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"London Bridge is falling down, falling down, falling down." We all know the first verse to that nursery rhyme. But have you ever heard this one: "Set a man to watch all night, watch all night, watch all night"? People once felt that a bridge required a human spirit. They sometimes buried a human sacrifice in the bridge's foundation, so that the spirit could "watch all night." Fortunately, 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. Span bridges consist of beams and/or trusses resting on supports or piers. When the span length would require a very large or heavy beam to support the loads, a truss system is often used. Since a truss is composed of triangles (the strongest polygons because their shapes cannot be distorted), a truss bridge can support heavy loads with its relatively small weight. A suspension bridge hangs from cables firmly anchored at each of its ends. Towers positioned at regular intervals along the span also support it. The main elements of suspension bridges, the cables, are in tension. The trusses hang from the cables and the trusses, in turn, support the deck. Trusses in these bridges provide stiffness to their decks. Suspension bridges are generally used for long spans. Engineers need to accommodate torsion in their bridge designs. What causes a bridge to twist? Wind. An example of torsion was the Tacoma Narrows Bridge in the state of Washington. A suspension bridge without open trusses, this bridge twisted in the breeze. One day, the bridge twisted so violently in the wind that its center span collapsed. Since then, engineers test bridge models in wind tunnels prior to construction and build their decks more stiffly. When determining what type of bridge to build, engineers evaluate terrain and length of span. In addition, bridges change as our needs and resources change. Engineers constantly look for ways to improve bridge designs and materials. During the recent earthquakes in California, for example, engineers analyzed highway overpasses to find ways to make them stronger. Why is it important to know whether parts of a bridge will be subjected to tension or compression?