What is a portal frame?
A portal frame is a structural frame of two columns with a rafter or beam above, in which the corners between column and rafter are moment-resisting — that is, rigid in bending. Because those corners are not pins, the frame can take sideways load on its own. No bracing is needed in the plane of the frame, which leaves a completely clear, obstruction-free space.
That is precisely why practically every steel industrial building, warehouse, riding arena and farm shed consists of a row of portal frames with purlins and side rails spanning between them.
Pins and redundancy
| Type | Bases | Corners | Characteristic |
|---|---|---|---|
| Two-pinned frame | Pinned to the foundation | Rigid | Most common; simple pad foundations, since the base takes no moment |
| Three-pinned frame | Pinned | Pin at the apex | Statically determinate; insensitive to differential settlement |
| Fixed-base portal | Fixed, moment-resisting | Rigid | Slimmer steel, but much heavier foundations |
The more moment-resisting connections, the lighter the steelwork can be — but the heavier the foundations, and the more sensitive the structure to uneven settlement.
The haunch
The bending moment is largest at the eaves corner. That is why the rafter is often deepened there with a haunch: a tapered length of section under the rafter at the column. You get extra depth exactly where it is needed without making the whole member heavier. The connection itself is usually an end plate with preloaded bolts.
Points to watch
- Horizontal thrust. Under load a portal pushes outwards at the bases. With pinned bases that thrust has to go somewhere — into a tie rod, a tie in the slab, or the floor slab itself.
- Buckling and lateral torsional buckling. Slender rafters and columns can move sideways. Purlins and side rails do double duty: they carry the cladding and they restrain the frame. Never remove them without checking.
- Stability along the building. A frame is only stiff in its own plane. At right angles to it you need bracing or a stiff wall diaphragm.
- Torque the bolts. Moment connections are only moment connections if the bolts have been tightened to the specified value.
- Have it designed. Span, section size, end plate thickness and bolt size follow from a design to the Eurocodes. A catalogue building is designed for particular snow and wind loads; depart from those and the justification no longer applies.
Related terms
- Portal
- Truss
- Steel structure
- Haunched portal frame
- Haunch
- Bracing
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