Clause 6.3.4 introduces the importance factor for cyclonic regions (k4). This factor was newly added in the 2015 revision of IS 875 Part 3. For structures in non-cyclonic regions, k4 = 1.0. For structures in cyclonic regions (as identified in Appendix B and Fig. 1A), k4 = 1.15 for important post-cyclone functioning structures (hospitals, cyclone shelters, communication towers, power plants, fire stations) and k4 = 1.0 for other structures. The cyclonic region roughly covers the east coast (from West Bengal to Tamil Nadu), the west coast of Gujarat, and parts of coastal Maharashtra and Goa.
Key Requirements
•k4 = 1.0 for all structures in non-cyclonic regions
•k4 = 1.0 for general structures (not post-disaster critical) in cyclonic regions
•k4 = 1.15 for important post-cyclone functioning structures in cyclonic regions
•Cyclonic regions are identified in IS 875 Part 3 Fig. 1A and Appendix B
•Important structures include hospitals, cyclone shelters, communication towers, power plants, water supply systems, fire stations, and emergency facilities
Reference Tables
k4 Values — Importance Factor for Cyclonic Region (Clause 6.3.4)
Region
Structure Type
k4
Non-cyclonic
All structures
1
Cyclonic
General buildings
1
Cyclonic
Post-cyclone important structures
1.15
Cyclonic regions include: east coast (Odisha, Andhra, Tamil Nadu, West Bengal), Gujarat coast, parts of Maharashtra coast. k4 = 1.15 represents a 15% increase in design wind speed.
Practical Notes
✓k4 was a new addition in the 2015 revision — it did not exist in IS 875:1987. Designers working with the old code miss this factor.
✓k4 = 1.15 adds 15% to wind speed, which translates to approximately 32% increase in wind pressure (since pz ∝ Vz²). This is a significant increase.
✓The cyclonic region definition covers about 100 km inland from the coast on the east coast and parts of the Gujarat coast.
✓For buildings that serve as cyclone shelters or emergency facilities, k4 = 1.15 is mandatory — these structures must remain functional after a cyclone.
Common Mistakes
⚠Ignoring k4 entirely because it was not present in the 1987 edition of IS 875.
⚠Not applying k4 = 1.15 for hospitals in cyclonic regions — hospitals are critical post-disaster facilities.
⚠Applying k4 = 1.15 for a general residential building in a cyclonic region — k4 = 1.0 is correct for non-critical buildings.
⚠Not checking whether the site falls in the cyclonic region as defined in Fig. 1A — some coastal cities are on the boundary.