Clause 6.3.1 defines the risk coefficient (k1) which accounts for the desired safety level based on the mean probable design life of the structure. It is also called the probability factor. Table 1 gives k1 values for different structure classes and mean probable design lives. For general buildings and structures with a 50-year design life, k1 = 1.0. For important structures like hospitals, power plants, and communication towers, k1 = 1.08.
Key Requirements
•k1 shall be selected from Table 1 based on structure class and mean probable design life
•For temporary structures with a 5-year life, k1 = 0.82 (Class 1) to 0.90 (Class 4)
•For general buildings with 50-year design life, k1 = 1.0
•For important structures (hospitals, power stations, communication towers) with 100-year life, k1 = 1.08
•If the structure has a design life other than listed values, k1 may be interpolated or calculated using the formula in Appendix A
Reference Tables
Table 1 — Risk Coefficients for Different Classes of Structures (Clause 6.3.1)
Structure Class
Mean Probable Design Life (years)
k1
Class 1 — All general buildings and structures
50
1
Class 2 — Temporary sheds, structures during construction
5
0.82
Class 3 — Buildings presenting low risk to life (farm buildings, etc.)
25
0.94
Class 4 — Important structures (hospitals, communication, power)
100
1.08
k1 values are based on a statistical analysis of extreme wind speeds assuming a Type I (Gumbel) distribution. For design lives between the listed values, linear interpolation may be used.
Formulas
k1 = (A − B × ln(−ln(1 − 1/N))) / (A − B × ln(−ln(0.98)))
General formula for k1 based on return period N (simplified from Appendix A)
k1 = Risk coefficientN = Return period in years (approximately equal to mean probable design life for common values)A = Coefficient based on mode of the extreme wind distributionB = Coefficient based on dispersion of the extreme wind distributionln = Natural logarithm
Practical Notes
✓For 95% of building design, k1 = 1.0 (general buildings, 50-year life). Only change it for temporary structures or critical infrastructure.
✓k1 = 1.08 for hospitals and emergency buildings — this 8% increase in wind speed translates to roughly 17% increase in wind pressure (since pressure ∝ V²).
✓Temporary construction-stage structures (scaffolding, formwork) can use k1 = 0.82, significantly reducing design wind loads.
✓Nuclear, chemical, and defence installations may require return periods beyond 100 years — use the Appendix A formula.
Common Mistakes
⚠Using k1 = 1.0 for all structures without checking if the building is a hospital, school, or other important structure requiring k1 = 1.08.
⚠Using k1 = 0.82 for a permanent warehouse by incorrectly classifying it as a temporary structure.
⚠Not increasing k1 for structures in cyclonic regions that also serve as cyclone shelters.
⚠Confusing design life with structural life — k1 relates to the desired wind return period, not the expected physical life of the building.