The foundations are the part of a building nobody sees and everybody relies on. For a steel portal frame building, they do two jobs: carry the weight down into the ground, and hold the frames down when wind tries to lift or push them over. Here’s how it’s usually done.
How a portal frame meets the ground #
Each portal frame column stands on a steel base plate. It’s normally fixed to the concrete with mechanical anchor bolts, or with threaded rod set into the concrete with chemical anchoring. Where it’s beneficial, we’ll discuss using a cast-in bracket that extends into the floor instead: for example at a firewall for post-fire stability, on a very windy site, or on a very large span. Cast-in brackets take more set-up time, because they have to be secured so they don’t move while the concrete is poured. Most of the load in a steel building is concentrated at those columns, which is why the footings under them are bigger than the rest of the floor.
The common options #
| Option | What it is | Suits |
| Pad footings | A block of reinforced concrete under each column | Most portal frame buildings, with or without a floor slab |
| Slab with thickened footings | A reinforced floor slab with deeper, wider concrete under the columns and edges | Workshops, garages for machinery, commercial buildings |
| Bored piles | Concrete piles drilled deeper into the ground | Soft ground, fill, slopes, or high uplift loads; with or without a concrete floor |
| Hardfill floor with pad footings | Footings at columns, compacted hardfill between | Farm sheds and hay barns where a full slab isn’t needed (bored piles can also be used this way) |
What changes the design #
- Ground conditions. Soft or filled ground, peat and some clay soils need specific design, which is where a geotechnical report earns its keep.
- Wind uplift. Open-sided sheds, canopies and exposed sites need footings heavy or deep enough to stop the frame lifting.
- Floor use. Forklifts, racking, car hoists and heavy machinery need a thicker, more heavily reinforced slab.
- Slope. Sloping sites may need cut and fill, retaining or stepped foundations.
Getting the slab right #
- Vapour barrier under the slab, to stop ground moisture coming up and condensing on the roof.
- Reinforcing as specified by the engineer.
- Control joints cut at the right spacing to manage shrinkage cracking.
- Finish: power-floated for workshops, broom finish for wash-downs and ramps.
- Falls towards doors or drains where you’ll wash down.
Accuracy is everything #
The footings and slab must be the right shape, size and height, and accurately set out to the drawings. If they’re out, the frame won’t sit properly and the build slows down while things are fixed. Where cast-in brackets are used, they have to be positioned exactly right too. Careful set-out and a check before the pour avoid the problem.
Who does what #
On design & build projects, Steelspan designs and builds the foundations. On kitset orders, our drawings show exactly what’s needed, and your concrete contractor builds to them. Every plan set, and every construction set we send, includes a set-out layout. We normally specify a flat, floated slab or footing top with nothing cast in, so there’s nothing to float around; cast-in brackets are a last resort for firewalls or large loads.
If you’re arranging the concrete yourself, choose a contractor you know, whose work you’ve seen, or who can show you similar jobs they’ve done. The finish matters, so this isn’t a job for someone just having a go. Engineers will usually specify certified concrete from a plant, at a set strength such as 25 MPa, rather than mixed on site.
Talk to Steelspan #
Tell us about your ground and how you’ll use the floor, and we’ll design foundations to suit. Use our Easy Shed Quote or call 0800 787 726.
