Different materials and shapes are used in bridge construction. The material and shape used depends on the distance the bridge must cover and the amount of traffic and elemental forces it must withstand. Most bridges are constructed using at least two supports that are set into the ground. These supports, called abutments or piers, help to hold the bridge up. The distance between two supports is called a span. Bridges supported by two piers or abutments, are called single-span bridges. Longer bridges with more than two supports are called multi-span bridges.
Beam bridges, also called girder bridges, are the most simply designed type of bridge. Typically, beam bridges are constructed with beams that run horizontally with each end of the beam supported by piers. The beam's weight pushes directly down on the piers. When a load presses down on the bridge's beam, its top edge is pushed together while the bottom edge is stretched. The beams used in this type of bridge must be extremely strong so that it doesn't bend under its own weight or the added weight of traffic crossing over it.
One of the oldest types of bridges are arch bridges. Instead of pushing straight down on the supports, the weight of this type of bridges travels outward along its curve to abutments at each end of the bridge. These carry the load and keep the ends of the bridge from spreading out. The span of each arch can range up to 1,700 feet.
Suspension bridges can span 2,000 to 7,000 feet, which is longer than any other type of bridge. Light and aesthetically pleasing, these bridges are also often the most expensive to build. The roadway of a suspension bridge hangs from large main cables that extend from end to end. These cables are placed on top of towers and secured at each end by anchorages (massive constructions of concrete or masonry). A large portion of the bridge's weight travels through the cables to these anchorages. The cables are distributed over a large area inside the anchorages to prevent them from breaking free from the concrete and to evenly spread the load.
Similar to suspension bridges in that they use towers and are suspended from cables, cable-stayed bridges, however, support the load they carry differently. In this type of bridge construction, the towers to which the cables are connected withstand the load entirely. Cable-stayed bridges attach the cables to the roadway in either a radial or a parallel pattern. In a radial pattern, the cables run from several points on the road to one point on the tower. In a parallel pattern, the cables are connected to the tower at different heights along the tower and run parallel to each other.
