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Compression and Tension
The beam bridge is able to remain stable despite the weight of the cars, people or trains passing over it, because there are two opposing forces at work in the bridge and they balance each other. The weight of the bridge's traffic pushes down on the top surface and compresses it. At the same time, the bottom of the bridge expands under the tension. The two forces are in balance, so the bridge remains stable.
Length Between Spans
The balance between compression and tension in a beam bridge will only work for bridges of about 250 feet or less between the piers or support pillars -- any more than this and the forces become too great, destabilizing the bridge. By comparison, an arch bridge can be up to 1,000 feet between spans, and a suspension bridge can be 7,000 feet between spans. So there is a limitation to the beam bridge design. However, this limitation can be minimized by using the strongest shapes for the beam.
Trusses
The possible span of any beam bridge increases up to a point as stronger and heavier beams are used. However, if the beam turns out to be too heavy, it can collapse. Triangles are stronger shapes than squares because the sides support each other against any force exerted against one of them. Beam bridges can be constructed out of triangles or "trusses." A truss bridge is much stronger than an ordinary beam bridge.
Cantilevers
A cantilevered bridge is a more complex type of beam bridge. Cantilevered bridges have diamond-shaped support structures between the piers and each of these has additional straight supports with diagonal supports of their own. Every part of the structure helps to hold up every other part of the structure, allowing the bridge to span a greater distance. The cantilevered bridge is based on the principle of a lever and a fulcrum, with each of the bridge piers acting as a fulcrum for two levers, one on each side.