Deflections in Concrete ETM

Published: 20/12/2012

On the evening of 29 November 2012, one hundred engineers gathered at the Institution headquarters in London to hear about “Deflections in Concrete”.

This inaugural event was a joint meeting between fibUK and the Institution, and was held to recognise the contributions of Professor Andrew Beeby to both organisations and to concrete engineering in general.

Professor Steve Denton, Chairman of fibUK, addressed the audience, alluding to Professor Beeby’s mythical status to concrete designers. The subsequent three speakers, Robin Whittle, Dr Richard Scott and Dr Robert Vollum, enthusiastically explained some of Professor Beeby’s contributions in the field of deflection. The event was recorded; if you would like to view the webinar, please click here.

Robin Whittle spoke about tension stiffening and the problems of determining deflections in the reality between uncracked and fully cracked behaviour. Professor Beeby’s early tension stiffening model had been incorporated into BS8110 Structural use of concrete and Robin had used this to predict deflections on a test slab, predictions that have subsequently been proven wrong. It was only through a suggestion by Professor Beeby some years later that research was undertaken on the rate of decay of tension stiffening. This research showed that tension stiffening dropped 50% in 19 days – just as Professor Beeby had predicted.

Dr Richard Scott discussed bumping into his friend Professor Beeby on a train who resultantly implored him to do some work on service loads at “Cardington”. Cardington was the full scale in-situ test building being constructed inside an airship hanger. Dr Scott explained how funding was quickly arranged to enable measurement of in-situ strains in the sixth of seven flat slabs. With the help of Arup, different tension stiffening models were investigated and found to give very different answers. With the caveats that only transient loads and one highly reinforced slab (1.7%) were investigated, Dr Scott concluded that ICE Technical Note 372 appeared most appropriate - at least for slabs on the cusp of being cracked.

Dr Robert Vollum talked about span: effective depth (L/d) ratios, where Professor Beeby was a pioneer, developing rules for BS8110. Over time these rules proved to work very well. It became increasingly clear that tension stiffening has most influence on uncracked sections. Therefore load history and cracking become very important in determining deflection. This was recognised in the background to the Eurocode 2 L/d rules. Unfortunately some of the assumptions made have now proved to be unconservative for lightly loaded slabs in multi-storey construction. Dr Vollum explained how improvements could be made.

Questions and discussion followed the talks with the Chairman, Professor Denton, closing proceedings by leading a warm round of applause.

The full report of this event is available as a downloadable PDF here.

Report-Deflections-in-Concrete-ETM-(1).pdf Report-Deflections-in-Concrete-ETM.pdf

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