| 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 80ÃÂÃÂ80ÃÂÃÂ8 to 150ÃÂÃÂ150ÃÂÃÂ15 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 | 356ÃÂÃÂ171 UB 51 | 203ÃÂÃÂ203 UC 46 | 500 mm | 6-7.5 m |
| 20 | 6-7 | 406ÃÂÃÂ178 UB 60 | 203ÃÂÃÂ203 UC 71 | 700 mm | 6-7.5 m |
| 25 | 7-8 | 457ÃÂÃÂ191 UB 67 | 254ÃÂÃÂ254 UC 73 | 800 mm | 6.0 m |
| 30 | 8-9 | 533ÃÂÃÂ210 UB 92 | 254ÃÂÃÂ254 UC 89 | 1000 mm | 6.0 m |
| 40 | 9-10 | 610ÃÂÃÂ229 UB 101 | 305ÃÂÃÂ305 UC 97 | 1200 mm | 5.0-6.0 m |
| 50 | 10-12 | 762ÃÂÃÂ267 UB 173 | 356ÃÂÃÂ368 UC 153 | 1500 mm | 5.0-6.0 m |
Based on S355 steel, typical UK loading (0.6 kN/mÃÂò roof dead + 0.6 kN/mÃÂò imposed + UK wind)
Frame Classification (EN 1993-1-1 Clause 5.2)
| Frame Type | Classification | ÃÂñcr = Fcr / FEd | Design Method |
|---|---|---|---|
| Non-sway (braced) | Rigid-plastic | ÃÂñcr âÃÂÃÂ¥ 10 | First-order (or 2nd order with amplification) |
| Non-sway (braced) | Elastic | ÃÂñcr âÃÂÃÂ¥ 15 | First-order elastic |
| Sway (unbraced) | — | ÃÂñcr âÃÂÃÂ¥ 10 | First-order with amplification |
| Sway (unbraced) | — | 3 âÃÂä ÃÂñcr < 10 | Second-order analysis required |
| Sway (unbraced) | — | ÃÂñcr < 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-ÃÂÃÂ): For sway frames, the amplification factor: [ \delta = \frac{1}{1 - 1/\alpha_{cr}} ]
Where ÃÂñcr = Vcr/Ved is the ratio of elastic critical buckling load to design vertical load.
Member second-order effects (P-ÃÂô): 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 ÃÂñcr parameter determines the need for second-order analysis. For ÃÂñcr âÃÂÃÂ¥ 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 ÃÂÃÂ 6 m to 7.5 m ÃÂÃÂ 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 ÃÂÃÂ 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 356ÃÂÃÂ171 UB 51 for 15 m spans to 762ÃÂÃÂ267 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.