Limitations and Uncertainties in the Long-Term Deflection Calculation of Concrete Structures

© 2014 American Society of Civil Engineers. There is no distinctive boundary in RC members when the short-term deflection ends and long term begins. Simplified procedures for predicting deflections in design codes are not compatible with actual deflection under service loads, especially in sensitive elements such as floor slabs. Design codes generally predict the time-dependent deflection by multiplying an empirical amplification factor by an instantaneous deflection. Despite the negligible difference in instantaneous and short-term deflection, the calculated value for long-term deflection that is due to decrease in stiffness overtime due to the inelastic deformation of concrete shrinkage and creep, is sometimes significantly less than the actual deflection. In this paper, simplified calculations for the ratio of long-term to short-term deflection in codes and some empirical works have been compared. Based on the comparison between simplified methods and data obtained from experimental investigations of Gilbert and Guo (2005), it is evident that the long-term/ short-term ratio is sometime significantly beyond the range of codes of practice