Panel fixing systems

Fibre Reinforced Facade Encyclopedia: Design and detailing

A panel fixing system is the set of anchors and connections that attach a cladding panel to its supporting rail or bracket. It is a different design problem from the substructure, which is the secondary framing behind the panel. The substructure decides how load gets back to the building; the fixing system decides how load gets out of the panel, and it is governed by what the panel material can take at a point rather than by what the rail can span.

At a glance

Gravity and restraint: the distinction that governs everything

A fixing that carries weight and a fixing that resists suction are not the same component and must not be detailed as though they are. A gravity fixing takes the panel dead load in bearing or shear and transfers it into the rail. It is stiff, it sits close to the panel centre of gravity, and there are normally exactly two per panel so the load path is determinate and calculable.

A restraint fixing takes wind pressure and suction out of the plane of the panel and nothing else. It has to be free to move in the plane of the panel, through a slotted hole, a sleeved oversized hole or a sliding profile, so the panel slides past the restraint as it expands rather than levering on it. A restraint point built as a rigid bolted connection becomes a third or fourth gravity point, load redistributes unpredictably, and the panel cracks at the anchor rather than anywhere a calculation predicted.

For a lightweight panel the restraint fixings usually govern. A GRC panel at 40 kg per square metre weighs a fraction of the suction acting on it, so anchor sizing, edge distances and fixing counts are driven by wind, and by the local pressure factors that make corner and parapet panels worse than field panels.

Anchor types and what they suit

Cast-in sockets are the standard fixing for GRC and precast. A stainless threaded ferrule is cast into a rib, a thickened boss or a bonding pad, and the connection is made with a bolt from the rail. Capacity depends on embedment depth, on the surrounding material and on the cone that would fail in pull-out, so edge distance and rib geometry matter as much as ferrule size. Socket positions are set when the mould is designed, not later.

Undercut anchors are drilled into the rear face of a dense panel to a controlled depth, with a widened base so the anchor keys into an undercut rather than relying on friction. They do not penetrate the front face, which is why they are standard for thin stone and dense composites. They need a minimum panel thickness, a calibrated rig and depth and torque control. An anchor set too deep, too shallow or into a void has no reliable capacity and cannot be identified once the panel is hung.

Flex anchors belong to stud frame GRC. A bent stainless rod has one leg embedded in a bonding pad on the back of the thin skin and the other fixed to the steel stud frame. The bend is deliberately flexible so the skin, which moves with moisture as well as temperature, can move relative to a frame that does not. Gravity anchors in the same panel are stiff and grouped centrally, so the skin hangs from one small stiff zone and is restrained everywhere else by flexible ones.

Hook-on rails carry a hook or cleat on the back of the panel that drops onto a continuous rail. They are quick, concealed, tolerant of small setting-out errors and demountable, so a single panel can be lifted out for replacement. Face fixings are visible rivets or screws through the panel into the rail: cheapest and most tolerant, but the hole must be oversized and sleeved so the panel can move, and the fixing head is a design element whether or not anyone intended it. Bonded systems using structural silicone or tape are used, but most jurisdictions also require mechanical retention plus a project-specific adhesion and compatibility assessment.

Adjustability in three axes

Every fixing system has to reconcile a factory-made panel with a site-built structure, using adjustment split across three connections. In and out adjustment, controlling the facade plane, is taken mostly at the bracket to structure connection. Vertical and horizontal adjustment, controlling level and joint width, is taken at the bracket to rail and rail to panel connections. A final few millimetres sits at the panel itself.

The principle is coarse to fine in that order. Typical ranges are ±10 to ±25 mm at the structure, ±10 to ±15 mm at the rail and ±3 to ±5 mm at the panel. Trying to take up a 30 mm structural deviation at the panel connection is how a facade ends up with anchors at the end of their slots, panels binding on their neighbours and joints that vary visibly across an elevation.

Adjustment also has to be locked. Serrated washers, toothed plates, keyed profiles or a designed friction connection at a specified torque all work. A plain washer in a slot, tightened by feel, can creep under cyclic wind load.

Designing and proving the fixings

Fixing layouts are engineering, not detailing preference. The panel engineer sets the number, type and position of every anchor from the panel geometry, the material properties, the zoned wind pressure for that part of the elevation and the movement the panel will undergo. Those positions are then reflected in the mould, the rib layout and the shop drawings, which is why fixing design has to precede mould manufacture rather than follow it.

Anchor design in brittle thin-section materials carries higher factors than steel-to-steel connections, because the failure mode is a cone of material pulling out rather than a ductile yield. Published capacities belong to a stated material, thickness, embedment and edge distance and do not transfer to another panel without assessment.

Verification happens in three places: the factory, where anchors are installed under production control and sample tested; the mock-up, where the complete system is loaded and, where required, tested within an AS 4284 sequence; and site, where a nominated sample of structural anchors is proof tested to an agreed multiple of design load.

• Specify anchor material and grade, not just a type
• Require fixing design certified by the panel engineer against the project wind pressures
• Require anchor positions on shop drawings before mould manufacture
• Nominate a site proof test rate, load and acceptance criteria
• Decide at design stage whether individual panels must be removable for replacement

Standards and references

• AS/NZS 1170.2 Wind actions; AS/NZS 1170.0 General principles and load combinations
• AS 5216 Design of post-installed and cast-in fastenings in concrete; AS 3600 Concrete structures
• AS/NZS 4673 Cold-formed stainless steel structures
• GRCA Specification for the Manufacture, Curing and Testing of GRC Products (anchor and bonding pad requirements)
• PCI MNL-128 Recommended Practice for Glass Fiber Reinforced Concrete Panels (anchor design)
• AS 4284 Testing of building facades, including proof loading of the complete system
• European Assessment Documents for kits for external wall claddings, including undercut anchor assessment
• ASTM C1354 Standard Test Method for Strength of Individual Stone Anchorages in Dimension Stone (US)

Where this matters on a ShapeShift project

Related terms

• Undercut anchor
• Hook-on rail system
• Substructure
• Flex anchor
• Gravity anchor
• Stud frame GRC
• Wind load
• Shop drawings

Questions specifiers ask

Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.