Calculation of Action Combination Factor (Kc,e) and Area Reduction Factor (Ka)
AS/NZS 1170.2:2011 • Clause 5.4.2 • Table 5.4 • External wind pressure
1. Select the surface and design area
Clause 5.4.2: Table 5.4 applies to roofs and sidewalls; other cases use Kₐ = 1.0.
Tributary area is the area contributing to the force/action being considered.
Use the effective tributary area of the member or component being designed, not necessarily the total building area.
Clause 5.4.2: A is the area contributing to the force being considered. For a beam, this may be its tributary width multiplied by the relevant span or length. For a cladding panel, it may be the supported panel area. The chosen area must be appropriate to the structural action being checked.
2. Area reduction coefficient — Table 5.4
| Tributary area A (m²) | Area reduction factor Kₐ | Interpretation |
|---|---|---|
| A ≤ 10 | 1.00 | No area reduction |
| A = 25 | 0.90 | Tabulated intermediate point |
| A ≥ 100 | 0.80 | Minimum Table 5.4 coefficient |
Table 5.4 Note: use linear interpolation between tabulated areas. Values are bounded between 0.80 and 1.00 for the applicable roof and sidewall cases; do not extrapolate beyond the table.
3. Live calculation and interpolation
Tributary area A—m²
Applicable surface—Clause 5.4.2
Reduction—Relative to Kₐ = 1.0
Calculated area reduction factor
Kₐ = —
4. Graph — Kₐ versus tributary area
Blue line: Table 5.4 for roof and sidewall cases. Orange marker: selected area. For W, L and other surfaces the final Kₐ is 1.0 regardless of this reference curve.
Reference: AS/NZS 1170.2:2011, Clause 5.4.2 and Table 5.4. This calculator determines only the external area reduction factor Kₐ.
5. Action combination factor — Kc,e
Select the combination relevant to the structural action, not merely the number of surfaces on the building. Confirm applicability against AS/NZS 1170.2:2011 Clause 5.4.3 and Table 5.5.
Use only when justified by the relevant standard provision. Leave blank for the selection above.
| Contributing surfaces (simplified selection) | Kc,e |
|---|---|
| One | 1.00 |
| Two | 0.90 |
| Three or more | 0.80 |
Combined factor check: For roof and sidewall cases, apply the minimum to the product Kₐ × Kc,e: Kcombined = max(Kₐ × Kc,e, 0.80). For other surfaces, Kₐ = 1.0; the roof/sidewall product floor is not automatically imposed. Check the complete Table 5.5 provisions for your actual load combination.
Area factor—
Selected Kc,e—
Unadjusted product—
Final combined factor Kₐ × Kc,e (after applicable minimum)
—