During the 1950’s and early 1960’s the use of stone bonder courses, seated on the concrete slab or edge beam and built into the brickwork, was favoured as a means of transferring the load of the stone back to the structure. Restraint to the brickwork backing and concrete frame was achieved using brass or bronze cramps and ties.

design fixings natural stone

The load was transferred back to the structure with corbels, often made from sand-cast brass and manganese bronze. It was soon discovered that some of these brass alloys suffered from stress corrosion, which resulted in cracking and some failures.

A change to copper, phosphor bronze and aluminium bronze solved the problem of stress corrosion. In the early 1970’s, and due to economic pressures, stainless steel replaced the copper alloys used to fabricate all types of fixings.This created new challenges as the fixings could not easily be adapted on site. A comparison with the former references is shown in table 1. The exact grade used will be selected to meet both the stresses required and level of exposure. It is not suitable for welding or working on site. For hot working or welding for use on coastal sites.

For stabilised welding if subsequent heat treatment is not required.

The austenitic group of stainless steels has superior corrosion resistance and can be readily formed and welded. Austenitic stainless steel also offers high ductility and strength, it is substantially non-magnetic, and is resistant to unsightly staining and has life cycle costing benefits.

Where the stainless steel fixings are attached directly to a mild steel frame, isolators must be used to ensure that corrosion of the base or less noble metal is avoided. The use of isolators ensures that galvanic corrosion between electrochemically dissimilar metals does not occur where moisture is present. The isolation materials selected will depend on the type of fixing being used, the load applied and the required life expectancy of the fixing. Special epoxy paint treatments can be considered for isolation where shorter life requirements exist. Applied loads – wind loads (positive and negative), any live service loads applied during maintenance and impact loads.

They may have a dual support and restraint capability. They can be secured in place with either mechanical expansion anchors resin bonded anchors or bolted to cast-in-channels.

There may also be corbel plates grouted into pockets in the structure. The support may be a simple 90° formed angle or angles with the projecting leg cranked at 15°, a flat corbel plate or complex multi-adjustable component. Where the stones are exposed on the lower edge, or because of the location of the structure, the supports may be set up from the bottom of the panel. In this case the distance up to the corbel slot or pocket should not exceed 150 mm. This is particularly relevant to open jointed back ventilated rainscreen cladding where stone thinner than that used for traditional handset cladding tends to be used. Generally these fixings will be located in the top and bottom joints and, unless proved by structural calculation or performance testing, the peripheral distance between fixings should not exceed 1200 mm. Restraint fixings can be secured in place with mechanical expansion or resin anchors, grouted into pockets in the structure or into cast-in-channels.

The use of resin anchors or grout-in fixings may be the only option in a number of situations.

Most restraint systems incorporate dowels set into the edge of the stone. Except with sedimentary stone 75mm thick and above, all holes, mortices and sinkings should be formed prior to delivery. If the holes, mortices and sinkings are to be formed on site the work should be carried out by a qualified mason using the appropriate equipment. Each stone should be attached to the structure using four bolts located at a distance from any edge equal to three times the minimum thickness of the stone behind the bolt fixing, but in no case less than 75mm. The diameter of the recessed washer should be at least twice the diameter of the hole through the stone. The choice of anchor will require careful selection based on both the size and type of structure and imposed loads.

There is a large range of expansion anchors and resin bonded anchors available, each having its own unique features.

design fixings natural stone 1

Expansion anchors and resin bonded anchors should always be installed in accordance with manufacturers’ instructions and tightened using calibrated torque spanners.

To cope with structural tolerances it is normal practice to place packing shims between the fixing and the structure. The packing shims should provide a sufficient bearing area. The maximum packing thickness should be carefully considered. For loadbearing situations this should not exceed the outside diameter of the anchor unless proven by shim pack analysis or testing. Each panel individually fixed to the structure. With rainscreen cladding it is usual to have a continuous insitu or precast concrete backing structure or metal sub-frame. In this situation the individual combined support and restraint fixings can be secured with any of the various options described above. When using the stone as rainscreen cladding it may be back fixed using stress free undercut anchors.

The fixing system normally comprises a stainless steel or aluminium frame. Fixings will be pre-set into the stone or the stone produced with continuous kerfs.

In all cases justification by testing will be required. All fixing installation details should specify the maximum acceptable tolerances, how this is achieved using shims, and the maximum thickness which must not be exceeded. The shims used should be stainless steel or a high density polyethylene and provide a full bearing for the back face of the angle or cramp. Under no circumstances should “plastic” shims be used. The drawings should show all the relevant manufacturing and installation details.

The location of the fixings should be shown on the stonework details in order to enable the mason to pre-form all holes, slots and mortices if required.

Stoneclip Installation

Installing stone panels using the “StoneClip” mechanical fixing system.

design fixings natural stone 2

During the 1950’s and early 1960’s the use of stone bonder courses, seated on the concrete slab or edge beam and built into the brickwork, was favoured as a means of transferring the load of the stone back to the structure. Restraint to the brickwork backing and concrete frame...