Trusses - Guidance on overall concept - Buildings.

For pitched-roof trusses such as the Pratt, Howe or Fink, Fig. 19.1(a), (b) and (c), the most economical span-to-depth ratio (at apex) is between 4 and 5, with a span range of 6m to 12m, the Fink truss being the most economical at the higher end of the span range. Spans of up to 15m are possible but the unusable roof space becomes excessive and increases the running costs of the building. In such circum- stances the span-to-depth ratio may be increased to about 6 to 7, the additional steel weight (increase in initial capital expenditure) being offset by the long-term savings in the running costs. For spans of between 15m and 30m, the mansard truss, Fig. 19.1(d), reduces the unusable roof space but retains the pitched appearance and
offers an economic structure at span-to-depth ratios of about 7 to 8.

Common types of roof trusses: (a) Pratt – pitched, (b) Howe, (c) Fink, (d) mansard, (e) Pratt – flat, (f) Warren, (g) modified Warren, (h) saw-tooth
Fig. 19.1 Common types of roof trusses: (a) Pratt – pitched, (b) Howe, (c) Fink, (d) mansard,
(e) Pratt – flat, (f) Warren, (g) modified Warren, (h) saw-tooth

The parallel (or near parallel) chord trusses (also known as lattice girders) such as the Pratt or Warren, Fig. 19.1(e) and (f), have an economic span range of between 6m and 50m, with a span-to-depth ratio of between 15 and 25 depending on the intensity of the applied loads. For the top end of the span range the bay width should be such that the web members are inclined at approximately 50° or slightly steeper.

For long, deep trusses the bay widths become too large and are often subdivided with secondary web members.

For roof trusses the web member intersection points with the chords should ideally coincide with the secondary transverse roof members (purlins). In practice this is not often the case for economic truss member arrangements, thus resulting in the supporting chord being subject to local bending stresses.

The most economical spacing for roof trusses is a function of the span and load intensity and to a lesser extent the span and spacing of the purlins, but as a general rule the spacing should be between 1/4 and 1/5 of the span, which results in a spacing of between 4m and 10m for the economic range of truss spans.

For short-span roof trusses between 6m and 15m the minimum spacing should be limited to 3–4m.

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