Wind Load Design in India — IS 875 (Part 3):2015 Complete Walkthrough

Wind is the second-largest lateral load on most Indian buildings after earthquake, and it's the dominant one for everything in cyclone-exposed coastal zones — Gujarat-Kutch, Odisha coast, Andhra coast, Tamil Nadu coast, West Bengal, and the islands. IS 875 (Part 3):2015 is the code every Indian structural engineer applies to compute that load. It supersedes IS 875 (Part 3):1987 — most usefully, the 2015 revision added Annex A (cyclone factor k₄), Annex B (terrain transitions), and revised the basic wind speed map.

This guide walks through the full design path: basic wind speed V_b → design wind speed V_z → design wind pressure p_z → force on the structure. Each step links to the underlying code clause, the lookup map, or the relevant calculator on InfraLens so you can verify against the source without leaving the page.

Code reference: IS 875 (Part 3):2015 — Wind Loads. Companion parts: Part 1 (Dead Loads), Part 2 (Live Loads), Part 4 (Snow Loads), Part 5 (Special Loads — temperature, hydrostatic). The full IS 875 family is what NBC 2016 Part 6 (Structural Design) cites for non-seismic loading.

Step 1 — Pick the Basic Wind Speed V_b from the Map

The basic wind speed V_b is a 3-second gust speed at 10 m height in open country (terrain category 2), with a 50-year return period. It's read directly from Fig. 1 of IS 875 Part 3:2015 — the same map InfraLens publishes as the interactive Basic Wind Speed Map of India, with per-city and per-state lookup.

India has six wind-speed zones: 33, 39, 44, 47, 50, and 55 m/s. Roughly:

For projects on the east or west coast, also check the Cyclone-Prone Regions Map — it determines whether you apply the additional k₄ factor (Step 2 below).

Step 2 — Compute the Design Wind Speed V_z

The design wind speed V_z at any height z and terrain is V_b multiplied by four factors:

k₂ Lookup — Terrain Categories

IS 875 defines four terrain categories. The most common pair on Indian projects: Category 2 (open terrain with few obstructions) and Category 3 (well-built-up area). At 10 m height, k₂ = 1.00 for Cat 2 and 0.91 for Cat 3. Tall buildings need the height-dependent factor — sample values:

For values not in the table, IS 875 Cl. 6.3.2.1 mandates linear interpolation. The official table covers heights up to 500 m — adequate for everything except India's tallest towers.

Step 3 — Design Wind Pressure p_z

The design wind pressure (in N/m²) follows from the design speed by Bernoulli's law:

Where:

• K_d = wind directionality factor — typically 0.90 for buildings (Cl. 7.2.1). 1.0 for circular cross-sections.
• K_a = area averaging factor (Table 4). Ranges 1.0 (≤10 m²) to 0.8 (≥100 m²) — captures the fact that gusts don't peak everywhere simultaneously.
• K_c = combination factor — applied when wind acts in combination with other loads.

Quick-Reference Pressures (Cat 2 terrain, 10 m height, K_d=0.9, K_a=1.0)

These are the cladding pressures most Indian buildings see at low rise without cyclone factor. For the cyclonic strip (k₄ = 1.15 to 1.30), multiply V_z by k₄ first; pressure scales as the square, so a 1.15× speed = 1.32× pressure, and 1.30× speed = 1.69× pressure. That's why coastal industrial structures see roughly 2× the design pressure of inland equivalents.

Step 4 — Force on the Building (F = Cf × A × pd)

The total wind force on a member or surface is:

For pressure-sensitive cladding (glazing, fixings, rooftop equipment), IS 875 also defines external and internal pressure coefficients (C_pe, C_pi) — Cl. 7.3 + Annex D. Pressure on a face = (C_pe − C_pi) × p_d. Internal pressures swing −0.5 to +0.7 depending on opening configuration — significant for industrial sheds with large doors.

Worked Example — 8-Storey Apartment in Mumbai

Apartment block in suburban Mumbai (Maharashtra) — typical residential build. Inputs:

• Location: Mumbai → V_b = 44 m/s (from Wind Speed Map)
• Height: 8 storeys × 3 m = 24 m
• Terrain: suburban → Category 3
• Topography: flat (no significant hill/escarpment nearby) → k₃ = 1.00
• Importance: residential building, not on coast → k₁ = 1.00, k₄ = 1.00

Computation:

1. k₂ at 24 m, Cat 3 ≈ 0.99 (interpolated between Cat 3 rows for 20 m = 0.97 and 30 m = 1.01)
2. V_z = 44 × 1.00 × 0.99 × 1.00 × 1.00 = 43.6 m/s
3. p_z = 0.6 × 43.6² = 1,141 N/m² = 1.14 kN/m²
4. p_d = 0.90 × 1.0 × 1.0 × 1.14 = 1.03 kN/m² (basic design pressure)
5. For a 200 m² façade, with C_f = 1.2 (typical rectangular building): F = 1.2 × 200 × 1.03 = 247 kN wind force on that face

This 247 kN is the lateral input to the frame analysis. It combines with seismic per IS 1893 (whichever governs — see our Earthquake Zones of India guide) and goes into the structural design per IS 456 (concrete) or IS 800 (steel).

Pro tip: For irregular shapes (L-plan, podium-and-tower, large podium overhangs), the standard force coefficient is conservative but can miss vortex shedding and torsion. IS 875 Cl. 9 recommends wind-tunnel testing for buildings taller than 200 m, or any slender structure with height-to-width > 5. Codal estimation is for primary design only — final cladding pressures on tall residential and commercial towers in Mumbai, Bengaluru, and Hyderabad almost always come from CFD or BLWT (boundary-layer wind tunnel) studies.

Cyclone Factor k₄ — The 2015 Update Most Engineers Miss

The biggest change in IS 875 Part 3:2015 over the 1987 version is the explicit cyclone importance factor k₄ (Annex A). Pre-2015 designs in Visakhapatnam, Cuttack, and Kandla used V_b = 50 or 55 m/s without an additional multiplier — that turned out to be inadequate during Cyclones Hudhud (2014), Fani (2019), and Tauktae (2021). The 2015 revision added k₄ values:

To check whether your site qualifies, look up the Cyclone-Prone Regions Map. It overlays the 60 km cyclone strip on India's coastline. Combined with the basic wind speed map, k₄ = 1.30 raises the effective design pressure by 69% — a 50 m/s site becomes equivalent to a 65 m/s site in design impact.

Wind vs Earthquake — Which One Governs?

For low- and mid-rise buildings (up to ~10 storeys) in IS 1893 Zone IV/V, the earthquake load typically governs the base shear. For tall buildings (>20 storeys), wind generally governs the upper stories because earthquake forces decrease with height while wind pressures grow.

For zone classification by city, see our Earthquake Zones of India — IS 1893 Guide, or use the interactive Seismic Zones Map.

Loads on Cladding — Different from Loads on Structure

One of the most common errors in Indian structural practice: applying the structure-level design pressure to glazing or wall panels. IS 875 Cl. 7.3 makes the distinction explicit. Cladding pressures are higher because:

• Area-averaging factor K_a approaches 1.0 for small areas (a 1 m × 2 m glazing panel sees ~1.05× the structure-level pressure)
• External pressure coefficient C_pe reaches −1.8 to −2.0 at building corners and roof eaves (vs −0.5 typical for walls)
• Internal pressure C_pi can swing ±0.7 with door opening conditions

This is why curtain-wall design pressures in coastal Mumbai high-rises routinely exceed 2.5 kN/m² (250 kg/m²) even though the structure-level design pressure is 1.0 kN/m². For QA/QC of cladding installation, use the Concrete QA/QC family for structural items and explicitly cross-check facade pressure tests against IS 875 Cl. 7.3.

Related InfraLens Resources

• IS 875 (Part 3):2015 — Wind Loads — full code page with key clauses, key tables, FAQ, and PDF link
• Basic Wind Speed Map of India — interactive lookup by city / state
• Cyclone-Prone Regions Map — overlays the 60 km cyclone strip
• Wind Load — Concept — short definition page
• IS 875 (Part 1) — Dead Loads — material unit weights
• IS 875 (Part 2) — Live Loads — imposed loads by occupancy
• IS 875 (Part 4) — Snow Loads — for hill stations > 700 m elevation
• Earthquake Zones of India — IS 1893 — companion load article
• IS 875 vs ASCE 7 — Wind Code Comparison — for engineers working across both codes
• IS 800 — Steel Design — applies the wind force to steel structures
• IS 456 — Concrete Design — applies the wind force to RCC structures

FAQ

What is the basic wind speed for Delhi as per IS 875?

V_b = 44 m/s for Delhi per Fig. 1 of IS 875 (Part 3):2015. The same value applies to most of north India — Haryana, Punjab, western UP. At 10 m height in suburban terrain (Cat 3), the basic design pressure works out to roughly 1.0 kN/m² before factoring for area and direction. Use the Wind Speed Map for any other city.

Where does the cyclone factor k₄ apply?

Within 60 km of the coast in cyclone-prone regions — primarily the east coast (West Bengal, Odisha, Andhra Pradesh, Tamil Nadu) and the Gujarat-Saurashtra-Kutch coast. Inland sites, including the entire central plateau and the south Indian interior, do not require k₄. The exact boundary is shown on the Cyclone-Prone Regions Map.

Is IS 875 Part 3:1987 still valid?

No — IS 875 (Part 3):2015 supersedes the 1987 edition. The 1987 version is still available for reference but should not be used for new design. The key changes in 2015 were: the cyclone factor k₄ (Annex A), revised wind speed map (no major zone changes but boundary updates), terrain transition rules (Annex B), and updated pressure coefficient tables.

What's the difference between V_b and V_z?

V_b is the basic wind speed — read straight from the IS 875 map. It's a reference value at 10 m height in open country. V_z is the design wind speed at your specific height z and terrain after applying k₁, k₂, k₃, k₄. The design pressure p_z = 0.6 × V_z², not 0.6 × V_b².

Do I need wind-tunnel testing for tall buildings?

IS 875 Cl. 9 recommends boundary-layer wind tunnel (BLWT) testing for buildings taller than 200 m, or any slender structure with height-to-width ratio > 5. In practice, Indian developers commission BLWT studies for towers above ~120-150 m. CFD is also increasingly used as a complement, especially for podium-tower configurations and clustered developments where building-to-building interference effects matter.

How do I find the k₂ value for an intermediate height?

IS 875 Cl. 6.3.2.1 specifies linear interpolation between the tabulated values in Table 2. For example, for 24 m height in Category 3: k₂(20m) = 0.97, k₂(30m) = 1.01 → k₂(24m) = 0.97 + (24-20)/(30-20) × (1.01-0.97) = 0.986. Most structural-analysis software (ETABS, STAAD, MIDAS) does this automatically when you set the wind code to IS 875.

How does IS 875 wind load compare to ASCE 7?

The general framework is similar — both codes start from a 3-second gust at 10 m height and apply height, terrain, topography, and importance factors. Key differences: ASCE 7 uses 700-year return period for the basic wind speed (vs 50-year in IS 875), but applies lower load factors in combinations; ASCE 7 doesn't have an explicit cyclone factor (uses zone-based map instead). For a side-by-side, see our IS 875 vs ASCE 7 — Wind Code Comparison.

Summary

IS 875 (Part 3):2015 lays out a four-step design path: (1) Look up V_b from the map → (2) Apply k₁k₂k₃k₄ to get V_z → (3) Compute p_z = 0.6V_z² → (4) Apply K_dK_aK_c and force coefficients C_f / C_pe / C_pi to get the force on the structure. The 2015 cyclone factor k₄ is the most-missed update; if you're designing within 60 km of the east or Gujarat-Kutch coast, applying it correctly is the single highest-impact thing you can do. Use the Wind Speed Map and Cyclone Map together — they're the canonical lookups for the codified inputs.