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Wind load to SANS 10160-3

Peak wind pressure, wall and duopitch roof pressures for a 20 × 12 m building, 10 m high, step by step. The numbers come from the free STATIX wind load calculator and were checked by hand.

1. The problem

Input	Value
Building	20 m long (b) × 12 m wide (d) × 10 m high (h), duopitch roof at 15°
Basic wind speed	v_b = 32 m/s (see the warning below)
Terrain	Category C (suburban)
Site altitude	0 m (air density 1.20 kg/m³)
Internal pressure	C_pi = +0.2 / −0.3

Building with wind zones A to J and the side-wall and roof zone layouts from the STATIX wind load calculator — The free wind load calculator with these inputs: the wall zones A to E and the roof zones F to J for wind on the long face.

Check the wind speed first. 32 m/s is the lowest basic wind speed in SANS 10160-3 Annex A. Most of South Africa is 36 or 40 m/s. Pressure grows with the square of the speed, so a 40 m/s site designed at 32 m/s is under-designed by 56%. Look up your municipality in Table A.1.

2. Basic velocity pressure (cl. 7.4)

q_b = ½ ρ v_b² = 0.5 × 1.20 × 32² = 614 Pa

3. Peak velocity pressure at 10 m (cl. 7.3, Table 1)

Terrain C: z_g = 350 m, z_o = 3 m, z_c = 5 m, α = 0.12 c_r(z) = 1.36 × ((z − z_o)/(z_g − z_o))^α = 1.36 × (7/347)^0.12 = 0.851 c_o(z) = 1.0 (flat terrain) v_p = c_r c_o v_b = 0.851 × 32 = 27.2 m/s q_p = ½ ρ v_p² = 0.5 × 1.20 × 27.2² = 0.445 kPa

The SANS map already gives a 3-second gust speed, so there is no gust factor to add.

4. Walls (cl. 8.3.2, Table 6)

h/d = 10 / 12 = 0.83 → interpolate between h/d = 0.25 and 1 Windward wall D: C_pe = +0.78 Leeward wall E: C_pe = −0.46 Side walls A / B / C: C_pe = −1.20 / −0.80 / −0.50 Net pressure w = q_p (C_pe − C_pi), taking the worse C_pi: D: 0.445 × (0.78 + 0.30) = +0.48 kPa E: 0.445 × (−0.46 − 0.20) = −0.29 kPa A: 0.445 × (−1.20 − 0.20) = −0.62 kPa ← largest

5. Duopitch roof at 15° (cl. 8.3.5, Table 10)

Wind on the long face (θ = 0°), suction case: F = −0.90 G = −0.80 H = −0.30 I = −0.40 J = −1.00 Zone J: 0.445 × (−1.00 − 0.20) = −0.53 kPa (largest roof uplift) Pressure case (a separate load case): F = G = H = +0.20

6. Cladding and fixings

For small loaded areas (1 m² or less: sheeting, fixings, purlin connections) use the local coefficients C_pe,1. They are much higher at corners and edges:

Wall corner A: C_pe,1 = −1.40 → w = 0.445 × (−1.40 − 0.20) = −0.71 kPa Roof zone F: C_pe,1 = −2.00 → w = 0.445 × (−2.00 − 0.20) = −0.98 kPa

7. Result

Item	Net pressure
Peak velocity pressure q_p at 10 m	0.445 kPa
Windward wall	+0.48 kPa
Wall edge zone A	−0.62 kPa
Roof uplift, zone J	−0.53 kPa
Roof corner F, cladding (≤ 1 m²)	−0.98 kPa

Repeat for wind on the gable (θ = 90°) and for the other internal pressure. The worst of all cases governs each element.

Run this example yourselfThe free STATIX wind load calculator opens with exactly these inputs. Change the wind speed, terrain or building size and every pressure updates.
Open the free wind load calculator

A worked example for learning, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.

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