A truss consists of a set of members that act in compression or tension as a unit, and truss bridges stay up because of the forces that pull outward on the ends of the tension member, forming a triangular structure that keeps the bridge in place. A triangle is the ideal shape to hold this tension because a triangle does not experience distortion from the pushing and pulling of stress.
When building a truss bridge, engineers put two long chords in place: these are straight members that form the bridge's top and bottom, and they are linked by a structure of diagonals and vertical posts. At both ends, the bridge has the support of abutments, and in many cases, piers also support the bridge in the middle. When designed effectively, a truss distributes stress through the entire structure, permitting the bridge to support not only its own weight but also the weight of traffic on it, as well as loads from the wind. While arch bridges provide support from beneath and suspension bridges provide support from above, a truss bridge strengthens the road itself, making it stiffer and holding it in place against the many different stresses it encounters while keeping itself in place.