Why calculate deflections?

Clause 7.4.2(1) of BS EN 1992-1-1 notes that

"Generally, it is not necessary to calculate the deflections explicitly.."

As noted in the Concrete Centre publication TR58 and EC2, satisfying span-to-depth limits means the deflections may be considered to satisfy the deflection limits set out in the code. However, Clause 7.4.2 goes on to note that:

"More rigorous checks are necessary for members which lie outside such limits, or where deflection limits other than those implicit in simplified methods are appropriate."

The limits inherent in a span-to-depth check are not explicitly stated, nor does the code provide guidance on when members lie outside the limits of application. Therefore, there are not clearly defined limits on when deemed to satisfy criteria are appropriate, nor clearly defined parameters on when deflections should be explicitly calculated. However, in the following cases, span-to-depth ratio based acceptance criteria may no longer be valid and the calculation of deflections may be advised:

•End user deflection limits are specified which are more onerous than those recommended in the design codes

•brittle elements or finishes require enhanced protection by limiting absolute deflection values

•determining cambers on members

•long span structures where span-to-depth ratios would result in large absolute values of deflection and deflection needs to be limited to a maximum value

•use of thinner and so lighter structures that would not comply with span-to-depth ratios would result in a more economic structure

•evaluating structures under specialist or unique loading conditions

•construction sequences require early age loading to be applied to the structure and induced cracking may lead to increased long term deflections

•the use of high moduli concrete results in higher early strengths enabling early age loading

•the presence of glazed curtain walling where tolerance on vertical movements can be small

With a tendency towards longer span structures, the use of span-to-depth ratios may no longer be appropriate since the magnitude of the span may result in a deflection limit that is itself of sufficient magnitude that it may be detectable visually. Even in cases where span-to-depth ratios would be deemed appropriate, the geometry of the structure may make the determination of the appropriate span difficult. For example, irregular column layouts or curved edges where the slab is part cantilevering may lead to difficulty in determining the required span.

It needs to be recognised, however, that the rigorous method of calculation of deflections is not exact and thus the resulting deflection calculations are still subject to uncertainty. This uncertainty needs to be considered when considering the magnitudes of the calculated deflections.