Portal frame structures

In the plane of the frame, stability comes from the rigid corners themselves; the moment connections stop the frame folding flat. Out of plane, the frame is slender and would buckle sideways without restraint, so the secondary steelwork does double duty. Purlins on the roof and side rails on the walls hold the frame at regular intervals, and where the inside flange goes into compression, under wind uplift, or near the haunch under gravity, we add fly braces from the purlins and rails to restrain that flange directly. Getting the restraint right is usually what decides the member sizes, more than the raw bending.

Along the length of the building, wind on the gables and drag from the roof are carried by bracing. A braced bay, typically cross bracing in the plane of the roof and down one or both end walls, collects the longitudinal load and takes it to the foundations, and eaves struts tie the frames together so they act as a set rather than individually. Gable frames are often lighter posts and rails because they only span between ground and roof bracing rather than carrying a full bay of roof.

Portal frames are the default for warehouses, distribution centres, factories, agricultural barns, retail sheds and hangars, anywhere a large column-free floor and a simple pitched roof are wanted at low cost. They are economical because the system is repetitive, the members are efficient, and fabrication and erection are quick. Eaves height, bay spacing and frame spacing can all be tuned to the use, from a low farm building to a high-bay logistics unit with racking to the underside of the haunch.

The limits follow from the form. The pitched shape and the depth of the haunch are not always wanted architecturally, and a shallow roof pitch increases the forces and the horizontal spread at the base, which the foundations must resist. Long spans mean real horizontal thrust at the feet, so the foundations often need tie rods across the floor or substantial pads to hold the bases in. As with all steel, fire and corrosion protection are designed to suit the use, and single-storey industrial buildings often benefit from relaxed fire requirements that keep the steel exposed.