Span/Thickness Limits for Deflection Control

Predicting the deflection serviceability of reinforced concrete members 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 specified allowable limits. 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 ACI 318-08, CSA A23.3-04, BS 8110-97, AS 3600-2009, Eurocode 2 (2004), ACI Committee 435 revisions, and the proposals of numerous other authors.