IS Standards Series · Part 7 of 9

Wind Load Calculation per IS 875 Part 3:2015

Step-by-step wind load determination per IS 875 Part 3:2015 — from basic wind speed V_b through design wind pressure p_z to pressure coefficients on building surfaces. Includes k₁–k₄ factor tables and a worked example.

Basic Wind Speed V_b
Design Wind Speed V_z
k₁ — Risk / Probability Factor
k₂ — Terrain, Height & Structure Size Factor
k₃ — Topography Factor
k₄ — Importance Factor for Cyclone Regions
Design Wind Pressure and Forces
Worked Example

1. Basic Wind Speed V_b

IS 875 Part 3:2015 gives the basic wind speed V_b in m/s as the 3-second gust speed at 10 m height in Terrain Category 2 for a 50-year return period. Values are read from the wind map (IS 875 Part 3 Fig. 1).

V_b (m/s)	Representative Regions
33	Southern tip of India, Andaman & Nicobar
39	Western Rajasthan, Gujarat interior, Himalayas
44	Most of interior India including Delhi, Bhopal, Nagpur
47	Mumbai coast, Gujarat coast, Kolkata, Bhubaneswar
50	Coastal Odisha, northern Andhra Pradesh
55	Coastal Andhra Pradesh and Tamil Nadu

2. Design Wind Speed V_z

IS 875 Part 3 Cl.5.3 — Design Wind Speed at Height z

IS 875-3V_z = V_b · k₁ · k₂ · k₃ · k₄m/s

Each factor modifies the basic wind speed to account for different conditions specific to the site and structure:

k₁ — Risk / probability factor (design life and return period)
k₂ — Terrain category, height above ground, and structure size
k₃ — Topographic effects (hills, escarpments, cliffs)
k₄ — Importance factor for structures in cyclone regions

3. k₁ — Risk / Probability Factor

k₁ adjusts V_b for structures whose design life differs from the 50-year reference period or where different reliability levels are required (IS 875 Part 3 Table 1):

Class	Structures	k₁
Class 1	Temporary structures, greenhouses, small farm buildings (design life < 5 yr)	0.82
Class 2	Industrial structures, storage tanks (design life 25 yr)	0.94
Class 3	Buildings & residential structures (50 yr — standard)	1.00
Class 4	Important buildings (power plants, hospitals, bridges — 100 yr)	1.05
Class 5	Critical facilities (nuclear, dams, post-disaster infrastructure)	1.08

4. k₂ — Terrain, Height & Structure Size

IS 875 Part 3 defines four terrain categories:

Category 1: Open sea, open plain, no obstructions
Category 2: Open terrain with scattered obstructions (standard terrain)
Category 3: Urban/suburban areas, industrial sites, forests
Category 4: City centres with large, closely-spaced buildings

Height z (m)	Cat 1	Cat 2	Cat 3	Cat 4
10	1.05	1.00	0.91	0.80
15	1.09	1.05	0.97	0.80
20	1.12	1.07	1.01	0.80
30	1.15	1.12	1.06	0.97
50	1.20	1.17	1.12	1.10
100	1.26	1.24	1.20	1.20

Interpolation is required for intermediate heights. Class A, B, or C structure size (IS 875 Part 3 Table 2) also affects k₂; the table above applies to Class B (medium structures).

5. k₃ — Topography Factor

k₃ = 1.0 for flat terrain. For structures on hills, cliffs, or escarpments, k₃ amplifies the wind speed. IS 875 Part 3 Cl.5.3.3 gives k₃ as a function of slope angle and position on the feature:

Upwind slope < 3° (essentially flat): k₃ = 1.00
Upwind slope 3°–17°: k₃ varies linearly from 1.00 to 1.36 near the crest
Upwind slope > 17°: k₃ = 1.36 (at crest), reducing downwind

6. k₄ — Cyclone Importance Factor

k₄ applies only in cyclone-prone coastal regions (IS 875 Part 3 Cl.5.3.4). For structures in these regions:

Structure Type	k₄
Structures below the level of cyclone inundation	1.0
All residential, industrial, commercial structures	1.15
Hospitals, schools, post-disaster structures	1.30

For non-cyclone zones: k₄ = 1.0.

7. Design Wind Pressure and Forces

IS 875 Part 3 Cl.5.4 — Design Wind Pressure

IS 875-3p_z = 0.6 · V_z²N/m²

V_z in m/s; p_z in N/m² (Pa). Factor 0.6 = ½ρ where ρ = 1.20 kg/m³ (standard air density)

Design Wind Force on Surface

IS 875 Part 3 Cl.6.2 — Pressure on Building Surfaces

IS 875-3F = (C_pe − C_pi) · A · p_zN

C_pe = external pressure coefficient; C_pi = internal pressure coefficient; A = surface area

Pressure Coefficients C_pe — Rectangular Buildings

Surface	h/w ≤ 0.5	h/w = 1	h/w ≥ 2
Windward wall	+0.7	+0.7	+0.8
Leeward wall	−0.2	−0.25	−0.5
Side walls	−0.5	−0.6	−0.7

Internal Pressure C_pi

Buildings with small openings (<5% of wall area): ±0.2
Buildings with large openings on one face: +0.7 or −0.5 (depending on wind direction)
Permeable cladding throughout: ±0.5

8. Worked Example

Problem: 4-storey office building, Mumbai (V_b = 44 m/s), H = 15 m, Terrain Category 3, k₁ = 1.0, k₃ = 1.0, k₄ = 1.0. Find design wind pressure at roof level.

V_b = 44 m/s
k₁ = 1.0 (50-year return, Class 3)
k₂ = 0.97 at z = 15 m, Category 3 (interpolated)
k₃ = 1.0 (flat terrain)
k₄ = 1.0 (non-cyclone zone)
V_z = 44 × 1.0 × 0.97 × 1.0 × 1.0 = 42.7 m/s
p_z = 0.6 × 42.7² = 0.6 × 1823 = 1094 N/m² ≈ 1.09 kN/m²

For windward wall: net pressure = (C_pe − C_pi) × p_z = (0.7 − (−0.2)) × 1.09 = 0.9 × 1.09 = 0.98 kN/m²

Next: Foundation Design →

Preliminary design only. Wind pressures require site-specific terrain assessment. Always consult IS 875 Part 3:2015 directly and engage a licensed structural engineer for final design.

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Wind Load Calculation per IS 875 Part 3:2015

Contents

1. Basic Wind Speed Vb

2. Design Wind Speed Vz

3. k1 — Risk / Probability Factor

4. k2 — Terrain, Height & Structure Size

5. k3 — Topography Factor

6. k4 — Cyclone Importance Factor