Bridges and Other “static” Structures

How can you reduce the bending of a beam bridge?

The simplest way to keep the roadbed from bending is to use a king post. In the illustration below, the downward force of the center of the bridge pulls down on the vertical post. This places the diagonal braces under compression. They transmit the force to the piers. The upward force on the post makes the net force on the post zero. It is under tension.

While the king-post bridge can reduce the bending in the center of the bridge, it can do nothing about bending between the pier and the bridge center. One solution is to add a second vertical post and connect the two by a horizontal member, creating a queen-post bridge. But a method that allows much more support on a longer bridge is the truss.

Similar methods of translating downward forces to compression forces exerted on the piers on the ends of the bridge were known to the Romans who worked in stone and concrete. They were famous for the arches used in their massive aqueducts. One such aqueduct, the Pont du Gard, was completed in 18 B.C.E. and was used to carry water a length of 270 meters (886 feet) over the Gardon river valley in southern France.


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