How to Calculate Wind Load (EN 1991-1-4): Step-by-Step Guide

Wind load is one of the governing loads for roofs, façades, steel structures, silos, signs and machines working outdoors. This guide explains step by step how wind load is calculated to EN 1991-1-4 (Eurocode 1, Part 1-4). The numbers here only illustrate the method; in a real project the values depend on the site, the national annex and the project specification.

Key terms in wind load calculation

• Basic wind velocity (v_b): The reference wind speed at 10 m height given for the region in the national annex.
• Terrain category and roughness length (z₀): Describe how rough the surroundings of the structure are.
• Peak velocity pressure (q_p(z)): The design pressure at height z, including gusts.
• Pressure coefficients (c_pe, c_pi): Coefficients for external and internal pressure on a surface.
• Structural factor (c_sc_d): Accounts for size and dynamic behaviour; 1.0 for most ordinary structures.

Step 1: Basic wind velocity

v_b = c_dir · c_season · v_b,0

v_b,0 is taken from the wind map in the national annex. The directional factor c_dir and season factor c_season are usually 1.0.

Step 2: Basic velocity pressure

q_b = ½ · ρ · v_b²

Air density ρ is usually taken as 1.25 kg/m³.

Step 3: Terrain category

• Category 0: Sea or coastal area exposed to the open sea (z₀ = 0.003 m)
• Category I: Lakes or flat area without obstacles (z₀ = 0.01 m)
• Category II: Low vegetation and isolated obstacles (z₀ = 0.05 m)
• Category III: Regular cover of vegetation or buildings, villages and suburbs (z₀ = 0.3 m)
• Category IV: At least 15% of the surface covered with buildings averaging over 15 m in height (z₀ = 1.0 m)

Step 4: Peak velocity pressure q_p(z)

q_p(z) = [1 + 7 · I_v(z)] · ½ · ρ · v_m²(z)

Here the mean wind velocity is v_m(z) = c_r(z) · c_o(z) · v_b, the roughness factor c_r(z) = k_r · ln(z / z₀), the terrain factor k_r = 0.19 · (z₀ / 0.05)^0.07 and the turbulence intensity I_v(z) = 1 / (c_o(z) · ln(z / z₀)). On flat terrain the orography factor c_o = 1.0.

Step 5: Wind pressure on surfaces

w_e = q_p(z_e) · c_pe and w_i = q_p(z_i) · c_pi

External pressure coefficients are tabulated for zones A, B, C, D, E on walls and F, G, H, I on flat roofs. The net pressure on a surface is the difference between external and internal pressure. Suction at corners and edges is considerably higher than in the middle zones.

Step 6: Total wind force

F_w = c_sc_d · c_f · q_p(z_e) · A_ref

The force coefficient c_f depends on the shape of the structure or member (rectangular section, cylinder, lattice and so on).

Worked example

To illustrate the method with example values: v_b = 25 m/s, terrain category II, height z = 10 m.

• q_b = 0.5 × 1.25 × 25² ≈ 391 N/m² ≈ 0.39 kN/m²
• k_r = 0.19; ln(10 / 0.05) ≈ 5.30; c_r ≈ 1.01; v_m ≈ 25.2 m/s
• I_v ≈ 1 / 5.30 ≈ 0.19
• q_p(10) ≈ (1 + 7 × 0.19) × 0.5 × 1.25 × 25.2² ≈ 0.92 kN/m²
• On the windward wall (zone D) with c_pe,10 = +0.8: w_e ≈ 0.8 × 0.92 ≈ 0.74 kN/m²

Common mistakes

• Choosing the wrong terrain category for the surroundings
• Ignoring internal pressure (c_pi)
• Using c_pe,10 instead of c_pe,1 for cladding and fixings
• Missing the high suction at roof corners and edges (zones F and G)
• Not checking uplift from wind suction on light roofs

This guide is for information only; design calculations should be carried out by a competent engineer to the national annex and project specification. For calculation and reporting of wind, snow and seismic loads, see our structural analysis and load calculations service.

Frequently asked questions

Which standard is used to calculate wind load?

In Europe EN 1991-1-4 (Eurocode 1); in Türkiye it is published as TS EN 1991-1-4, and older Turkish projects may refer to TS 498. The standard to use depends on the project specification.

Can wind load be calculated in Excel?

Yes, the formulas can be put into a spreadsheet, but the basic wind velocity, terrain category and pressure coefficients must be chosen correctly. For critical structures the calculation should be checked by an engineer.

How do I choose the terrain category?

By the obstacles around the structure: open sea coast is category 0, flat open land category I, land with isolated obstacles category II, villages and suburbs category III, and dense city centres category IV.

Can you do the wind load calculation for us?

Yes. Send us the dimensions and location of the structure and we will calculate and report the wind, snow and seismic loads.

Related pages

• Structural analysis and load calculations
• How to calculate snow load
• Machine design
• Engineering services

Get your load calculation done

Send us the dimensions, location and any drawings of the structure, and we will calculate and report the wind, snow and seismic loads. WhatsApp: +90 547 909 99 90 · info@dokuti.com