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What is the purpose of a bridge? To provide a safe, stable road surface over a waterway or ravine. A bridge supports the load of the vehicles that move across it. Because loads are moving and constantly changing, they’re known as live loads.

A bridge must also support another load — its own weight. This is known as a dead load. Dead loads always remain constant.

Both live and dead loads exert forces on the structure of the bridge. Two important forces are compression and tension.