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Dr. P. A. Jackson (M) (Gifford & Partners)
I have long thought it odd that the conventional design methods for ground supported slabs (including concrete paving slabs) and for suspended slabs are so fundamentally different. The latter are designed on the assumption that they are cracked and that the tensile strength of concrete contributes little to their behaviour and nothing to their flexural strength; the former are designed on the assumption that they are not cracked and that they work by the tensile strength of concrete. This difference would be logical if the stresses in ground supported slabs were lower than in suspended slabs and this often, although not always, appears to be the case if one considers only load-induced flexural stresses. However, cracks can also be induced by stresses from other effects such as shrinkage and temperature and, because of differences in the restraint, these stresses are often higher in ground supported slabs.
A unique facility for structural and wind engineering research on steel portal framed building behaviour is described. The Silsoe Structures Building is an intensively instrumented full-scale building structure, the frames of which can be connected to their pad foundations by any one of three interchangeable bolted joint arrangements which are representative of those joints used in practice and also of the pinned condition normaily assumed in design.
Mr J. K. Botterill of Chessington has some queries about the lapping requirements for fabric in the concrete Codes, which he sees as giving quite inadequate guidance. Would any of our readers feel emboldened to offer their own opinions as to what should be done, and whether what Mr Botterill suggests can be justified?