What Is an Arch Bridge?
An arch bridge is a type of bridge that relies on a curved, semi-circular structure for its support. Unlike post and beam bridges, which support loads on columns placed in the water along the entire bridge length, an arch bridge can span great distances while still leaving wide openings for water flow and water traffic. Dating back to ancient Greece and Rome, this bridge structure has been in use for thousands of years. In fact, because this type of structure is so strong, many ancient bridges can still be seen today in the cities of Europe and Asia.
The strength of the arch bridge comes from its shape. As the arch presses downward from its top, the ground pushes upward with an equal force. Extra support is also often placed along the abutments, or sides, of the structure to keep the arch from bending outwards when heavy loads press down on it. As a result, the arch is always under compression, causing it to become rigid and maintain its strength.
To be able to withstand the strong compression forces that help the bridge keep its shape, the arch bridge must be built out of materials that can handle such compression. Ancient Romans used cut stone, one of the best materials for withstanding compression, to build their bridges. Today, stone and brick are still used in bridge construction. Materials such as steel and poured concrete, which can be more easily shaped, are now also being used.
An arch bridge is initially constructed around a wooden form that helps maintain the arch shape during construction. Ancient stone bridges relied on the final top stone, or keystone, to be put into place for the bridge to experience the compression that would make it strong. Supports along the side of the bridge also help keep its shape as loads press down from above. Once the complete arch is in place, the temporary wooden structure can be removed.
Modern bridges that rely on reinforced poured concrete can have their temporary wooden form removed once the concrete sets. Some of these new structures can maintain their strength from the reinforced concrete of the arch. In this case, no additional supports are needed along the sides.
There are many types of arch bridges. One traditional arrangement has the roadway on top of the arch, while in others the arch rises above the roadway. Arch bridges used for aqueducts or certain roadways can even be built on top of each other to span a much higher elevation.
Written by Anna Brooks
