Structures
Design a steel portal frame building
Step-by-step steel portal frame design in 3D: span, eaves height, roof pitch, purlins, bracing, wind and roof loads, rafter and column checks, and auto-sizing.
The steel portal frame is the workhorse of single-storey buildings — warehouses, factories, barns and sports halls. A row of identical frames, each two columns and two sloping rafters joined rigidly at the eaves and the ridge, gives a clear span of 15–40 m with very little steel. This tutorial designs one with the Portal frame shed template.
How a portal frame works
The roof sheeting carries snow, maintenance load and wind to the purlins, which span between frames. The purlins load the rafters. Because the rafters are rigidly connected to the columns at the eaves, the frame works as one bent member: the rafters hog at the eaves and sag near mid-span, the columns bend, and the frame pushes outwards at the bases. That outward thrust and the bending at the eaves are why portal design is mostly about the eaves moment.
Along the building, the frames are only linked by purlins and side rails, which cannot resist sway. The bracing in the end bays — X-braces in both side walls and in the roof — gives longitudinal stability and carries the wind on the gables down to the foundations.
Step 1 — Pick the template
- In the gallery choose Industrial › Portal frame shed.
- The default is a 20 m span, six 6 m bays, 6 m to the eaves and a 10° roof, in steel with pinned bases.
Step 2 — Set the size
| Parameter | Section | Default | Range |
|---|---|---|---|
| Span | Span | 20 m | 8–40 m |
| Bays | Plan | 6 | 2–15 |
| Frame spacing | Plan | 6 m | 4–10 m |
| Eaves height | Height | 6 m | 3–12 m |
| Roof pitch | Roof | 10° | 3–25° |
| Purlin spacing | More options | 1.5 m | 1–2.5 m |
Purlins are placed at no more than the purlin spacing along each rafter, plus one at each eave and at the ridge. Each rafter is one member from eave to ridge; the purlins frame into it, and the analysis mesh cuts it there.
Step 3 — Materials and sections
- Pick the Design code and a steel Grade.
- Set sections for Columns, Rafters, Purlins and Bracing. I-sections suit columns and rafters; bracing works in tension and compression as truss members.
- Leave Size members for me on.
The roof and wall sheeting is cladding: it has no stiffness in the model, but it collects the roof and wind loads and passes them to the members along its edges.
Step 4 — Supports and loads
- Bases: Pinned is the default and the usual choice — cheaper foundations, but more sway and larger eaves moments. Fixed bases stiffen the frame against sway but need bigger base plates and foundations.
- Switch on Pad footings to size the foundations from the reactions.
- The roof gets the code's roof live load (for example 0.4 kN/m² in Eurocode, 0.96 kN/m² in ASCE 7) and the Finishes and services load (sheeting, insulation, services — 0.25–0.5 kN/m² is typical for a light roof).
- Wind acts in ±X and ±Y on the side and gable walls: pressure on the windward wall, suction on the leeward one.
The wind cases load the walls; they do not add uplift on the roof. On light roofs uplift often governs the rafters, purlins and holding-down bolts. Add it by hand as line loads on the purlins (in the Full interface, select a member and use Add a line load), and add snow the same way if your site has it.
Step 5 — Analyse
Press Run analysis (F5). Check:
- Moment on the envelope: the eaves hogging moment should be the largest, with sagging in the rafters about a third of the way from the ridge.
- Displacement: vertical deflection at the ridge (service) and horizontal sway at the eaves under wind. A common limit for sheds is eaves height/150 to /300 for sway — check your code.
- Reactions (table in the Full interface): the horizontal thrust at the pinned bases, which the foundation must take.
- Axial in the end-bay bracing under the ±X wind cases — this is the longitudinal load path.
Step 6 — Design
- Press Run design (F6). Rafters and columns are checked for combined bending and axial load, flexural buckling, lateral-torsional buckling and deflection. The buckling lengths follow from where purlins and braces frame in.
- Steel connections are designed: base plates, end plates at the eaves and ridge, fin plates at the purlins, gussets at the braces. Show Steel connections in 3D and click a joint for its details.
- Show Utilization: each part of a member is coloured by its own check along its length, so you can see where the rafter is working hardest.
- Produce the Calculation report and the Shop drawings.
Typical problems and fixes
- Rafters governed by lateral-torsional buckling. The bottom flange is in compression near the eaves and is not restrained by the purlins. Reduce the Purlin spacing or choose a section with a wider flange.
- Weak-axis bending in the columns. The columns are not rotated, so frame action can land on their minor axis. Check Minor moment; if it is large, set the columns' Rotation in the Full interface so the strong axis lies in the plane of the frame.
- Sway too large. Fix the bases, increase the column section, or reduce the span.
- Brace fails in compression. X-bracing designed as truss members still takes some compression; use a stockier section (a tube) or accept heavier bracing.
Rules of thumb
- Rafter depth ≈ span / 50 to span / 60 (with haunches); without haunches, span / 35 to / 40.
- Column depth ≈ rafter depth or slightly more.
- Steel weight for a light shed: about 20–35 kg per m² of floor.
- Frame spacing of 6–7.5 m usually gives the cheapest purlin and frame combination.
For a crane building see Warehouse with crane; for longer spans see Steel truss roof.