Long-term deflection of RC beams under constant loads

Abstract

A method of constructing a time-dependent bilinear moment-curvature curve for reinforced concrete beams is proposed and its use for estimating long-term deflection of statically determinate beams under constant loading is demonstrated. The moment-curvature relationship defined herein is meant for a beam length subjected to a constant moment profile as opposed to a section subjected to a varying moment. The time-dependent concrete material behaviour subjected to the environmental humidity and temperature conditions is assumed to be characterised by the shrinkage strain and an effective modulus. The beam behaviour is then derived by considering uncracked and fully cracked sections. A linearised moment-curvature relationship is adopted in the post-cracking range, for taking into account the tension stiffening effect. Deflections of four example test beams are calculated, adopting an available concrete model to determine the shrinkage strain and effective modulus using concrete mix proportions and other beam data together with assigned constant values of humidity and temperature for the test duration. Comparison with measured deflections show that a reasonable level of accuracy can be achieved by the proposed approach. © 1998 Elsevier Science Ltd. All rights reserved.