Fibre Reinforced Facade Encyclopedia: Design and detailing
Wind load on a facade is the pressure and suction the wind exerts on the external wall, determined in Australia and New Zealand under AS/NZS 1170.2 Structural design actions, Part 2: Wind actions. For a thin panel it is almost always the governing action. A GRC or FRP rainscreen weighs 5 to 50 kg per square metre, so its self weight is trivial beside a design suction of 2 to 4 kPa trying to pull it off the building.
A design suction of 3 kPa on a 1.2 by 2.4 metre panel is a pull of about 8.6 kN, roughly 880 kg, on a panel that weighs about 90 kg. Nothing about the panel holds it on. The fixings do.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Governing standard | AS/NZS 1170.2:2021 | Used with AS/NZS 1170.0 for importance levels and load combinations |
| Wind regions | A, B, C and D | The 2021 edition subdivides A into A0 to A5 and B into B1 and B2 |
| Regional wind speed VR | about 45 m/s in Region A to about 80 m/s in Region D | 500 year return period; indicative, read the standard |
| Terrain category | TC1 open water to TC4 city centre | Sets the terrain and height multiplier Mz,cat |
| Importance level | IL1 to IL4 per AS/NZS 1170.0 | IL2 ordinary buildings use a 1/500 annual probability for ultimate wind |
| Pressure equation | p = 0.5 × 1.2 × Vdes² × Cfig × Cdyn | p in Pa with Vdes in m/s and air density 1.2 kg/m³ |
| Typical cladding ULS pressure | 1.0 to 4.0 kPa | Suburban low rise to coastal high rise; higher in cyclonic regions |
| Local pressure factor Kl | 1.5 to 2.0 on wall cladding | Applies to small areas at corners, edges and parapets |
| Area reduction factor Ka | 1.0 for a single panel | Reductions begin above about 10 m² of tributary area |
| Serviceability wind | commonly a 1/25 year event | Used for deflection limits and water penetration test pressures |
How a facade pressure is arrived at
AS/NZS 1170.2 works in two steps. First it produces a site wind speed, by taking the regional gust wind speed VR and modifying it for wind direction, for terrain and height, for shielding by upwind buildings and for topography such as a hill or escarpment. The governing result becomes the design wind speed Vdes.
Second it converts speed to pressure: half the air density of 1.2 kg/m³ times the square of Vdes, multiplied by an aerodynamic shape factor Cfig and a dynamic response factor Cdyn. For an external wall, Cfig is built from the external pressure coefficient for that face, the area reduction factor Ka, the combination factor Kc and the local pressure factor Kl.
The panel is designed for the net pressure across it: the external pressure less whatever acts on its back face. For a sealed wall that is the building internal pressure, which depends on porosity and on whether there is a dominant opening. For a rainscreen it is the cavity pressure, covered on the rainscreen page. Unless the facade engineer has established a cavity pressure, design the panel for the full external pressure.
Region, terrain and importance level
Region A covers most of the continent, including Melbourne, Sydney, Adelaide, Perth and Canberra. Region B is an intermediate non-cyclonic band that includes Brisbane and the Gold Coast. Region C is the tropical cyclonic coast, taking in Darwin, Cairns, Townsville and Rockhampton. Region D is the severe cyclonic north west coast of Western Australia around Onslow and Port Hedland. Read the boundaries off the maps in the standard rather than from a list of cities; several capitals sit close to a line.
Terrain category describes the roughness upwind of the site, from TC1 open water through TC2 open grassland and TC3 suburban to TC4 city centre. Rougher terrain slows the wind near the ground, so a low rise building in TC4 sees far less pressure than the same building in TC1. The benefit fades with height: by around 100 metres the multipliers converge.
Importance level comes from AS/NZS 1170.0 and sets the annual probability of exceedance for the ultimate limit state: 1/500 for an ordinary IL2 building, 1/1000 for IL3, 1/2000 for a post-disaster IL4 facility. Because pressure varies with the square of speed, a modest change in importance level is not a modest change in pressure.
Why cladding sees more than the wall average
Pressures used to design a building frame are averaged over large areas. Pressures used to design cladding are not. Where flow separates from a sharp edge, at a windward corner, along a parapet, at a deep reveal or around a fin, it forms conical vortices producing intense short-duration suctions over small patches of wall. The local pressure factor Kl accounts for this, commonly 1.5 on wall cladding in defined edge zones.
The area reduction factor works the other way and is why the gap is so wide. Ka is 1.0 up to about 10 m² of tributary area and falls to about 0.8 at 100 m² and above, because a gust never covers a large surface uniformly. A single 2 to 4 m² panel gets no reduction at all, so a corner panel can carry 1.5 to 2 times the pressure of an identical panel mid-elevation. Deep reveals, blades and parapet cappings are worse again, being loaded on two faces.
The practical consequence is zoning. The panel schedule is split into field, edge and corner zones, and the corner and parapet zones get more fixings and heavier anchors rather than a different panel.
Ultimate and serviceability limit states
The ultimate limit state asks whether anything breaks. It uses the wind speed for the annual probability set by the importance level and governs the panel section, the anchors, the rails, the brackets and the anchorage into the structure. For a lightweight panel the critical case is usually suction, where self weight is factored down because it is helping to hold the panel on and cannot be relied on.
The serviceability limit state asks whether the facade still works. It uses a much lower wind speed, commonly a 1/25 year event, and governs deflection and water penetration. Typical facade deflection limits are span over 180 to span over 240 with an absolute cap around 19 to 20 mm, tighter where a member supports glass or a sealed joint. The serviceability pressure also sets the water penetration test pressures in AS 4284, while the structural proof test in that standard derives from the ultimate pressure.
A specification quoting one pressure and calling it the design wind load is incomplete. The facade package needs both, by zone.
Indicative numbers, and who sets the real one
The figures below are illustrative only, calculated for simple rectangular buildings to show order of magnitude. They are not a design method and must not be used on a project.
- Two storey building, Region A, TC3 suburban, IL2, panel at 8 m: corner panel design suction in the order of 0.8 to 1.0 kPa
- Twenty five storey building, Region B, TC2, IL2, panel at 80 m: corner panel design suction in the order of 2.0 to 2.5 kPa
- Mid rise building, Region C cyclonic, TC2, IL2, panel at 30 m: 4 to 6 kPa is not unusual once a dominant opening internal pressure is included
For tall, slender or unusually shaped buildings, a boundary layer wind tunnel study commonly replaces the code method and produces zoned cladding pressures directly. Where one exists it governs the panel schedule.
Standards and references
- AS/NZS 1170.2:2021 Structural design actions, Part 2: Wind actions
- AS/NZS 1170.0:2002 Structural design actions, Part 0: General principles, importance levels and load combinations
- NCC 2022 Volume One, Part B1 Structural provisions (check the edition adopted in your jurisdiction)
- AS 4284 Testing of building facades (serviceability and ultimate test pressures derived from the design wind load)
- Australasian Wind Engineering Society guidance on boundary layer wind tunnel testing of buildings
- EN 1991-1-4 Eurocode 1: Actions on structures, Part 1-4: Wind actions
- ASCE/SEI 7, Chapter 30 components and cladding (US)
- CWCT Standard for Systemised Building Envelopes, wind loading and testing sections (UK)
Related terms
Questions specifiers ask
How is facade wind load calculated in Australia?
Under AS/NZS 1170.2. The regional wind speed is modified for direction, terrain and height, shielding and topography to give a design wind speed, which is converted to a pressure and multiplied by aerodynamic shape factors.
Why do corner panels need more fixings?
Because flow separates at the corner and produces intense local suction, and a single panel is too small to attract any area reduction. A corner panel can see 1.5 to 2 times the pressure of an identical panel mid-elevation.
What is the difference between ultimate and serviceability wind pressure?
The ultimate pressure is for strength: nothing may fail. The serviceability pressure is much lower, commonly a 1/25 year event, and governs deflection and water penetration, including AS 4284 test pressures.
What wind pressure should go in the facade specification?
Zoned ultimate and serviceability pressures issued by the project engineer, not a single figure. Give field, edge, corner and parapet zones, and state the importance level and annual probabilities used.
Does a ventilated cavity reduce the wind load on the panel?
Only where the cavity is compartmented and vented so that pressure equalises, and only where the facade engineer has established that. Otherwise design for the full external pressure. The mechanism is covered on the rainscreen page.
Who is responsible for the wind load figure?
The project structural or facade engineer determines it. The panel engineer designs the panel, its anchors and its substructure to that figure. A panel manufacturer should never be the source of the design wind pressure for a building.
Designing panels to a project wind pressure? ShapeShell panels are supplied with an engineered fixing system designed to the zoned pressures in the project brief. Open the facade calculators
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.