Bridges

The simplest type of bridge corresponds to the felled tree mentioned above. It consists of a single piece of material that stretches from one side of the gap to be bridged to the other side. That piece of material—the beam, or girder—rests directly on the ground on each side or is supported on heavy foundations known as piers. The length of a beam bridge of this kind is limited by the weight of the beam itself plus the weight of the traffic it has to bear. Longer beam bridges can be constructed by joining a number of beams to each other in parallel sections.

The concept of a beam bridge can be extended to make a stronger product, the continuous bridge. A continuous bridge differs from a beam bridge in that the latter has at least one additional point of support beyond the two found in a beam bridge. The longest existing continuous bridges now in use are the Astoria Bridge that crosses the Columbia River near Astoria, Oregon, and the Oshima Bridge that connects Oshima Island to the mainland in Japan.