David finds out that a properly designed bridge made from dry pasta can actually support his weight. We all know the first verse to that nursery rhyme. People once felt that a bridge required a human spirit. Fortu-nately, we no longer sacrifice a night watchman when we build these structures. But there is still more to bridges than meets the eye.
Bridge builders choose from one of several types of bridges or combine two or more.
Three basic types are the arch, span, and suspension. The structural elements in a bridge are subjected to various compression and tension forces. Something bearing weight or being pushed together is under compression while something being pulled apart is under tension. In an arch bridge, compression pushes the weight away from the arch and against the side walls and the stones of the arch itself. The Romans were first to build arch bridges, and some of their bridges and aqueducts still stand today.