Clause 7.3 provides the method for calculating the total wind force (F) on a building or structure. The force is computed as F = Cf × Ae × pd, where Cf is the force coefficient (drag coefficient) depending on the shape of the structure, Ae is the effective projected area normal to the wind direction, and pd is the design wind pressure. For enclosed buildings, the wind force can alternatively be computed using internal and external pressure coefficients (Cpe and Cpi) on individual surfaces.
Key Requirements
•Wind force on the building shall be calculated as F = Cf × Ae × pd
•Force coefficient Cf shall be obtained from Tables 23–28 based on building shape and aspect ratio
•Ae is the effective frontal area of the structure projected normal to the wind direction
•pd is the design wind pressure at the appropriate height
•For buildings with openings, internal pressure coefficients (Cpi = ±0.2 for normal permeability, ±0.5 for large openings) shall be considered in addition to external pressure coefficients
Formulas
F = Cf × Ae × pd
Total wind force on a structure or structural element
F = Wind force in NewtonsCf = Force coefficient (drag coefficient) from Tables 23–28Ae = Effective projected area in m²pd = Design wind pressure in N/m² (pd = Kd × Ka × Kc × pz)
F_surface = (Cpe − Cpi) × A × pd
Wind force on a surface of an enclosed building using pressure coefficients
F_surface = Wind force on the surface in NewtonsCpe = External pressure coefficient (from Tables 5–20)Cpi = Internal pressure coefficient (±0.2 normal, ±0.5 large openings)A = Surface area in m²pd = Design wind pressure in N/m²
Practical Notes
✓For preliminary design of low-rise rectangular buildings, Cf ≈ 1.2 (for height/breadth ratio around 1) is a reasonable starting point for overall drag.
✓The F = Cf × Ae × pd method gives the total force on the entire structure (for stability/overturning checks). For cladding and individual member design, use the Cpe/Cpi surface pressure method.
✓Internal pressure (Cpi) is critical for buildings with large openings — a dominant opening on the windward side can cause Cpi = +0.7, significantly increasing roof uplift.
✓Always check both wind directions (parallel and perpendicular to the long axis of the building) — the critical direction depends on the building's proportions.
Common Mistakes
⚠Using Cf for overall building force when designing individual cladding panels — cladding design requires local Cpe values which can be much higher (especially at corners and edges).
⚠Ignoring internal pressure Cpi — for buildings with openings, the net pressure on a surface is (Cpe − Cpi), and the worst combination of signs must be considered.
⚠Not accounting for both positive and negative Cpi — the sign that produces the worst effect on each element must be used.
⚠Using a single pd value for the entire building height instead of computing pd at different levels.