Fibre Reinforced Facade Encyclopedia: Properties
Deflection limits are the serviceability criteria that cap how far a facade panel, rail or bracket may bend under wind and its own weight, written as a fraction of the span such as span over 180 or span over 360. For thin composite panels they, and not strength, almost always decide panel size, rib depth and rail spacing.
A 1.5 m span at span over 180 may move 8.3 mm. The same span at span over 360 may move 4.2 mm. Halving the allowance roughly doubles the stiffness the section has to provide.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Facade panel, general | span/180 to span/240 | Under serviceability wind pressure |
| Panels adjacent to or supporting glass | span/360 | Protects sealed units, gaskets and seals |
| Mullions and support rails | span/175 up to about 4.1 m span, then span/240 plus 6.4 mm | Common curtain wall practice, AAMA TIR-A11 |
| Cantilevers | limit applied to twice the projecting length | A 600 mm cantilever is checked as a 1,200 mm span |
| Serviceability wind pressure | commonly the 1 in 25 year event | Annual probability set by AS/NZS 1170.0, pressure by AS/NZS 1170.2 |
| Flexural modulus, FRP laminate | 7 to 15 GPa typical | A fraction of steel; this is why deflection governs |
| Flexural modulus, sprayed GRC | 10 to 20 GPa typical | For comparison |
| Aluminium and steel | 70 GPa and 200 GPa | Substructure comparison |
| Deflection under uniform load | proportional to load x span to the fourth, divided by modulus x second moment of area | Span dominates everything else |
Why deflection governs thin panels
Deflection under a uniform load varies with the fourth power of the span and inversely with the product of elastic modulus and second moment of area. Composites bring a low modulus to that equation. An FRP laminate at 7 to 15 GPa and a glass fibre reinforced concrete (GRC, GFRC) skin at 10 to 20 GPa sit an order of magnitude below steel, so a panel sized for strength alone is usually well past its deflection limit before it is near its failure stress.
Panel sizing is therefore a stiffness problem. The design levers are span, the most powerful because of the fourth power, and second moment of area, which is raised by depth rather than by more material in the skin. Ribs on the back of a GRC panel, returned edges and folds on an FRP panel, and a stud frame behind the skin all buy second moment of area without buying weight.
What the limit is protecting
- Appearance: a visibly bowing panel reads as a defect whether or not it is safe, which is why the criterion is often set by eye at around span/200 to span/250
- Seals and sealants: joint movement from panel deflection adds to thermal and structural movement, and a sealant has a finite movement accommodation factor
- Adjacent glass: glazing and its gaskets tolerate very little imposed movement, hence the tighter span/360
- Clearance: the panel must not touch the substructure, services or the wall behind it at full deflection
- Water path: deflection must not flatten a fall, close a drainage gap or pond water in a soffit or sill
- Fixings: excessive rotation at the panel edge loads anchors in ways they were not designed for
The load case matters as much as the limit
A deflection limit means nothing without the load it applies to. Serviceability checks use a serviceability wind pressure, commonly the 1 in 25 year event, derived from AS/NZS 1170.2 with the annual probability chosen under AS/NZS 1170.0 for the importance level of the building. Ultimate limit state pressures are much higher and are checked against strength.
Dead load is a separate case, and for soffits, sunshades and sills it is often the one that matters, because it acts permanently. Thermal bow is a third: a panel hot on the face and cool on the back curves, and on a dark thin panel that curvature can be of the same order as the wind deflection. Where a facade is tested, AS/NZS 4284 measures deflection at serviceability pressure directly.
Creep in polymer panels
A thermoset laminate is viscoelastic. Under a sustained load it keeps deflecting over time, and the effect accelerates with temperature. Where the load is permanent, principally self weight and anything imposed on a horizontal element, deflection is calculated with a reduced long-term or creep modulus rather than the short-term coupon value. Wind is transient enough for the short-term modulus to apply.
GRC creeps far less but is not immune, and it also carries an irreversible drying shrinkage in its first months. On both materials the response is the same: keep permanent bending out of thin skins, and give the section depth rather than relying on the material.
Meeting the limit
The order of effectiveness is nearly always the same. Reduce the span by adding a rail or a fixing point, which pays off to the fourth power. Add depth to the section with a rib, a return or a fold. Add a stud frame behind the skin. Only then consider thickening the skin, which adds weight and cost for the smallest gain, or changing material, which changes the fire pathway and the finish as well.
The practical sequence is to fix the serviceability pressure first, then the limit, then test the proposed panel geometry against both, before the elevation is set. Panel size, wind load and substructure carry the rest.
Standards and references
- AS/NZS 1170.0 Structural design actions, General principles, including serviceability criteria and annual probability of exceedance
- AS/NZS 1170.2 Structural design actions, Wind actions
- AS/NZS 4284 Testing of building facades, including deflection measurement at serviceability pressure
- AAMA TIR-A11 Maximum Allowable Deflection of Framing Systems for Building Cladding Components
- CWCT Standard for Systemised Building Envelopes, deflection and movement criteria
- NCC 2022 Volume One, structural provisions calling up the AS/NZS 1170 series (check the edition adopted in your jurisdiction)
Related terms
Questions specifiers ask
What deflection limit applies to facade panels?
Commonly span over 180 to span over 240 under serviceability wind pressure, tightening to span over 360 where the panel supports or adjoins glazing. The project specification governs.
Why does deflection govern instead of strength?
Because composites have a low elastic modulus. An FRP or GRC panel reaches a visible or serviceable deflection limit long before its flexural strength is approached.
What wind pressure is used for a deflection check?
A serviceability pressure, commonly the 1 in 25 year event derived from AS/NZS 1170.2, not the ultimate limit state pressure used for strength.
What is the most effective way to reduce deflection?
Reduce the span. Deflection varies with the fourth power of span, so an extra support does more than any realistic increase in thickness.
Sizing panels against a deflection limit? The ShapeShift facade calculators cover span, wind pressure and panel geometry for GRC and FRP. Open the facade calculators
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.