Wind Load on Structures
Wind exerts pressure on building surfaces. Design wind loads account for basic wind speed, height above ground, exposure category, and building shape.
Basic Velocity Pressure
q = 0.613 × V² (SI, Pa)
q = 0.00256 × V² (Imperial, PSF)
V = design wind speed (m/s or mph)
Example: V=45 m/s (162 km/h):
q = 0.613 × 45² = 1241 Pa ≈ 1.24 kPaDesign Wind Pressure (ASCE 7 simplified)
p = q × G × Cp
G = gust factor (0.85 for rigid structures)
Cp = pressure coefficient (shape factor)
Windward wall: +0.8
Leeward wall: -0.5
Side wall: -0.7
Roof (flat): -0.7 to -1.3Exposure Categories
- B — Urban/suburban: trees/buildings ≥10m — lowest winds at low height
- C — Open terrain: scattered obstructions — most common
- D — Flat/open: coastlines, lakes — highest design pressures
Calculate wind loads: Free Wind Load Calculator
Basic Wind Pressure Formula
Dynamic wind pressure: q = ½ρV², where ρ = air density (~1.25 kg/m³), V = design wind speed (m/s). At 45 m/s (162 km/h): q = ½ × 1.25 × 45² = 1,266 Pa ≈ 1.27 kPa. Applied wind force: F = q × Cd × A, where Cd = drag (force) coefficient (flat plate normal to wind: Cd ≈ 1.3; circular cylinder: Cd ≈ 0.5; streamlined body: Cd ≈ 0.04) and A = exposed area. Building codes (Eurocode 1 Part 4, ASCE 7) add terrain factors, height factors, and pressure coefficients for different surfaces (windward wall, leeward wall, roof).
Design Wind Speeds
- UK (Eurocode 1): Basic wind speed v_b ≈ 22–27 m/s depending on region; exposed upland sites may reach 50 m/s gust.
- Hurricane categories: Cat 1: 119–153 km/h. Cat 3: 178–208 km/h. Cat 5: >252 km/h (70 m/s).
- Return period: Building codes typically design for 50-year return period wind (2% annual probability). Critical structures use 1,000 or 2,000 year return.
Frequently Asked Questions
What are pressure coefficients and why do they matter?
Wind creates positive pressure on windward faces and negative pressure (suction) on leeward faces and roofs. Pressure coefficients (Cp) from wind tunnel tests or code tables adjust q_ref to local surface pressure: p = q × Cp. Roof edges and corners experience amplified suction (Cp up to -2.0) — this is why cladding and fixings must be strongest at building corners. Overall structural forces sum positive and negative pressures across the whole building.
How does building height affect wind load?
Wind speed increases with height (logarithmic boundary layer profile). Building codes apply height correction factors: wind speed and pressure at 50 m height is significantly higher than at ground level. A 200 m tower faces pressures 3–4× higher than a single-storey building in the same location. For tall buildings, dynamic analysis is required to check resonance — building sway frequency must not match wind gust frequency.
What is the difference between along-wind and cross-wind response?
Along-wind response is drag in the wind direction — addressed by static equivalent load methods in codes. Cross-wind response is oscillation perpendicular to wind direction, caused by vortex shedding (alternating vortices cause lateral force oscillations). Vortex-induced vibration is critical for slender structures (chimneys, pylons, bridges) and requires dynamic analysis or aerodynamic shaping to avoid resonance.