Free Portal Frame Design Guide — AISC 360, AS 4100, EN 1993-1-1
Portal (rigid) frames are the most common structural system for single-storey steel buildings. This guide covers the design workflow: preliminary sizing, second-order analysis, member capacity checks, and connection design.
Analysis Methods
Elastic analysis (first-order) is acceptable when P-Δ effects are less than 10% of first-order moments. Use the stability function or LeMessurier method to amplify moments. For slender frames (H/(Δ/Ω) > 10), switch to second-order analysis.
Second-order analysis includes both P-Δ (story drift) and P-δ (member curvature) effects. Most software automates this, but hand methods using the B1/B2 amplifier per AISC 360 Chapter C are still valid for regular frames.
Rafter Sizing
Typical span-to-depth ratios for portal frame rafters:
- Light industrial: L/20 to L/25
- Medium industrial: L/25 to L/30
- Heavy industrial: L/30 to L/35
Rafter depth governs deflection. For a 20m span, start with a 700-900mm deep UB or welded plate girder section.
Column Base Conditions
| Base Type | Fixity | Frame Action | Foundation Demand |
|---|---|---|---|
| Pinned base pins | Nearly pinned (< 20% fixity) | Braced frame | Low overturning |
| Moment base plate | 20-60% fixity | Partially restrained | Moderate |
| Fixed base | > 80% fixity | Rigid frame | High overturning |
Pinned bases are simplest but create larger rafter moments. Fixed bases reduce rafter size but require substantial foundations.
Deflection Limits
| Standard | Lateral Drift (H) | Rafter Deflection (Span) |
|---|---|---|
| AISC 360 (LRFD) | H/400 (wind) | L/240 (live load only) |
| AS 4100 | H/500 (service) | L/250 (total) |
| EN 1993-1-1 | H/150 to H/300* | L/200 to L/300* |
*National Annex values vary. UK NA uses H/300 for wind.
Haunch Design
Haunches (tapered sections at the rafter-to-column connection) are a defining feature of portal frame design. A haunch increases the section depth at the eaves where bending moments are highest, allowing the rafter to be shallower over the rest of the span.
Typical haunch proportions:
- Length: 8-15% of the frame span (e.g., 1.6-3.0 m for a 20 m span)
- Depth ratio: Haunch depth at column face = 1.5 to 2.5 times the rafter depth
- Flange: Same flange width and thickness as the rafter section
- Web stiffeners: Required at the haunch tip and at the column-rafter intersection where concentrated forces occur
The haunch reduces the required rafter section by 15-30% compared to a constant-depth rafter and shifts the zone of maximum stress away from the welded moment connection.
Frame Stability Checks
Sway stability must be verified per the governing standard:
Per AISC 360 Chapter C: The direct analysis method accounts for both P-Δ and P-δ effects through reduced stiffness (0.8EI for steel members) and notional loads (0.002 Yi for gravity-only combinations). The required second-order drift ratio Δ2nd/Δ1st should not exceed 1.5 for typical industrial frames.
Per AS 4100 Clause 4.3: Amplified sway moment method using δs = 1/(1-1/λs) where λs is the elastic buckling load factor for the sway mode. The sway amplifier should not exceed 2.0.
Per EN 1993-1-1 Clause 5.2: Frame classification uses αcr = HEd/VEd × h/δH, where δH is the first-order sway displacement under horizontal loads. If αcr ≥ 10, first-order analysis suffices. If 3 ≤ αcr < 10, amplified sway moments are required. If αcr < 3, full second-order analysis is mandatory.
Plastic hinge analysis requires that the frame can form a mechanism with sufficient rotational capacity at each hinge location. For Class 1 (plastic) sections, hinges should have minimum rotation capacity of 3-4 times the elastic rotation at Mp.
Connection Design for Portal Frames
The eaves connection (rafter-to-column) is the most highly stressed connection in a portal frame. Design considerations:
- Bolted end-plate connections: Most common for erected frames. Flush end-plates for lighter frames, extended end-plates for frames with deeper sections or higher moments. Bolt rows in the tension zone carry the majority of the moment.
- Welded connections: Common for shop-fabricated frames. Full-penetration welds at flange-to-end-plate joints. Fillet welds may be sufficient for lighter sections.
- Apex connection: At the ridge, the moment is typically 30-50% of the eaves moment. A bolted or welded connection with lapped or spliced rafters. Purlin continuity across the apex requires careful detailing.
- Column base: Pinned bases use 2-4 anchor bolts with minimal base plate projection. Fixed bases require 4-8 anchor bolts with stiffened base plates, designed for combined axial and moment demand.
Crane Load Considerations
Portal frames supporting overhead cranes require additional design checks:
- Vertical crane load: Wheel loads from the crane bridge, including impact factors (typically 25% increase for electric overhead cranes per ASCE 7-22).
- Horizontal crane load: Lateral surge (20% of lifted load + trolley weight) and longitudinal traction (10% of maximum wheel loads).
- Fatigue: Crane runway beams and supporting connections require fatigue design per AISC 360 Appendix 3 or EN 1993-1-9. Stress ranges at weld details govern the fatigue life.
- Crane girder deflection: Vertical deflection limited to L/600 to L/1000 depending on crane class. Lateral deflection limited to L/400.
The frame member sizes are often governed by crane loads rather than environmental loads for industrial buildings with cranes over 10-ton capacity.
Purlin and Girt Design
Purlins (roof) and girts (wall) span between the main portal frames, transferring cladding loads to the primary structure:
Purlin design considerations:
- Section types: Cold-formed Z-sections (for continuous lapped systems), C-sections (for simple spans), or channel sections. Z-purlins with overlapping at supports provide approximately 15-20% greater capacity than simple spans due to continuity
- Bridging: Lines of lateral restraint (bridging) at third-points or quarter-points prevent purlin rotation and increase capacity. Without bridging, purlin capacity is limited by lateral-torsional buckling of the unbraced top flange
- Sag rods: Required between purlin lines to resist the gravity component parallel to the roof slope. Typically 5/8-inch or 3/4-inch diameter threaded rods at midspan for roof slopes above 2:12
Girt design follows similar principles but must also resist wind suction on wall cladding, which reverses the bending direction and can control the design for end-wall girts.
Seismic Design Considerations
Per ASCE 7-22 Chapter 12 and AISC 341-22, portal frames in seismic design categories D, E, and F must comply with special seismic provisions:
Response modification factors (ASCE 7-22 Table 12.2-1):
- Ordinary moment frames (OMF): R = 3.5
- Intermediate moment frames (IMF): R = 4.5
- Special moment frames (SMF): R = 8.0
Detailing requirements for SMF:
- Reduced beam section (RBS) or other pre-qualified connections per AISC 358
- Panel zone shear checks at beam-to-column connections
- Strong-column weak-beam (SCWB) verification: ΣMpc/ΣMpb ≥ 1.0
- Lateral bracing at anticipated plastic hinge locations (within d/2 of the hinge)
- Maximum axial load on columns limited to 0.6ϕcPn for SCWB compliance
Most industrial portal frames qualify as ordinary moment frames (OMF) due to their single-story configuration, but facilities in high-seismic regions may require IMF or SMF detailing.
Calculate Your Frame
Use the Portal Frame Calculator to run second-order analysis on your specific frame geometry, section sizes, and loads.
Frequently Asked Questions
What's the difference between a portal frame and a braced frame? A portal (rigid) frame resists lateral loads through moment connections between beams and columns, creating a rigid bent. A braced frame uses diagonal bracing members to resist lateral loads, with simple (pinned) beam-column connections. Portal frames are preferred when clear openings are needed (no bracing to obstruct doorways or windows).
When do I need second-order analysis? Second-order analysis is required when P-Δ effects exceed 10% of first-order moments. For typical industrial portal frames with spans over 15m and column heights over 6m, second-order effects become significant. Use the B1/B2 amplifier method per AISC 360 Chapter C or a P-Δ analysis per AS 4100 Clause 4.3.
Can I use a pinned base for a portal frame? Yes, pinned bases are common for lighter industrial buildings with spans under 20m. The pinned base simplifies foundation design but increases rafter depth. For taller columns or higher crane loads, consider fixed or moment-resisting base plates to control drift.
What is a haunch and why is it used in portal frames? A haunch is a tapered deepening of the rafter section at the eaves connection where bending moments are highest. It is typically 1.5-2.5 times the rafter depth and extends 8-15% of the span length. Haunches reduce the required rafter depth over the rest of the span by 15-30% and shift the maximum stress zone away from the welded moment connection at the column face.