Empirical design of trusses

Trusses are often a convenient and economical way to increase the span capacity of wood or steel. Assembled of relatively short pieces of material into a system of triangles, a truss resists load by developing axial forces, compression and tension, within the bars of the truss. In building work, trusses are commonly used for spans of up to 60–80 ft or 20–30 m in commercial construction and for any roof spans commonly used in residential construction. In residential construction, trusses are usually assembled from pieces of 2 × 4 or 2 × 6, joined with gang-nail plate panel connectors, known as truss plates. These trusses are designed to be placed at a spacing of approximately 24 in. or 600 mm and concealed within the roof assembly. Larger trusses may be made of glued laminated wood (glulam), solid sawn wood, or steel and used in commercial or industrial buildings, and observe wider spacing. Some of the terminology used in describing trusses is illustrated in Figure 6.1.