Wind load is a critical lateral force for tall and slender structures. IS 875 Part 3:2015 is the Indian standard for wind loads on buildings and structures. Errors in wind load estimation can lead to structural failures, especially in cyclone-prone coastal zones of India.
IS 875 Parts Overview
| Part | Covers |
|---|---|
| Part 1 | Dead loads |
| Part 2 | Imposed loads |
| Part 3 | Wind loads |
| Part 4 | Snow loads |
| Part 5 | Special loads and combinations |
Basic Wind Speed Zones in India (IS 875 Part 3)
| Vb (m/s) | Zone | Cities Covered |
|---|---|---|
| 33 | Zone I | Parts of central India (lowest risk) |
| 39 | Zone II | Delhi, Pune, Lucknow, Bhopal |
| 44 | Zone III | Mumbai, Kolkata, Hyderabad, Bengaluru |
| 47 | Zone IV | Chennai, Raipur, Ranchi, Guwahati |
| 50 | Zone V | Coastal Odisha, Andaman & Nicobar (highest risk) |
These are 3-second gust speeds at 10 m height in open terrain with 50-year return period.
Design Wind Speed Formula
Vz = Vb × k1 × k2 × k3 × k4
| Factor | Meaning | Value Range |
|---|---|---|
| k1 | Risk coefficient (return period factor) | 0.71–1.35 (50yr = 1.0) |
| k2 | Terrain & height factor | Depends on category (A/B/C/D) and height |
| k3 | Topography factor | 1.0–1.36 (for hills/cliffs) |
| k4 | Importance factor for cyclonic regions | 1.0 or 1.15 |
k1 Values (Risk/Return Period)
| Mean Probable Life (years) | k1 |
|---|---|
| 25 | 0.90 |
| 50 | 1.00 |
| 100 | 1.07 |
Terrain Categories
- Category 1: Open terrain — sea coast, flat grassland, no obstructions
- Category 2: Open terrain — scattered obstructions up to 10 m height (suburbs on outskirts)
- Category 3: Terrain with numerous closely spaced obstructions — forests, suburbs, industrial areas
- Category 4: Large city centres with tall closely spaced buildings
k2 Values (Sample — Category 2)
| Height z (m) | k2 (Cat 2) | k2 (Cat 3) | k2 (Cat 4) |
|---|---|---|---|
| 10 | 1.00 | 0.91 | 0.80 |
| 15 | 1.05 | 0.97 | 0.80 |
| 20 | 1.07 | 1.01 | 0.83 |
| 30 | 1.12 | 1.06 | 0.90 |
| 50 | 1.17 | 1.12 | 1.00 |
| 100 | 1.24 | 1.20 | 1.11 |
| 150 | 1.28 | 1.24 | 1.17 |
Design Wind Pressure
pz = 0.6 × Vz² (in N/m², with Vz in m/s)
This is the dynamic wind pressure (velocity pressure). Actual design pressure considers shape and exposure.
Wind Force on a Structure
F = Cf × A × pz
where Cf = force coefficient, A = area of element normal to wind, pz = design wind pressure.
Pressure Coefficients for Enclosed Buildings
Design wind pressure on surfaces:
p = (Cpe − Cpi) × pz
- Cpe = external pressure coefficient (depends on H/W, L/W ratios, surface location)
- Cpi = internal pressure coefficient: ±0.2 for buildings with negligible openings; −0.5 or +0.7 for permeable buildings with dominant face opening
Worked Example — 10-Storey Building in Mumbai
Given Data
- Location: Mumbai, Vb = 44 m/s (Zone III)
- Building: 35 m tall, 20 m × 15 m plan, rectangular
- Terrain: Category 3 (suburban area)
- k1 = 1.0 (50-year life), k3 = 1.0 (flat ground), k4 = 1.0
Design Wind Speed at Different Heights
| Height z (m) | k2 (Cat 3) | Vz = 44×1×k2×1×1 (m/s) | pz = 0.6Vz² (N/m²) |
|---|---|---|---|
| 0–10 | 0.91 | 40.0 | 961 |
| 15 | 0.97 | 42.7 | 1093 |
| 20 | 1.01 | 44.4 | 1184 |
| 30 | 1.06 | 46.6 | 1303 |
| 35 | 1.08 | 47.5 | 1354 |
Wind Pressure on Windward Wall (Cpe = +0.7, Cpi = −0.2)
Net p = (0.7 − (−0.2)) × pz = 0.9 × pz
At 30 m: p = 0.9 × 1303 = 1173 N/m² = 1.17 kN/m²
Total Wind Force at 30 m storey (3 m height)
F = p × width × storey height = 1.17 × 20 × 3 = 70.2 kN per storey
Gust Factor Method vs Static Method
| Aspect | Static Method | Gust Factor Method |
|---|---|---|
| Applicable for | Rigid structures (T < 1 sec) | Flexible/tall structures (T > 1 sec) |
| Accounts for | Mean wind only | Mean + dynamic gust component |
| Gust Factor G | Not used | Calculated from IS 875-3 Appendix B |
| Complexity | Simple | Complex, needs natural frequency |
Along-Wind and Across-Wind Response
- Along-wind (drag): Response in the direction of wind flow; estimated by gust factor method
- Across-wind (lift): Vortex shedding perpendicular to wind; critical for towers, chimneys, tall slender buildings
- Vortex shedding frequency: fs = St × Vz / b, where St = Strouhal number (~0.2 for square sections)
- If fs ≈ natural frequency of structure → resonance → amplified response
Special Structures
- Chimneys: IS 6533 gives wind load procedure for circular and square chimneys
- Lattice towers: IS 802 gives force coefficients for transmission towers
- Bridges: IRC 6 gives wind loads for highway bridges
- Solar structures: Use IS 875 Part 3 with adjusted Cpe for inclined panels
Frequently Asked Questions
What is the basic wind speed in Delhi as per IS 875?
Delhi falls in Zone II with Vb = 39 m/s (3-second gust, 10 m height, 50-year return period).
Which is more critical for tall buildings — wind or earthquake?
In India, seismic forces tend to govern below ~100 m in most zones. Above 100 m or in high-wind coastal areas (cyclone-prone zones), wind loads often govern lateral design. Always check both IS 875-3 (wind) and IS 1893 (seismic).
What is the k4 factor in IS 875 Part 3?
k4 is the cyclone importance factor applicable to buildings in cyclone-prone coastal strips (within 60 km of coast). For critical structures in these zones, k4 = 1.15, increasing design wind speed by 15%.