BS 8298-2:2010 pdf download – Code of practice for the design and installation of natural stone cladding and lining – Part 2: Traditional handset external cladding
4.4 Impact resistance
The risk of impact on the surface of the cladding system should be considered at the design stage and the recommendations of BS 8298-1:2010, 6.14 should be followed.
4.5 Cramp mortices and sunken dressed margins
The minimum thickness of stone in front or behind a slot or mortice should be suitable for its intended application. In the absence of any specifc testing or calculation, the minimum thickness should be 12 mm for stones of 15 MPa mean fexural strength or greater, and 25 mm for all stones below 15 MPa.
The location and type of different fxings should also be assessed using performance testing and/or calculation. Where stone might be vulnerable to impact damage, such as at plinth (ground) level, additional measures, e.g. using thicker stones in the affected area, or sealing the backs of the stones before fxing and then flling the cavity with a lightweight fne concrete mix after fxing, should be used. Rebates can extend the full length of the stone (particularly where the stone is seated on a continuous angle). The quantity of stone left in front of the rebate is dictated by the type of stone: the performance under lateral load should then be determined by testing.
In the absence of any specifc testing or calculation, the minimum thickness should be 12 mm for stones of 15 MPa mean fexural strength or greater, and 25 mm for all stones below 15 MPa. Where the design of the cladding incorporates a sunken dressed margin to provide a recessed joint (rustications) where it coincides with any fxings, its size should not compromise the performance of the stone. Where fxings are located in the same joint as the rustication, the overall thickness of the stone should be increased or specifcally tested in accordance with BS EN 13364.
4.6 Sofft stones Sofft stones should not exceed 900 mm × 600 mm. If stones of a greater size are required, they should be justifed by calculation. In the absence of any specifc calculation, the minimum thickness should be 40 mm for stones of 15 MPa mean fexural strength or greater, and 75 mm for all stones below 15 MPa. In all cases, the thickness of stone behind the fxing should be 50% of the total thickness of the stone.
5 Methods of attachment and support
NOTE 1 Typical loadbearing, restraint, face and sofft fxings are shown in Figure 1, Figure 2 and Figure 3 respectively. Allowance should be made for articulation of cladding components, to take account of differential thermal movements. NOTE 2 The methods of attachment of units to the structure vary widely, depending upon the type of stone, the size of the units and the structure of the building. Fixing devices fall into fve groups:
a) loadbearing fxings (see 5.5);
b) restraint fxings (see 5.6);
c) combined loadbearing and restraint fxings (see 5.7);
d) face fxings (see 5.8);
e) sofft fxings (see 5.9).
It is not normally possible to ascertain the conditions of fxings or to undertake maintenance once the cladding has been completed; therefore, the metals described in BS 8298-1:2010, 5.2, should be used, as they offer high resistance to corrosion and do not require maintenance.
5.2 Design Fixings should be able to withstand imposed loads; these could be dead load from the cladding together with lateral loads from wind pressure and suction, which are equal to the basic wind pressure appropriate to the degree of exposure and the height above ground level of individual units.
NOTE Guidance on determining wind loads can be found in Table 1 and BS EN 1991. The design of the fxings should:
a) allow for three-way adjustment to enable proper ft within holes or mortices in the stone and to the substrate;
b) take into account the difference between the specifed tolerances for the fnished cladding and the likely structural tolerances as set out in BS 5606.