------------------ | ---------- | -------------------- | ------------------ | | London | 21.7 | 1.0 | 21.7 | | Birmingham | 21.0 | 1.0 | 21.0 | | Manchester | 22.5 | 1.0 | 22.5 | | Edinburgh | 23.0 | 1.0 | 23.0 | | Cardiff | 22.7 | 1.0 | 22.7 | | Belfast | 23.5 | 1.0 | 23.5 | | Glasgow | 24.0 | 1.0 | 24.0 | | Inverness | 26.0 | 1.0 | 26.0 | | Coastal (NW Scotland) | 28.0 | 1.0 | 28.0 |

Values from UK NA Figure NA.1. For sites above sea level, multiply by caut = 1.0 + 0.001 × altitude (m).

Terrain Categories and Roughness Factors (UK NA)

Terrain Factor kr and Roughness Factor cr(z)

Terrain factor: ( kr = 0.19 \times (z_0 / z{0,II})^{0.07} )

Roughness factor: ( c_r(z) = k_r \times \ln(z / z_0) ) for z ≥ zmin

Peak Velocity Pressure (EN 1991-1-4 Clause 4.5)

[ q_p(z) = c_e(z) \times q_b ]

Where:

• ( q_b = 0.5 \times \rho \times v_b^2 ) — basic velocity pressure
• ( \rho = 1.25 , \text{kg/m}^3 ) — air density (UK NA)
• ( c_e(z) = [1 + 7 I_v(z)] \times 0.5 \times [c_r(z)]^2 ) — exposure factor

Peak Velocity Pressure Values (London, vb = 21.7 m/s)

qb = 0.5 × 1.25 × 21.7² = 294.3 Pa = 0.294 kN/m². Terrain categories per UK NA.

Pressure Coefficients for Typical UK Building Shapes

Vertical walls (EN 1991-1-4 Table 7.1):

Roof pressures for flat roofs (EN 1991-1-4 Table 7.2):

Worked Example — 30m × 20m × 15m Office Building in London

Given:

• Building: 30 m wide × 20 m depth × 15 m eaves height
• Flat roof, suburban location (Terrain III)
• London: vb,0 = 21.7 m/s

Step 1 — Basic wind velocity: vb = 21.7 m/s (cdir = cseason = 1.0)

Step 2 — Peak velocity pressure at roof level (z = 15 m): From table above, Terrain III at 15 m: qp(15) = 0.67 kN/m²

Step 3 — External pressure coefficients: Windward wall (Zone D, h/d = 15/20 = 0.75): cpe = +0.8 Leeward wall (Zone E, h/d = 0.75): cpe = -0.5 Roof (Zone H, flat roof): cpe = -0.7

Step 4 — Net wind pressure on windward frame: Windward wall: ww = 0.67 × 0.8 = 0.54 kN/m² Leeward wall: wl = 0.67 × (-0.5) = -0.34 kN/m² (suction) Total horizontal pressure on frame: 0.54 + 0.34 = 0.88 kN/m²

Step 5 — Total wind load on frame at 5 m grid: Tributary width = 6 m (assumed frame spacing) Wind load per floor level (3.5 m storey height): Fw = 0.88 × 6 × 3.5 = 18.5 kN per floor Total base shear: for 4 storeys: ~74 kN (ignoring roof contribution for conservatism)

Step 6 — Check dynamic response (EN 1991-1-4 Clause 6): Building height = 15 m < 50 m → no vortex shedding check required Building height/width = 15/30 = 0.5 < 6 → no galloping check Natural frequency > 1 Hz (typical for steel frame) → no dynamic amplification needed

UK NA Modifications Summary

Design Resources

• UK Snow Load — Snow loading to EN 1991-1-3
• UK Seismic Design — Seismic loading
• UK Framing Systems — Lateral stability systems
• UK Connection Design — Bracing connections
• UK Steel Beam Sizes — Section properties
• All UK References

Frequently Asked Questions

How is UK wind load calculated per EN 1991-1-4?

Basic wind velocity vb = cdir × cseason × vb,0 per EN 1991-1-4 Clause 4.2. UK NA gives vb,0 values for each wind zone from its wind map. Peak velocity pressure qp(z) = ce(z) × qb, where ce(z) is the exposure factor accounting for terrain roughness and orography. The UK wind map divides the country into zones with vb,0 ranging from 21 m/s (London and southeast) to 28 m/s (northwest Scotland).

What are the key UK NA wind load modifications?

UK NA specifies: basic wind velocity map with iso-velocity contours for the whole UK, simplified orography factor co = 1.0 except for isolated hills with specific shape criteria, terrain categories A-D with specific UK examples (Category 0 = sea/coast, Category I = open countryside), and air density ρ = 1.25 kg/m³ (matching the recommended value). The UK NA also provides specific altitude adjustment caut = 1.0 + 0.001 × site altitude (metres).

When is dynamic response analysis required for UK buildings?

Per EN 1991-1-4 Clause 6, dynamic response (gust factor method or vortex shedding) is required for: buildings taller than 50 m, structures with fundamental frequency < 1 Hz, slender structures with height/width > 6, and flexible roofs and canopies. Most UK steel-framed buildings below 30 m height are sufficiently stiff that a quasi-static approach with the gust factor cscd = 1.0 is appropriate.

What internal pressure coefficients apply to UK buildings?

Internal pressure coefficient cpi depends on the distribution of openings. For buildings with dominant openings (e.g., open loading bays): cpi = 0.75 × cpe (where cpe is the external pressure at the opening). For buildings without dominant openings: cpi = ±0.2 (the more onerous of positive or negative should be applied). For sealed buildings (no openings): cpi = 0.0. The UK NA does not modify these values.

How is wind load combined with other loads per UK NA?

The UK NA to EN 1990 specifies combination factors for wind with other variable actions. For ULS STR/GEO: 1.5 × wind + 1.5 × imposed (if unfavourable). For wind as the leading variable: ψ0 = 0.5 (imposed), ψ0 = 0.7 (snow). These are consistent with EN 1990 recommended values. Wind load effects are typically multiplied by 0.75 when combining with temperature effects.

Reference only. Verify all values against the current edition of EN 1991-1-4:2005 and UK NA (BS NA to EN 1991-1-4). This information does not constitute professional engineering advice.