Wind Load Analysis for Purlins: AS/NZS 1170.2 Explained

Wind load analysis for purlins is the hardest part of the design, and most tools either approximate it or make the engineer calculate it and type it in. Both choices push the problem back onto the person least able to check it.

Purlins and girts are, structurally, unremarkable members. Bent cold-formed sections, spanning between frames, doing an ordinary job. What makes them awkward is not the section. It is the load.

A roof beam sees gravity. A purlin sees gravity, then it sees a wind event that tries to peel the roof off, and in most of Australia the second case is the one that governs. Getting that case right is the whole design.

Most other tools or software publicly available either approximates wind loading on the members, or requires the user to manually calculate and enter the loads.

Utsav Desai — Industrial & Manufacturing Facilities, Peritas

What wind load analysis for purlins actually requires

AS/NZS 1170.2 does not hand you a pressure. It hands you a chain of factors, each of which has to be resolved for the specific building, on the specific site, for the specific member. The procedure is set out in Clause 2.1: site wind speed, then design wind speed, then design wind pressure, then wind actions.

Site wind speed

Site wind speed (Clause 2.2) starts from the regional gust wind speed for the site’s wind region and applies a climate change multiplier and a wind direction multiplier (Section 3), then the site exposure multipliers for terrain category and height, shielding from upwind buildings, and topography (Section 4). Two identical warehouses, one on a coastal escarpment and one behind a suburban windbreak, do not get the same number.

There is a wrinkle here specific to purlins. The standard defines immediate supports as the members cladding is fixed directly to, and names purlins and girts among them (Clause 1.4). In the cyclonic regions, the wind direction multiplier is set to 1.0 for cladding and its immediate supporting structure (Clause 3.3). The direction multiplier that is correct for the portal frame is not necessarily the one that applies to the purlins it carries — a distinction easy to lose when the loads are worked out once, elsewhere, and typed in.

Aerodynamic shape factor

This is where purlins get interesting. The aerodynamic shape factor (Section 5) is not one coefficient. For external pressures on an enclosed rectangular building it is assembled (Clause 5.2) from the external pressure coefficient (Clause 5.4.1) together with:

• an area reduction factor (Clause 5.4.2), which recognises that a large tributary area does not experience peak pressure everywhere at once;
• a combination factor (Clause 5.4.3), accounting for the unlikelihood of peak external and peak internal pressures coinciding;
• a local pressure factor (Clause 5.4.4), which does the opposite, and raises the design pressure at roof edges, ridges and corners;
• a permeable cladding reduction factor (Clause 5.4.5), where it applies to the roof or side walls.

Note where the local pressure factor sits. Clause 5.4.4 gives it for cladding, and purlins are what cladding fixes to. It is not a factor that lives somewhere else in the model.

Internal pressure

Internal pressure (Clause 5.3) is a function of the openings and leakage in the building envelope. Once an opening on a surface exceeds 0.5% of that surface’s area — whether designed in, left open, or created by a roller door that fails under pressure — the internal pressure coefficients come from a different table, and the net uplift on every purlin in the building changes with them. Internal pressure is not a detail. It is frequently the difference between a section that works and one that does not.

Why edge zones drive the design

The local pressure factor (Clause 5.4.4) is the reason a purlin design cannot be resolved with a single load case. Near the windward edge of a roof, and in the corner regions, flow separation produces suction peaks above the general roof value. The purlins in those zones carry a different load from the ones four bays in.

Which means a correct design is zoned. Different purlin spacings, different sections, or different bridging arrangements across a single roof plane. Do it by eye and you either over-specify the whole roof, or under-specify the corner.

The extent of those zones follows a reference dimension defined in Clause 5.4.4, and on the large, low-rise roofs that purlins are used for, the edge and corner regions are not a trivial share of the plan area. Treating the roof as uniform is an approximation in a place where the consequences are not symmetrical: over-specify and you have wasted steel, under-specify and you have a problem at the corner where the suction is highest.

This is precisely the calculation that gets approximated. It is tedious, it is geometry-dependent, and it changes every time the roof changes.

Ultimate and serviceability are different problems

Uplift governs strength. Deflection under serviceability wind governs whether the cladding and its fasteners survive a normal winter. They rarely point at the same critical member, and they never point at the same load combination.

The software gives confidence that all loading conditions have been considered to create the worst case loading for ultimate and serviceability conditions.

Andrew Jens — Commercial & Education, Peritas

The value of automating this is not that the arithmetic is hard. It is that the enumeration is hard. There are a lot of combinations, they interact with member position, and a human working through them at 6pm on a Thursday will miss one.

Derived loads, not entered loads

PurlinIQ derives wind loads from the building geometry and site inputs rather than asking the engineer to arrive with a pressure in hand. The distinction matters for three reasons:

• The inputs are auditable. A reviewer can see the terrain category, not just the resulting kilopascals.
• Revisions propagate. Move the ridge, and the edge zones move with it.
• Nothing gets retyped. The load that was calculated is the load that was analysed and the load that appears in the report.

Capacity checks then run against AS/NZS 4600, the primary design standard for light-gauge cold-formed steel in Australia, with the methodology checked against ColdSteel, the cold-formed steel design software developed by the University of Sydney's Centre for Advanced Structural Engineering.

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