IS 875 Part 3 provides the standardized procedures for calculating wind loads on buildings and structures in India. Structural and facade engineers use it to determine design wind speeds, pressures, and aerodynamic forces based on geographic location, terrain category, building geometry, and cyclonic significance.
Provides methodology for calculating wind loads on buildings and structures, considering various factors like terrain, height, and shape.
Quick Reference Values
Basic wind speed (Vb) range in India33 m/s to 55 m/s
Pressure constant for pz0.6
Internal pressure coefficient (Cpi) for low permeability (< 5% openings)±0.2
Internal pressure coefficient (Cpi) for medium permeability (5% to 20% openings)±0.5
Internal pressure coefficient (Cpi) for large openings (> 20% openings)±0.7
Wind directionality factor (Kd) for solid framing/rectangular buildings0.90
Cyclonic importance factor (k4) for hospitals/schools/lifeline structures1.30
Cyclonic importance factor (k4) for industrial structures1.15
Key Formulas
Vz = Vb * k1 * k2 * k3 * k4 — Design Wind Speed
pz = 0.6 * Vz^2 — Design Wind Pressure
pd = Kd * Ka * Kc * pz — Design Wind Pressure with Factors
F = (Cpe - Cpi) * A * pd — Wind force on a building component
Practical Notes
The 2015 revision significantly changed wind pressure calculations from the 1987 version by introducing new factors like Kd (wind directionality), Ka (area averaging), Kc (combination factor), and k4 (cyclonic importance).
Always calculate forces using both positive and negative Internal Pressure Coefficients (Cpi) and pair them with External Pressure Coefficients (Cpe) to find the most critical design case.
For designing cladding, glass, and fasteners, local pressure coefficients must be applied. These localized pressures at edges, corners, and ridges are significantly higher than overall structural wind pressures.