Portal frame building design
The portal frame is the workhorse of single-storey commercial building, giving wide clear spans from a small amount of steel. We design portal frame buildings from the rafters and haunches to the bases and foundations, for clients across the UK.
What span can a portal frame achieve?
Single-span portal frames commonly reach thirty metres or more with no internal columns, and multi-bay frames cover much larger footprints. We choose the span and bay spacing that carry your loads with the least steel while leaving the internal layout you need.
How a portal frame works
A portal frame is a pair of columns and a pitched rafter joined by stiff, moment-carrying connections at the eaves. That rigid corner is what lets the frame span thirty metres or more with no internal columns, because the eaves connection turns the rafter and column into a single bending structure that carries the roof load out to the feet. The haunch, the tapered piece of steel at each eaves, exists to strengthen the frame exactly where the bending moment is highest.
Because the joints are stiff, the frame is sensitive to how it deflects and how it buckles sideways. We check the rafters and columns for combined bending and axial force, restrain the inner flange against lateral-torsional buckling with the purlins and stays, and control the sideways spread of the frame under load. The design is carried out to BS EN 1993, using elastic or plastic analysis depending on the span and loading.
Loads that govern the design
Portal frames are light structures, so wind uplift often governs rather than gravity load. We calculate wind actions to BS EN 1991-1-4 and its UK National Annex, and in many cases the critical case is the roof lifting off in a storm, which reverses the forces in the frame and sizes the holding-down bolts and the roof fixings. Snow load to BS EN 1991-1-3 also matters, especially asymmetric snow and drifting in the valleys of multi-bay frames.
The frame feet generate a large horizontal thrust as they try to spread apart under load, and dealing with that thrust is central to the design. We either provide a tie across the frame at floor level, design the ground slab and its reinforcement to hold the feet together, or design pad foundations big enough to resist the thrust in the ground. The choice affects the floor, the foundations and the cost, so we settle it early.
Secondary steel, bracing and foundations
The purlins and side rails do more than carry the cladding. They restrain the main frame against buckling and carry wind load back to the bracing, so we design the secondary steel and the frame together rather than treating the cold-rolled sections as an afterthought. Cranked eaves beams, gable posts and cladding rails are all sized to suit the panel type and the wind pressure across the elevation.
Stability comes from the bracing. We design plan bracing in the roof and vertical bracing in at least one bay of each wall to carry wind load down to the foundations and to hold the frames square during erection. The foundations are then designed for the combined vertical load, uplift and horizontal thrust at each base, usually as pad foundations with tie beams, or piles where the ground is poor.
What we check.
The points our calculations resolve for a project like this.
- Rigid eaves connections and haunch design where bending is highest
- Wind uplift often governing over gravity load, sizing bolts and fixings
- Asymmetric and drifted snow to BS EN 1991-1-3 on multi-bay valleys
- Base thrust resisted by ties, slab reinforcement or pad foundations
- Purlins and rails as both cladding support and frame restraint
- Plan and vertical bracing for stability and erection
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What span can a portal frame achieve?
Single-span portal frames commonly reach thirty metres or more with no internal columns, and multi-bay frames cover much larger footprints. We choose the span and bay spacing that carry your loads with the least steel while leaving the internal layout you need.
Why is wind uplift so important on a portal frame?
Portal frames are light, so in a storm the wind can lift the roof and reverse the forces in the frame. That uplift case often governs the holding-down bolts and roof fixings, so we design for it to BS EN 1991-1-4 rather than just for gravity load.
How is the outward thrust at the feet dealt with?
The frame feet push outwards under load. We resist that with a tie across the frame, by designing the ground slab to hold the feet, or with pad foundations sized to take the thrust. We decide the approach early because it affects the floor and foundation cost.
Do you cover projects across the UK?
Yes. We design portal frame buildings for clients throughout the UK, delivering calculations and drawings remotely and arranging site visits by region.
Related in commercial, industrial & agricultural structural engineering.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
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