Practical considerations

Real circumstances are generally different from the idealised conditions assumed in the analysis of trusses.

The basic theory behind truss analysis assumes a number of features which are not practical in reality. These concern principally the position of loads and joint details.

Loads rarely coincide with joints, and certain load conditions can cause forces in otherwise unloaded members.

The strength of the roof sheeting governs the best spacing of the supporting purlins. It is possible that this spacing will not coincide with the desired truss joint positions. Under such circumstances, bending occurs within the truss member. This is best avoided since it increases the size of the member involved.

There is a great temptation to assume that if a member is not loaded, it need not be included in the truss. However, it is usual for several load cases to be considered in design and it is unlikely that any member will be unstressed under all possible load conditions.

Joint details are rarely pinned, and the main chords are often a single continuous member.

To simplify analysis, all joints are assumed to be pinned, and the centroid of all members must meet at the centre of the joint. In practice there is no such thing as a perfect pin and the physical positioning of members will not always satisfy the latter condition. However, experience shows that an analysis based on such simplifying assumptions gives acceptably accurate results. In addition support conditions are idealised as pinned, with one of the two supports also free to move sideways (a 'roller' support). Again these ideal end conditions are not realised in practice but such an assumption enables a relatively simple and sufficiently accurate solution.