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HeidelbergCement in the United Kingdom
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Structural use

Thermalite® blocks will provide an excellent structural solution for most

Structural design

Dwellings of up to 3 storeys

For residential buildings of up to 3 storeys, the Building Regulations, along with BS 8103: Part 2, provide guidance in the form of ‘simple rules’. This enables the designer to determine the required strength and wall thicknesses of masonry to be used without the need for structural calculations.

The dimensions of the building layout will influence the strength and thickness of the masonry. For residential buildings of up to 2 storeys a compressive strength of 2.9N/mm² will meet the criteria of the ‘simple rules’, and can be achieved using any of the Thermalite® block products.

For the upper two storeys of 3 storey residential buildings 2.9N/mm² will meet the criteria, whilst at ground floor level and below d.p.c. the ‘simple rules’ require a compressive strength of 7.3N/mm². This can be achieved by using Thermalite® Hi-Strength 7. As an alternative to following the ‘simple rules’ guidance, the required thickness and compressive strength of the masonry can be determined by calculation.
Other types of building
For residential buildings of over 3 storeys and commercial and industrial buildings, the structural design should be carried out in accordance with BS 5628: Part 1 by a suitably qualified engineer.

When carrying out the design in accordance with BS 5628: Part 1, the structural engineer will take into account general factors when calculating the load the  particular wall will safely support. In respect of the Thermalite® product range the following information will be required:

  • the compressive strength of the product
  • the partial safety factor for the material
Hi-Strength 10 and ‘Special Category’
All Thermalite® blocks are manufactured to Special Category of Manufacturing Control in accordance with BS EN 771-4 and BS 5628: Part 3. Therefore, γm = 3.1 can be used, if required in calculations.

This applies to the 8.7N/mm² Hi-Strength 10 material that can be shown by design calculation to provide an equivalent wall strength to 10.4N/mm² concrete blocks manufactured under the less onerous ‘normal category’ conditions.

For further advice on the structural design of Thermalite® masonry walls, contact Hanson’s Product Services department.

Disproportionate collapse
Approved Document A 2004 contains important revisions to the application of Requirement A3, Disproportionate Collapse, to bring all buildings under control. The guidance refers to 4 Building Classes that are tabulated and described therein. In Class 1, for houses not exceeding 4 storeys, agricultural buildings or buildings into which people rarely go (subject to conditions), provided that they are designed and constructed in accordance with the ‘simple rules’ or other Section 1 guidance, no additional measures are likely to be necessary.

Buildings in other classes will be subject to additional measures as described. A Technical Guidance Note has been produced by the NHBC in consultation with the DCLG to supplement the guidance in Section 5 of the Approved Document.

Lateral restraint
The Thermalite® block walls should be provided with adequate lateral restraint at floor and roof levels. Clauses 28.2 and Appendix C of BS 5628: Part 1 provide detailed guidance on methods of restraint.

Joist hangers
The use of joist hangers to support timber floor joists is becoming increasingly popular, particularly when the supporting wall is a party wall. It is important that the joist hangers used comply with BS EN 845-1 and are marked with the minimum compressive strength of the block with which they are to be used. The type and size of hanger should be selected to suit the span and the size of the joist to be supported.

It is important to install the joist hanger with the back plate tight against the surface of the supporting wall and in accordance with the manufacturer’s instructions. The masonry flanges and the joist hanger should be held in position by sufficient masonry above the joist hanger support and should be allowed to achieve adequate strength before any load is applied to the hanger.

Manufacture and testing
The European Standard BS EN 771-4 is the standard for the specificaton of aircrete blocks. It addresses the essential requirements of the construction products directive and contains guidance on CE marketing of products.

This is reflected in the Building Regulations, Approved Document A, ‘Structure’ (2004 and amendments), which contains tables of minimum compressive strengths of masonry units. A revised BS 8103 was published early in 2005, which reflects these circumstances.

The Thermalite® product range provides compressive strength that reflects both the regulatory requirements and the need of the designer to satisfy the structural requirements of a wide range of building types.

Modulus of elasticity
Typical values for the Modulus of Elasticity (E) of Thermalite® blocks are as follows:
  • Turbo: E = 1700 N/mm²
  • Shield: E = 2100 N/mm²
  • Floorblock: E = 2600 N/mm²
  • Party Wall: E = 2600 N/mm²
  • Hi-Strength 7: E = 2900 N/mm²
  • Hi-Strength 10: E = 3100 N/mm²