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180 Barry’s Advanced Construction of Buildings
Steel portal
frames
Purlins fixed across
frames support roof
covering
Portal
frame
Figure 4.23 Long-span steel portal frames.
Rafter of steel portal frame
Ridge of
portal frame
Beam cuttings
welded to the
underside of
rafters
End plates welded
to rafters and bolted
together
Figure 4.24 Stiffening at ridge of steel portal frame.
The junction of the rafters at the ridge is often stiffened by welding cut I-sections to the
underside of the rafters at the bolted site connection as shown in Figure 4.24.
Steel portal frames may be fixed or pinned to bases to foundations. For short-span portal
frames, where there is relatively little spread at the knee or haunch, a fixed base is often
used. The steel plate, which is welded through gusset plates to the post of the portal frame,
is set level on a bed of cement grout on the concrete pad foundation and is secured by
four holding-down bolts, which are set or cast into the concrete foundation (illustrated in
Figure 4.25 and Photograph 4.4). A pinned base is made by positioning the portal base
plate on a small steel packing piece on to a separate base plate, which bears on the concrete
foundation. Two anchor bolts, either cast or set into the concrete pad foundation, act as