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The Stone Arch Bridge

The semicircular arch developed by the Romans was a great technological achievement in architectural design. The stability of this true (or voussoir) arch depends on the compression between its wedge-shaped stones. (That is, the stones are forced to squeeze against each other.) This results in horizontal outward forces at the springing of the arch (where it starts curving), which must be supported by the foundation (abutments) on the stone wall on the sides of the arch. It is common to use very heavy walls (buttresses) on either side of the arch to provide the horizontal stability. If the foundation of the arch should move, the compressive forces between the wedge-shaped stones may decrease to the extent that the arch collapses. The surfaces of the stones used in the semicircular arches constructed by the Romans were cut, or "dressed,'' to make a very tight joint; it is interesting to note that mortar was usually not used in these joints. The resistance to slipping between stones was provided by the compression force and the friction between the stone.

How arch bridges are built

Arch bridge diagram
The wooden form, or template (in brown). The stone is built up on top of the form until the arch is complete, after when the form is knocked out or, sometimes, burnt away

Building an arch bridge isn't easy, since the structure is completely unstable until the two spans meet in the middle. For years, engineers used a technique called centering, in which a wooden form supported both spans until they locked together at the top. Before an arch can be built a wooden frame is built. This allows the stones to be put in position. The frame is removed after the stones are in position. A newer method today supports the spans using cables anchored to the ground on either side of the bridge.

Arched Bridges

Arched Bridges are one of the oldest forms of bridges. An arched bridge channels the weight from above to the foundations below. Arch bridges don't need additional supports or cables - they will stand on their own for a long time. In fact, there are arch bridges still standing today that were built by the Romans!

An arch bridge has a semicircular design with abutments on each end. An arch is known as a compression structure. Stone arches are built over a temporary wooden form called a centering. Once the last piece of stone, called the keystone has been set in place, the arch is pretty much finished. But if the centering is removed before the keystone is set, the uppermost stones would want to fall, and this would push the sides of the arch outward. To prevent this from happening, more material must be built up against the sides of the arch before the centering is removed. This prevents movement between the stones, and also compresses them together. The more the wedge-shaped stones are compressed, the stronger the arch.

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