Wind Load Calculator

Wind pressure goes with the square of velocity, so going from 40 to 50 m/s — a 25% increase — lifts pressure from 1.00 to 1.56 kPa, a 56% increase. That is why a 5 m/s difference in basic wind speed changes the steel tonnage so much in cyclone regions. The height correction is not linear either: on the same site the pressure at 50 m can be 40% above that at 10 m, because ground friction slows the wind near the surface and the profile approaches the gradient wind with height. Exposure matters even more — at 45 m/s and 10 m, Exposure C (open terrain) gives Kz around 1.00 while Exposure B (suburban, wooded) gives only 0.72, a 28% difference. These are ASCE 7 exposure categories, not the A/B/C/D terrain categories of GB 50009 — the two systems invert each other: GB C is a built-up city centre, ASCE C is open country. Classify the site to the ASCE definition before using this tool, or the pressure comes out backwards. Take shape coefficients per face, never a blanket 0.8: windward is +0.8 and leeward −0.5, which only add to 1.3 as a total push, while roof suction can exceed −1.0. That suction is what peels off light steel roofs, so purlins and roof fixings must be designed on the negative value; using +0.8 under-designs them by more than half. Internal pressure is separate, around ±0.18 for enclosed buildings, and it flips the moment a window blows in — corners fail first. Two things get left out. ASCE 7 gives p = qz G Cp, so dropping the gust factor G of 0.85 for a rigid building over-predicts pressure by 18%. And from ASCE 7-16 onward there is a ground elevation factor Ke = exp(-0.0000362 H) with H the site altitude in feet; this tool takes Ke = 1.0 at sea level, so at 1000 m the true value is about 0.89 and high altitude projects must apply it themselves. Below 15 ft (4.6 m) Kz does not keep falling either — Table 26.10-1 lists a single value for the whole 0-15 ft band.