Portal Frame Design: A Practical Guide

Portal frames dominate the UK industrial building market. They’re economical, fast to erect, and provide clear spans that warehouses and manufacturing facilities need. After working on several F1 and industrial facilities, here’s my practical approach to portal frame design.

Why Portal Frames?

The economics are compelling:

For spans up to 50m, portal frames typically win on:

• Material cost
• Fabrication simplicity
• Erection speed
• Maintenance access

Initial Sizing

Frame Geometry

Eaves height: Driven by functional requirements

• Clear height needed for operations
• Add depth of rafter and purlins
• Consider future flexibility

Span: Column-free width required

• Plus twice wall thickness
• Check planning constraints

Roof pitch: 6° is common

• Steeper pitches reduce horizontal thrust
• Shallower pitches may need deflection checks
• Affects drainage design

Rule-of-Thumb Sizing

Rafters:

Span 15-20m: 457 UKB
Span 20-30m: 533 UKB
Span 30-40m: 610 UKB
Span 40-50m: 686 UKB or plate girder

Columns:

Height 6-8m: 457 UKC
Height 8-10m: 533 UKC
Height 10-12m: 610 UKC

Haunches:

• Length: ~10% of span
• Depth: ~2× rafter depth at column face

These are starting points - analysis will refine them.

Loading

Dead Loads

Typical cladding system:

Roof sheeting:          0.10 kN/m²
Insulation:             0.05 kN/m²
Purlins:               0.03 kN/m²
Services:              0.10 kN/m²
Total:                 0.28 kN/m² (use 0.30)

Imposed Loads

Per EC1-1-1 and UK NA:

• Category H (roof): 0.6 kN/m²
• Plus local concentration: 0.9 kN

Note: Don’t reduce for large areas in portal frame design - the frame reacts to total roof load.

Wind Loads

Critical for portal frames. Follow EC1-1-4:

1. Determine basic wind velocity (vb)
2. Calculate peak velocity pressure (qp)
3. Apply pressure coefficients (Cpe)
4. Consider internal pressure (Cpi = ±0.2 typically)

Wind load cases:

• Wind on gable
• Wind on side wall
• Wind at angle (corner suction)

Wind often governs rafter design (uplift) and always affects foundations.

Crane Loads

For buildings with overhead cranes:

• Vertical wheel loads
• Horizontal surge (acceleration/braking)
• Horizontal crab (across span)

Design consideration: Crane loading induces moment in columns. May need deeper columns or crane brackets.

Frame Analysis

Elastic vs Plastic Design

Elastic analysis:

• Simpler, more conservative
• Required for reversing moments (wind cases)
• Good for initial sizing

Plastic analysis:

• More economical (10-15% lighter frames)
• Requires Class 1 or 2 sections
• Needs stability checks at hinge locations

Key Checks

In-plane stability (sway):

• Check λcr (elastic critical load factor)
• If λcr > 10, first-order analysis OK
• If 3 < λcr < 10, use amplified moments
• If λcr < 3, second-order analysis required

Out-of-plane stability (LTB):

• Rafters restrained by purlins
• Columns restrained by side rails
• Check unrestrained lengths

Deflection limits:

• Eaves spread: height/150 (serviceability)
• Rafter deflection: span/200
• Crane runway: specific limits per crane spec

Haunched Connections

Why Haunches?

Haunches serve multiple purposes:

1. Increase moment capacity at column junction
2. Move plastic hinge away from connection
3. Reduce connection forces
4. Improve frame stability

Haunch Design

Cutting angle: Match rafter flange to haunch flange

• Typically 10-15° to horizontal
• Don’t exceed 45° (web slenderness issues)

Length:

• Long enough to reach sagging moment location
• 10-15% of span is typical

Stability:

• Haunch is an unrestrained compression flange
• Check LTB capacity
• May need restraints at haunch end

Column Bases

Pinned Bases

Standard for most portal frames:

• Smaller, cheaper foundations
• Base plate with 4 holding-down bolts
• Designed for shear and light tension (uplift)

Design checks:

• Base plate bending
• Bolt tension (wind uplift)
• Shear transfer (friction or bolts)
• Foundation design for overturning

Fixed Bases

Used when:

• Very high frames (reduces sway)
• Heavy crane loads
• Architectural requirements

Significantly larger foundations required - often not economical.

Bracing Systems

Roof Bracing

Purpose: Transfer wind loads to braced bays, provide rafter restraint

Layout:

• Braced bays at each end of building
• Additional braced bays every 30-40m
• Circular hollow sections common

Wall Bracing

Purpose: Resist wind on gables, provide column stability

Types:

• X-bracing (most efficient)
• K-bracing (allows doorways)
• Portal bracing (no diagonals)

Plan Bracing

At eaves level:

• Connects all rafters to braced bays
• Essential for load path continuity

Fire Design

Industrial buildings often have reduced fire requirements:

• Single-storey: 30 minutes boundary condition
• Boundary condition: collapse away from boundary

Portal frame fire engineering:

• Calculate rafter failure temperature
• Ensure collapse is inward (moment reversal)
• Check column base moment for reversed case

Practical Design Tips

From F1 Facility Experience

1. Services coordination: Mechanical equipment loads can be significant. Get input early.

2. Future flexibility: Size frames for potential future loads (PV panels, additional equipment).

3. Foundation conditions: Poor ground can shift economics toward lighter frames with bigger foundations.

4. Cladding interface: Ensure structure can support chosen cladding system and its movement.

5. Crane requirements: Crane supplier specifications override standard assumptions. Get crane data before finalizing design.

Common Mistakes

1. Undersizing columns for crane buildings - crane loads often govern
2. Forgetting in-plane restraints at haunch ends
3. Ignoring temperature effects in long buildings
4. Using incorrect pressure coefficients for wind loading
5. Not checking deflection under crane vertical loads

Software Options

Simple Portal Frames

• Tekla Structural Designer: Good for standard frames
• CSC Fastrak Portal Frame: Purpose-built, fast
• Excel spreadsheets: For initial sizing

Complex or Non-Standard

• Robot Structural Analysis: Full FE capability
• SAP2000/ETABS: When integration with other structures needed
• SCIA Engineer: Good for unusual geometries

Conclusion

Portal frames are deceptively simple. The structural form is clear, but getting optimal design requires balancing:

• Material efficiency
• Fabrication simplicity
• Erectability
• Building physics (thermal bridging, condensation)
• Future adaptability

Start with rules of thumb, refine with analysis, and always consider the complete building system - not just the frame in isolation.