Forces are everywhere. They lift us up and drag us down; they push us forward and pull us back. Whether it's getting out of bed in the morning, running a marathon, or hammering a nail, forces are at work in all the things we do. But forces are also acting on objects and structures that seem never to move at all.

Because we feel forces most obviously when we're very active, it may be surprising to learn that structures such as buildings and bridges withstand tremendous forces simply standing where they are built. A structure's own weight pushes down with nearly unimaginable force, while, at the same time, the ground supporting a structure pushes up with equal force. In addition, the weight and movement of equipment, vehicles, and people, and environmental factors such as temperature change, wind, and movement of the earth, combine to challenge structures further.

A structure's parts are constantly being squeezed, stretched, bent, and twisted. However, the design of these parts and their position relative to one another serves to balance these forces. The deck of a bridge, for example, is pulled downward by the force of gravity. The weight of the deck, in turn, pushes downward on the columns beneath it and pulls downward on the cables attached to it. In opposition, the columns and cables push and pull up on the deck with a force equal and opposite to that of the force of gravity. This keeps the deck and the rest of the structure in a state of equilibrium, and when forces cancel each other out in such a way, an object or structure maintains its shape and stays in place.