A. The beam or truss bridge is, in effect, a pair of girders supporting a deck spanning the gap between two piers. Such a beam has to withstand both compression in its upper parts and tension in its lower parts. Where it passes over supports, other forces come into play. A beam may be a hollow box girder or an open frame or truss. B. An arch bridge can be designed so that no part of it has to withstand tension. Concrete is well suited to arched bridge design. When reinforced concrete is used, a more elegant and sometimes less costly arch can be designed and most concrete arch bridges are reinforced.
C. A suspension bridge consists, basically, of a deck suspended from cables slung between high towers. The cables of high tensile steel wire can support an immense weight. The towers are in compression and the deck, often consisting of a long slender truss (used as a hollow beam), is supported at frequent intervals along its length.
D. A cantilever bridge is generally carried by two beams, each supported at one end. Unlike a simple beam supported at both ends, the cantilever must resist tension in its upper half and compression in its lower.
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