Explanatory Handbook on Indian Standard Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures
2001 Edition

The basic wind speeds are established through statistical analysis of peak 3-second gust wind data collected from 47 DPA stations spanning 1948 to 1983. These speeds are normalized to a height of 10 meters over flat, open terrain (Terrain Category 2), and extreme values are derived using the Gumbel distribution for a 50-year return period. Regional adjustments account for geographic features such as mountain ranges, coastal cyclone impacts extending inland, funneling effects like the Palghat Gap, and localized dust storms. The country is segmented into six wind zones with speeds ranging from 33 m/s to 55 m/s. For sites near zone boundaries without local data, the higher adjacent zone speed is recommended. Wind speed variation with height and terrain roughness follows established velocity profiles.

Terrain categories, as per IS 875 Part 3, classify surface roughness into four categories: Category 1 represents open sea or smooth flat areas with minimal obstructions; Category 2 covers open terrain with sparse obstacles; Category 3 includes suburban areas with low-rise buildings and scattered trees; and Category 4 pertains to urban areas with dense, tall buildings. Each category has an associated roughness length and power law exponent influencing the wind velocity profile with height. Wind velocity increases with elevation, and rougher terrains produce lower speeds near ground but a steeper increase with height. These factors directly affect wind pressure calculations, as pressure is proportional to the square of velocity, thus impacting the structural wind loads.

Pressure coefficients for external surfaces are detailed based on building shape, roof slope, and wind direction. For walls, coefficients vary with orientation relative to wind; windward walls experience positive pressures, while leeward walls exhibit suction effects. Roof pressure coefficients depend on slope angle and type (gable, hip, flat), with corners and edges subjected to higher negative pressures due to airflow separation. Internal pressure coefficients depend on the size and location of openings, with partially open buildings having elevated internal pressures. The net wind pressure on a surface is calculated by combining external and internal coefficients as p = 0.6 × V² × (Cpe - Cpi), ensuring accurate load representation.

The standard employs a stepwise approach where wind velocity is considered constant up to 10 meters and varies above this height following a power law profile characterized by a terrain-dependent exponent. The design wind speed at height z is calculated as Vz = Vb × k1 × k2 × k3, where Vb is the basic wind speed for the zone, k1 is the risk/probability factor, k2 accounts for terrain roughness, height and structural size, and k3 adjusts for topography. The design wind pressure is then determined by pz = 0.6 × Vz², providing a height-dependent pressure value vital for structural load analysis.

Dynamic wind effects are incorporated using the gust factor method, which modifies the static wind pressure to account for fluctuating wind speeds and structural response. The along-wind load is calculated as Fz = Cf × Ae × Pz × G, where Cf is the force coefficient, Ae is the effective frontal area, Pz is the design wind pressure based on hourly mean wind speed, and G is the gust factor. The gust factor accounts for parameters like natural frequency, damping, and turbulence characteristics, calculated via empirical relations and charts provided in the handbook. This method simplifies dynamic loading into an amplified static equivalent, suitable for structures within specified height and terrain categories, facilitating practical design without full dynamic analysis.