Bridges

Three kinds of forces operate on any bridge: the dead load, the live load, and the dynamic load. The first of these terms refers to the weight of the bridge itself. Like any other structure, a bridge has a tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is constructed (i.e., the wood, concrete, steel, or aluminum). The second term refers to traffic that moves across the bridge as well as normal environmental factors such as changes in temperature, precipitation, and winds. The third factor refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes. All three factors must be taken into consideration in the design of a bridge.

For example, suppose that it is necessary to build a bridge across a span that is 325 ft (100 m) wide. It would not be possible to build a beam bridge, one that consists of a single slab of steel 325 ft (100 m) long, of that length. The weight of the material used to construct the bridge plus the weight of the traffic on the bridge would be too great for the bridge to remain standing. An engineer would have to design some other kind of bridge-an arch or suspension bridge, for example-that would be able to hold up that amount of weight.