A truss is an assembly of interconnected triangles most commonly used to strengthen bridge decks, which are subject to great stress. Horizontal bars, called chords, run along the top and bottom of the length of the deck, usually on both sides. These upper and lower chords are joined by diagonal and sometimes vertical braces -- called struts and ties, respectively -- which create triangular configuration. A triangle is a particularly rigid shape that will not bend, twist, or otherwise deform because a force pulling on one of the three sides is balanced by pushing forces on the other two sides. Trusses derive their strength from triangles: the bigger its triangles, the more strength and rigidity a truss will provide.

When the force of a load is exerted on a truss bridge -- as when a train crosses over it -- the upper chords experience compression, as pushing forces act to shorten that surface. The lower chords, by contrast, are stretched or lengthened by a pulling force called tension. The diagonal and vertical braces that connect the upper and lower chords carry these forces so that tension or compression acting on one member of the truss is countered by a balancing force acting on an adjoining member. In this way, the load is distributed evenly through the truss' framework design.

Trusses have many advantages over girders and solid beams, which can also provide stiffness to a structure. A truss is especially useful because its construction is so lightweight. Since the individual pieces are small, trusses can be built even in places where heavy equipment cannot be used. Trusses can support heavy loads over long spans and can be incorporated into almost any bridge design. Trusses create a hollow skeletal structure, so that a roadway may pass either over or through it, leaving room below. In addition, a truss offers a minimum of surface area on which wind can act, thereby diminishing another force that could potentially destabilize a structure.