Forces at Work: Arch Bridges

Arch bridges are subject to the two basic forces – compression and tension – that are in effect for all bridges. In an arch bridge, the tension is so minimal that we say it is only in compression. This is because the force at work in an arch bridge is the squeezing force (–> <–)of compression. The pulling force of tension (<– –>) is negligible. How is this possible?

archbridge PBSThe arch stays together by being squeezed. This squeezing, or compressive, force moves outward along the curve of the arch. The arch can’t spread very far because it is being pushed back by the abutments  – or legs – that serve as the foundation.

The compressional stress of the arch bridge is dissipated along the abutments and into the earth. If this were not the case, the bridge would collapse.

43890771 – illustration of an igloo

Let’s look at it in a less scientific way. At the entry of the igloo, you see an arch. It has a keystone – that block of ice that is trapezoid in the shape of a four-sided triangle. This keystone is the piece that secures the sides of the entrance. As they were built up and then across the top of the entrance – probably using falsework to support their weight and hold them in place until the keystone was put in place. If you were to climb atop the entranceway, the force of your weight would be distributed down the sides of the entranceway. Likewise, the igloo itself is an arch that uses the sides to support the structure. There is a key component in the roof – the keystone – that keeps all the forces in check.

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More: Types of Bridges: Arch More about bridges at PBS

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