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Mr. D. Shackley: ' I hope that, as a visitor to your proceedings and representing British Gypsum Ltd., I may be permitted to contribute to the discussion on Mr. Cracknell's paper. It is not generally appreciated that there is indeed a very large industrial complex located in the woods to the west of the London to Hastings road
just north of Battle. Gypsum has, in fact, been mined from the Purbeck beds in this locality for almost 100 years. The growth of this local industry has closely followed
the general growth in building materials based on gypsum plaster. It is quite significant that the site of the development about which you have just heard is designated on the Ordnance Survey Map as Lime Kiln Wood. The whole area of this wood is dotted with shallow bellpits from which limestone was extracted in past years and
no doubt there was a thriving lime-burning operation associated with them. The steady growth of gypsum plasters which are now largely replacing lime-based plasters is one of the many factors that stimulated the development that has been the subject of this paper. '
The work described in this paper provides a rational basis for the determination of effective widths taking the parameters relevant to the geometry and materials properties in the section into account, assuming the absence of slip between the steel joist and the concrete slab. Harmonic series solutions are employed in the determination of effective widths for various side ratios of the slab and various ratios of the slab thickness to the depth of the steel beam. These solutions incorporate the variation of the neutral axis depth across the width of the wide concrete flange due to shear lag effect. Three standard steel sections were used as the basis of the computations. A general computer program that requires the dimensions of the steel joist and the slab has been developed. The case considered is that of a simply supported symmetrically loaded system of a slab spanning several equally spaced identical steel ribs. The loading consists of central point loads on all ribs, the dead load of the composite section being taken into account in the computations.
The collapse mechanisms and the ultimate load equations to be considered in the limit design of uniformly loaded continuous slab and beam floors are examined. It is shown that the beams may be designed on the basis of the distribution of the loading on the adjacent segments of the yield-line pattern for collapse of the panels alone, rather than by the consideration of composite collapse mechanisms. The use of approximate loading distributions is shown to lead to unsafe design of the beams.