G.J. EDGELL British Ceramic Research Limited
The British Standard Code of Practice, BS5628:Part 1, allows a designer to enhance the stiffness of an inner leaf of a cavity wall loaded in compression to account for the increased resistance to lateral deflection supplied by the outer leaf. The majority of previous work on the compressive strength of cavity walls has concentrated on walls with vertical twist wall ties with only a few tests on cavity walls with butterfly wall ties. The tests with butterfly wall ties indicate that the unloaded leaf of the cavity wall has no effect on the stiffness of the wall. To extend the available data, a small test programme was carried out to investigate the compressive strength of cavity walls containing butterfly wire wall ties, and the results were compared with previous work. Initiatly five cavity walls each with a 50mm cavity were tested, the inner leaf being loaded axially. This indicated that the presence of the outer leaf gave no strength increase above that of a similar single leaf As a result further tests investigated whether the conclusion differed if the load on the inner leaf was eccentric, if the cavity were narrower and if high strength brickwork were used in the outer leaf. The overall conclusion is that an unloaded brickwork leaf connected by butterfly wire ties to a concrete blockwork leaf does not enhance its strength.