Gravity anchor

Fibre Reinforced Facade Encyclopedia: Manufacturing

A gravity anchor is the anchor in a stud frame panel that carries the self weight of the thin glass fibre reinforced concrete (GRC, GFRC) skin into the steel frame behind it. Unlike the flex anchors around it, it is designed to take load in the plane of the panel and is made stiff enough to do so. There is normally one effective gravity anchor location per panel, usually built as a symmetrical pair, and that restriction is the whole logic of the system: the skin hangs from one point and is free to move everywhere else.

At a glance

The load path, and where it stops

A stud frame panel is a thin concrete skin hung on a light steel frame. The skin weighs 30 to 50 kg per square metre, so a 3 by 3 metre panel carries 270 to 450 kg of its own material before the frame is counted. That weight has to reach the frame somewhere, and the gravity anchors are that somewhere.

The weight passes from the skin into the bonding pad by shear across the bond line, from the pad into the anchor leg, from the anchor into the stud frame by a weld or bolted connection, and out through the frame brackets into the structure. The frame end of that path is ordinary structural engineering. The skin end is not, because the material carrying the shear is 13 to 15 mm of thin section concrete whose aged tensile capacity is modest, which is why the pad is generous in area and why the bond is made by spraying rather than by adhesion to a cured face.

Everything past the frame belongs to the panel fixing systems and substructure pages.

Why only one effective gravity anchor location

This is the point on which stud frame panels succeed or fail. In the plane of the panel, the skin must be supported at exactly one effective location. Support it at two and it is statically indeterminate in plane: as the skin expands or contracts relative to the frame, the two supports pull against each other and the difference is taken as stress in the skin.

The skin cannot take it. Glass fibre reinforced concrete moves 0.5 to 1.5 mm per metre with moisture and a further 0.6 to 1.2 mm per metre with temperature, while the steel frame in the shaded cavity moves very little. Its limit of proportionality, the stress at first crack, is typically 7 to 11 MPa for sprayed GRC, and restraining a few millimetres of movement across a thin section reaches that easily. The crack appears at or between the restraining anchors and it is permanent.

The symmetrical pair normally built is not a contradiction of this. The two anchors sit equidistant either side of the vertical centre line, so together they fix the skin vertically and fix its centre line horizontally while leaving it free to expand outward in both directions. Functionally that is one support location plus rotational restraint. Every other anchor on the panel is a flex anchor: slender, free to bend, carrying wind out of plane and nothing in plane.

Making it, and the hold point that matters

The gravity anchor pad is the most important single operation in making a stud frame panel, and it cannot be inspected once it is done.

The pad has to be sprayed wet on wet, applied while the skin beneath it is still green, with the same material as the skin and compacted into it. Done that way the pad and the skin become one piece of material. Applied to a cured skin, even hours later and even with a bonding agent, the pad is stuck to a surface rather than integral with it, and the failure mode is a pad that debonds under sustained load, which means a panel that falls off its frame.

• Pad spraying is a hold point, witnessed and recorded against the batch record
• The anchor leg must be fully encapsulated, with no voids
• Anchors are welded to the frame before the pad is cast, never after, because weld heat destroys the bond
• Pull-off testing of sample pads is done at the prototyping and first-off stage
• Anchor and pad positions are checked against the shop drawings
• Anchors, welds and the frame are inspected for corrosion protection before the panel is closed up

The quality control page covers how these checks sit inside factory production control.

Failure modes

• Too many in-plane supports. A second gravity anchor pair, or a restrained flex anchor, produces map or line cracking across the skin, usually within the first year
• Pad debonding. A pad sprayed onto a cured skin loses its bond under sustained load, sometimes years later, and the panel drops
• Undersized pad, or welding to an anchor after its pad is cast
• Anchor corrosion. A mild steel or lightly galvanised anchor in a damp cavity corrodes, expands and spalls the pad off the back of the skin, invisibly until the panel moves
• Frame contact. A frame member touching the back of the skin becomes an unintended in-plane support

Standards and references

• GRCA Practical Design Guide for the design of GRC stud frame panels and anchor systems
• GRCA Specification for the Manufacture, Curing and Testing of GRC Products
• AS/NZS 1170.1 permanent and imposed actions; AS/NZS 1170.2 wind actions
• PCI MNL-128 Recommended Practice for Glass Fiber Reinforced Concrete Panels (United States anchor practice)

Related terms

• Stud frame GRC
• Flex anchor
• Panel fixing systems
• Dead load per square metre
• Quality control
• Limit of proportionality (LOP)
• Installation

Questions specifiers ask

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