Wind 2: pressure coefficients and a portal frame

Lesson 4 found the peak pressure q_p. This lesson spreads it over the building: external coefficients for walls and roofs, internal pressure, the worst net pressures, local coefficients for cladding, and the line loads on a portal frame, with the STATIX wind load calculator.

1. Learning outcomes

After this lesson you can:

name the wall and roof zones and read C_pe from SANS 10160-3;
explain internal pressure and choose the worse C_pi for each element;
find net pressures, including the local values for cladding and fixings;
turn zone pressures into the wind loads on a portal frame;
check every zone in the STATIX wind load calculator.

2. Before you start

Lesson 4: the peak velocity pressure q_p.
Your copy of SANS 10160-3 (clause 8.3, Tables 6 to 12) and the free wind load calculator.
A pocket calculator and about 50 minutes.

3. Walls

Wall zones (SANS 10160-3 8.3.2, Table 6). The windward wall D is pushed, the leeward wall E is sucked, and the side walls are sucked hardest near the windward corner (A), less further back (B, C). The zone widths depend on e = the smaller of b and 2h.

The coefficients change with the building's shape. For a squat building (small h/d) the windward wall gets a little less pressure and the leeward wall a little less suction; between the table's rows STATIX interpolates.

4. Roofs

Duo-pitch roof zones (Table 10, wind on the long face). The windward eave strip (F at the corners, G between) is sucked hardest, then H on the rest of the windward slope; J is a strip just past the ridge and I the rest of the leeward slope.

Roof pitch matters. On a low roof the whole roof is sucked up. On steeper roofs the windward slope can be pushed down, so Table 10 gives a negative and a positive set and both are separate load cases. STATIX lists both.

Mono-pitch roofs have their own tables (8 for wind on the low or high eave, 9 for wind along the slope). Hipped roofs use Table 12. The STATIX calculator offers duo-pitch, mono-pitch and hipped roofs, and flat roofs below 5°.

5. Internal pressure and net pressure

Internal pressure. Air leaks in and out through gaps and openings, so the inside is pressurised or depressurised and every inside face is pushed or pulled. Without better knowledge both C_pi = +0.2 and −0.3 are checked. A large opening in one face (a door left open) makes it dominant and changes C_pi; the calculator can work that out from the opening areas.

Net pressure. A wall or roof sheet feels the outside pressure minus the inside pressure: w = q_p (C_pe − C_pi). Positive pushes inwards, negative pulls outwards.

The worst combinations. Positive internal pressure adds to external suction; negative internal pressure adds to external pressure. Each element is designed for the worse of the two, and for each wind direction (on the long face and on the gable).

net pressure: w = q_p (C_pe − C_pi) C_pi = +0.2 and −0.3, the worse governs frames, purlins: C_pe,10; sheets and fixings (≤ 1 m²): C_pe,1; e = min(b, 2h)

6. Cladding and frames

Local coefficients. Small areas such as a sheet, a fixing or a purlin cleat feel the gusty peaks at corners and edges. For loaded areas of 1 m² or less the code gives C_pe,1, much higher than the overall C_pe,10. The calculator lists both.

From pressures to a frame. A portal frame collects the wind from half a bay on each side, so its line load is the net pressure times the frame spacing: on the columns (D and E) and on each part of the rafters (the roof zones it crosses). STATIX gives the pressures; the frame loads are a short hand step.

7. In STATIX

In the STATIX wind load calculator: after q_p, choose the roof type and pitch and the internal pressure (or derive it from openings). It lists every wall and roof zone with C_pe and the net pressure for both C_pi, for wind on the long face and on the gable, and marks the governing ones. It has no topography input (c_o = 1.0).