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Steel portal frame buildings

A portal frame is a row of rigid steel frames, each made of two columns and two rafters joined by stiff, haunched connections. Purlins on the roof and side rails on the walls tie the frames together and carry the cladding.

The main dimensions

• Span: the width between column centres. Most single-span frames are 12 to 40 m.
• Bay spacing: the distance between frames, normally 4.5 to 7.5 m. Wider bays mean fewer frames but deeper purlins.
• Eaves height: from floor to the top of the wall, usually 4 to 10 m.
• Roof pitch: usually 4° to 15° on a duo-pitch roof. Profiled steel sheets need at least about 4°, and fibre cement about 5°.

Typical member sizes

As a rough guide, rafters are universal beams about span ÷ 55 deep and columns are one size deeper. For a 15 m span that means about 305 mm columns and 254 mm rafters; for a 30 m span about 610 mm columns and 533 mm rafters. Haunches at the eaves and apex stiffen the joints. Final sizes depend on the snow and wind loads for your site, so they must come from a structural engineer.

What else is in the steel

The portal frames are often only about half the steel. Gable posts, eaves beams, roof and wall bracing, door framing, lean-to rafters and posts, and any mezzanine all add to the tonnage.

Options

• Duo-pitch or mono-pitch roofs.
• Lean-to canopies on the side, usually at a lower eaves height than the main building.
• Mezzanine floors and offices inside, on their own columns and beams.
• Crane beams on brackets on the columns, which need heavier frames.

What drives the cost

The main costs are the steel tonnage, the cladding area and type, the doors, the floor slab and erection. Wide spans, tall eaves, exposed sites and insulated panels all add cost. Use the Pricing tab in the designer to enter your own supplier's rates.

Standards

The frame should be designed to the Eurocodes (BS EN 1991 for loads, BS EN 1993 for steel) and fabricated to BS EN 1090 with UKCA or CE marking.