Clause 6.4 introduces the wind directionality factor (Kd), which accounts for the reduced probability that the maximum wind speed will occur from the most aerodynamically critical direction. For buildings and structures that are not oriented to receive the worst wind from all directions equally, Kd provides a reduction. Kd = 0.90 for buildings (rectangular, square, or other), Kd = 0.90 for chimneys, tanks, and similar structures, and Kd = 1.0 for circular cross-sections and for conservative design. Kd is applied to the wind pressure, not the wind speed.
Key Requirements
•Kd shall be applied as a multiplier on the design wind pressure (pz), not on the wind speed (Vz)
•Kd = 0.90 for rectangular, square, and other plan-shape buildings
•Kd = 0.90 for chimneys, tanks, and communication towers with non-circular cross-sections
•Kd = 1.0 for circular cross-sections (no directional reduction since wind from all directions is equally critical)
•Kd = 1.0 if the designer chooses not to apply directionality reduction (conservative approach)
Reference Tables
Wind Directionality Factor Kd (Clause 6.4)
Structure Type
Kd
Buildings — all plan shapes (square, rectangular, L-shape, etc.)
0.9
Chimneys, tanks, communication towers (non-circular)
0.9
Circular cross-section structures
1
Conservative design / when directionality is uncertain
1
Practical Notes
✓Kd = 0.90 is a straightforward 10% reduction in wind pressure for most buildings. This can translate to meaningful savings in structural members.
✓Kd was introduced in the 2015 revision — the 1987 edition did not have this factor. Some designers may not be aware of this beneficial reduction.
✓Kd applies to the pressure (pd = Kd × pz), not to the velocity. The effective reduction in wind force is 10%.
✓For critical structures or when the designer is unsure about the wind directionality at the site, it is prudent to use Kd = 1.0.
Common Mistakes
⚠Applying Kd to the wind speed Vz instead of the pressure pz — Kd is a pressure multiplier, not a speed multiplier.
⚠Using Kd = 0.90 for circular tanks or chimneys — circular sections should use Kd = 1.0 since they are equally loaded from all wind directions.
⚠Not applying Kd at all because it was absent in the 1987 edition — the 2015 code permits this beneficial reduction.
⚠Double-counting Kd with other load combination reduction factors.