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188 Barry’s Advanced Construction of Buildings
Apex brace fixed
between ridge
purlins
Angle brackets
welded to ends of
angle braces bolted
between purlins
Zed purlin
Figure 4.34 Purlin braces.
The secret fixing for standing seam roof sheeting for low-pitch roofs does not provide
lateral restraint for cold-formed purlins; thus it is necessary to use a system of braces
between purlins. The braces are manufactured from galvanised steel sections and bolted
between purlins with purpose-made apex braces, as illustrated in Figure 4.34.
To support the wall sheeting (cladding), sheeting rails are fixed across, or between, the
steel columns and/or vertical frame members (Figure 4.31 and Figure 4.32). Zed or Sigma
section rails are bolted to cleats and then bolted to the structural frame. A system of side
rail struts is fixed between rails to provide strength and stability against the weight of the
sheeting. The side rails are fabricated from lengths of galvanised mild steel angle, with a
fixing plate welded to each end, thus enabling the rails to be bolted to the sheeting rails
(Photograph 4.6). A system of tie wires is also used to provide additional restraint as shown
in Figure 4.35.
Timber provides an alternative material for short- and medium-span purlins between
structural frame members. The ease of cutting and simplicity of fixing make treated timber
a convenient and economic alternative to steel.
Precast reinforced concrete portal frames
Following the end of the Second World War (1945), there was a shortage of steel, which
led to the widespread use of reinforced concrete portal frames for single-storey structures,
such as agricultural sheds, storage and factory buildings. A limited range of standard frames
is cast in standard moulds under factory conditions. The comparatively small spans, limited
sizes and bulky nature of the frames resulted in this method being used much less than
steel. The advantages of concrete are its good fire resistance and relative freedom from
maintenance.