11.06 Fixings

Categories: Stone Cladding

Introduction
This section describes the different forms of fixings used for the attachment of natural stone cladding panels.  It describes the mechanical performance of the fixings but does not discuss the different materials that may be used to manufacture the fixings.  Guidance on the selection of materials for fixings is available in BS 8298 and Wilson and Harrison 1993.  Materials are primarily selected to resist corrosion and materials such as stainless steel are often specified.  However, material selection can affect structural performance.  Most fixings have to have a degree of flexibility if they are to accommodate movement.  Many fixings will displace under load and the deformed shape may alter the load path and load transfer into the stone.  Substitution of one metal for another may lead to a more flexible fixing that will deflect too far or a more rigid fixing that will not accommodate movement.

Fixings for stone cladding fall into two broad categories.  Those that carry in plane loads and provide some restraint to in-plane movement and those that are designed to carry only forces normal to the plane of the cladding.  These offer restraint to movement normal to the cladding panel but allow in-plane movement.  This latter category of fixings has traditionally been called restraint fixings whilst fixings that carry in-plane loads have been termed support fixings. This report continues to use the term support fixing.  However, the term non-support fixing is used to describe those fixings that cannot carry in plane load.

For panels mounted in the vertical plane this is a convenient distinction as support fixings carry gravity loads and possibly wind loading whilst non-support fixings carry only wind load.  However for soffit panels and inclined panels all fixings will carry both gravity loads and wind loads.  A full structural consideration of the panel, fixing system and degrees of restraint is necessary for any panels that are not in the vertical or horizontal plane.  Guidance on structural calculation is given in Stein, 1993.
 

Support fixings
These fixings provide restraint and carry loads in the plane of the panel.  Some, but not all, also provide restraint and carry loads normal to the wall. The basic forms of fixing are described but each can be detailed in many different ways.
 

Angles and shelves
These consist of hot rolled or cold formed angles that are fixed to the floor edge or a beam of the primary structural frame.  These fixings are used to support large areas of cladding and are necessarily large to carry the self weight of a whole storey height of individual stone panels stacked one above the other.

These supports contact cladding panels along the whole of their lower edge and have to be detailed to allow dishing of the panels.  It follows that lateral restraint cannot be provided along the whole lower edge of the panel and the lateral restraint is usually provided by separate non-support fixings, image.
 

Edge of panel support brackets
Discrete brackets may be used in place of a continuous angle.  These provide support at discrete points and may be used to provide restraint normal to the plane at the same points without preventing the panels from dishing, image.  To provide restraint the brackets have to be engaged in the edge of the panel by means of dowels, kerfs or mortices.  Support brackets may permit in plane movement if they are able to flex in that direction or sliding may occur between the bracket and the panel.  Brackets of this form may be used at each floor level to support the bottom course of a large area of stone cladding, on which the upper courses are stacked.  However support brackets can be provided for each panel in every course of panels.  They are then attached either to a structural back wall or a metal carrier frame.
 

Corbel plates
These fixings comprise angled metal plates that locate into inclined slotted holes cut in the inner face of the cladding panels, image.  They provide support and restraint in the same way as support brackets but have the advantage that the point of support is remote from the edge of the panel hence the bending stresses in the stone panel are reduced.  Corbel plates provide support and restraint at discrete points and do not prevent dishing of the panels.  Corbel fixings may allow in-plane movement by flexing or by sliding of the plate in the slot.
 

Back face support brackets
Corbel plates have traditionally been used for fixing to the back face of stone panels.  Other methods of fastening brackets and plates to the back face of panels are now in use, image.  These include the use of undercut anchors, expansion bolts, resin anchors and through bolting.
 

Channels
There have been examples of stone panels fixed into frame rebates in the same way that glass is installed into a glazing frame.  This provides a continuous support to the edges of the panel, which will restrict dishing of the panel.  These systems of support are complex in their behaviour and invariably induce stresses in the stone panels.  They should only be used after full appraisal of movement, restraint and induced stresses in the panels.  This will involve testing.
 

Non-support fixings
Fixings provided to carry only the out-of-plane loading will generally be of lighter construction than support fixings.  They are articulated or flexible so that they allow in-plane movement to occur and do not attract in-plane loads.

The following forms of non-support fixing are used.
 

Ties or cramps
Ties or cramps fabricated from metal sheet are frequently used to provide restraint normal to the plane of the cladding and transfer wind loading.  They attach to the edge of the panel by means of dowels, kerfs or mortices.  They may be deformed, as shown in this image, to allow complete freedom of movement in the plane of the cladding or they may be as shown in this image in order to resist movement and carry load in one of the principal in-plane directions.
 

Wire ties
Wire ties may be used to provide lateral restraint.  They locate into the edge of the stone panels in the same way as dowels and are able to accommodate movement in the plane of the cladding because of their inherent flexibility in this direction.  The load carrying capacity of these fixings is limited and they are not normally used to fix large and heavy stone panels.

Wire ties may also be used as a connection between adjacent panels and provide restraint at corners and returns in the wall, image.
 

Brackets
Corner brackets may be used to connect adjacent panels and provide out-of-plane restraint at corners, image.  They are attached to the inner face of the stone panels using either expansion, undercut or resin anchors.
 

Connection to stone panels
Kerfs
Kerfs and mortices cut into the edge of stone panels are used to locate kerf plates or cramps that carry normal to plane loads and may also carry in plane loads, image.  Kerfs are slots extending the full length of the edge of the stone.  Mortices are similar in performance but are cut over a limited edge distance.  Kerf or mortice fixings may be used on any edge of the stone but should only be used to provide fixing at discrete points.  Continuous fixing or fixing at more than two points along one edge will restrain dishing of the panel.  In plane movement of the panel may be accommodated by slippage of the kerf plate or cramp in the kerf or flexure of the tie or cramp.

The strength of a kerf or mortice fixing will vary with the dimensions of the kerf slot or mortice and its distance from the face of the panel.  Allowance for deviations must be made when specifying kerf dimensions.  The failure mode and strength of a kerf fixing is highly dependent on the geometry of the kerf plate or cramp.  The use of kerfs or mortices with thin stone panels may create insurmountable difficulties of tolerance, machining and achievable strength.
 

Dowels
Dowels are used to locate ties and brackets on the edge of stone panels.  The dowels provide a positive location for the tie or bracket and transfer normal to plane and in-plane loads, image.  Dowels are inserted into pre drilled holes or, for softer stones, site drilled holes.  The strength of the connection depends on the distance from the dowel hole to the face of the stone panel and, as with kerfs and mortices, some difficulties may be experienced with thin stone panels.
 

Expansion bolts
Brackets and other fixings may be attached to the stone panels by expansion bolts placed in the back face of the panels.  Expansion anchors expand in a pre-drilled hole with parallel bore.  They can only resist pull out by developing friction or mechanical interlock as a result of radial forces developed at the interface.  This leads to a complex stress field in the stone and tensile zones that may lead to fracture.  Proper consideration has to be given to the possibility of edge spalling and complex failure patterns in the stone panels.
 

Undercut anchors
Undercut anchors may be used instead of expansion bolts.  They locate in under reamed holes, hence their name.  Holes are drilled with parallel bore and then under reamed using a proprietary drilling system.  The undercut anchor is then expanded into the undercut chamber.  The anchor is expanded only until it contacts the undercut conical face and does not at this stage induce stresses in the stone panel.  The undercut anchor resists pull out by the obvious mechanical interlock and fails by pulling a symmetrical cone from the stone.

Undercut anchors in diamond drilled holes are available for stone panel thicknesses down to 20mm. They may also be used with ceramic and glass panels down to 10mm thick.
 

Resin anchors
Resin anchors are used in a similar way to expansion and undercut anchors to secure brackets and other fixings to the inner face of the stone panels.  Resin anchors consist of metal pins secured into pre-drilled holes with a resin adhesive.  Concerns have been raised about the long term strength of resin anchors and their performance in the case of fire.  However cranked pins may be used so that there is always a residual mechanical interlock, image.
 

Through bolts
Stone panels may be connected by bolting through the stone from the outer face.  The bolt head is subsequently concealed by a plug of stone fitted flush in the outer face of the panel.  These fixings may be used to fix brackets onto the inner face of the stone panel.  Traditionally they have been used for fixing soffit units and making connections at corners and returns.