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Deflections in slabs and beams

Deflections must always be considered in the design of slabs and beams. Both Eurocode 2 and BS 8110 include deemed-to-satisfy span-to-depth ratio methods for ensuring compliance with acceptance criteria. These rules will be perfectly adequate and provide economic solutions for the vast majority of designs. However, such methods are semi-empirical and, while rough deflection estimates may be made, they are not intended to predict how much a member will actually deflect.

There can be circumstances where the calculation of deflections is desirable, such as where specified deflections are more onerous that the code recommendations, if long-term estimates of deflection are required, where the amount of movement can have a significant effect on the economics (e.g. the cost of allowing for movements in cladding) or where early-age loading can have a significant effect.

Concrete Society Technical Report 58, Deflections in concrete slabs and beams, reviews and compares the span-to-depth ratio methods given in Eurocode 2 (BS EN 1992) and BS 8110 (withdrawn), with flow charts to demonstrate the application of the approach. It then considers the more rigorous methods contained in the Codes, which consider the curvatures induced by loading and shrinkage and translate these into deflections. Again flowcharts that demonstrate the application of the methods are included. There are also worked examples of the deflection calculation process.

The final chapters of Technical Report 58 deal with the factors influencing deflection (such as the concrete properties, the environment, time of loading etc) and the effects of early-age loading. The latter can be significant, leading to unacceptable cracking and overloading of a slab.  

Acknowledgement: The Concrete Society

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TR58 Deflections in concrete slabs and beams