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Fixings For Stone Cladding |
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New Build |
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DESIGN OF FIXING SYSTEMS
The objective is to achieve safe and reliable fixing of the cladding and full consideration at an early stage of the design work is essential. Fixings must be capable of three-dimensional adjustment to accommodate construction tolerances. Fixings are subjected to two main forces.
1. Dead Loads (Permanent)
The actual weight of the supported stonework, which can be conservatively assessed using a density of 3 tonnes per cubic metre.In some cases, the stone may be offset from the centre of support and the dead load will then act eccentrically on the fixings.
2. Applied Loads (Transient)
Caused by wind pressure and suction, window cleaning equipment,thermal and structural movements etc. These loads will probably need to be assessed by a Structural Engineer. Fixings must be produced from stainless steel or non-ferrous metal, must be of sufficient size and strength to resist the relevant forces and must be readily available to meet the construction programme.
TYPE OF FIXINGS
The precise type of fixing will vary widely, depending upon the type
of stone, the size of the unit, the materiai of the main structure and the supplier, but four main groups can be recognised.
1. Loadbearing Fixings. These carry the self weight of the stonework and often take the form of corbel plates and angles or channels.
2. Restraint Fixings. These resist the applied loads and consist of wire ties dowels or cramps. Properly designed, they should be free from stress in the absence of any applied load.
3. Combined Fixings. It is possible to combine the loadbearing and restraint function in a single fixing, for example a face bolt or a bolted cramp and dowel.
4. Soffit Fixings. Soffit stones require particular attention to the size and type of fixings, which are always subjected to combined loading and must be capable of fine adjustment to produce a smooth soffit. A bolted hanqer plate is typical.
MOVEMENT OF BUILDING AND CLADDING
The stone cladding may be subject to thermal and moisture movement.The structure to which it is fixed can also move due to compression of columns, deflection of beams etc.
The design of the cladding system should take into account anticipated movement, incorporating compression and movement joints, filled with a flexible sealant, and tolerances.
1. Horizontal Compression Joints
These should normally occur at each floor level and be a minimum of 15mm wide. The cladding above this joint should be supported by load bearing fixings.
2. Vertical Movement Joints
These should be provided to accommodate the differential movement between structure and cladding. They should occur at about 6m centres and about 1.5 to 3m from corners. These should be not less than 10mm wide.
3. Other Joints
These should not exceed 13mm in width, and are normally filled
with mortar.
4. Structural Tolerance
Fixing schemes should be designed to accommodate the specified structural tolerance. Careful consideration at the design stage should be given to meet the contractor's tolerances. This can assist in faster and safer fixing.
It is important that the cavity is large enough to accommodate variations of the structure, together with, where relevant, the insulation, the fixings themselves and any services.
It is essential to consult a Stone Federation member at an early stage to advise on both natural materials and fixing methods.
For further guidance refer to BS 8298:1989 "British Standard Code Lining of Practice for Design and Installation of Natural Stone Cladding and Lining”. |
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