Title: Prediction of Short-term Deflections in Reinforced Concrete One-way Continuous Members

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Publication: CIA

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Date: 2/28/2011

Abstract:
The control of deflections in reinforced concrete members is achieved either by calculating deflections and comparing them with limiting values or by not exceeding maximum span to depth ratios. This paper examines the current provisions of AS3600 for beam and slab deflection by simplified calculation. These provisions advocate the use of Branson’s equation to determine the effective second moment of area Ief at a cross-section and, for continuous members, a weighted average Ief.av of values calculated at two or three cross-sections along the member. In the case of one-way simply supported slabs, it has been demonstrated recently that AS3600 significantly underestimates the measured short-term deflections. Much better predictions have been shown to be obtained using a modified formula for Ief based on the Eurocode 2 (EC2) provisions. In this paper, predictions using the AS3600 provisions and the EC2 modified formula for Ief are compared with the results of recent tests on continuous one-way slabs. It is shown that the current AS3600 provisions again significantly underestimate the deflections and better predictions are obtained using the modified EC2 formula. The weightings used to calculate Ief.av for continuous members are also examined.