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184 Barry’s Advanced Construction of Buildings
Eaves bracing
at tie level
Rafter
bracing
Gable wind girder between
ties of roof trusses
Valley beam
Longitudinal
ties at tie level
Eaves bracing
at tie level
Rafter
bracing
Figure 4.28 Wind bracing to steel truss roof on steel columns.
necessary to use a system of eaves bracing to assist in the distribution of horizontal loads.
The system of eaves bracing shown in Figure 4.28 consists of steel sections fixed between
the tie or bottom chord of roof frames and columns. To transfer the loads from wind pressure
on the gable ends, a system of horizontal gable girders is formed at tie or bottom chord
level.
Structural bracing and wind bracing
Additional bracing is used to assist in setting out the building, to stabilise the roof frames,
square up the ends of the building and offer additional resistance to the wind. The rafter
bracing between the end frames, illustrated in Figure 4.28, serves to stabilise the rafters of
the roof frames. Longitudinal ties between roof frames stabilise the frames against probable
uplift due to wind pressure. The vertical bracing in adjacent wall frames at gable end corners
hold the building square and serve as bracing against wind pressure on the gable ends of
the building (Photograph 4.5).
Purlins and sheeting rails
Purlins are fixed across rafters and sheeting rails across the columns to provide support
and fixing for roof and wall cladding and insulation (Figure 4.29 and Figure 4.30). The
spacing and size of the purlins and the sheeting rails are determined by the type of roof
and wall cladding used. As a general rule, the deeper the profile of the sheeting, the greater
its safe span and the further apart the purlins and sheeting rails may be fixed.
Mild steel angles and purlin rails are sometimes used, but these tend to have been
replaced by a range of standard sections, purlins and rails in galvanised, cold-formed steel
strip. The sections most used are Zed and Sigma (Figure 4.31), with more complex sections
with stiffening ribs also produced. These thin section purlins and rails help to facilitate
direct fixing of the sheeting by self-tapping screws.
Purlins and sheeting rails are fixed to structural supports with cleats, washer plates and
sleeves as illustrated in Figure 4.32. Anti-sag bars are fixed between cold-formed purlins
to stop them twisting during the fixing of roof sheeting and to provide lateral restraint to