Steel Portal Frame Design Guide -- Sizing, Haunches & Stability

Portal frames are the dominant structural system for single-story industrial buildings, warehouses, retail sheds, and agricultural structures. This reference covers rafter and column sizing, eaves haunch geometry, connection design, stability checks, fly bracing, pinned versus fixed bases, horizontal thrust, and code-specific provisions across AISC 360, AS 4100, EN 1993, and CSA S16.

How portal frames work

A portal frame is a rigid-jointed, single-story structure where rafters and columns act together as a continuous frame to resist gravity and lateral loads. The rigid knee (eaves) connection 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.

Unlike a simply supported beam, the portal frame distributes bending moments into the columns, reducing the rafter peak moment. This redistribution is the fundamental efficiency of the portal frame system -- it allows lighter rafters than a simply supported beam of the same span.

The three primary force paths:

1. Vertical loads -- roof sheeting to purlins to rafters to eaves connections to columns to foundations.
2. Lateral loads -- cladding to girts/purlins to portal frames to foundations. Longitudinal wind is carried by braced bays in the roof and wall planes.
3. Horizontal thrust -- the rafter pushes outward at the eaves under gravity load, resisted by tie rods, ground beams, or soil friction.

Typical clear spans range from 15 m to 50 m (up to 60 m with tapered sections). Frame spacing is 6 m to 9 m.

Portal frame types

Single-span portal frame

Two columns and two rafters meeting at a ridge apex. The most common configuration, suitable for spans from 12 m to 50 m. The ridge can be symmetric (gable) or single-slope.

Multi-span portal frame

Multiple bays share internal columns, reducing total frames and foundations. Adjacent spans partially balance eaves moments, reducing internal column moments. Expansion joints are required at 50 m to 60 m intervals. The valley between spans requires careful detailing for drainage and snow accumulation.

Lean-to portal frame

A single-slope frame attached to the side of a primary building. Used for extensions, canopies, and loading docks. The connection to the main building must accommodate differential deflection and drift.

Crane buildings

Portal frames supporting overhead cranes require heavier columns (often built-up sections) to resist crane surge forces and wheel loads. Columns must be checked for fatigue, and frames typically use fixed or partially fixed bases to limit lateral deflection at crane rail level (height/400 to height/700).

Typical span ranges and bay spacing

Spans above 30 m favor haunched or tapered built-up sections. Spans above 50 m often use truss rafters or lattice portal frames.

Member sizing guidelines

Rafter depth-to-span ratios

• UB sections (AS 4100): span/40 to span/55. A 30 m span rafter might use a 530UB82.
• W sections (AISC): span/40 to span/50. A 30 m span might use a W21x62 or W24x68.
• HE sections (EN 1993): span/40 to span/50. A 30 m span might use an HEA 500 or HEB 450.
• W sections (CSA S16): span/40 to span/50.

Deeper rafters give greater stiffness but may not be the lightest solution. A shallower, heavier section can be more economical when deflection is not critical.

Column depth-to-height ratios

Column depth is typically height/15 to height/25. For pinned-base frames, the column section is usually one or two sizes larger than the rafter. For fixed-base frames, the column must also resist the base moment.

Section selection strategy

1. Estimate rafter depth from span-to-depth ratio.
2. Estimate column depth from height-to-depth ratio.
3. Run first-order elastic analysis for member forces.
4. Check capacity under worst load combination.
5. Iterate sections as needed.
6. Run second-order analysis to verify stability.

Haunch design

The eaves haunch deepens the rafter section locally at the knee connection to resist the peak eaves moment without upgrading the entire rafter.

Haunch length

Typical haunch length is span/10 to span/7, extending past the point of contraflexure under gravity loading. For a 30 m span, haunch length is 3.0 m to 4.5 m.

Haunch depth

Total depth at the column face is typically span/10 to span/8. For a 30 m span with a 530 mm rafter, the haunched depth is 750 mm to 900 mm. The haunch flange tapers from full depth at the column to zero at the rafter tip.

Haunch flange and web

The haunch lower flange should be at least as thick as the rafter flange. The haunch web should match the rafter web. Both are continuous welds. The haunched segment must be checked for LTB as a tapered member.

Haunch design checklist

• Haunch length extends past the point of contraflexure.
• Depth provides sufficient section modulus for peak eaves moment.
• Flange thickness is at least equal to rafter flange.
• Welds develop full flange capacity.
• Tapered segment checked for LTB.
• Fly braces provided at haunch end and at intervals along haunch.

Connection design

Eaves connection (knee joint)

The most critical joint in a portal frame. The typical connection uses a thick extended end-plate welded to the haunch and bolted to the column flange. Design follows the T-stub method (EN 1993-1-8) or yield-line method (AISC DG 4, AS 4100 Cl. 9.6).

Key checks: bolt tension with prying, end-plate bending, column flange bending, column web panel shear (doubler plates may be needed), column web bearing, weld capacity, and haunch web shear.

Welded eaves connections (CJP welds direct to column flange) are used for heavier frames but require shop fabrication.

Apex connection (ridge joint)

Joins the two rafters at the ridge. Typically a bolted end-plate connection, smaller than the eaves. The apex moment is small under symmetric gravity load but can be significant under wind uplift.

Base connection

• Pinned base (4-bolt plate): most common, assumes zero base moment. Shear transferred through anchor bolts or shear lugs.
• Fixed base (6--8 bolt plate): reduces frame moments and drift but requires larger foundations. Used for crane buildings and tall frames.

See Column Base Plate Design for detailed guidance.

Stability

In-plane stability

Portal frames are prone to second-order (P-delta) effects because vertical loads act through the laterally displaced frame.

• AISC 360 (Direct Analysis Method, Ch. C): Reduced stiffness (0.80 EI/EA) and notional loads (0.2% of gravity). Must capture P-delta and P-Delta effects.
• AS 4100 Cl. 4.4: Moment amplification delta_s = 1 / (1 - sum(N*) / sum(N_omc)). If delta_s > 1.5, full second-order analysis required.
• EN 1993-1-1 Cl. 5.2.2: alpha_cr = F_cr / F_Ed. First-order OK if alpha_cr >= 10; second-order mandatory if alpha_cr < 3.
• CSA S16 Cl. 8.4: Notional loads 0.2%--0.5% of factored gravity; second-order required.

Out-of-plane stability and fly bracing

The inner (compression) flange of the rafter near the eaves has no lateral support from roof sheeting, which attaches to the outer flange. Fly braces -- short diagonal members connecting the inner flange to purlins -- provide the necessary restraint. Similarly, column inner flanges need fly braces to girts.

Fly brace requirements:

• Capacity: resist 2.5% of the compression flange force at that location (2% for CSA S16).
• Location: at or near the haunch end, at the point of contraflexure, and at regular intervals (typically 1.5--3.0 m near eaves, 3.0--4.5 m toward mid-span).
• Missing a single fly brace can reduce rafter capacity by 30%--50%.

Sway versus non-sway classification

Most portal frames are sway frames -- second-order effects must be included. Non-sway classification (where amplification is negligible) is uncommon. First-order-only analysis is insufficient for typical portal frames.

Load paths and typical load combinations

Gravity loads: sheeting to purlins (1.2--1.8 m spacing) to rafters to eaves connections to columns to foundations.

Lateral loads: cladding to girts/purlins to portal frames to foundations.

ASCE 7 / AISC 360 load combinations

D = dead, L = live, Lr = roof live, S = snow, R = rain, W = wind.

AS/NZS 1170.0 load combinations (AS 4100)

G = dead, Q = live, Wu = wind (ultimate).

Wind uplift (LC5 ASCE 7, LC4 AS 1170) reverses the rafter moment diagram -- the bottom flange goes into compression at mid-span. Fly brace locations must account for both gravity and uplift cases.

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. Grade 300 (AS 4100) or Grade 50 (AISC).

Loading

Dead = 0.5 kPa, live/snow = 0.6 kPa. Factored UDL on rafter (ASCE 7 LC2): (1.2 x 0.5 + 1.6 x 0.6) x 7.5 = 11.7 kN/m.

Preliminary sizing

Simple beam moment = wL^2/8 = 11.7 x 30^2 / 8 = 1,316 kN-m. With portal action (pinned base), eaves moment ~ 0.6 x 1,316 = 790 kN-m; mid-span sagging moment ~ 526 kN-m.

Rafter check

Required Ze (Grade 300, phi = 0.9): 526 x 10^6 / (0.9 x 300) = 1,948 cm^3. Select 530UB82 (Ze = 2,060 cm^3). Utilization = 0.95 -- acceptable but tight.

Haunch

Total depth 800 mm (530 + 270 mm cut). Approximate Ze = 3,251 cm^3. Capacity = 0.9 x 300 x 3,251 / 10^3 = 878 kN-m > 790 kN-m. Haunch length = span/10 = 3.0 m.

Column

Axial load ~ 175 kN, eaves moment = 790 kN-m. Required Ze = 2,926 cm^3. Select 460UB97 (Ze = 3,400 cm^3).

Horizontal thrust

Thrust = eaves moment / column height = 790 / 7.5 = 105 kN per frame. Resist with ground beams, tie rods, or soil friction.

Code comparison -- portal frame checks

AS 4100 portal frame provisions

AS 4100:2020 is widely used for portal frame design in Australia. Key provisions:

Second-order analysis (Clause 4.4)

Moment amplification factor:

delta_s = 1 / (1 - (sum(N*)) / (sum(N_omc)))

If delta_s > 1.5, rigorous second-order analysis is recommended. N_omc can be found by eigenvalue buckling analysis.

Member moment capacity (Clause 5.1, 5.2)

Ms = fy * Ze

Ze is the effective section modulus (compact, non-compact, or slender classification).

Lateral-torsional buckling (Clause 5.6)

Mob = alpha_m * Ms * alpha_s

alpha_m is the moment modification factor (1.0 for uniform moment to 2.4 for double curvature). alpha_s is the slenderness reduction factor:

alpha_s = 0.6 * [sqrt((Ms/Moa)^2 + 3) - (Ms/Moa)]

For haunched sections, Clause 5.6.1.1(b) provides guidance on equivalent section properties.

Effective length for LTB (Clause 5.6.3)

le = k_t * k_l * k_r * L

k_t = torsional restraint factor, k_l = load height factor, k_r = rotational restraint factor.

Portal frame deflection limits (Appendix B)

Common mistakes

1. Ignoring the haunch in the stability check. The haunched segment must be checked for LTB as a tapered member. Using rafter properties alone is unconservative.

2. Under-bracing the inner flange at the eaves. Without fly bracing within 500 mm of the haunch start, the effective unbraced length includes the full haunch.

3. Neglecting horizontal thrust on foundations. A 30 m span frame generates 80--150 kN of thrust per column. Pad footings without tie rods may slide on granular soils.

4. Using first-order analysis only. Portal frames are sensitive to P-delta effects. First-order analysis can underestimate moments by 10%--30%.

5. Ignoring wind uplift moment reversal. Uplift puts the bottom flange into compression at mid-span. Fly braces designed for gravity only will not restrain the correct flange under uplift.

6. Incorrect effective length for the haunched rafter. Ignoring the taper effect and using mid-span rafter properties is unconservative.

7. Omitting notional loads. All codes require minimum lateral forces for out-of-plumb. Omitting them is unconservative, especially for frames with low wind loads.

8. Inadequate column web panel zone check. Concentrated haunch flange forces produce high shear in the column web. Without a doubler plate, the web can yield or buckle.

FAQ

What is the typical span range for a steel portal frame?

Economical for 12 m to 50 m clear spans. Up to 60 m with tapered sections. Below 12 m, simply supported beams are usually cheaper. Above 50 m, truss portal frames become competitive.

How do I estimate the rafter size?

Use depth-to-span ratio of span/40 to span/55. For 30 m span, rafter depth ~ 550--750 mm. Select a standard section (530UB82, W21x62) and verify. The haunch reduces mid-span moment, so the rafter can be lighter than a simply supported beam.

When do I need fly braces?

Whenever the compression flange is not restrained by cladding. Near the eaves, the inner flange is in compression and roof sheeting restrains the outer flange, so fly braces are mandatory. Provide at every second purlin near eaves and at the haunch end.

What is the purpose of the haunch?

It deepens the rafter at the eaves where moment is highest, increasing section modulus locally. This allows a lighter rafter for the mid-span region. Without the haunch, the entire rafter would need to be sized for peak eaves moment.

How much horizontal thrust does a portal frame generate?

Approximately equal to eaves moment divided by column height. For a 30 m span with 7.5 m columns: 80--150 kN per column under factored gravity. Resist with tie rods, ground beams, or soil friction.

Should I use pinned or fixed bases?

Most frames use pinned bases. Fixed bases reduce moments and drift by 15%--25% but require heavier foundations. Use fixed bases for crane buildings, tall frames, or soft soil conditions.

What deflection limits apply to portal frames?

Rafter live load: span/250. Total load: span/180 to span/200. Eaves drift: height/150 to height/250. Crane rail: height/400 to height/700. Confirm with project specification.

How does AS 4100 treat portal frame stability?

Clause 4.4 requires moment amplification: delta_s = 1 / (1 - sum(N*) / sum(N_omc)). If delta_s > 1.5, full second-order analysis is recommended.

Run this calculation

• Portal Frame Calculator -- Size rafters, columns, and haunches for single-span portal frames.
• Beam Capacity Calculator -- Check bending, shear, and deflection for standard sections.
• Section Properties Database -- UB, UC, W, and HSS section properties and capacities.
• Bolt Capacity Calculator -- Design bolted eaves and apex connections.
• Base Plate Calculator -- Design pinned and fixed column base plates.
• Lateral Torsional Buckling Calculator -- Check LTB capacity between restraints.

Related references

• Moment Frame Design -- SMF, IMF, and OMF design per AISC 341.
• Braced Frame -- Concentric and eccentric braced frame systems.
• Lateral Torsional Buckling -- LTB theory and calculation methods.
• Column Base Plate Design -- Base plate design for pinned and fixed connections.
• Steel Building Envelope -- Roof and wall cladding systems.
• Deflection Limits -- Serviceability deflection criteria by member type.
• Load Combinations -- ASCE 7, AS 1170, and EN 1990 load combinations.
• Purlin and Girt Design -- Cold-formed purlin and girt design.
• Connection Design Workflow -- Step-by-step connection design process.
• Effective Length -- K-factor and effective length determination.
• Framing Systems -- Overview of steel framing systems.
• Roof Loading -- Dead, live, snow, and wind loads on steel roofs.

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 (AISC 360, AS 4100, EN 1993, CSA S16, or other governing code) and project specification before use. Portal frame design requires engineering judgment and should be performed by a licensed structural engineer. The site operator disclaims liability for any loss arising from the use of this information.

Design Resources

Calculator tools

• Portal Frame Calculator

Design guides

• Portal Frame Design Guide
• Portal Frame Worked Example