Same wind, two rulebooks
Every wind load calculation on Earth starts from the same physics: moving air carries kinetic energy, and stopping it against a wall converts speed into pressure at ½ρV². The American and European codes package that physics differently — ASCE 7-22 multiplies a squared wind speed by a chain of K factors, while Eurocode 1 (EN 1991-1-4) builds a peak velocity pressure from terrain roughness and turbulence — but both are answering the same three questions. How fast is the design wind at this site? How does the terrain change it at the building’s height? And how does the building’s shape turn that dynamic pressure into pushes and pulls on each surface?
The constants give the shared physics away. ASCE’s velocity pressure equation carries the coefficient 0.613, which is one-half of an air density of about 1.226 kg/m³; Eurocode uses ρ = 1.25 kg/m³ explicitly, so its half-density factor is 0.625. Nearly identical — the real differences live elsewhere. To keep the comparison honest, one building runs through both codes in this guide: an enclosed, rigid building with a 10 m (33 ft) mean roof height on flat, open terrain at sea level — Exposure C in American terms, Terrain Category II in European ones.
ASCE 7-22: the K-factor chain
ASCE 7-22 computes the velocity pressure at height z as qz = 0.613 · Kz · Kzt · Kd · Ke · V², with V in m/s and qz in pascals. Each K corrects the raw wind speed for one site effect. Kz, the exposure coefficient from Table 26.10-1, follows a power law Kz = 2.01 · (z/zg)^(2/α), where α and the gradient height zg come from the exposure category: B (urban and suburban, α = 7.0, zg = 365.76 m), C (open terrain with scattered obstructions, α = 9.5, zg = 274.32 m), or D (flat, unobstructed areas and open water, α = 11.5, zg = 213.36 m). Heights below 4.6 m are computed at 4.6 m. The example building sits in a sweet spot of the arithmetic: at z = 10 m in Exposure C, Kz = 2.01 · (10/274.32)^(2/9.5) = 1.00 almost exactly.
The rest of the chain: Kzt is the topographic factor, (1 + K1K2K3)² per Eq. 26.8-1, which stays 1.0 on flat sites and grows on hills and escarpments where the wind accelerates. Kd is the directionality factor from Table 26.6-1 — 0.85 for both main-system and cladding analysis — a statistical credit for the improbability that the worst wind arrives from the worst direction. And Ke, new in ASCE 7-22, credits thin air at altitude: Ke = e^(−0.0000362 · elevation), which is 1.0 at sea level and about 0.947 at 1,500 m. The basic wind speed V itself comes off risk-category hazard maps, so an essential facility designs for rarer, faster wind than a barn does.
The American side of the example
Run the numbers for a mapped V = 50 m/s (about 112 mph) at the example site. With Kz = 1.00 and every other factor at 1.0, the raw velocity pressure is 0.613 × 50² = 1,533 Pa — the ≈1.53 kPa figure a first-pass estimate gives. Applying the directionality factor Kd = 0.85 brings the design velocity pressure to 0.613 × 1.00 × 1.0 × 0.85 × 1.0 × 2500 ≈ 1.30 kPa. From there the building’s geometry takes over: design pressure is the velocity pressure times a pressure coefficient, p = qz(GCp − GCpi), where GCp describes the external suction or push on a zone and GCpi the internal pressure trying to help or hurt from inside.
ASCE splits the answer in two. MWFRS pressures — main wind-force resisting system — size the frames, shear walls, and diaphragms that carry the building’s total wind load, with a gust-effect factor of 0.85 for rigid buildings. Components-and-cladding pressures size the individual windows, panels, and fasteners, and they run higher because a small element can sit entirely inside a localized gust peak that the whole frame would average away. For an interior wall zone with GCp of 0.8/−0.5, the example building sees design pressures of roughly +1.2/−0.8 kPa — modest numbers that climb steeply in corner and edge zones, which is where cladding failures actually start.
Eurocode 1: roughness and turbulence instead of exposure
EN 1991-1-4 reaches its dynamic pressure by modeling the wind profile physically rather than by power-law category. The site’s terrain sets a roughness length z0 — from 0.003 m over open sea (Category 0), through 0.05 m for open farmland (Category II) and 0.3 m for suburbs and forest (Category III), up to 1.0 m in dense urban fabric (Category IV). The mean wind at height z is vm(z) = cr(z) · co · vb, where the roughness factor is logarithmic, cr(z) = kr · ln(z/z0) with kr = 0.19 · (z0/0.05)^0.07, co is the orography factor (1.0 on flat sites), and vb is the basic wind velocity.
Turbulence then enters explicitly. The turbulence intensity Iv(z) = 1/ln(z/z0) feeds the peak velocity pressure qp(z) = [1 + 7 · Iv(z)] · ½ρ · vm(z)², so gustiness is built into qp itself — where ASCE applies a separate gust-effect factor, Eurocode’s equivalent factor is already inside the pressure. At the example building’s 10 m height in Terrain II: cr = 0.19 · ln(200) = 1.007, Iv = 1/ln(200) = 0.189, and the bracket [1 + 7 × 0.189] = 2.32 — the peak pressure is 2.3 times the mean-wind pressure. Collapsed to one number, qp(10 m, Terrain II) ≈ 1.47 · vb² in pascals.
The European side — and why the two wind speeds aren’t the same wind
With a basic wind velocity vb = 26 m/s, the example gives qp = 2.32 × 0.625 × (1.007 × 26)² ≈ 994 Pa, call it 0.99 kPa. Surface pressures follow as w = qp · (cpe − cpi) using the simplified external coefficients — windward wall +0.8, leeward −0.5, sidewalls −0.8, windward roof −0.7, leeward roof −0.4 — against an internal coefficient of 0.2 for enclosed buildings. The example building’s net pressures: +0.60 kPa pushing on the windward wall, −0.70 kPa on the leeward wall, −0.99 kPa of suction on the sidewalls, and −0.89/−0.60 kPa lifting the roof.
Do not read 26 m/s versus 50 m/s as Europe designing for gentler wind — the two codes define the design wind speed over different averaging times. ASCE 7’s V is a 3-second gust; Eurocode’s vb is a 10-minute mean, and the same physical storm produces a much higher 3-second gust than 10-minute average, which is exactly why Eurocode’s [1 + 7Iv] term re-inflates the mean to a peak. Converting a speed from one code into the other without adjusting for averaging time is the classic cross-code blunder. The wind load calculator sidesteps it by running ASCE 7-22, Eurocode 1, and AS/NZS 1170.2 from each code’s own inputs and showing the results side by side — with the exposure-to-terrain mapping (B→III, C→II, D→0) handled for you.
Suction rules the envelope — the example in one view
Look back at the signs in both codes’ results: only the windward wall ever sees positive pressure. Every other surface — sidewalls, leeward wall, most roof zones — is in suction, and the sidewall and roof-edge suctions outweigh the windward push. Wind doesn’t blow buildings over so much as it peels them apart, which is why fastener schedules and cladding attachments in edge zones deserve more attention than the headline windward pressure suggests, and why the internal pressure coefficient matters: a breached opening on the windward face pressurizes the interior and adds to the suction demand on everything else.
One building, both rulebooks: at 10 m over open terrain, ASCE 7-22 turns a 50 m/s design gust into a velocity pressure of 1.53 kPa raw, 1.30 kPa after Kd = 0.85, and roughly +1.2/−0.8 kPa on an interior wall zone; Eurocode 1 turns a 26 m/s ten-minute mean into qp ≈ 0.99 kPa via cr = 1.007 and a 2.32 gust bracket, then +0.60 to −0.99 kPa across the walls. Different scaffolding, same destination: a site speed, a height-and-terrain correction, a shape coefficient — and an envelope designed mostly against being pulled, not pushed.