Site Wind Pressure

How it calculates

The Site Wind Pressure calculator follows the EN 1991-1-4:2005 procedure to derive the peak velocity pressure qp at a given reference height, using the project address or coordinates to source wind speed and altitude data automatically.

Inputs and configuration

Site location - Project latitude, longitude, and altitude are read from Project Defaults when you enter an address or coordinates. For supported National Annexes, the fundamental basic wind velocity vb,0 and altitude correction parameters are sourced from the corresponding wind maps without manual lookup.

National Annex - Select from UK (BS EN 1991-1-4), Sweden (EKS/BFS 2024), Denmark, Norway, or Ireland. For any other NA, use "Other / Manual" mode and enter the peak velocity pressure directly.

Height and terrain - Reference height above ground z (m) and displacement height h_dis (m) define the effective height z - h_dis used in roughness calculations. Terrain category (0 to IV for UK; I to IV for other NAs) sets the roughness length z0 and minimum height zmin from Table 4.1. Both can be auto-detected from GIS data or set manually.

Season and directional factors - Season factor c_season (default 1.0) and directional factor cdir (from the NA or default 1.0) scale the fundamental wind velocity. Orography tables per wind direction sector are available for sites in Significant Orography regions.

Calculation chain (EN 1991-1-4:2005)

Step 1 - Basic wind velocity (Cl 4.2)

vb = cdir x cseason x vb,0

where vb,0 is the fundamental basic wind velocity from the NA wind map for the project location.

Step 2 - Terrain roughness (Cl 4.3.2, Table 4.1)

The terrain factor kr, roughness length z0, and minimum height zmin are taken from Table 4.1 for the selected terrain category. The roughness factor at height z is:

cr(z) = kr x ln(z / z0) for z ≥ zmin

where kr = 0.19 x (z0 / z0,II)^0.07 and z0,II = 0.05 m (Terrain Category II reference).

Step 3 - Turbulence intensity (Cl 4.4)

Iv(z) = kI / (co(z) x ln(z / z0))

where kI is the turbulence factor from the National Annex (default 1.0) and co(z) is the orography factor.

Step 4 - Mean wind velocity (Cl 4.3.1)

vm(z) = cr(z) x co(z) x vb

Step 5 - Peak velocity pressure (Cl 4.5)

qp(z) = (1 + 7 x Iv(z)) x 0.5 x rho x vm(z)^2

where rho is the density of air (1.25 kg/m3 default). This is equivalent to:

qp(z) = ce(z) x qb

where qb = 0.5 x rho x vb^2 is the basic velocity pressure and ce(z) is the exposure factor combining roughness and turbulence.

Orography factor (Annex A.3)

When the site lies within a Significant Orography region (upwind slope greater than 0.05 per Cl 4.3.3), the orography factor co is computed per Annex A.3 for each wind direction sector. The calculator evaluates the shape factor s and the effective upwind slope phi, then applies:

co(z) = 1 + 2 x s x phi for hills and ridges (co ≥ 1.0)

A per-direction table of peak velocity pressures is produced and the governing maximum qp is reported as the design value.

Output and assumptions

The primary output is Maximum Peak Velocity Pressure qp (kN/m2), tabulated by wind direction and used directly as the characteristic wind action in EN 1991-1-4 external and internal pressure calculations.

Key assumptions:

• No allowances for local wind accelerations (edge effects or funnelling) - see EN 1991-1-4 Cl 4.3.4 and relevant NA clauses
• Torsional effects from asymmetric wind forces are not considered (Cl 7.1.2)
• Only listed National Annexes are fully supported; for any other NA, use "Other / Manual" mode