Clause 6.3 gives the design wind speed (Vz) at any height z above ground. It is obtained by multiplying the basic wind speed (Vb) by four modification factors: k1 (risk coefficient based on mean probable design life and structure class), k2 (terrain roughness and height factor), k3 (topography factor), and k4 (importance factor for cyclonic region). The design wind speed varies with height, terrain, topography, and the desired level of safety.
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Key Requirements
•Design wind speed Vz shall be calculated as Vz = Vb × k1 × k2 × k3 × k4
•k1 shall be obtained from Table 1 based on structure class and desired mean return period
•k2 shall be obtained from Table 2 based on terrain category, structure class, and height
•k3 shall account for topographic features (hills, ridges, cliffs) as per Appendix C
•k4 shall be taken as 1.0 for non-cyclonic regions and per Clause 6.3.4 for cyclonic regions
Formulas
Vz = Vb × k1 × k2 × k3 × k4
Design wind speed at height z above ground level
Vz = Design wind speed at height z in m/sVb = Basic wind speed from Fig. 1 / Appendix A in m/sk1 = Risk coefficient (probability factor) from Table 1k2 = Terrain roughness and height factor from Table 2k3 = Topography factor (1.0 for flat terrain)k4 = Importance factor for cyclonic region (1.0 for non-cyclonic)
Practical Notes
✓For most inland buildings (general category, flat terrain, non-cyclonic): k1 = 1.0, k3 = 1.0, k4 = 1.0, so Vz = Vb × k2. The only variable is height via k2.
✓k2 increases with height — at 10 m in Category 2 terrain, k2 = 1.0 (the reference condition). At 50 m it is approximately 1.14.
✓In cyclonic coastal areas (k4 = 1.15) with important structures (k1 = 1.08), the combined amplification over Vb can be 25–40%.
✓Always compute Vz at multiple heights for tall buildings — wind loads increase significantly with elevation.
Common Mistakes
⚠Forgetting k4 in cyclonic regions — many designers default all factors to 1.0 without checking the cyclone map.
⚠Using k2 for the wrong terrain category — urban sites are Category 3, not Category 2.
⚠Applying k3 = 1.0 on a hilltop site without checking topography provisions in Appendix C.
⚠Not recalculating Vz at each floor level for multi-storey buildings and instead using a single value.