UK Steel Framing Systems — EN 1993 Design Guide
Steel framing systems provide the primary lateral and gravity load-resisting structure for UK steel buildings. EN 1993-1-1 and UK practice covers design requirements for various system types.
Frame selection depends on building height, seismicity, wind loads, and architectural requirements. Each system has specific ductility, stiffness, and detailing requirements.
Code Reference: EN 1993-1-1 and UK practice
Framing Systems Overview
| System | Typical Height | Span Range | Lateral Stability | UK Common Use |
|---|---|---|---|---|
| Braced frame | 2-30 storeys | 6-15 m | X-bracing, K-bracing | Office buildings |
| Portal frame (pinned base) | 1-3 storeys | 15-60 m | Frame action + bracing | Industrial, warehouses |
| Portal frame (fixed base) | 1-5 storeys | 15-40 m | Frame action | Sports halls, retail |
| Moment-resisting frame | 2-15 storeys | 6-12 m | Beam-column connections | Perimeter stability |
| Concrete core + steel frame | 10-50+ storeys | 6-18 m | Concrete core wall | High-rise towers |
| Diagrid | 20-60+ storeys | 6-24 m | Triangular grid | Landmark buildings |
Braced Frame Systems
Braced frames are the most common system for UK multi-storey buildings, providing efficient lateral stability through vertical truss action.
Bracing configurations:
| Type | Efficiency | Ductility | Architectural Impact | UK Preference |
|---|---|---|---|---|
| X-bracing (tension-only) | High | Moderate | High | Typical, angles |
| Chevron (V-bracing) | Moderate | Moderate | Moderate | Crane bays |
| K-bracing | Moderate | Moderate | Moderate | Less common |
| Eccentric brace (EBF) | High | High | Moderate | Seismic zones |
| Buckling-restrained (BRB) | Very high | Very high | High | Specialised |
Typical bracing member design:
For X-bracing in tension only per EN 1993-1-1 Clause 6.2.3: [ N*{t,Rd} = A \times f_y / \gamma*{M0} \quad \text{(yield on gross section)} ] [ N*{u,Rd} = 0.9 A*{net} \times fu / \gamma{M2} \quad \text{(fracture on net section at connections)} ]
Angles for bracing: typically 80×80×8 to 150×150×15 in S355.
Portal Frame Design (EN 1993-1-1)
Portal frames are the dominant system for UK single-storey industrial buildings. Key design aspects:
- Rafter stability: Haunched at eaves, lateral restraint from purlins at 1.5-2.0 m centres
- Column stability: Major axis — frame sway. Minor axis — fly bracing at 1.5-2.5 m intervals
- Base fixity: Nominally pinned for economy, fixed for taller frames or higher crane loads
- Plastic design: Generally used for pinned-base portals (Class 1 sections required)
Quick Portal Frame Sizing Guide
| Span (m) | Eaves Height (m) | Rafter Section | Column Section | Haunch Depth | Frame Spacing |
|---|---|---|---|---|---|
| 15 | 5-6 | 356×171 UB 51 | 203×203 UC 46 | 500 mm | 6-7.5 m |
| 20 | 6-7 | 406×178 UB 60 | 203×203 UC 71 | 700 mm | 6-7.5 m |
| 25 | 7-8 | 457×191 UB 67 | 254×254 UC 73 | 800 mm | 6.0 m |
| 30 | 8-9 | 533×210 UB 92 | 254×254 UC 89 | 1000 mm | 6.0 m |
| 40 | 9-10 | 610×229 UB 101 | 305×305 UC 97 | 1200 mm | 5.0-6.0 m |
| 50 | 10-12 | 762×267 UB 173 | 356×368 UC 153 | 1500 mm | 5.0-6.0 m |
Based on S355 steel, typical UK loading (0.6 kN/m² roof dead + 0.6 kN/m² imposed + UK wind)
Frame Classification (EN 1993-1-1 Clause 5.2)
| Frame Type | Classification | αcr = Fcr / FEd | Design Method |
|---|---|---|---|
| Non-sway (braced) | Rigid-plastic | αcr ≥ 10 | First-order (or 2nd order with amplification) |
| Non-sway (braced) | Elastic | αcr ≥ 15 | First-order elastic |
| Sway (unbraced) | — | αcr ≥ 10 | First-order with amplification |
| Sway (unbraced) | — | 3 ≤ αcr < 10 | Second-order analysis required |
| Sway (unbraced) | — | αcr < 3 | Significant second-order effects, more rigorous analysis |
Composite Floor Systems
UK multi-storey buildings use composite steel-concrete floors:
| System | Slab Depth (mm) | Span (m) | Typical Use | Advantages |
|---|---|---|---|---|
| Metal deck composite | 130-200 | 2.5-4.5 | Offices | Fast construction, shallow |
| Precast hollowcore | 150-400 | 6-18 | Car parks, residential | Long spans, no propping |
| Composite beam (shallow) | 300-500 | 6-12 | Offices, hospitals | Reduced floor zone |
| Stub girder | 600-1000 | 12-24 | Long-span areas | Service integration |
| Slim floor | 280-400 | 6-10 | Residential, offices | Flat soffit |
Stability and Second-Order Effects
Global second-order effects (P-Δ): For sway frames, the amplification factor: [ \delta = \frac{1}{1 - 1/\alpha_{cr}} ]
Where αcr = Vcr/Ved is the ratio of elastic critical buckling load to design vertical load.
Member second-order effects (P-δ): Evaluated through buckling checks to Clause 6.3.
Design Resources
- UK Beam Design — Beam design
- UK Column Design — Column buckling
- UK Connection Design — Frame connections
- UK Wind Load — Lateral loading
- UK Seismic Design — Seismic considerations
- UK Steel Beam Sizes — Section data
- All UK References
Frequently Asked Questions
What framing systems are typical in UK steel buildings?
UK steel buildings commonly use braced frames (with cross bracing or K-bracing) for low/mid-rise, portal frames for industrial buildings, and moment frames for perimeter stability. For multi-storey buildings (5-20 storeys), a reinforced concrete core with surrounding steel frame is increasingly common. For high-rise (20+ storeys), concrete core or diagrid systems with outrigger trusses provide the necessary stiffness.
How are UK steel frames designed for stability?
EN 1993-1-1 Clause 5.2 classifies frames as sway (elastic or rigid) or non-sway. UK practice typically uses braced bays with vertical X-bracing and horizontal diaphragm action through composite floors. The αcr parameter determines the need for second-order analysis. For αcr ≥ 10 (plastic) or 15 (elastic), first-order analysis is sufficient. Braced frames are classified as non-sway, allowing simpler first-order design and stability checks per Clause 6.3.
What are the typical bay sizes for UK office buildings?
UK office buildings typically use 6 m × 6 m to 7.5 m × 9 m grid patterns. The primary beams span between columns, with secondary beams at 3 m centres (decking span). For cellular office layouts, 7.5 m × 7.5 m is common. For open-plan offices, spans of 12-15 m are possible with deeper beams or cellular beams (with service openings). The floor-to-floor height is typically 3.5-4.0 m for offices, allowing 300-500 mm floor zone depth.
How do UK portal frames differ from braced frames?
Portal frames (EN 1993-1-1 Clause 5.2) rely on flexural action at connections (eaves and apex) for stability, using moment-resisting joints. They are typically pinned at the base. Braced frames rely on triangulated vertical bracing for lateral stability with simple (pinned) beam-to-column connections. Portal frames dominate single-storey construction (warehouses, factories, sports halls). Braced frames dominate multi-storey construction (offices, apartments, hotels).
What steel sections are used for UK portal frame rafters?
Portal frame rafters typically use UB sections in S355 steel: from 356×171 UB 51 for 15 m spans to 762×267 UB 173 for 50 m spans. Rafters are haunched at the eaves (typically 10-15% of span) to resist the peak moment. Haunches are fabricated from the same section or a deeper section, with stiffened web panels at the change of depth. Purlins are cold-formed Z-sections (C200 to C300) spaced at 1.5-2.0 m centres with bridging at third points.
Reference only. Verify all values against the current edition of EN 1993-1-1 and UK practice. This information does not constitute professional engineering advice.