Steel Portal Frame Design — Rafter, Haunch & Bracing Guide
Portal frame design: rafter and column sizing, eaves haunch geometry, fly bracing, pinned vs fixed bases, horizontal thrust, and code-specific checks.
How portal frames work
A portal frame is a rigid-jointed single-story structure where the rafters and columns act together as a continuous frame to resist gravity and lateral loads. The rigid knee connection at the eaves transfers moment between column and rafter, creating a characteristic bending moment diagram with peak negative moment at the eaves and peak positive moment near mid-span of the rafter.
Portal frames are the most common structural system for industrial buildings, warehouses, retail sheds, and agricultural buildings worldwide. Typical clear spans range from 15 m to 50 m, with spans up to 60 m achievable using tapered or haunched sections. Frame spacing is typically 6-9 m.
Key design parameters
- Span-to-depth ratio — rafter depth is typically span/40 to span/55 at mid-span. A 30 m span rafter might use a 530UB82 (UB) or W21x62 (AISC).
- Eaves haunch — the haunch deepens the rafter at the knee to approximately span/10 in depth and extends span/10 to span/7 along the rafter. The haunch reduces the required rafter size by shifting the critical section away from the peak moment zone.
- Column height — typically 6-10 m for industrial frames. Column sections are usually deeper than rafters because they resist combined axial load and moment.
- Roof pitch — 5 to 10 degrees is standard. Steeper pitches reduce horizontal thrust but increase rafter length and surface area.
Worked example — 30 m span portal frame
Frame: 30 m clear span, 7.5 m column height, 6-degree roof pitch, 7.5 m frame spacing. Pinned bases.
Loading per frame (ULS): dead = 0.5 kPa, live/snow = 0.6 kPa on plan area. Factored UDL on rafter = (1.2 x 0.5 + 1.6 x 0.6) x 7.5 = 11.7 kN/m.
Simple beam moment at mid-span = wL^2/8 = 11.7 x 30^2 / 8 = 1,316 kN-m. With portal action (pinned base), the eaves moment is approximately 0.6 x simple span moment = 790 kN-m, and the mid-span sagging moment reduces to approximately 1,316 - 790 = 526 kN-m.
Required rafter Zx at mid-span (Grade 300, phi = 0.9): Zx = 526 x 10^6 / (0.9 x 300) = 1,948 cm^3. A 530UB82 (Zx = 2,060 cm^3) works. At the eaves, the haunch section must resist 790 kN-m — the haunched depth of approximately 750 mm provides the required capacity without upgrading the rafter section.
Horizontal thrust at base = eaves moment / column height = 790 / 7.5 = 105 kN per frame. This thrust must be resisted by the foundation (ground beam, tie rod, or pad footing friction).
Code comparison — portal frame checks
| Check | AISC 360-22 | AS 4100:2020 | EN 1993-1-1 | CSA S16-19 |
|---|---|---|---|---|
| Member capacity | Ch. F (flexure), Ch. E (compression) | Cl. 5.1-5.6 (member capacity) | Cl. 6.3.3 (combined) | Cl. 13.8 (beam-column) |
| LTB of rafter | Eq. F2-2 to F2-4 with Cb factor | Cl. 5.6.1 with alpha_m | Cl. 6.3.2.2 with chi_LT | Cl. 13.6 with omega_2 |
| Haunched segment | Treat as tapered, Cb per DG 25 | Cl. 5.6.1.1(b) for non-uniform | EN 1993-1-1 Cl. 6.3.2.4 | CSA S16 Cl. 13.6(e) |
| In-plane stability | Direct Analysis Method Ch. C | Cl. 4.4 amplified moments | Cl. 5.2.2 second-order | Cl. 8.4 notional loads |
| Deflection | L/150 to L/240 (project-specific) | AS 4100 App. B (L/250) | L/200 to L/250 | L/180 to L/240 |
Fly bracing and restraint
Lateral-torsional buckling of the rafter compression flange is the most critical stability check. The inner (compression) flange of the rafter at the haunch region has no direct lateral support from the roof sheeting, which attaches to the outer (tension) flange. Fly braces — short diagonal members connecting the inner flange to the purlins — provide the necessary restraint.
Fly braces are required at or near: the end of the haunch, at purlin locations near the point of contraflexure, and at any location where the compression flange force is high. Missing a single fly brace can reduce rafter capacity by 30-50 percent.
Pinned vs fixed bases
Pinned bases reduce foundation cost and allow simpler base plate details, but they increase the eaves moment by 15-25 percent and column deflection. Fixed bases reduce frame moments and sway but require larger base plates, more anchor bolts, and stiffer foundations. Most portal frames use nominally pinned bases with 4-bolt base plates.
Common pitfalls
- Ignoring the haunch in the stability check. The haunched segment has varying cross-section and must be checked for LTB as a tapered member. Using the rafter properties alone is unconservative.
- Under-bracing the inner flange at the eaves. The maximum compression flange force is at the knee. Without fly bracing within 500 mm of the haunch start, the effective unbraced length includes the full haunch.
- Neglecting horizontal thrust on foundations. A 30 m span frame can generate 80-150 kN of horizontal thrust per column. Pad footings without tie rods may slide, especially on granular soils.
- Using first-order analysis only. Portal frames with slender columns are sensitive to P-delta effects. Second-order analysis or amplified first-order moments (per the Direct Analysis Method) is required by all modern codes.
Run this calculation
Related references
- Braced Frame
- Moment Frame Design
- Steel Building Envelope
- Column Base Design
- Lateral Torsional Buckling
- How to Verify Calculations
- steel beam capacity calculator
Disclaimer
This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.