The truss is an engineering design that dates back thousands of years. A planar truss design is a simple triangle, or connected triangles, while a space truss has members that extend in a different direction to establish a third dimension. Trusses are used for framing bridges, roofs, floors, subfloors, and many other structures. The best truss design typically depends on the application.
A planar truss design is used for bridges and framed roofs. For a roof, the triangles connect the roof rafters and the ceiling joists; the load is spread between the rafters and joists. For a bridge structure, the top and bottom of the joist are parallel and are called chords. Solid girders to support the load on a bridge would add greatly to the weight and cost; the truss supports the load at varying degrees, depending on how far apart the chords are spaced.
Tried-and-true planar truss designs include the Pratt truss — with boxes that contain one diagonal member per box — and the king post truss — with one vertical member and two angled members connecting to the top of the vertical. Queen post trusses have two vertical members and a top chord comprising a box that are supported by one angled member on each vertical. Lenticular trusses, commonly seen on bridges, have the top chord of the truss running at a gentle arch, thereby giving a lens shape to each structure of the truss.
For a space truss, a tetrahedron or pyramidal shape is the most common truss design. More complicated space trusses will link and configure tetrahedrons in various ways to distribute load, torsion, compression, and tensile forces. Space truss designs can be seen in structures, such as high-tension line pylons. Also, space truss webs are used for commercial building roof structures.
Analysis of truss designs is a complex process, but usually analysis is done at the joints, or hinges, of the structure rather than along the length of the members. The load that is exerted by a passing car or train, for instance, is considered a moment in which the stresses are mainly on the joints and the stress exerted along the lengths of the members are negligible. Graphical diagrams, such as the Cremona diagram or Culmann diagram, are used to calculate the stresses on the truss members themselves. To further examine these types of loads, formulas such as the analytical Ritter method can be used.
