Simplified analysis of creep in prestressed concrete beams

This paper describes a simple, step-by-step method for evaluating long-term creep effects in prestressed concrete beams. The method has also been used in a parallel study of shrinkage in prestressed slabs and beams. By introducing several simplifying assumptions into a one-step analysis, approximate, closed-form equations are derived for the loss of prestress and the curvature in a prestressed concrete beam section due to long-term creep. The long-term creep curvature is obtained by applying a correction term to the free creep curvature, which is the initial elastic curvature multiplied by the long-term creep coefficient. This correction allows for the restraining effect of the tendon and the reinforcing steel on concrete creep in the section. Creep deflections are obtained by integrating creep curvatures in key cross-sections. A numerical study shows that corrections to the free creep curvature of up to about 30% are needed to account for the presence of the reinforcing steel and tendon. This suggests that the free creep curvature is adequate for many preliminary design calculations, and for detailed calculations where the design is not deflectionsensitive. The calculations also show that reinforcement has an important effect on loss of prestress and long-term deformations.