In structural engineering, a truss beam is a beam made from a truss. A truss is a network of bars that only experience tension or compression. This is made possible because the bars are connected to each other using only pin joints, which let the bars pivot. A beam is a type of structural element designed to resist bending. The truss beam is a popular component of bridges because of its efficient use of materials.
A truss can be thought of as a skeleton of straight bars that have a particular type of connection with each other. Each bar is connected with a pin joint, which allows it to pivot in any direction. The effect of using only pin joints is that each bar can only be in tension or compression. They cannot be bent, because a bar in bending will simply relax if its end is allowed to pivot. When connected into triangles, these bars form a rigid truss structure that acts as a solid piece.
The advantage of a truss is its strength-to-weight ratio. Typically, most of the space within a truss 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 only subjected to tension or compression 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 smaller size than the length of the beam. The outer circumference, on the other hand, is stretched out to a larger size than the beam’s original length. Both of these deformations serve to resist the bending applied. Unless permanently damaged, the beam will tend to return to its “natural” state.
A truss beam can be used in the same way as a solid beam. If a truss is built with the same outer dimensions as a beam, it could be subjected to the same bending loads. The truss beam, which would consist of triangles of bars with pin joints at the ends of them, would resemble the same circular arc shape as the solid beam. Here, also, the bars near the inner circumference would get compressed. Combined with the tension of the bars near the outer circumference, the truss beam would be capable of resisting bending.
Engineers might choose to use a truss beam in a bridge because it is more economical and easier to construct than other designs. Like with all trusses, a truss beam is a very efficient way to use materials. The cost of raw materials, manufacturing and transport can be much lower with a truss beam than with a solid beam. It may also be more feasible to build certain structures, such as bridges, with truss beams.