Simplified calculation of the long-term deflection of reinforced concrete flexural members

In the structural design of many reinforced concrete members, the deflection of a given span is controlled by limiting the calculated deflection to a preselected maximum value. AS3600 permits the estimation of both the short-term and long-term components of the final deflection by either refined calculation or simplified calculation, the latter of which is the focus here. In this option, the short-term deflection component has, for many years, been predicted using Branson's equation to determine an effective second moment of area I-ef for the span which is then used in standard elastic deflection formulae. However, it has been demonstrated that this equation underestimates the short-term deflection of laboratory specimens, significantly so for lightly reinforced concrete slabs. Much better predictions have been obtained using a modified formula for I-ef derived from the Eurocode 2 approach for deflection calculation, and this has been proposed for inclusion in the next revision of AS3600. The long-term deflection component can be predicted using either a simple multiplier k(cs) or by calculating the deflections due to shrinkage and creep separately. Both of these approaches are examined here. Predicted long-term deflections are compared with the measured results of long-term tests on beams and one-way slabs reported in the literature, and conclusions are drawn.