bridge load down into the ground, with less force placed on the anchorages.

5                                  In the twentieth century, suspension bridges achieved greater spans and more graceful designs. One of the best known examples is San Francisco’s magnificent Golden Gate Bridge, completed in 1937. Several engineers worked on the project, although Charles Ellis is given credit as the principal designer. The scale of the Golden Gate Bridge is greater than that of any other suspension bridge at the time. The length of the main span is 1,300 meters, the height of the towers is 64 meters from the surface of the water, and the width of the roadway is an impressive 27 meters. Each of the cable anchorages has three main components: the base block, which is set in the bedrock; the anchor block, which is keyed into the base block; and a weight block, which rests on top of the anchor block. The great weight of the anchorages counteracts the pull of the cables.

6                                  The bridge designers created a structure that would control, rather than try to prevent, the movement caused by high winds and extreme temperatures. Each of the two towers is constructed with cross–bracing to stiffen the structure against the force of the wind. The ends of the roadway assembly are hinged to the towers, enabling the entire structure to rotate slightly on a horizontal plane. Sets of trusses along the roadway serve to stiffen the curvature of the cables and to minimize bending caused by wind pressure. The bottom corners of the stiffening trusses rest on flexible vertical columns called rocker arms. In very hot weather, the roadway will expand, and the ends of the bridge will slide toward the towers. To prevent this movement from affecting cars on the bridge, expansion joints are built into the road surface on both sides of each tower.