Wind Load

Wind load is the lateral pressure exerted on a structure by wind, calculated as design wind pressure times area. The Indian code IS 875 Part 3:2015 (revised from the 1987 edition) governs wind load calculation, providing a basic wind speed map of India (Vb = 33-55 m/s across zones), terrain category factors (k2 = 0.93-1.06 for Cat 1-4), height factors (k3 increases with elevation above ground), and pressure coefficients (Cp) for common building shapes. For a typical 30-metre office building in Mumbai (Vb = 44 m/s, terrain 3), the design wind pressure at roof level works out to approximately 1.5-2.0 kN/m², generating substantial lateral force on a tall slender structure.

The core design equation per IS 875 Part 3 Cl. 5: design wind pressure pz = 0.6 × Vz², where Vz = Vb × k1 × k2 × k3 × k4 (m/s), and k1 = 1.0-1.07 (return period), k2 = terrain (Table 2), k3 = topography, k4 = importance for cyclone zones. Force on each face = Cpe × Cpi × pz × A, with Cpe (external pressure coefficient) from Tables 5-9 and Cpi (internal) from Table 18 for specific building shapes. Modern tall buildings in India (40+ floors) require wind tunnel testing per IS 875 Part 3 Cl. 8.4 because the code's static method is inadequate for slender, asymmetric, or torsionally sensitive shapes.

Wind load governs lateral design for low-to-mid-rise buildings up to about 6-8 floors in seismic Zone II/III, after which seismic loads typically dominate. For tall buildings, wind serviceability (cross-wind acceleration limits per IS 875 Part 3 Annex C, typically 0.10-0.15 m/s² rms for residential) often controls the design more than ultimate strength. Cyclone-prone coastal regions (Andhra coast, Odisha, Tamil Nadu, Gujarat) require special attention to roof uplift, wall cladding fixings, and connection details — IS 875 Part 3 Annex D gives guidance for cyclone-resistant detailing in vulnerable zones.