Introduction

A truss, or lattice structure, is a structural assembly of small interconnected elements.

Trusses are formed by an interconnected assembly of relatively small elements, which create a lattice arrangement. The overall form, size and shape of the truss is as important as the strength of the individual components, and a wide variety of design options is available.

A truss acts like a beam, with bending resisted by the couple created by forces in the top and bottom members.

When an I-shaped beam is subjected to simple bending it can be seen that the bulk of resistance to bending moment is offered by a couple consisting of the forces in the flanges multiplied by the distance between them. Accepting that little error is involved in assuming that all resistance to bending is offered in this way, the most efficient system will be one in which the flange forces are reduced to a minimum to save material, and the distance between increased accordingly.

Shear forces are resisted by the internal members.

This, however, leaves the problem of accommodating shear forces which, in the case of the I-beam, were absorbed by the web. To continue to provide a solid web would waste a lot of material. As an alternative, discrete members running diagonally, and sometimes vertically, are used to carry the shear. The resulting system, given the generic name of a truss, will carry loads over large spans with great economy.

Trusses are structurally efficient and particularly popular for long span roofs.

Trusses are highly efficient, able to span considerable distance, and they are visually light. The economy in use of materials has led to the use of truss systems in preference to beams in a large number of situations, both in the vertical and horizontal planes. They became popular solutions to the problem of roofing in large clear areas such as factories, workshops and railway stations. In these situations the increased structural depth inherent in truss structures was not unduly punitive. In contrast, their use as floor support systems was hampered by their increased depth.

Trusses are typically used for spans of between 10 - 100m, with a span:depth ratio of between 10:1 and 15:1.

There are practical limits, and it is generally recognised that steel trusses are a viable structural form for spans from 10 metres to 100 metres. Typically parallel boom trusses have a span to depth ratio of approximately 15:1 for light loading to approximately 10:1 for heavier loading. For other shapes of truss the overall depth is most often a function of the roof slope.