Eurocode Wind Load Calculator

What it calculates

This wind load calculator works to Eurocode EN 1991-1-4. It takes the basic wind velocity for your site and returns the peak velocity pressure q_p at the reference height, then applies the external and internal pressure coefficients to give the design wind pressure on each zone of the building - walls, flat and pitched roofs, monopitch and duopitch, canopies.

The output is a pressure per zone rather than a single number, because Eurocode wind loading is zonal by construction: the suction at a windward corner is several times the pressure on the middle of a wall, and it is the corner that governs the fixings.

What you enter

Basic wind velocity v_b

The 10-minute mean at 10 m in open country, from your National Annex wind map, in m/s

Terrain category

0 (open sea) to IV (dense urban) - sets the roughness length z0

Reference height z

Usually the building height, in m

Orography factor c_0

1.0 on flat ground; higher on a hill or escarpment

Air density rho

1.25 kg/m3 unless the National Annex says otherwise

Building geometry

Width b, depth d and height h - these set the zone widths

The basic wind velocity is the one input you cannot derive from the geometry: it comes from the wind map in your country National Annex, and it already includes the directional and seasonal factors unless you set them separately.

Worked example - peak velocity pressure

Take the tool defaults: a basic wind velocity of 27 m/s, terrain category II, reference height 6 m, flat ground and standard air density. Category II has a roughness length z0 = 0.05 m, which is also the reference roughness, so the terrain factor comes out at exactly 0.19.

Terrain factor k_r = 0.19 (z0/z0,II)^0.07

0.190

Roughness factor c_r = k_r ln(z/z0)

0.19 x ln(120) = 0.910

Mean wind velocity v_m = c_r c_0 v_b

0.910 x 1.0 x 27 = 24.56 m/s

Turbulence intensity I_v = 1 / (c_0 ln(z/z0))

0.209

Peak velocity pressure q_p = (1 + 7 I_v) 0.5 rho v_m^2

0.928 kN/m2

Basic velocity pressure q_b = 0.5 rho v_b^2

0.456 kN/m2

Exposure factor c_e = q_p / q_b

2.04

The exposure factor of 2.04 is worth noting: turbulence and gusting roughly double the pressure relative to the mean wind speed. That factor is where most of the wind load comes from, and it is why using a mean-velocity pressure directly would under-design the structure by half.

Note also that the mean velocity at 6 m is 24.6 m/s, below the 27 m/s basic value - the basic velocity is defined at 10 m, and the wind is slower nearer the ground.

Zone geometry and the parameter e

Eurocode divides each wall and roof into zones, and the zone widths come from a single geometric parameter e = min(b, 2h), where b is the crosswind width and h the height. Everything else follows from it: on a wall, zone A is the first e/5 from the windward corner, zone B runs to e, and zone C covers the remainder.

With the default 20 m by 10 m building at 5 m high, e = min(20, 10) = 10 m, so zone A is only 2 m wide and zone B is 8 m. That narrow zone A carries the highest suction on the whole building, which is why cladding fixings are specified at closer centres near corners than in the middle of a facade.

e = min(b, 2h)

min(20, 10) = 10 m

Zone A width, e/5

2.0 m

Zone B width, to e

8.0 m

h/d ratio (sets c_pe from Table 7.1)

0.50

Internal pressure

Internal pressure coefficient c_pi acts on the inside face and combines with the external coefficient, so the net pressure on a cladding panel is (c_pe - c_pi) q_p. The tool defaults to the standard envelope of -0.3 and +0.2, which is what EN 1991-1-4 permits when the opening distribution is not known.

The two values must be checked as separate load cases, not averaged. For a roof already in suction, the +0.2 internal pressure adds to the uplift and governs the fixings; for a wall in pressure, the -0.3 case governs. A building with a dominant opening - a large door that could be open in a storm - falls outside the default envelope entirely and needs c_pi calculated from the opening areas.

Formula reference

Terrain factor

$k_r = 0.19\left(\dfrac{z_0}{z_{0,II}}\right)^{0.07}$

Roughness factor

$c_r(z) = k_r \ln\!\left(\dfrac{z}{z_0}\right)$

Mean wind velocity

$v_m(z) = c_r(z)\,c_0(z)\,v_b$

Turbulence intensity

$I_v(z) = \dfrac{k_I}{c_0(z)\ln(z/z_0)}$

Peak velocity pressure

$q_p(z) = \bigl[1 + 7 I_v(z)\bigr]\,\tfrac{1}{2}\rho\,v_m^{2}(z)$

External pressure

$w_e = q_p(z_e)\,c_{pe}$

Net pressure

$w_{net} = q_p\,(c_{pe} - c_{pi})$

Wind force

$F_w = c_s c_d \, c_f \, q_p(z_e)\, A_{ref}$

Assumptions and limits

The peak velocity pressure expression assumes the recommended k_I = 1.0 and the standard logarithmic profile. National Annexes vary these - the UK Annex in particular uses a different procedure with its own terrain and altitude treatment - so check your Annex before using the recommended-value output for a UK project.

The structural factor c_s c_d is taken as 1.0 unless you set it. That is valid for the great majority of buildings under 15 m and for framed buildings under 100 m with normal stiffness, but a slender or dynamically sensitive structure - a tower, a chimney, a long-span roof - needs c_s c_d evaluated properly, and vortex shedding and aeroelastic instability are outside this tool entirely.

Pressure coefficients are for the standard building forms in EN 1991-1-4 Section 7. An unusual geometry, a building with a dominant opening, or a site with significant funnelling between adjacent buildings falls outside the tabulated cases and needs judgement or wind-tunnel data.

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