UK Steel Warehouse Design -- Portal Frame Construction to EN 1993

The single-storey steel portal frame is the default structural form for UK warehouses, distribution centres, and industrial buildings. Its dominance -- approximately 95% of single-storey industrial buildings in the UK -- reflects its structural efficiency, speed of construction, and adaptability to future expansion. This reference presents the complete design process for a UK steel warehouse portal frame, from scheme design through to detailed member verification, with a worked example for a 30 m span x 60 m long warehouse.

Warehouse Layout and Dimensional Conventions

A typical UK warehouse portal frame has the following dimensional parameters:

A 30 m span frame with 7.5 m bays and 8 m eaves height is a common UK configuration. The roof pitch of 6 degrees provides adequate drainage without excessive steel weight.

Design Loading for UK Warehouses

Dead Loads

• Roof cladding (built-up system, 0.7 mm steel outer, insulation, liner): 0.12-0.15 kN/m^2 on slope
• Purlins (Z-section at 1.8 m centres): 0.04-0.06 kN/m^2
• Services (lighting, sprinklers, cable trays): 0.05-0.10 kN/m^2
• Ceiling (if suspended): 0.10-0.20 kN/m^2
• Bracing self-weight: negligible for scheme design

Total roof dead load: typically 0.20-0.35 kN/m^2 on slope for a standard warehouse.

Imposed Loads

• Snow load per BS EN 1991-1-3 + UK NA:

  • Ground snow load map: varies from 0.25 kN/m^2 (London) to 1.00 kN/m^2 (Scottish Highlands)
  • Shape coefficient for monopitch 0-30 deg: mu_1 = 0.8
  • For Leicester: s = 0.8 x 0.50 = 0.40 kN/m^2 on plan

• Wind load per BS EN 1991-1-4 + UK NA:

  • Basic wind speed vb,map: 21.0-23.5 m/s for inland UK
  • Peak velocity pressure qp for a 10 m building in open country: approximately 0.75-0.90 kN/m^2
  • External pressure coefficients for duo-pitch roof depend on the ratio h/d and roof angle
  • Cpe for 6 deg roof, h/d = 0.3, windward slope: -1.2 to -1.4 (suction) for the critical wind direction

Wind uplift is typically the governing load case for portal frame rafter design, not gravity (dead + snow). The combination of external suction on the roof and internal pressure (door open) produces net uplift pressures of 1.0-1.5 kN/m^2.

Load Combinations

Per EN 1990 + UK NA, the following ULS combinations govern portal frame design:

• EQU 1: 1.10 x Gk + 1.50 x Qk,snow (snow governing, wind suction beneficial)
• EQU 2: 1.10 x Gk + 1.50 x Qk,wind (wind governing, snow beneficial or absent)
• STR 1: 1.35 x Gk + 1.50 x Qk,snow + 1.50 x psi_0 x Qk,wind (snow primary)
• STR 2: 1.35 x Gk + 1.50 x Qk,wind + 1.50 x psi_0 x Qk,snow (wind primary)

The UK NA specifies psi_0 = 0.5 for snow (altitude <= 100 m) and psi_0 = 0.5 for wind.

Frame Sizing -- Rules of Thumb

For scheme design (RIBA Stage 2), the following span-to-depth ratios provide initial member sizes:

For a 30 m span portal frame:

• Column and rafter: 457 x 191 x 67 UB in S355J2 (typical for medium-height warehouse)
• For 8 m eaves height with a 5-tonne underslung crane: upgrade to 533 x 210 x 82 UB
• For 40 m span: 610 x 229 x 101 UB columns with 533 x 210 x 82 UB rafters

These are indicative -- the final section must be verified by frame analysis (elastic or plastic) with second-order effects.

Portal Frame Analysis

Elastic vs Plastic Design

UK practice permits both elastic and plastic design of portal frames:

• Elastic design: First-order or second-order analysis with moment amplification factors. MEd <= Mel,Rd. Used for frames where plastic hinges would form in locations with low rotation capacity (e.g., adjacent to bolted splices).

• Plastic design: First-order plastic analysis with Merchant-Rankine second-order check, or second-order plastic analysis (advanced). Allows moment redistribution through plastic hinge formation. Requires Class 1 sections at hinge locations.

Most UK portal frames are designed elastically with second-order effects accounted for by the Merchant-Rankine formula or amplified sway method.

Frame Imperfections

EN 1993-1-1 Clause 5.3.2 requires frame imperfections to be modelled:

• Global sway imperfection: phi = phi_0 x alpha_h x alpha_m

  • phi_0 = 1/200
  • alpha_h = 2/sqrt(h) = 2/sqrt(8.0) = 0.707
  • alpha_m = sqrt(0.5 x (1 + 1/m)) = sqrt(0.5 x (1 + 1/1)) = 1.00 (single bay)
  • phi = (1/200) x 0.707 x 1.00 = 1/283

  Equivalent horizontal forces (EHF) = phi x NEd applied at each column top.

Serviceability Deflections

The SCI guide to portal frame design recommends:

• Vertical deflection at apex (characteristic snow): span/250 = 120 mm for 30 m span
• Horizontal deflection at eaves (characteristic wind): height/200 to height/300
• Total vertical deflection (dead + snow): span/200 (to avoid ponding on near-flat roofs)

Pre-camber is typically specified as 50-75% of the dead load deflection, milled into the rafter during fabrication.

Bracing Systems

Roof Bracing

The roof must be braced in its plane to transfer wind loads on the gable ends to the vertical bracing in the side walls. For a 30 m x 60 m warehouse:

• Wind girder at each end bay: Cross-bracing (flat pattern) using 20-25 mm diameter Macalloy bars or CHS sections. For a 7.5 m wide end bay, the wind girder transfers approximately 25-35 kN to the side wall bracing.
• Intermediate purlins: The roof sheeting acts as a diaphragm, transferring wind loads from intermediate purlins to the wind girders. The sheeting-to-purlin fasteners must be checked for the diaphragm shear flow.

Vertical Bracing (Side Walls)

Vertical bracing in the side walls transfers the roof wind girder reactions and longitudinal wind loads to the foundations:

• Cross-bracing in end bays: Typically 25 mm diameter Macalloy bars, angles, or CHS sections
• Portalised bays: Where cross-bracing would obstruct door openings, a moment-resisting bay (portalised frame) is used

Column Base Fixity

UK portal frames typically use nominally pinned bases to avoid transferring moments into the foundations (which increases foundation size and cost). The base plate is designed for shear and axial load only. If frame deflections are excessive with pinned bases, a semi-rigid base detail (4 holding-down bolts inside the column section, stiff base plate) can be used to provide partial rotational restraint.

Worked Example -- 30 m x 60 m Warehouse

A distribution warehouse in Leicester has:

• Span: 30 m clear (single-span, no internal columns)
• Length: 60 m (8 bays at 7.5 m)
• Eaves height: 8.0 m
• Roof pitch: 6 degrees
• No crane

Step 1 -- Scheme design:

• Frame spacing: 7.5 m (8 frames)
• Estimated rafter weight: 30 x 1.0 / 50 = 0.60 kN/m (approx 60 kg/m)
• Column: 457 x 191 x 67 UB, Rafter: 457 x 191 x 67 UB
• Purlin: Z 202 x 65 x 1.8 at 1.8 m centres (5 purlins per rafter) in S450GD

Step 2 -- Loading on frame:

• Dead: roof = 0.20 x 7.5 = 1.50 kN/m on plan; self-weight: frame sections generate 0.67 + 0.67 = 1.34 kN/m average along rafter
• Snow: 0.40 x 7.5 = 3.00 kN/m on plan
• Wind uplift: -1.2 x 0.80 x 7.5 = -7.20 kN/m on plan (suction)

Step 3 -- Steel tonnage estimate:

• Frames: 8 frames x (8 m columns x 2 + 15.2 m rafter x 2) x 67 kg/m = 8 x (16.0 + 30.4) x 0.067 = 8 x 46.4 x 0.067 = 24.9 tonnes
• Purlins: 60 m / 1.8 m = 34 purlin lines x 30 m x 5.2 kg/m = 34 x 30 x 0.0052 = 5.3 tonnes
• Side rails: 8 m x 1.8 m spacing x 60 m perimeter x 4.2 kg/m = 5.0 tonnes approx
• Bracing + connections + cleats: 15% allowance = 5.3 tonnes
• Total estimated steel: 40.5 tonnes (approximately 22.5 kg/m2 GFA)

Step 4 -- Budget cost:

• 40.5 tonnes at £2,000/t fabricated and erected = £81,000 for steelwork
• Add foundations, floor slab, cladding, M&E for total building cost

Design Resources

• UK Portal Frame Design — Detailed portal frame to EN 1993
• UK Cold-Formed Purlin Design — Purlin sizing and manufacturer tables
• UK Steel Framing Cost Guide — £/t and £/m2 cost data
• UK Wind Load Guide — Wind loading to BS EN 1991-1-4 + UK NA
• UK Snow Load Guide — Snow loading to BS EN 1991-1-3 + UK NA

Frequently Asked Questions

What is the most economical span for a UK steel portal frame warehouse?

The economic sweet spot is 20-35 m clear span. Below 20 m, the frame weight per m2 increases because the frame elements become proportionally heavier relative to the enclosed area. Above 35 m, the rafter section becomes disproportionately heavy due to second-order P-delta effects and lateral-torsional buckling of the deep slender section. For spans above 40 m, a tied portal (with a tie rod at eaves level) or a truss becomes more economical than a conventional portal frame.

How does a portal frame work structurally?

The portal frame resists vertical loads (dead + snow) through frame action, not beam action. The eaves moment connection transfers moment from the rafter into the column, reducing the rafter mid-span moment by approximately 40-50% compared to a simply supported beam of the same span. The apex connection is typically a moment-resisting splice, providing continuity but at a location of lower moment. Under wind uplift, the frame action reverses, with the eaves connection carrying moments of opposite sign. The haunch at the eaves provides the necessary depth for the bolt group to develop the eaves moment resistance.

When do I need a haunch at the eaves?

A haunch is required when the rafter depth alone is insufficient to accommodate the bolt group required for the eaves moment connection. The haunch increases the lever arm for the bolt group from the rafter depth (450-500 mm) to the haunch depth (750-1000 mm), increasing moment resistance by 50-100%. Haunches are standard in UK portal frames above 20 m span. Below 15 m span, a flush end plate on the column flange may suffice.

What fire resistance is required for a UK warehouse portal frame?

For a single-storey warehouse used for storage (Purpose Group 3 under Approved Document B), the fire resistance requirement is typically 30 minutes for the structural frame if the floor area is less than 20,000 m2 and the building is fitted with automatic fire detection. For larger warehouses or higher-risk storage (Purpose Group 4), 60 minutes may be required. Portal frames often use unprotected steel (zero fire resistance) where the portal rafter is more than 5 m above floor level and the building has adequate ventilation, relying on the boundary distances rather than fire resistance of the frame.

Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex, BS EN 1991-1-3, BS EN 1991-1-4, and BS EN 1990. 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.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.