BS 8103:Part 2:1996 and Approved Document A of the Building Regulations for England and Wales gives guidance on the strengths and wall thicknesses for residential buildings up to three storeys, without there being any need for structural calculations.
In general, for residential buildings of up to two storeys, a compressive strength of 2.8 N/mm² for the inner leaf is all that is required and can be met by the use of any Celcon block. In three or four storey buildings, a higher compressive strength may be required, particularly in the lower storeys, and in areas of concentrated loading such as piers. Here, Celcon Standard or Hi-Strength blocks would be more appropriate. For the upper two storeys, the designer can usually revert to blocks with a strength of 2.8 N/mm².
In cases where a structural design is required it should be carried out by a suitably qualified person and in accordance with BS 5628-1:1992 or ENV1996.1.1.
Restraint and Connections
Walls should be adequately restrained at the level of intermediate floors, and at the roof. Detailed guidance is contained in BS 5628:Part 1:1992, Clause 18.104.22.168 and 2, and in Appendix C.
Where floor joists run at right angles to the supporting walls, and there is no requirement for joist hangers to be used, Celcon blocks should be used to infill between the joists. The blocks can easily be cut to size on site.
Where joists run parallel to the wall, the three joists nearest the wall should be connected to it by metal restraining straps, 30 x 5mm in cross section, and located at maximum 2m centres. These straps should have an anchorage turn-down at the wall face of at least 100mm, and there should be diagonal strutting between the three joists. In order that the restraining straps are able to act in compression, as well as in tension, there should also be adequate packing fixed between the wall and the first joist.
Where areas of high stress or concentrated imposed loads occur in blockwork, it is good practice to consider the use of reinforcement, laid in the horizontal bed joints. The reinforcement should be of sufficient length to distribute the stresses to nearby movement joints, or into adjacent areas of blockwork. See Movement joints.
Bed-joint reinforcement should be positioned in the joint so that it has a minimum mortar cover of 25mm on external faces, and 13mm on internal faces.
Joist hangers should comply with BS 6178-1:1990, and should be clearly marked on the underside of the joist flange with the minimum crushing strength of the block for which they are intended to be used. The type and size of hanger must be selected to suit the span and dimensions of the joist to be supported.
When hangers are being built in, care should be taken to ensure that their back plates are tight up against the surface of the wall and, in accordance with the hanger manufacturer's recommendations, that the masonry flanges are securely held in position by a sufficient height of masonry above. Restraint type hangers should be provided every 2m OR the joists strapped as in Fig 9, BS 8103: Part 1 at 2m centres max. The above-flange masonry should be allowed to mature adequately before the hangers are loaded.
For roof/wall connections at gables and separating walls, restraining straps of the same specification should be used, and installed in the same manner as described previously. These straps should be positioned at maximum 2m centres at rafter (gable verge line) level, and at truss tie (ceiling) level.
Detailed guidance is contained in BS 5628-3:2001, Clause 5.3.3, in Section 1c of Approved Document A to the Building Regulations (England & Wales), and in BS 8103:Part 2:1996.
Walls should normally be bonded at returns and junctions unless a movement joint is required.
Where a section of wall has to be constructed after other work has been completed, the bonding can be replaced by a straight joint provided ties are built-in across the joint. This method of jointing is preferable to bonding in situations where differential movement is likely to occur, (for example where sections of wall are constructed on different foundations).
Designing with Hi-Tene Blockwork
The highest strength required for blocks in BS 8103 Part 2 and in Approved Document A (structural guidance) of the Building regulations is 7.0 N/mm² for three storey dwellings. Where the layout of the dwelling is outside this guidance or it is greater than three storeys a structural design calculation is necessary.
When designing in accordance with BS 5628-1:1992, the structural engineer will take into account several factors. To calculate how much load any given wall can take the following is used:
1. The basic strength of the masonry (fk), which depends upon the block strength.
2. The safety factor to apply (γm) which depends upon the workmanship and quality control of blocks used.
3. The wall configuration (which is independent of the block type).
The safety factor can vary according to the level of control and testing in manufacturers’ factories. For higher strength units, Celcon’s Quality Assurance System means we test at more frequent intervals and qualify for ‘Special Category of Manufacturing Control’ classification. Consequently, the designer can have greater confidence in achieving the desired wall strength with Celcon and can reduce the level of safety for design in accordance with the BS. In practice, the material safety factor reduces from 3.5 to 3.1 where normal site workmanship exists.
These safety factors are used to establish the effective strength of a blockwork wall. Normally the designer will use British Standard BS 5628: Part 1 to obtain the basic wall strength. This is then divided by the 3.5 safety factor. However with Celcon, the wall strength safety factor can be reduced to 3.1. Consequently, a block strength of only 8.4 N/mm² is required to achieve the same loadbearing capacity as a 10 N/mm² block from manufacturers who operate less onerous levels of testing.
Wall strength is related to the ratio:
Wall characteristic compressive strength (fk)
Partial Safety Factor for Material Strength (γm)