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Control of deflection in reinforced concrete A short paper ‘Span/depth ratios for concrete beams and slabs’, by Mr Alasdair Beal, published in The Structural Engineer for April 1983, dealt with the treatment of defection in CPs l10 and 114. Mr Francis Beale has written to us expressing great interest in the paper and suggesting that some modifications were required to Table 8. In his letter, which is quoted below, Mr Beale provided a revised table showing generally lower values for recommended span/depth ratios, together with some furthur comments: I have taken the opportunity to modify some of the figures to reflect the endspan or corner panel condition in all cases, so that a ready design tool is available; and in the case of flat slabs, I have assumed support by columns. Verulam
The design of slabs is dependent on an understanding of the manner in which internal forces and moments are mobilised. The paper examines linear slab behaviour in zones near free edges. It is shown that analysis based on ‘thick-plate’ theory provide predictions consistent with test data and 3-dimensional analysis, whereas analyses based on classical or ‘thinplate’ theory do not. Two cases are examined in which design against shear failure can be critical. These are a skew bridge slab and a flat building slab. In both cases, the internal actions are shown to be different to those commonly assumed. It is shown that finite element analysis, using the heterosis formulation for ‘thick-plate’ theory, requires a mesh considerably finer than that commonly used in edge zones for reliable results to be provided. R.J. Cope and P.V. Rao
I feel that the views and comments expressed by Mr McQuillan should not pass without comment. I would therefore like to make the following comments and pose certain queries that have not been covered adequately in the ‘Viewpoint’ article. Mr. K.R. Jackson