Bridge Design

A pedestrian bridge.

There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like rivers, streams, valleys, and railroad tracks. But do you know how they work? Engineers must consider many things — like the distance to be spanned and the types of materials available — before determining the size, shape, and overall look of a bridge.

Since ancient times, engineers have designed three major types of bridges to withstand all forces of nature: beam bridges, truss bridges and suspension bridges.

The beam bridge…consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet. NOTE: The blue arrows indicate tension (pulling apart), the red arrows represent compression (pushing together).