A beam bridge is basically a rigid horizontal structure that rest on two piers, one located at each end of the bridge. A simple beam bridge is flat across and supported by two ends or abutments. The beam bridges can be made longer by placing piers or towers in the middle of the bridge to support the beam structure and extending the support into the solid substrata below the surface.
Another way engineers make the beam bridge longer or have more span and accommodate more weight is to allow the deck to set upon a truss system. A truss system is composed of triangles and can support heavy loads with its relatively small weight. The beam must be strong enough so that it doesn't bend under its own weight and the added weight of the traffic crossing it. When the load pushes down on the beam the top edge is pushed together or compressed, while the bottom of the beam is stretched or is under tension. The force of compression on the upper side of the beam causes it to shorten because of the load pushing the beam inward. The result of the compression on the on the topside of the beam causes tension in the lower part of the beam. Tension causes the lower part of the beam to lengthen. The middle of the beam bridge experiences very little tension and compression.
There are several different types of beam bridges. One type of beam bridge is the truss bridge. A truss bridge is a series of connected triangles that distribute the weight or load to each member of the truss. It consists of a top chord, bottom chord and web members. The truss bridge is lightweight, but very strong due to the open triangular members along its sides. Two special features about the truss bridge are that the members that make up the triangles or diagonals do not bend. Secondly, a truss bridge is more efficient because the individual members carry axial load and minimize any bending. Therefore it requires less material than a simple beam bridge. The members get pulled apart during tension and pushed together during compression. While the simple beam bridge consists of a solid web member to carry the load, the truss has a top chord that is in compression and the bottom chord typically in tension. The other members of the truss are in different stages of tension and compression. As heavy loads travel across the bridge it may deflect vertically in the middle due to the individual members of the truss reacting to the forces of compression and tension.
There are many different types of truss. The design, location and composition of the truss determine the type. For example, the Warren Truss, Pratt Truss and Howe Truss differ based on the arrangement of the triangles. Their identifying names are attributed to the engineers who invented each of these particular arrangements.
The cantilever bridge is another type of beam bridge. This kind of bridge is supported on two levers that are continuous over piers. A simple cantilever bridge consists of two cantilever sections with abutments on each side to act as counterweights. The downward force at the center of the cantilevered end of the bridge is counteracted by the weight of the continuous adjacent span anchored at the far end. The opposite ends called arms reaches out and meet in the center. There is also a pier that supports each cantilever arm. The cantilever bridge can be made even longer by adding an additional simply supported section to the middle of a cantilever bridge.