due to the infrequent spacing of the primary
structural elements. So far as the overall
geometry of trusses is concerned, any shape is
theoretically possible so long as a stable
configuration is adopted. The most favoured
shapes are the traditional pitched truss and
the parallel-chord truss (Fig. 6.28). The
mansard shape is a commonly used variation
on the latter. Bowstring shapes are also used
but these are more expensive to construct
(especially if they are at the higher end of the
span range). Typical span ranges for all of
these types are given in Table 6.9.
The internal geometry of a truss must be
stable and this is achieved by adopting a fully
triangulated arrangement. Where, for any
reason, these types of geometry cannot be
adopted, stability can be provided by continu-
ity of elements through joints (Fig. 6.29). For
efficiency in the use of material, internal
angles of around 60° are best and angles which
are less than 30° are avoided. Economy in
construction results if the total number of
joints is minimised, but this must not be
achieved at the expense of ill-conditioned
triangles (i.e. triangles with angles less than
30°) or the creation of excessively long
compression elements. Adoption of a small
number of joints also reduces the deflection
which results from slipping at the fastenings.
Timber structuresFig. 6.28 Typical configurations for large one-off timber
trusses.Such slipping occurs with all types of mechan-
ical fastener and can increase the total deflec-
tion of the truss significantly.
The overall depth of a timber truss must be
such as to give a fairly small ratio of span to
depth (Table 6.9) and this is particularly
important where the span is long (greater thanPitchedParallel chordBowstring10-3015-4515-7558-106-8
207Table 6.9 Typical span ranges and proportions for one-off timber trusses
Configuration Span range (m) Span/depthPitched Pratt trussParallel chord Pratt truss Mansart trussPitched fink trussBowstring truss