Clause 7.2 provides the formula for converting design wind speed (Vz) into design wind pressure (pz). The relationship is pz = 0.6 × Vz², where Vz is in m/s and pz is in N/m² (Pascals). The constant 0.6 is derived from ½ × ρ, where ρ = 1.2 kg/m³ is the air density at sea level under standard conditions. The design wind pressure at a given height is the basis for computing wind forces on structures and cladding.
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Key Requirements
•Design wind pressure shall be calculated as pz = 0.6 × Vz² in N/m²
•For computing wind loads on individual elements/cladding, pd = Kd × Ka × Kc × pz
•Kd is the wind directionality factor (Clause 6.4)
•Ka is the area averaging factor for large tributary areas
•Kc is the combination factor (0.9) when wind loads are combined with other loads
Reference Tables
Design Wind Pressure at 10 m Height, Terrain Category 2 (Reference Values)
Vb (m/s)
Vz at 10 m, Cat 2 (m/s)
pz (N/m²)
pz (kN/m²)
33
33
653
0.653
39
39
912
0.912
44
44
1162
1.162
47
47
1325
1.325
50
50
1500
1.5
55
55
1815
1.815
Reference values with k1=1.0, k2=1.0 (Cat 2, 10 m), k3=1.0, k4=1.0. Actual pz at different heights and terrain categories will differ.
Formulas
pz = 0.6 × Vz²
Design wind pressure at height z
pz = Design wind pressure in N/m² (Pa)Vz = Design wind speed at height z in m/s0.6 = Half of air density ρ = 1.2 kg/m³ (i.e., ½ρ = 0.6)
pd = Kd × Ka × Kc × pz
Design wind pressure for load calculation on elements
pd = Design wind pressure for structural elements in N/m²Kd = Wind directionality factor (0.90 for most buildings)Ka = Area averaging factor (1.0 for small areas, < 1.0 for large cladding panels)Kc = Combination factor (0.9 when combining with other loads, 1.0 when wind is the primary load)pz = Design wind pressure at height z
Practical Notes
✓pz = 0.6 Vz² is the most frequently used formula in IS 875. For quick mental estimates: Vz = 50 m/s gives pz = 1.5 kN/m² (about 150 kgf/m²).
✓Wind pressure varies as the square of wind speed — a 10% increase in Vz produces a 21% increase in pz. This is why the k-factors are so important.
✓For Vb = 47 m/s (Delhi): pz ≈ 1.33 kN/m² at 10 m. At 30 m in Category 3 (suburban): Vz ≈ 47 × 1.06 = 49.8 m/s, pz ≈ 1.49 kN/m².
✓The air density of 1.2 kg/m³ is for standard sea-level conditions. At higher altitudes (e.g., Leh at 3500 m), actual air density is lower, but the code does not adjust for this.
Common Mistakes
⚠Using Vb directly in the pressure formula instead of Vz — the design speed Vz (modified by k1, k2, k3, k4) must be used.
⚠Expressing pz in wrong units — the formula gives N/m² when Vz is in m/s. Divide by 1000 for kN/m².
⚠Forgetting to apply Kd (0.90) when computing pd for design — this 10% pressure reduction is permitted by the 2015 code.
⚠Not computing pz at multiple heights for tall buildings — pz increases with height through the k2 factor in Vz.