Portal Frame Geometry — UK Practice
Typical UK single-storey portal frames use the following proportions:
| Parameter | Typical UK Range | Notes |
|---|---|---|
| Span | 15-50 m | 20-35 m most common for industrial buildings |
| Eaves height | 4-12 m | 6-8 m typical for warehouses |
| Roof pitch | 5-15 degrees | 6 degrees standard for profiled metal cladding |
| Frame spacing | 6-8 m | 6 m standard, 8 m for heavier sections |
| Haunch length | 0.1-0.15 x span | Approximately 10 % of span |
Roof types per UK practice:
- Pitched roof (symmetrical): Standard for most industrial buildings. Rafters inclined at 6 degrees with a ridge.
- Mono-pitch (single slope): For lean-to extensions or architectural preference. Rafter slope typically 5-8 degrees.
- Cellular beam rafters: For longer spans (35-50 m) or reduced steel weight. Castellated beams with circular or elongated openings for services integration (common UK practice for office buildings).
Structural Form — Member Types
The standard UK portal frame uses hot-rolled UB sections throughout, with haunches at the eaves to increase moment capacity at the critical section:
| Member | Typical UK Section | Grade | Notes |
|---|---|---|---|
| Rafter | 406UB to 610UB | S355 | Sized for combined bending + compression |
| Column | 356UB to 686UB | S355 | Heavier than rafter at same depth |
| Eaves haunch | Rafter section cut from next-size UB | S355 | 10-15 % of span length |
| Apex haunch | (optional) | S355 | Only for spans > 35 m |
Haunch geometry:
The eaves haunch is typically fabricated by cutting a UB section diagonally and welding it to the underside of the rafter at the column connection. The haunch depth at the column face is approximately 2 x rafter depth, tapering to rafter depth over a length of 0.1-0.15 x span.
Frame Stability — EN 1993-1-1 Clause 5.2
In-plane stability:
UK portal frames are typically analysed as sway frames. The UK NA to EN 1993-1-1 Clause 5.2.2 permits either:
- First-order elastic analysis with amplified sway moments — use when alpha_cr >= 3.0 (elastic critical buckling factor)
- Second-order elastic analysis (P-Delta) — required when alpha_cr < 3.0 (slender frames)
Typical alpha_cr for UK portal frames:
- 6 m frame spacing, 8 m eaves: alpha_cr approximately 8-12 (first-order sufficient)
- 8 m frame spacing, 10 m eaves: alpha_cr approximately 4-6 (first-order with amplification sufficient)
- 8 m frame spacing, 15 m eaves: alpha_cr approximately 2-3 (second-order required)
Loading — EN 1991 UK NA
Permanent actions (dead load):
- Steel self-weight: 0.3-0.6 kN/m2 (on plan area)
- Cladding and purlins: 0.15-0.25 kN/m2
- Services: 0.05-0.15 kN/m2
Variable actions (imposed and snow):
Per BS EN 1991-1-3 (Snow) with UK NA:
- UK snow load map zones 1-6, characteristic ground snow load sk = 0.25-1.0 kN/m2
- Roof snow load shape coefficient mu1 = 0.8 for pitched roofs to 30 degrees
Wind actions:
Per BS EN 1991-1-4 with UK NA:
- UK basic wind velocity vb,map = 21-28 m/s depending on location
- External pressure coefficients Cpe per Table 7.4a for duopitch roofs
- Internal pressure coefficient Cpi = +0.2 or -0.3 depending on openings
ULS Load Combinations for Portal Frames
UK NA Eq. 6.10b combinations for portal frames:
| Load Case | Description | Partial Factors |
|---|---|---|
| LC1 | Dead + Snow (leading) | 1.35Gk + 1.5Qk,snow (where xi = 0.925) |
| LC2 | Dead + Wind (leading, uplift) | 1.0Gk (favourable) + 1.5Qk,wind |
| LC3 | Dead + Snow + Wind | 1.35Gk + 1.5Qk,snow + 1.5 x psi_0 x Qk,wind |
| LC4 | Dead + Wind + Snow | 1.35Gk + 1.5Qk,wind + 1.5 x psi_0 x Qk,snow |
UK NA psi factors (combination coefficients):
- Snow (H <= 100 m): psi_0 = 0.5
- Wind: psi_0 = 0.5
- Imposed (roof): psi_0 = 0.7
Rafter Design — Worked Example
Problem: Design the rafter for a 30 m span portal frame at 6 m frame spacing with 8 m eaves height and 6-degree roof pitch.
Loading (plan area):
- Dead: gk = 0.50 kN/m2 (steel + cladding + services)
- Snow: sk = 0.60 kN/m2 (ground), on roof: 0.60 x 0.8 = 0.48 kN/m2
- Wind uplift (zone G, corner): we = -1.40 kN/m2 (net suction)
Line loads on rafter pair (6 m spacing):
- UDL dead along rafter slope: gk = 0.50 x 6.0 = 3.0 kN/m
- UDL snow along plan: sk_plan = 0.48 x 6.0 = 2.88 kN/m
Rafter section (trial): 457 x 191 UB 67 in S355
The rafter is subject to combined bending and axial compression (from frame action). The axial force in the rafter is typically highest near the eaves haunch and lowest at the apex. For a 30 m span frame, the maximum rafter axial compression is approximately 60-120 kN, which should be checked in combination with the bending moment per Clause 6.3.3.
Stability check (Clause 6.3.3 — members in bending and axial compression):
The rafter is laterally restrained by purlins at 1.8 m centres (typical UK purlin spacing). The bottom flange at the haunch is in compression and must be restrained at the haunch-to-rafter inflection point per SCI P399.
Typical UK rafter sections for 30 m span (6 m spacing):
| Span | Rafter Section | Utilisation Guide |
|---|---|---|
| 20 m | 356 x 171 UB 51 | ~80 % |
| 25 m | 406 x 178 UB 60 | ~75 % |
| 30 m | 457 x 191 UB 67 | ~80 % |
| 35 m | 533 x 210 UB 82 | ~75 % |
| 40 m | 610 x 229 UB 101 | ~80 % |
Column Design — Portal Leg
The portal frame column is subject to combined bending and axial compression. The maximum moment occurs at the eaves haunch connection, and the axial force is the vertical reaction from the rafter plus column self-weight.
Base fixity options:
| Base Type | Effect on Frame | Base Moment | UK Usage |
|---|---|---|---|
| Nominally pinned | Reduces foundation size; increases rafter moment ~15 % | ~0 | Most common |
| Fixed (moment-resisting) | Reduces rafter moment; increases foundation cost | 30-50 % of eaves moment | Larger spans (>35 m), crane buildings |
| Semi-rigid (partial fixity) | Intermediate | ~20 % of eaves moment | Rare in UK practice |
Typical UK column sections for portal frames:
- 30 m span, 8 m eaves: 356 x 368 UC 129 (pinned base) or 356 x 406 UC 235 (fixed base)
- 25 m span, 6 m eaves: 305 x 305 UC 97 (pinned base)
- 20 m span, 5 m eaves: 254 x 254 UC 89 (pinned base)
Deflection and SLS Checks
SCI P399 deflection limits:
- Rafter vertical deflection (total load): span/200 maximum (SCI P399)
- Eaves deflection (horizontal): height/150 to height/300 depending on cladding
- Snow load deflection: not explicitly limited unless brittle finishes are present
UK portal frames are typically deflection-governed. The rafter depth is often determined by SLS deflection rather than ULS strength. A span-to-depth ratio of 55-65 for rafters (L/D) is common for SLS-satisfactory frames at typical 6 degree roof pitches.
Construction Stage Considerations
UK practice for portal frame erection (BS EN 1090-2):
- Columns erected first, temporarily propped until rafters connected
- Rafter pairs assembled on ground and lifted as a pair (common for spans <= 30 m)
- First two frames erected, plumbed, and braced (provide longitudinal stability)
- Purlins and longitudinal ties installed sequentially
- Temporary bracing removed only after permanent bracing (roof bracing + wall bracing) is complete
Temporary works responsibility: The frame designer must specify any temporary restraint requirements during erection. The erector's method statement should address lateral stability of partially completed frames.
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Frequently Asked Questions
What is the most economical span for a UK portal frame?
UK industry experience indicates 25-35 m is the sweet spot for single-span portal frames with 6 m bay spacing. Spans below 20 m: the frame cost per square metre increases because fixed costs (connections, base plates, bracing) are spread over fewer square metres. Spans above 40 m: rafter sections become heavy (610UB+) and may require cellular beams or trusses to remain economical. For multi-span frames, 15-20 m internal spans with 25-30 m end spans are typical.
How do I size the eaves haunch for a UK portal frame?
SCI P399 provides haunch sizing guidance. The haunch length is typically 7-10 % of the span for standard frames. The haunch cutting depth (at the column face) is typically 1.8-2.2 x the rafter depth. The haunch is fabricated by cutting a UB section (usually one or two sizes larger than the rafter) diagonally, with the deeper end at the column face. The haunch flange and web are welded to the column flange and end plate. A 10 mm continuous fillet weld is standard for haunch-to-column and haunch-to-rafter connections.
When is second-order analysis required for UK portal frames?
The UK NA to EN 1993-1-1 requires second-order analysis (P-Delta) when alpha_cr < 10 for plastic analysis or alpha_cr < 3.0 for elastic analysis. Most UK portal frames use elastic analysis, so the 3.0 threshold applies. Slender frames (high eaves, narrow spans) and frames with heavy crane loads are most likely to require P-Delta analysis. SCI P399 provides a simplified alpha_cr estimation: alpha_cr approximately = (H / delta_H,Ed) x (N_Ed / H_Ed), approximately equal to the inverse of the first-order sway.
Related Pages
- UK Beam Design Guide — EN 1993-1-1 rafter and beam design
- UK Column Design Guide — UC section design to EN 1993-1-1
- UK Moment Connection Design — Eaves and apex connections
- UK Base Plate Design — Column base design per EN 1993-1-8
- UK Wind Load Reference — EN 1991-1-4 UK NA wind loads
- UK Snow Load Reference — EN 1991-1-3 UK snow loads
- UK Framing Systems — Portal, braced, and moment frames
Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK NA, SCI P399, and BS EN 1991 with UK National Annexes. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.