Stud frame construction
Liquidstone GRC Cladding Solutions
Glass Reinforced Concrete (GRC) is a versatile composite material composed of portland cement, sand, and alkali-resistant glass fibres. It’s ability to be formed into complex shapes with relatively lightweight panels, offers significant advantages in handling, transportation, and installation leading to savings on the supporting building structure. GRC combines the high compressive strength of cement mortars with enhanced impact, flexural, and tensile strength provided by the fibre reinforcement. It is also inorganic, non-combustible, and offers good fire resistance. While GRC is considered "quasi-structural," meaning it can be structural within limits, its primary use in cladding is often non-structural, transferring loads to a supporting frame.
Two common types of GRC rain screen cladding construction are widely utilized;
Stud Frame Construction: The most common type of GRC panel construction involves a single GRC skin, which may incorporate an architectural facing, attached to a prefabricated metal frame. This frame is typically made from light-gauge steel or a combination of structural sections and light-gauge steel. The metal frame supports and stiffens the GRC skin and provides the means to connect the panel to the main building structure6. In this system, the GRC skin is never attached directly to the main building structure. The stud frame system allows for the manufacture, transport, and erection of relatively large panels, sometimes up to 10-20m².
Ribbed Panels: These panels incorporate integral ribs, which are stiffening members located behind the GRC skin. These ribs provide structural rigidity and stiffness to the panel. The ribs can be formed using materials like expanded polystyrene foam or polyurethane foam as rib formers or by spraying GRC against removable moulds. The use of GRC stiffening ribs introduces considerations regarding restrained movement during casting and potential shadowing or wrinkling effects on the finished panel surface.
Mechanical fixings are crucial components of GRC cladding systems, serving to transfer loads and accommodate movement. Several types are employed, particularly in the common stud frame system:
Flex Anchors: These are typically L-shaped flexible anchors that connect the GRC skin to the metal panel frame. They are designed primarily for lateral support, transferring wind loads from the skin to the frame. Flex anchors are engineered to allow the GRC skin some degree of rotation and movement, accommodating shrinkage and moisture changes while providing minimum restraint in the plane of the panel (except for gravity load). Flex anchors are attached to the GRC skin, often embedded within GRC bonding pads integrated into the backing mix.
Gravity Anchors: These anchors are specifically positioned, usually along the bottom edge of the panels, to support the self-weight (gravity load) of the GRC skin, transferring it to the panel frame. Similar to flex anchors, they are attached to the skin and can be embedded in bonding pads.
Threaded Inserts: Inserts are threaded connecting or handling devices that are cast into the GRC units or the supporting structure during manufacture. They are used to facilitate the attachment of other components to the GRC panels. Inserts should be properly embedded, typically in built-up bosses or bonding pads for adequate anchorage. For effective load transfer and to prevent pull-out, attachments should bear directly onto the insert itself, rather than solely on the surface of the GRC.
Direct Fixings (Through Panel): Direct fixing methods can be used for attaching GRC elements or other components through the GRC panel. When fixing through the panel, it is recommended to use an oversize hole in the GRC combined with oversize washers or bearing plates on both sides to effectively spread the load and avoid damage or pull-through. Examples of fixings that can be used through the panel include nails, screws (into pilot holes, masonry plugs, or cast-in blocks), bolts, and blind fixings.
Mechanical Anchors for Veneers: For unbonded veneer facings like brick or natural stone attached to the GRC skin, mechanical anchors are used to provide the connection while accommodating the potential relative movement between the veneer and the GRC due to differential volume changes (thermal and moisture). These anchors, often made from stainless steel, are embedded in the GRC and designed to allow for this movement. A bond breaker is typically required between the veneer and the GRC backing when using these anchors
Figure A: TYPICAL arrangement of Stud Frame
NOTE: All gravity and flex anchors point towards the centre of the panel to allow free shrinkage
The flex anchors allow vertical and horizontal shrinkage of the cladding panel. Vertical shrinkage is accommodated by rotation of the flex anchors, by virtue of using a hand tight, lock nut arrangement, whilst horizontal shrinkage is allowed by the debonding sleeve placed on the flex anchor before bonding it to the face. The self-weight of the panel is supported solely on the fixed gravity anchors which are also sleeved to allow horizontal shrinkage of the GRC towards the centre of the panel. Gravity anchors behave as a strut and tie system to support the weight of the panel (Figure D).
Figure B: Stud Frame fabricated from rectangular hollow Sections
Figure C: Typical Flex Anchor indicating degrees of freedom
Figure D: Typical Gravity anchor acting as a strut and tie system