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A truss girder is a beam made of a truss, which is a network of bars connected by pin joints that can only experience tension or compression. Trusses have a high strength-to-weight ratio and are cheaper to build than solid girders, making them popular in bridge construction.
In structural engineering, a truss girder is a beam made up of a truss. A truss is a network of bars that experience only tension or compression. This is made possible because the bars are connected to each other using only pin joints, which allows the bars to rotate. A beam is a type of structural member designed to resist bending. Truss girder is a popular component of bridges due to its efficient use of materials.
A truss can be thought of as a skeleton of straight bars that have a particular kind of connection to each other. Each bar is connected with a pin joint, which allows it to rotate in any direction. The effect of using only pin joints is that each rod can only be in tension or compression. They cannot be bent, because a bent rod will simply relax if its end is allowed to rotate. When connected in triangles, these bars form a rigid lattice structure that serves as a solid piece.
The advantage of a truss is its strength-to-weight ratio. Typically, most of the space inside a trellis is empty; it is the skeleton of bars that forms the structure. The relatively high strength of a truss can be maintained because the bars are subjected only to tensile or compressive stresses.
A beam is used specifically to resist bending by deforming into a circular arc shape. The inner circumference of a beam in bending is compressed to a dimension less than the length of the beam. The outer circumference, on the other hand, is stretched to a greater dimension than the original length of the beam. Both of these deformations serve to resist the applied bending. Unless permanently damaged, the beam will tend to revert to its “natural” state.
A truss beam can be used in the same way as a solid beam. If a truss is built to the same outside dimensions as a beam, it may be subjected to the same bending loads. The truss girder, which would consist of triangles of bars with pin joints at the ends, would resemble the same circular arc shape as the solid girder. Here too the bars near the inner circumference would be compressed. Combined with the tension of the bars near the outer circumference, the truss would be able to resist bending.
Engineers might choose to use a truss girder in a bridge because it is cheaper and easier to build than other designs. As with all trusses, a truss beam is a very efficient way to use materials. The cost of raw materials, manufacturing and transportation can be much lower with a truss girder than with a solid girder. It may also be more feasible to build certain structures, such as bridges, with trusses.
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