The Romans were some of the most important innovators in structural design. Of their contributions, the arch and the bridges they built using this elegant shape stand out as the most creative and enduring. In this video segment adapted from Building Big, series host David Macaulay describes the forces and design features that give arches their strength.
Arch Bridge (Audio Description) (Video)
The simplest type of bridge is called a beam bridge. As the name implies, beam bridges are made up of long horizontal supports, called beams, which stretch from one side of a gap to the other. These supports are anchored to solid ground at each end and carry the entire weight of the bridge deck and its loads. Because of this, the distance a beam bridge can span is limited unless it is reinforced underneath by evenly spaced vertical columns, called piers. In a long bridge, however, the piers obstruct the flow of water and traffic along a waterway.
While the beam bridge relies on an abundance of material to span long distances, the arch provides a much more elegant solution. It arcs high above the gap it spans, leaving shipping lanes unobstructed. Despite the arch's delicate appearance, it is remarkably rigid and strong, especially with the extra support commonly placed along its sides.
An arch derives its strength directly from its shape. Downward force from the top of an arch is carried along the curving form all the way to the base. At the same time, the ground pushes up with equal force. As a result, each of the arch's sections are tightly squeezed, or compressed, by adjacent sections, making the structure very rigid. In addition, the curvature of the arch causes the lower sections to push up more steeply than the sections above them push down. This difference in vertical force between upper and lower sections allows an arch bridge to carry loads in excess of its own weight.
In the construction of an arch bridge, especially a very large one, the choice of materials is critical. Not all materials are equal in their ability to withstand compression, the force that squeezes a material from opposite sides, and the force that is most important in an arch bridge. Despite its curved shape, the individual components of an arch bridge undergo very little bending and stretching. They do, however, experience tremendous compressive forces, which are generated by the weight of the load above and the resistant force of the ground below. For good reason, the Romans chose stone for their arch bridges. Very few materials have greater compressive strength than cut stone. Contemporary builders use concrete for arch bridge construction because it can be readily molded into a variety of shapes and it is nearly as strong as, and much less expensive than, stone.
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