Industrial buildings and portal frames

Industrial buildings, warehouses and workshops need large clear floor areas, high eaves and often an overhead crane. Steel portal frames supply that cheaply: a row of rigid frames, each made of two columns and a rafter connected by moment-resisting knees, spaced typically 5 to 8 m apart and linked by purlins that carry the roof sheeting and side rails that carry the wall cladding.

Loads are dead load of sheeting and purlins, imposed roof load, wind and, where present, crane loads. Wind usually governs light roofs because it can lift them: internal pressure through open doors adds to external suction, and the critical case is often uplift reversing the stresses in the rafter, so that its bottom flange goes into compression and needs bracing. Wind pressures and coefficients come from IS 875 (Part 3); dead and imposed loads from Parts 1 and 2.

Portal frames are well suited to plastic analysis. With pinned bases, a sway mechanism forms with hinges at both column tops; a beam mechanism forms with hinges at the knees and near mid-span; and a combined mechanism usually governs when gravity and wind act together. Haunches at the knees strengthen the most heavily loaded zone. Fixed bases make the frame stiffer but need bigger foundations.

Stability comes from bracing: roof bracing in the end bays carries wind on the gable to the side walls, and vertical wall bracing takes it down to the foundations. Crane girders need checks for vertical wheel loads with impact, lateral surge, longitudinal braking and fatigue.