Span/Thickness limitations for Deflection Control


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Title: Span/Thickness limitations for Deflection Control

Author(s): N. J. Gardner

Publication: Special Publication

Volume: 203


Appears on pages(s): 95-114

Keywords: code provisions; deflections; long term deflections; serviceability

Date: 8/1/2001

Predicting the deflection serviceability of reinforced concrete members in service is fraught with uncertainties which include imperfect knowledge of the limiting serviceability criteria, the material properties and the load history including construction loads and the service load. The serviceability criteria can be immediate deflection/curvature or incremental deflection/curvature. Most codes offer two methods for control of deflections. The designer may choose to calculate the deflections and check that these computed deflections are less than arbitrary, specified allowable limits. Calculating the immediate deflections of reinforced concrete members is difficult due to the concrete cracking in the tension zones of such members. Calculating the additional deflections due to the shrinkage and creep of the concrete and the consequent redistribution of stress is extremely difficult. Alternatively the codes give specified maximum span/depth ratios for which serviceability can be assumed to be satisfied and deflections do not need to be calculated. This paper compares the deemed-to-comply span/thickness limits of AC1 318-99, CSA A23.3-94, BS 8110-85, AS 3600-94, Eurocode 2 (1992 draft), AC1 435-78 and the proposals of Gardner and Zhang (1995), Thompson and Scanlon (1988), Asamoah and Gardner (I 997) and Scanlon and Choi (1999).