Timber structures
Fig. 6.3 Primary-secondary
system. The primary
elements here are laminated
timber portal frames. These
carry purlins which in turn
support the roof and wall
cladding.
the full width of the space, are trusses. Purlins
(secondary elements) span between these and
support rafters, which are tertiary elements. A
characteristic of the hierarchical arrangement
of elements is that the primary elements carry
the greatest amount of load and must normally
therefore be relatively complex built-up
arrangements such as triangulated trusses. The
secondary and tertiary elements are closely
spaced and carry relatively small amounts of
load across small spans, as in the case of the
purlins in Fig. 6.3 and the rafters in Fig. 6.44.
The elements of which timber structures are
composed may take a number of forms (Fig.
6.4). The simplest of these is the solid beam.
This carries load by pure bending action and is 181
