Methods for checking deflection

This chapter describes the use of Eurocode 21 to check deflection by calculation. The alternative method for complying with the code requirements is to use the deemed-to-satisfy span-to-effective-depth ratios, which are appropriate and economic for the vast majority of designs. Further guidance on the span-to-effective-depth method is given in Chapters 3, 4 and 7, originally published as Beams2, Slabs3 and Flat slabs4. However, there are situations where direct calculation of deflection is necessary, as listed below:

■ When an estimate of the deflection is required.

■ When deflection limits of span/250 for quasi-permanent actions (see reference 5 for Eurocode terminology) or span/500 for partition and/or cladding loads are not appropriate.

■ When the design requires a particularly shallow member, direct calculation of deflection may provide a more economic solution.

■ To determine the effect on deflection of early striking of formwork or of temporary loading during construction.

In the past structures tended to be stiff with relatively short spans. As technology and practice have advanced, more flexible structures have resulted.

There are a number of reasons for this, including:

■ The increase in reinforcement strength leading to less reinforcement being required for the ultimate limit state (ULS) and resulting in higher service stresses in the reinforcement.

■ Increases in concrete strength resulting from the need to improve both durability and construction time, and leading to concrete that is more stiff and with higher service stresses.

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