USE OF STAINLESS-STEEL FIXINGS IN STRUCTURES
Life-cycle costing is increasingly recognised as the true way to establish the cost of building components. Although stainless steel may be considered expensive, its maintenance-free status and integrity eliminate the need for remedial or refurbishment measures during the life of the structure.
In cavity-wall construction, stainless steel is now exclusively used to manufacture wall ties, restraint fixings, masonry support systems and windposts.
As the trend towards higher specification and longer life continues, stainless steel could also provide cost-effective long-term solutions in other architectural applications.
SIGNIFICANT CHANGES TO WALL-TIE REGULATIONS Wall ties are an essential element in the stability of masonry panels. The correct selection, spacing and installation of ties is essential to avoid damp penetration and the distortion, cracking or even collapse of brickwork.
Wall-tie selection depends on many factors, including type of brick/block to be tied, cavity width, type and height of building, location and design life. There are several documents which need to be consulted and some recent changes in regulation.
BS 5628: The Use of Masonry: Part 1: 1992 provides recommendations on length of tie, embedment, density and positioning. Wall ties conforming to BS 1243 (withdrawn) or DD140 will meet the requirements of this standard.
BS 1243: Metal Ties for Cavity Wall Construction: 1978 was withdrawn in January 2005. This document specified the shape and material specification of butterfly ties, double triangle ties and vertical twist ties (pictured above right).
Wall ties should now be specified to DD140-2.
DD140: Part 2: 1987 gives recommendations for the design of wall ties for use in masonry and timber construction in the UK. Ties are classified by type.
The relevant classification is determined by strength, function and use. Types 1 to 4 cover masonry-to-masonry ties, while types 5 and 6 cover masonry-to-timber construction. Unlike BS 1243, DD140-2 enables architects to select wall ties to suit the performance criteria of the intended application.
DD140-2 enables wall tie manufacturers to design cost-effective wall ties that offer superior performance.
BS EN 845-1: 2003 Specification for Ancillary Components for Masonry specifi es the requirements for wall ties used for interconnecting masonry and for connecting masonry to beams, columns or other parts of the building.
Materials, tolerances, tie types and the requirements for declared values are all covered in this standard.
Approved Document E: Resistance to the Passage of Sound was amended in 2003 to raise the performance requirements of walls for their resistance to the passage of sound. To meet the new requirements the dynamic stiffness of ties must be measured to test the product's ability to transmit airborne sounds. Document E classifies wall ties into two types: type A and type B.
Type A ties: For separating (party) walls and external walls.
Taking both cavity width and tie density into account, these ties must have a measured dynamic stiffness of less than 4.8MN/m 3.Type B ties: For external walls where a type A tie is unsuitable. Taking both cavity width and tie density into account, these ties must have a measured dynamic stiffness of less than 113MN/m 3.Robust Details Limited has approved the use of several standard construction details as a means of complying with Document E when building new houses and flats. Use of these details eliminates the need for pre-completion sound testing. Ties complying with type A requirements must be used with the standard details for masonry separating walls.
DENSITY AND POSITIONING OF TIES For walls in which both leaves are 90mm or thicker, ties should be used at not less than 2.5 per square metre (900mm horizontal x 450mm vertical centres). Ties should be evenly distributed over the wall area, except around openings, and should preferably be staggered.
In cases where insulation board is incorporated within the cavity and restrained by ties with insulation-retaining clips, it may be necessary to reduce the horizontal spacing of the ties to 600mm.
At vertical edges of an opening, unreturned or unbonded edges and vertical expansion joints, additional ties should be used at a rate of one per 300mm height, located not more than 225mm from the edge.
LENGTH OF TIE AND EMBEDMENT Wall ties should be of the correct length to ensure that they are properly embedded in the masonry.
The tie should have a minimum embedment of 50mm in each leaf, but also take site tolerances into account for both cavity width and centring of the tie. For this reason, tie lengths which achieve an embedment of between 62.5mm and 75mm are recommended.