UK Seismic Design — EN 1998-1 & UK NA Guide
This reference covers seismic design for UK steel structures per EN 1998-1:2004 and UK NA. The UK is classified as a region of low seismicity, but seismic design provisions still apply to certain building types and locations.
Design requirements, worked examples, and practical design guidance are provided for common design office applications.
Code Reference: EN 1998-1:2004 and UK NA
UK Seismic Zones (UK NA to EN 1998-1)
| Zone | Peak Ground Acceleration agR (g) | Geographical Area | UK NA Map Zone |
|---|---|---|---|
| 0 | 0.02g | Most of England, Wales | Zone 0 |
| 1 | 0.04g | Parts of NW England, Scotland | Zone 1 |
| 2 | 0.08g | Scottish Highlands | Zone 2 |
| 3 | 0.15g | Extreme NW Scotland | Zone 3 |
The UK NA to EN 1998-1 provides a simplified seismic hazard map dividing the UK into 4 zones. Most UK building projects fall in Zones 0-1 and qualify for low-seismicity exemptions.
Low Seismicity Exemptions (UK NA Clause NA.2.5)
Seismic design may be waived when the design ground acceleration ag ≤ 0.05g AND one of:
- Importance Class I or II
- Building ≤ 15 m height
- No public assembly use
This exempts approximately 80% of UK building projects from formal seismic design.
Ductility Classes
| Ductility Class | Behaviour Factor q (MRF) | Behaviour Factor q (CBF) | Design Requirements |
|---|---|---|---|
| DCL (low) | 1.5 | 1.5 | No specific ductility detailing |
| DCM (moderate) | 4.0 | 4.0 | Capacity design, specific rules |
| DCH (high) | 6.0 (MRF), 5.0 (EBF) | 4.0 (V-bracing) | Strict capacity design, testing |
UK NA modifications: DCM is the maximum permitted without special studies. DCH requires national approval per UK NA.
Steel Moment-Resisting Frames (EN 1998-1 Clause 6)
DCM moment frames design requirements:
| Requirement | DCM | DCH | Notes |
|---|---|---|---|
| Beam section class | Class 1 | Class 1 | Plastic hinge capacity |
| Column section class | Class 1 | Class 1 | Except at base |
| Strong column-weak beam | Ωmin > 1.3 | Ωmin > 1.3 | ΣMpl,c ≥ 1.3 ΣMpl,b |
| Panel zone shear | Check | Check | Web doubler plate if needed |
| Beam-to-column connections | Full-strength | Full-strength | Overstrength factor γov = 1.25 |
| P-Δ effects | θ ≤ 0.2 | θ ≤ 0.2 | Inter-storey drift sensitivity |
Design Response Spectrum (EN 1998-1 Clause 3.2)
The elastic response spectrum for UK zones:
- Type 1 spectrum (M > 5.5): appropriate for UK (low-to-moderate seismicity)
- Ground types A-E per EN 1998-1 Table 3.1
For UK Zone 1 (agR = 0.04g) on Ground Type B:
- S = 1.2 (soil factor)
- TB = 0.15s, TC = 0.5s, TD = 2.0s
- Peak spectral acceleration: Se(T) = ag × S × 2.5 = 0.04 × 1.2 × 2.5 = 0.12g
Worked Example — Seismic Check of a UK Office Building
Given:
- 6-storey braced steel frame, London (Zone 0: agR = 0.02g)
- Building height: 22 m
- Importance class II (γ1 = 1.0)
- Ground Type B
Step 1 — Check low-seismicity exemption: ag = γ1 × agR = 1.0 × 0.02g = 0.02g 0.02g < 0.05g, height = 22 m > 15 m → exemption does NOT fully apply But ag < 0.05g means simplified check only.
Step 2 — Base shear calculation (simplified lateral force method): For DCL (q = 1.5): Building period T ≈ 0.085 × H^0.75 = 0.085 × 22^0.75 = 0.085 × 10.2 = 0.87s
For TB = 0.15, TC = 0.5, TD = 2.0, Type 1: Se(T) = ag × S × 2.5 / q × (TC/T) = 0.02 × 1.2 × 2.5 / 1.5 × (0.5/0.87) = 0.023g
Step 3 — Base shear: Building weight W ≈ 6 × 5000 kN = 30,000 kN Fb = Se(T) × W / g = 0.023 × 30,000 = 690 kN
Compared to wind base shear (typically 1000-1500 kN for this building): Seismic governs only ≈ 50-70% of wind. Wind design is likely to govern for this London office.
Steel Braced Frame Design for Seismic (EN 1998-1 Clause 6.7)
Concentric braced frames (CBF) — DCM requirements:
- Diagonal braces: Class 1 cross-section, slenderness λ¯ ≤ 2.0
- Brace overstrength: Npl,Rd,brace ≥ NEd,seismic (capacity design)
- Brace-to-gusset connections: designed for 1.1 × γov × Npl,Rd,brace
- Columns: verified for 1.1 × γov × Npl,Rd,brace from all braces at level
- Beams in V-braced bays: designed for unbalanced forces after brace buckling
Behaviour factors for braced frames:
| Frame Type | DCL (q) | DCM (q) | Notes |
|---|---|---|---|
| Cross-bracing (X) | 1.5 | 4.0 | Tension-only OK? Yes |
| V-bracing | 1.5 | 2.5 | Compression buckling governs |
| Inverted V (Chevron) | 1.5 | 2.5 | Beam must resist unbalanced load |
| K-bracing | 1.5 | NOT permitted | Per EN 1998-1 Clause 6.7 |
Capacity Design Principles
Capacity design ensures a controlled ductile mechanism by:
- Weak beam / strong column: Plastic hinges form in beams, not columns
- Overstrength factor: γov = 1.25 for steel (actual yield > nominal)
- Connections: Designed for plastic capacity of connected member × 1.1 γov
- Non-dissipative members: Remain elastic under seismic combination
Design Resources
- UK Framing Systems — Structural systems
- UK Connection Design — Ductile connection detailing
- UK Wind Load — Lateral load comparison
- UK Column Design — Column buckling
- UK Steel Beam Sizes — Section data
- All UK References
Frequently Asked Questions
Does the UK require seismic design of steel structures?
UK NA to EN 1998-1 defines the UK as a region of low seismicity. Seismic design is required for building importance classes II-IV in certain regions. Ground acceleration agR ranges from 0.02g to 0.15g. The UK NA provides a low-seismicity exemption: when ag ≤ 0.05g and the building height ≤ 15 m, no seismic design is required. For most UK building projects (offices, residential up to 5 storeys), seismic does not govern compared to wind loading, but the design must still document the exemption.
What ductility classes apply per UK NA?
UK NA permits DCL (low ductility, q = 1.5) for buildings up to 15 m height. DCM (moderate ductility) with q = 4.0 for moment-resisting frames and q = 4.0 for concentric braced frames can be used for taller buildings. DCH (high ductility, q = 6.0 for MRF) requires special national approval per UK NA. Most UK buildings that require seismic design use DCL since the low seismicity demand is adequately accommodated within normal elastic design with q = 1.5.
What are the key capacity design requirements for UK steel seismic frames?
Capacity design requires: (a) strong column-weak beam verification (ΣMpl,c ≥ 1.3 ΣMpl,b at connections), (b) connection overstrength: joints designed for 1.1 × γov × member plastic capacity, (c) brace overstrength for braced frames: gusset plates and connections designed for 1.1 × γov × Npl,Rd of the brace, and (d) beam design in V-braced bays for the unbalanced force after one brace buckles in compression (0.3 × Npl,Rd on compression side, Npl,Rd on tension side).
How does UK seismic compare to wind design?
For UK conditions, wind loading typically governs the lateral design for buildings up to 50-80 m height, depending on location and structural form. For a 10-storey London office (Zone 0, ag = 0.02g): seismic base shear ≈ 0.02 × building weight ≈ 400 kN, while wind base shear ≈ 1500 kN. Wind governs by a factor of 3-4×. For a similar building in NW Scotland (Zone 3, ag = 0.15g): seismic base shear ≈ 0.15 × building weight ≈ 3000 kN, which may equal or exceed wind. In practice, low-seismicity exemptions cover most UK construction.
What detailing requirements apply for DCM steel frames?
DCM steel frames require: (a) Class 1 cross-sections for all dissipative members (to allow plastic hinge formation), (b) beam slenderness limits L/b ≤ 60 for lateral stability, (c) column slenderness λ¯ ≤ 1.5 under seismic combination, (d) panel zone shear resistance checked using EN 1993-1-8 rules with overstrength, (e) brace slenderness λ¯ ≤ 2.0 and bracing member cross-section Class 1 for CBF, (f) beam-to-column connections verified for 1.1 × γov × Mpl,Rd of the connected beam.
Reference only. Verify all values against the current edition of EN 1998-1:2004 and UK NA. This information does not constitute professional engineering advice.