1. Basic Wind Velocity (vb)
Eurocode 1 Part 1-4 (EN 1991-1-4) determines wind forces based on site location, elevation, and terrain roughness. Fundamental basic wind velocity vb,0 is taken from National Annex maps (10-minute mean wind speed at 10 m height in open countryside, 50-year return period).
Where cdir is direction factor (typically 1.0) and cseason is season factor (1.0 for permanent structures).
2. Peak Velocity Pressure qp(z)
Mean wind velocity vm(z) varies with height z due to terrain roughness cr(z) and topography co(z):
Accounting for turbulence intensity Iv(z) = kI / [co(z) · ln(z/z0)], the peak velocity pressure qp(z) is calculated as:
Where air density ρ = 1.25 kg/m3.
3. External & Internal Pressure Coefficients
Wind pressures acting on external surfaces (we) and internal surfaces (wi) are evaluated separately:
Net pressure on wall or cladding element is wnet = we - wi.
| Building Zone | Description | Typical cpe,10 Value |
|---|---|---|
| Zone A | Windward edge of wall / roof (high suction) | -1.2 to -1.4 |
| Zone B / C | Intermediate suction zones on side walls | -0.8 to -1.0 |
| Zone D | Windward front vertical wall (positive pressure) | +0.8 |
| Zone E | Leeward rear vertical wall (suction) | -0.5 to -0.7 |
4. Total Wind Force on Global Structure
The total overall wind force Fw on a building structure or structural frame considering dynamic response is:
Where cscd is the structural factor combining size effect (cs) and dynamic resonance effect (cd). For buildings under 15 m height, cscd = 1.0 is generally conservative.
Calculation Checklist
- 1
Determine Site Factors
Establish vb,0, select Terrain Category (0 to IV), and calculate cr(z).
- 2
Calculate Peak Velocity Pressure
Compute qp(z) across building height sections.
- 3
Map Zoning & Pressure Coefficients
Divide walls/roof into EN 1991-1-4 zones (A through I) and select cpe,10 and cpi.
- 4
Apply Structural Factor & Combinations
Combine wind loads with ULS combinations per EC0 (1.35Gk + 1.5Qk,wind).