Long-term deflection of RC beams

Abstract

Long-term deflection of statically determinate reinforced concrete beams under service loads is analytically studied. The BP-2 creep and shrinkage model is adopted and extended to accommodate concrete tensile behavior by assuming a tensile stress-strain relation. The creep rate is related to the stress in the same manner for both tension and compression. Beam deflection is determined using curvatures computed by assuming that the total strain varies linearly across depth (Bernoulli-Navier hypothesis). Available test results for 37 beams are compared with deflection predictions by this numerical scheme and by applications of the same model after simplification using effective modulus methods. Further comparisons are made with the 1983 ACI Building Code method and a recent method proposed by Bazant and Oh. The direct implementation of the extended BP-2 model is observed to be the most accurate. The results demonstrate that a realistic model of creep strains can lead to good estimates of deflection. Furthermore, the results indicate that the contribution of shrinkage to overall deformation can be significant.