Time-dependent deflections of reinforced concrete beams revisited

This paper proposes a method to describe the stress, strain, and deflection behavior of reinforced concrete beams. The role of compression reinforcement is included via an enhanced rational approach and an iterative technique that are designed to converge quickly, normally within two to three cycles. Creep deflections are computed by applying a direct procedure similar to that currently used by ACI318-95 to calculate immediate deflections. The regular routine of computing deflections is thus basically preserved. A slight modification is only needed to model concrete and section properties under sustained service load. The results of the creep model are superposed with those from a shrinkage model, and the combined effect is found to well represent the actual time-dependent deflections. Beams with high compression steel ratios are particularly shown to yield calculated results in good correlation with experimental results. This represents an improvement in the ACI318-95 building code procedure, which is known to exaggerate the role of compression steel in reducing deflections, particularly for lightly reinforced beams with high compression to tension steel ratios. For the examined cases it was determined that the calculated deflections using the proposed method are 9% less to 12% more than the measured values.