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The President: This must have been a most exciting job with many problems to overcome. It was a congested site, with restrictions of allowable pressures on the ground. The factors which influence the engineering design are quite amazing; I think you said that it was imperative to obtain a uniform pressure on the ground and that that particular restriction to some extent led you to adopt the curve columns.
Mr. J. H. H. Williams(F) : Mr. Thomas has mentioned the importance of wall ties in cavity construction, but he did not mention the question of durability. It is my concern that we should not build into structures members which may become dangerous in sixty or even one hundred years' time because of the failure of some small but vital part which cannot easily be seen by future surveyors. Wall ties in cavity walls fall into the category of a humble but essential part contributing to the stability of large
panels of masonry and as they cannot easily be inspected I believe that they should be designed for a period well in excess of the anticipated life of the building. Some engineers and authorities insist that non-ferrous or stainless steel ties are used in cavity walls in high rise structures, while others, less informed, may be misled by the compliance of galvanized wire ties with the British Standard.
In his paper, the author has modelled the behaviour of compression members by assuming that they will have a buckling load given by Rankine's formula, and that they can undergo unlimited contraction at this load. It is well known, however, that compression members generally show some drop of load-carrying capacity after the critical load has been reached. On the other hand, it is quite reasonable to assume that tension members will elongate plastically under a constant yielding load (ignoring strain-hardening effects), provided that premature joint failure is prevented. Results of experimental work on compression members with varying degrees of end restraint have been plotted in Fig Cl. These values were mostly taken from a paper published by Francis and Stevens1 describing work carried out at the University of Melbourne, but also include results from Paris and van den Broek. It can be seen from Fig C1 that the critical loads of compression members are more closely related to the squash and Euler loads than to the Rankine load.