How is the design wind speed calculated?

It is calculated using the formula Vz = Vb * k1 * k2 * k3, where Vb is basic wind speed, and k1, k2, k3 are risk, terrain/height, and topography factors respectively.

What is the formula for calculating design wind pressure?

Design wind pressure pz is calculated as 0.6 * (Vz)^2, resulting in pressure measured in N/m2.

What internal pressure coefficient should I use for a closed building?

For buildings with normal permeability (openings less than 5% of wall area), use +0.2 and -0.2.

What terrain category applies to dense city centers?

Terrain Category 4 applies to areas with numerous large, closely spaced obstructions like central business districts.

IS 875 Part 3

: 1987

Design loads for buildings and structures - Part 3: Wind loads

CurrentEssentialCode of PracticeBIMStructural Engineering · Construction Safety and Practices

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This part of the IS 875 series specifies the wind loads to be considered when designing buildings, structures, and components. It dictates the methodology to calculate design wind speeds and pressures using regional basic wind speeds modified by terrain, topography, and building geometry.

Specifies wind loads for the design of buildings and structures, which are critical for tall temporary structures such as large scaffolding systems.

Quick Reference Values

Basic Wind Speed (Vb) Range33 to 55 m/s depending on geographical location

Multiplier for Design Wind Pressure0.6 (pz = 0.6 * Vz^2)

Internal Pressure Coefficient (Normal Permeability < 5%)+/- 0.2

Internal Pressure Coefficient (Medium Permeability 5-20%)+/- 0.5

Internal Pressure Coefficient (Large Permeability > 20%)+/- 0.7

Key Formulas

Vz = Vb * k1 * k2 * k3 — Design Wind Speed

pz = 0.6 * Vz^2 — Design Wind Pressure

F = (Cpe - Cpi) * A * pd — Wind load on individual building members/cladding

Practical Notes

Internal pressure coefficients (Cpi) of +0.2 and -0.2 must be evaluated as separate load cases to determine the most critical design forces.

IS codes dictate that maximum wind load and maximum earthquake load should not be considered to act simultaneously in structural load combinations.

Terrain categories (1 to 4) significantly affect the k2 factor; incorrectly assessing the terrain can lead to severe underestimation or overestimation of wind forces.