Large structures, like bridges, buildings, and dams, remain standing because they are designed to resist the forces exerted on them. These forces come in many forms. Of course, the water of a reservoir exerts force on a dam, and cars push down on the deck of a bridge as they pass over. But other things, including the weight of the structure itself, wind, snow, and rain, and changes in temperature that cause materials to expand or contract, can all apply force or change the force applied to a structure's parts.

The internal and external forces that structures must counteract to remain standing are commonly called loads. Loads fall into two broad categories: static and dynamic; and static loads are further subdivided into dead loads and live loads. A structure's dead load includes its own weight and the weight of anything permanently attached to it. Live loads include the forces a structure bears during normal use -- the weight of cars, trucks, and office equipment, for example -- and normal weather conditions -- including low to moderate winds, temperature fluctuations, and precipitation.

Dynamic loads are events that have a sudden impact on structures. They include earthquakes, sonic booms, and very large waves. These loads are often unpredictable. Still, unpredictable events do happen, and engineers must design structures that have reserve strength to counteract these forces. This reserve of strength is called a structure's "margin of safety."