What wind load is and why it must be computed

Wind load is one of the primary variable actions on buildings, alongside imposed loads and snow. It is not just a horizontal force on the building: wind generates positive pressure (thrust) on exposed façades, suction on leeward walls and roofs, local stresses on roofing, tiles, PV panels, signs, chimneys. Wind failure — partial (loss of tiles, uplift of light roofs, window deformation) or total (collapse of cooling towers, suspension bridges like Tacoma Narrows in 1940) — is one of the main causes of structural damage in many regions. The standards EN 1991-1-4 (Eurocode 1 part 1-4, international), NTC 2018 §3.3 (Italy), ASCE 7 (US) provide standardised methods to compute design pressures as functions of local wind velocity, height, terrain exposure and building geometry. This calculator implements the simplified EN 1991-1-4 / NTC 2018 method.

The fundamental formula: q_b = ½·ρ·v_b²

The reference velocity pressure is defined by Bernoulli's law applied to airflow stopping against a surface:

q_b = ½ · ρ · v_b²

where ρ = 1.25 kg/m³ is air density at 15 °C and sea level, and v_b is the reference wind velocity. In Italy, NTC 2018 §3.3.1 defines v_b,0 (10-min mean velocity at 10 m height in open country, 50-year return period) for 9 geographical zones: from 25 m/s (Po Plain, Aosta Valley) to 31 m/s (southern Sicily). The value is then corrected for altitude (c_a) above 500-1000 m ASL, for return periods other than 50 years, and for seasonality. Example: v_b = 25 m/s → q_b = 0.5·1.25·625 = 390.6 Pa (0.39 kN/m²).

Exposure coefficient c_e: terrain roughness and height

Wind velocity and pressure vary with height above ground (increase) and with roughness of terrain (increase on smooth ground, decrease on rough). The exposure coefficient c_e integrates both dependencies:

c_e(z) = [1 + 7·I_v(z)] · c_r²(z)

with c_r(z) = k_r·ln(z/z_0) roughness coefficient, I_v = 1/ln(z/z_0) turbulence intensity, k_r = 0.19·(z_0/0.05)^0.07 category factor. Roughness length z_0 and minimum height z_min depend on terrain category (EN 1991-1-4 Table 4.1):

• Cat. I Open sea, lakes, desert: z_0 = 0.01 m, z_min = 2 m. Maximum exposure.
• Cat. II Open country, meadows with isolated obstacles: z_0 = 0.05 m, z_min = 4 m. Reference category.
• Cat. III Suburbs, forests, scattered industrial: z_0 = 0.3 m, z_min = 8 m.
• Cat. IV Dense urban centres, forests: z_0 = 1.0 m, z_min = 16 m. Minimum exposure.

Example (z = 10 m, cat. II): c_e = 2.35. Peak pressure q_p = c_e·q_b = 2.35·390.6 = 919 Pa. This is the dynamic pressure accounting for peak instantaneous gusts, not just mean wind.

Aerodynamic coefficients c_p: positive and negative pressures

The effective pressure on a building surface is w = q_p·c_p, where c_p is the aerodynamic coefficient (or shape coefficient) depending on building geometry and surface location. Indicative values (EN 1991-1-4 §7.2):

• Windward façade (wall directly facing wind): c_pe = +0.7 to +0.8 (pressure).
• Leeward façade (opposite wall): c_pe = −0.3 to −0.5 (suction).
• Side walls: c_pe = −0.5 to −0.8 (suction).
• Flat roof: c_pe = −0.7 to −1.4 at edges, −0.2 at centre (suction).
• Duo-pitch roof (30°): windward c_pe = −0.5 to +0.7 depending on pitch; leeward c_pe = −0.3 to −0.5.
• Corner zones of tall buildings: c_pe up to −1.4 (intense local suction). Roofing/cladding fixings must be carefully designed.
• Internal coefficient c_pi: +0.2 to +0.3 if windward openings dominate; −0.3 to −0.5 if leeward. Net pressure on a panel is w = q_p·(c_pe − c_pi).

Suctions are often more critical than pressures for cladding elements: tiles ripped by wind are widespread damage because local suction can reach 2-3 kN/m² at roof edges.

Wind mapping in Italy (NTC 2018)

NTC 2018 §3.3.1 divides Italy into 9 zones with different reference velocity v_b,0 and reference altitude a_0. Summary (v_b,0 in m/s at sea level):

• Zone 1: Aosta Valley, Piedmont, Liguria, Lombardy, Trentino, Veneto, Friuli, Emilia-Romagna → v_b,0 = 25 m/s.
• Zone 2: Tuscany, Marche, Umbria, Lazio → v_b,0 = 25 m/s.
• Zone 3: Abruzzo, Molise, Puglia, Campania, Basilicata, Calabria → v_b,0 = 27 m/s.
• Zone 4: Sicily and Reggio Calabria → v_b,0 = 28 m/s.
• Zone 5-6: Sardinia → v_b,0 = 28 m/s.
• Zone 7: Liguria coast → v_b,0 = 28 m/s.
• Zone 8: Alpine region → v_b,0 = 30 m/s.
• Zone 9: Southern Sicily (Trapani, Agrigento) → v_b,0 = 31 m/s.

For altitudes above a_0, altitude correction c_a = 1 + k_s·(a_s − a_0)/a_0 can increase v_b by 20-50% at high mountain.

Total force on surface and structural checks

Total force on a panel or façade: F = w·A = q_p·c_p·A. Example: 100 m² windward wall (10×10 m) with q_p = 919 Pa and c_pe = 0.8: F = 0.8·919·100 = 73,520 N = 73.5 kN. This force must be transferred by cladding panels (via calibrated anchors) to the load-bearing structure (columns, beams) and then to foundations. Required design checks (NTC 2018 §4.1.4, EC1):

• Global stability: overturning, base sliding under total horizontal wind thrust (vector sum on all façades).
• Structural element resistance: columns, beams, bracing with wind actions in ULS combination (γ_Q = 1.5).
• Cladding check: light walls, masonry, curtain-wall — deflection limited to L/300 (SLS) under design pressure.
• Roofing check: uplift of tiles, sandwich panels, membranes. Corner and edge zones require extra fixings.
• Resonance check (slender buildings): towers, chimneys, bridges with natural frequency < 1 Hz may enter resonance with wind (vortex shedding). Specific dynamic analysis per EN 1991-1-4 §6.

Common mistakes and dynamic effects

• Underestimating roof suctions: negative pressure (uplift) on light roofs is often more critical than gravity load. In tornado zones most damage is from roof uplift, not lateral collapse.
• Ignoring internal pressure c_pi: an open door or window drastically changes the distribution: from c_pi = −0.3 to +0.3 the net roof load can change by 60% instantly.
• Ignoring local shape coefficients: corner panels, cornices, overhangs need local c_p up to −2.0. Standard edge fixings are often insufficient.
• Confusing v_b with instantaneous velocity: v_b is the 10-min mean velocity at 10 m height in open country, 50-year return period. Peak gust velocity (3-second duration) can be 1.5-1.7× v_b, and this is already included in q_p via c_e.
• Ignoring dynamic effects for slender buildings: buildings taller than 60 m or with period > 1 s need dynamic analysis per EC1 §6, introducing a "dynamic factor" c_dyn often > 1.2 amplifying static forces.
• Aerodynamic effects: circular towers (periodic vortex shedding causing transverse oscillations), suspension bridges (aeroelastic flutter), dome roofs (edge vortices). CFD or wind tunnel analysis for major projects.

How to use the calculator

Enter the reference wind velocity v_b in m/s (Italy: 25 north, 27 centre-south, 28 Sicily/Sardinia, 30 Alps, 31 south Sicily — see NTC 2018 zoning). Enter the height z in m of the surface of interest above ground (typically the top of the building or the centre of the considered façade). Choose the terrain category based on site morphology: 1 = open sea/lake (max exposure); 2 = open country with few obstacles (reference); 3 = suburbs, scattered residential; 4 = dense city centre. Enter the aerodynamic pressure coefficient c_p: +0.7-0.8 for windward façades, -0.3-0.5 for leeward, -0.7-1.4 for roof suctions (use negative sign for suction). Enter the area A of the surface in m². The calculator returns reference velocity pressure q_b, exposure coefficient c_e at height z, equivalent peak velocity v_p, peak pressure q_p, design pressure w = q_p·c_p, total force F in kN and qualitative assessment.