The portal frame building

How a single-storey steel building stands up: the frames, purlins, girts and bracing, the layout that sets them, and the paths that carry gravity and wind to the ground, with the loads of EN 1990, EN 1991-1-1 and EN 1991-1-4 as the STATIX wind calculator applies them on its Eurocode route.

1. Learning outcomes

After this lesson you can:

name the parts of a portal frame building and say what each one carries;
set out a layout: span, bay, eaves height and pitch;
follow the gravity and wind load paths to the bases;
place the bracing, and find the force in a wall brace;
combine the loads with EN 1990 for a light roof, including uplift.

2. Before you start

Steel Design 1 (Eurocode) (members and connections).
The free wind load calculator, set to EN 1991-1-4.
The portal frame showcase, to see where the course ends up.
A pocket calculator and about 45 minutes.
The same lesson on the SA codes: Steel Design 2, lesson 1 (SANS 10160).

3. The building

A portal frame building: rigid frames across the span at a regular spacing, purlins on the rafters and girts on the columns to carry the sheeting, and bracing in the roof and walls to carry loads along the building. The end frames (gable frames) are often lighter, with gable posts.

Layout. The span, the eaves height, the roof pitch and the bay (the frame spacing) set almost everything else: the frame moments, the purlin and girt spans, and the sheeting. The STATIX showcase frame is an 18 m span with 6 m bays, 6 m eaves and a 10° roof; the course follows a 20 m warehouse.

4. Load paths

The gravity load path. The roof load goes from the sheeting to the purlins, from the purlins to the rafters as point loads close enough to treat as a line load (the pressure times the frame spacing), round the frame to the columns, and down to the bases and footings.

Wind across the building. The girts take the wall pressure to the columns, the purlins take the roof suction to the rafters, and the frame resists both by bending in its own plane through its rigid eaves (and apex) joints. On a light roof the suction is larger than the weight: the frame and its bases go into uplift.

Wind on the gable. The frames cannot resist load along the building, so the gable posts pass the wind to a horizontal roof girder (the roof bracing in the end bay), the eaves struts carry it along the eaves, and the wall bracing takes it down to the bases. Every frame relies on that system for its stability out of plane.

5. Bracing and restraint

Wall bracing. Crossed flats, rods or angles working in tension only are the lightest; a K-brace leaves room for a door; a portal (a stiff frame in the bay) keeps the bay open. Brace at least one bay in each wall line, usually the end bays or one near the middle.

Frame action under gravity. The rigid eaves joints make the rafters and columns work together: the largest moments are at the eaves (the haunch is there to carry them), with a smaller sagging moment near the apex. Most portal frames have pinned bases, so the columns carry no moment at the foot, and the footings stay small.

Restraint. Purlins and girts hold the outer flange of rafters and columns, but under gravity near the eaves (and under uplift along the rafter) the inner flange is in compression. Fly braces from purlins or girts to the inner flange restrain it; Steel Design 1 (Eurocode) lesson 3 explains the lateral-torsional buckling they prevent.

6. Loads and combinations

imposed load on a roof not accessible except for maintenance (category H): q_k 0.4 kN/m², Q_k 1.0 kN (EN 1991-1-1 Table 6.10, the recommended values; a national choice) wind: q_p(z) = c_e(z) × ½ ρ v_b²; w = q_p (c_pe − c_pi) (EN 1991-1-4 4.5, 5.2) gravity: 1.35 G + 1.5 Q; uplift: 1.0 G + 1.5 W (EN 1990 6.10, Table A1.2(B))

The combinations that govern a light steel roof (EN 1990 6.4.3.2, equation 6.10 with the recommended factors of Table A1.2(B)). Gravity with the imposed roof load sizes the eaves; the uplift case with the permanent load favourable (1.0 G) reverses the moments, puts the inner flange in compression along the rafter and lifts the bases. The imposed load on a roof is not combined with wind or snow.

Two internal pressure cases. Positive internal pressure adds to the roof suction (the uplift case); negative internal pressure adds to the windward wall load and eases the uplift. Each governs different members, so the frame is analysed for both.

What the wind load calculator’s EN 1991-1-4 route uses: the basic wind speed v_b you enter (it includes c_dir and c_season), the terrain category 0 to IV with c_o = 1, ρ = 1.25 kg/m³, the reference height at the top of a duo-pitch roof, the zones with e = min(b, 2h), and the overall force with the factor 0.85 for lack of correlation (7.2.2(3)). The quick tools give imposed loads to SANS 10160-2 only; this lesson takes EN 1991-1-1 by hand.

7. The course

The course: one building from layout to the last joint, each lesson tied to a STATIX calculator on its Eurocode route where it has one. The cold-formed purlin and member calculators work to SANS 10162-2 only, so lesson 2 is by hand. It builds on Steel Design 1 (Eurocode).