IS 875 Part 3 : 2015Design Loads (Other than Earthquake) for Buildings and Structures - Wind Loads

IS 875 (Part 3):2015 specifies wind loads for buildings and structures in India. It is the mandatory reference for any structural design where wind may govern — tall buildings, steel-frame industrial sheds, transmission towers, highway bridges (combined with IRC 6), chimneys, signage, cladding, roof sheeting, and rooftop equipment.

You need IS 875 Part 3 for: - Any building ≥ 10 m tall (residential G+2 and above) - All industrial and warehouse roofing (wind often governs over gravity for roofing) - Cladding design — wall sheeting, curtain walls, window mullions - Free-standing structures — billboards, solar panel arrays, hoardings - Coastal construction (design wind speed 50+ m/s cyclone zone requires additional care)

Pair with: - IS 875 Part 1:1987 — dead loads - IS 875 Part 2:1987 — imposed (live) loads - IS 875 Part 5:1987 — special loads (temperature, soil, erection) - IS 1893 Part 1:2016 — seismic loads, for comparison (often wind governs for tall buildings in Zone II-III; seismic governs in Zone IV-V)

IS 875 Part 3:2015 computes design wind speed V_z at any height via:

V_z = V_b × k_1 × k_2 × k_3 × k_4

• V_b — basic wind speed at 10 m height in open terrain, from Figure 1 map. 6 zones from 33 m/s to 55 m/s across India.
• k_1 — risk factor (importance). 1.0 for ordinary buildings, 1.07 for important (hospitals, schools, fire stations), 0.71 for temporary structures.
• k_2 — terrain-height factor from Table 2. Increases with height, decreases with terrain roughness (Category 1 = open sea, Category 4 = dense urban).
• k_3 — topography factor. 1.0 for flat terrain; 1.1-1.36 for hills, cliffs, ridges per Appendix C.
• k_4 — cyclone factor (2015 revision addition). 1.30 for coastal strip within 60 km for post-cyclone critical buildings; 1.0 for all other cases.

Design wind pressure: p_z = 0.6 × V_z² (kN/m²). A V_z of 50 m/s gives p_z = 1.5 kN/m².

Force coefficient C_f from Table 4-7 translates pressure to force on specific building shapes (rectangular buildings, cylindrical towers, lattice structures, cladding).

The 2015 revision introduced the k_4 cyclone factor after the Phailin (2013) and Hudhud (2014) cyclones highlighted inadequate design wind speeds in coastal Andhra, Odisha, and Tamil Nadu.

Problem: G+8 residential RCC building, 30 m height, 20 × 15 m plan, in Mumbai (mid-suburb, Category 3 terrain — Mumbai metropolitan has mixed mid-rise density). Flat terrain, not in cyclone strip. Ordinary occupancy.

Step 1 — Basic wind speed V_b: From Figure 1: Mumbai lies in 44 m/s zone.

Step 2 — Risk factor k_1: Ordinary residential → k_1 = 1.0 (Table 1).

Step 3 — Terrain-height factor k_2: Category 3, height 30 m. From Table 2: k_2 at 30 m in Category 3 = 0.97 (linear interpolation).

Step 4 — Topography factor k_3: Flat Mumbai suburb → k_3 = 1.0.

Step 5 — Cyclone factor k_4: Not in designated cyclone strip → k_4 = 1.0.

Step 6 — Design wind speed at height 30 m: V_z = 44 × 1.0 × 0.97 × 1.0 × 1.0 = 42.7 m/s

Step 7 — Design wind pressure: p_z = 0.6 × 42.7² = 1.09 kN/m² at 30 m height.

Step 8 — Force coefficient (Table 4 for a rectangular building): For a rectangular building with h/w = 30/15 = 2.0 and b/d = 20/15 = 1.33: C_f (wind perpendicular to long face) ≈ 1.2 (face pressure) + 0.5 (side suction) = 1.7 net

Step 9 — Horizontal wind force on long face (20 m wide × 30 m tall): F = p_z × A × C_f = 1.09 × (20 × 30) × 1.7 = 1,111 kN total lateral force

Distribute to floors per Clause 6.3.3.3 (proportional to area per floor), check against base shear from seismic analysis (IS 1893 Part 1:2016) — whichever is larger governs lateral design.

Note: This is simplified. For detailed design, also check: - Along-wind + across-wind response per Clause 10 (vortex shedding for slender buildings) - Local cladding pressure coefficients C_pe for zones A, B, C, D, E on the building envelope (Table 5) — edge and corner zones can have suction coefficients -1.8 to -2.4 - Dynamic response factor G per Clause 9.4 (required for buildings > 50 m or aspect ratio > 5:1)

1. Using V_b at the wrong height. V_b is basic wind speed at 10 m above ground in open terrain Category 2. Some engineers apply V_b directly to their design without the k_2 adjustment — this under-designs for tall buildings (k_2 > 1 above 10 m) or over-designs for ground-level structures in rough terrain.

2. Wrong terrain category. Category 3 (mid-rise urban) and Category 4 (dense city centre) differ by ~10-15% on k_2. Mumbai's Bandra-Kurla Complex is Category 4 but many designers use Category 3. For tall buildings, this matters.

3. Missing the cyclone factor k_4. Added in 2015 but often missed by designers using pre-2015 templates. For critical buildings (hospitals, fire stations, schools used as shelters) in cyclone-prone coastal districts (Andhra, Odisha, Tamil Nadu coastal, parts of Gujarat, West Bengal), k_4 = 1.30 is mandatory. Without it, wind forces are under-designed by 30%.

4. Ignoring local cladding zones. Table 5 pressure coefficients for corners (Zone A) and edges (Zone B) can reach -1.8 to -2.4 (suction). Engineers often design cladding and roof sheeting using face-average pressures (~-0.8 to -1.2), leading to cladding failures at corners during storms. This is a known failure pattern for industrial sheds.

5. Skipping across-wind / dynamic response. For slender buildings (h/b > 5), vortex shedding induces across-wind oscillations that can exceed along-wind forces. IS 875 Part 3 Clause 10.3 mandates dynamic analysis. Designers stopping at static wind force analysis miss a critical failure mode — vortex-induced lateral oscillation has caused chimney and transmission tower failures.

• IS 875 Part 1:1987 — dead loads
• IS 875 Part 2:1987 — imposed / live loads
• IS 875 Part 4:1987 — snow loads (for Himalayan regions)
• IS 875 Part 5:1987 — special loads (temperature, soil pressure, erection)
• IS 1893 Part 1:2016 — seismic loads (often compared against wind for lateral design)
• IRC 6:2017 — specifies wind loads for bridges (different from building wind loads)
• IS 802 Part 1 Sec 1 — loads and stresses for steel transmission towers (uses IS 875 Part 3 base wind speed)
• IS 4998 — chimney design (supplements IS 875 Part 3 for chimneys)
• IS 14732 — wind loads for membrane structures (tensile fabric roofs)

IS 875 Part 3:2015 is a major revision over the 1987 edition. Key changes: cyclone factor k_4 added, terrain categories refined, dynamic response more clearly mandated, and pressure coefficients updated for modern building shapes.

Field reality: many pre-2015 building designs in Mumbai, Chennai, Kolkata, Visakhapatnam were re-verified after the revision; some needed retrofit (particularly critical buildings that now fall under k_4 = 1.30).

For tall buildings above 100 m, IS 875 Part 3 is conservative but less detailed than international codes. Indian high-rise projects typically supplement IS 875 with wind tunnel testing (follow Eurocode 1 Part 1-4 Annex E methodology) — the results usually show actual wind loads 5-15% lower than IS 875 prescriptive values for streamlined building shapes, but significantly higher for buildings with unusual massing (stepped, twisted, notched). Wind tunnel testing is mandatory for buildings above 150 m in most Indian cities' DCR.

For cladding and roofing on tall buildings, pressure equalization and internal pressure coefficients from Clause 6.3 can significantly change the design; this is often delegated to a facade consultant rather than the structural engineer, leading to interface issues. Specify pressure zones explicitly in your structural drawings so the facade contractor cannot later claim the structural pressures were 'conservative' and over-spec.

A revision is expected around 2028 to align with ISO 4354 framework and incorporate climate-change-adjusted design wind speeds (which may rise 5-10% for some coastal zones based on IPCC projections).