TRUSS, in engineering and architecture, a combination of structural members, usually straight, and so arranged in one plane that by being connected at their ends they form a rigid frame on which to support a roof, floor, bridge or other similar weight. In the theoretically perfect truss the component parts are so designed as to enclose between themselves rigid triangles and so arranged that every member is stressed only in the direc tion of its own length, either in compression or tension, with no bending forces present. This, the controlling ideal of modern truss design, governs in the design of those trusses which are purely structural, or designed primarily from the utilitarian standpoint. In ordinary architectural practice the exigencies of individual building problems, or the demands of aesthetic effect frequently render complete following out of the engineering theory impractical, and in one important class of architectural trusses, the hammerbeam (q.v.) trusses, considerable side thrust
is exerted upon the supporting walls or piers. Although the basic principle of truss design, that of the rigidity of a triangle, is so simple, the scientific design of trusses is a matter of comparatively recent development; and in the greater number of classic, Roman esque and Gothic trusses, despite the ingenuity of their construc tion and, especially in late Gothic work, the beauty of their design and execution, it is only rarely, and as it were by accident, that a perfect triangulation of the members is achieved.
A truss diagram is a graphic method of discovering the stresses in the component members of a truss, based upon the mechanical theory of the resolution of two angular forces into the diagonal of the parallelogram constructed upon them. (See BRIDGES; MECHANICS ; ROOFS.)