The Design of Fixings for Natural Stone Cladding

From the early 1900’s few fixings were used for restraint as the natural stone ashlar was raised simultaneously and built into the backing brickwork. 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.

With the introduction of cavities in the 1950’s and 1960’s the ashlar courses became thinner, e.g. cladding, and relied entirely on fixings for support and restraint.

Restraint to the brickwork backing and concrete frame was achieved using brass or bronze cramps and ties. 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.

Stainless steel has been used in the construction industry throughout Europe and the UK for over 50 years and currently all fixings used with natural stone are manufactured from austenitic stainless steel. complying with the requirements of BS EN 10088 part 2 Sheet strip and plate and BS EN 10088 part 3 rod and bar.

These standards supersede BS 1449 part 2 and BS 970 part 1. 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.

Table 1

BSEN 10088

Formerly known as  Application  
1.4301 304S15
304S16
For general fixings.
1.4035 303S31 If significant machining is required (i.e. cast-in sockets and some bolts). It is not suitable for welding or working on site.
1.4307 304S11 For hot working or welding.
1.4401 316S31 For enhanced resistance to pitting corrosion (e.g. for use on coastal sites).
1.4404 316S11 For hot working or welding for use on coastal sites.
1.4435 316S16 For hot working or welding for use on coastal sites.
1.4541 321S31 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.

Neoprene may be used for restraint fixings whilst synthetic resin bonded fibre such as “Tufnol” or high density polyethylene (HDPE) should be used for fixings carrying permanent loads. Special epoxy paint treatments can be considered for isolation where shorter life requirements exist.

The design of the fixing system will be dependent on the cladding type (handset, rainscreen or stone on a metal frame) and the backing structure, taking due note of the requirements of the relevant British Standards.

THE DESIGN OF THE FIXING COMPONENTS.

When designing fixings, or fixing systems, the requirements of the relevant codes and standards should be followed.

Guidance on the selection and design of fixings for cladding is set out in the following publications:

  • BS 8298 2010 code of practice for design and installation of natural stone cladding and lining
  • BS 8298 Part 1 General
  • BS 8298 Part 2 Traditional handset external cladding
  • BS 8298 Part 3 Stone-faced pre-cast concrete cladding systems
  • BS 8298 Part 4 Rainscreen and stone on metal frame cladding systems
  • BS EN 1469 Natural stone Products- Slabs for cladding – Requirements
  • BS 8200: 1985 code of practice for Design of non-loadbearing external vertical enclosures
  • BS 6180: 1995 code of practice for barriers in and about buildings
  • BS 6399: part 1:1996 code of practice for dead and imposed loads
  • BS 5080 Tests of structural fixings in concrete
  • Design of stainless steel fixings and ancillary components – The Steel Construction Institute
  • Section property and member capacity tables for cold-formed stainless steel
  • Performance and testing of fixings for thin stone cladding – Centre for window and cladding technology
  • Guidance notes published by the Construction Fixings Association (CFA)
    • Anchor installation
    • Anchor selection
    • Introduction to bonded anchors
    • Fixings to brickwork and blockwork
  • Guidance on the design, selection and installation of exterior dimension stone anchors and anchoring systems is given in the American code ASTM C 1242
  • Cladding Fixings – good practice guide CIRIA


Designing a fixing scheme
The fixing scheme is not normally considered at an early enough stage in the design of the natural stone cladding.The design of a competent fixing scheme is a highly specialised subject and should only be undertaken by those who have a full knowledge, not only of the performance of the stone and backing structure, but of the properties of stainless steel and the anchors being used.

All stone used for cladding must comply with the requirements of BS EN 1469, which forms the basis of CE marking. All stone placed on the market must be CE marked.

The testing set out in these standards includes flexural strength (BS EN 13161) and breakout load at dowel hole (BS EN 13364) which forms the basis of fixing design.

testing stone flexural strength
Flexural strength testing to BS EN 13161

The quality and performance of the backing structure and interfaces with dpc’s, fire stops, insulation and other elements, including widows, must also be considered.
Fixings are subjected to two principal forces:
Dead load – the self-weight of the stone cladding and any live service loads applied during maintenance.
Applied loads – wind loads (positive and negative), any live service loads applied during maintenance and impact loads.

Loadbearing fixings
With hand set cladding a series of short-formed loadbearing supports will normally be located at each floor level. 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.

The effective length of the mortice cut into the back face of the stone should not exceed 1/6th of the width of the panel and the support angle should be designed to comply with this requirement.

Where the supports are positioned at the joints, and therefore carrying two stones, the width of the support, unless proven otherwise, should not be less than 75 mm wide.   At vertical movement joints, or where individual supports are used, they should generally not be less than 50 mm wide.

Combined loadbearing and restraint fixing by kind permission of ASC Stainless Steel Fixings Ltd
Combined loadbearing and restraint fixing
By kind permission of ACS Stainless Steel Fixings Ltd

Restraint fixings

Restraint fixings will be designed to safely sustain the imposed loads, which are mainly due to the wind. The design of the fixings may also need to take account of impact.
As noted in BS 8928-1:2010 cladding may be liable to impact whether accidental or deliberate and impact risks are set out in Table 10. 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.

BS 8298-2:2010 clause 5.5 notes that unless proven by structural calculation or performance testing the peripheral distance between fixings should not exceed 1200mm and that fixings should be located at ¼ or 1/5 points depending on the bond pattern of the cladding. This requirement restricts the maximum size of the panel to approximately 0.8m2.  Larger panels can be used providing that structural calculations and performance testing as defined in BS EN 1469 is used to justify these.

BS 8298-2 clause 5.6 states there should be a maximum of four restraint fixings per stone. 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. This requirement restricts the maximum size of the panel to approximately 0.8m2.

In the absence of any specific testing or calculations the minimum thickness of stone in front or behind the cramp mortice should be 12mm for stone with a mean flexural strength of 15 MPa or greater and 25mm for stone below 15MPa, all as set out in BS 8298-2:2010 clause 4.5.
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. The design should take into account the performance of the stone as noted above. 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. All holes, mortices and sinkings must be formed within ± 2 mm of the specified position (BS EN 1469).

Typical loadbearing and restraint fixings (handset cladding) Fixing details by kind permission of ASC Stainless Steel Fixings Ltd
Typical loadbearing and restraint fixings (handset cladding)
Fixing details by kind permission of ACS Stainless Steel Fixings Ltd

Soffit fixings

The fixing of stones located on soffits or sloping surfaces, and where permanent loads are imposed, presents the designer with a need to make special considerations.

Where soffit stones are larger than 600mm x 900mm (the maximum size specified in BS 8298) performance calculations and structural testing of the stone will be required.

Face fixings
Face fixings can play a special role in the fixing scheme design, particularly to ensure that the last fixing can be installed.

The design of the fixing requires special attention to ensure that when tightened the expansion bolt does not impose any stress into the stone. 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.

Attachment to the structure
The fixings will be attached to the structure using:
expansion anchors
resin bonded anchors (pumped resin or encapsulated system)
cast-in channels
grout-in ties

When choosing anchors careful comparison of their performance figures and the type of background structure is critical.

Many stone masonry designers and fixer masons will favour the use of post-drilled expansion anchors. 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. Only anchors supported with European Technical Assessment (ETA) certificate or CE marking should be used.

All manufacturers’ literature states the safe or maximum working loads based on specific data such as concrete strength, minimum edge distance, bolt spacing, drill hole diameter and depth, embedment depth and tightening torque.

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. However, complex fixings designed to cope with these tolerances are available. 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.

Back Ventilated rainscreen cladding
Rainscreen cladding comprises natural stone panels with 10mm wide open joints. Each panel individually fixed to the structure. Guidance on the design is set out in BS 8298 Part 4. 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.

Curtain wall Cladding
Stone panels are used as part of a curtain wall system and will generally be post-fixed at works. The design of the fixings will be part of the frame design. Fixings will be pre-set into the stone or the stone produced with continuous kerfs. Again guidance is set out in BS 8298-4. In all cases justification by testing will be required.

Designing for construction tolerances
The tolerances to which the structure is built can vary from those specified and can have an adverse effect on the installation of the fixings and the cladding. It is essential that the cladding and fixing designers take care in making the design capable of accepting the stated tolerances. Failure to address this problem at the initial design stage can have an adverse effect on the programme and costs.

When tolerances encountered on site exceed those shown in the design, the mason fixer must be aware of the limitations of the fixings and where necessary use replacement fixings designed to cope. 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. Some fixings manufacturers now offer composite shims that are designed to deal with thermal bridging.

Under no circumstances should “plastic” shims be used. When the fixings are secured to a structural background with a bolted connection (Expansion anchor, resin anchor or cast-in-channel) the drawings and the method statement should clearly specify the correct tightening torque. The drawings should also clearly show the diameter and depth of any holes that are to be drilled together with the preparation that may be required to accept the post fixed anchor, i.e. clean the drilled hole properly by blowing and brushing.

Details and designs
All fixings, whether restraint or loadbearing, should be justified by calculations. 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.

Conclusion

Time spent on the fixing design, careful detailing and setting out, taking into account the anticipated structural deviations, sequence of erection and any site imposed restrictions, will result in a secure system and reduce time lost in construction.

 

Author:  Peter Harrison