Prediction of Long-Term Prestress Losses

Abstract

This paper presents an analytical method to predict long-term prestress losses in precast, pretensioned, or posttensioned concrete members. Design aids are presented for cast-in-place, posttensioned concrete bridges. The proposed method is based on equilibrium and compatibility principles of solid mechanics and can be used for multistage loading and prestressing. In deriving some of the equations, however, it is assumed that prestressing and dead loads are simultaneously applied and that long-term effects of concrete creep, shrinkage, and relaxation of prestressing steel occur gradually thereafter. Empirical equations in current bridge codes either underestimate or overestimate long-term prestress losses in concrete members, depending on the concrete creep and shrinkage properties and on the nonprestressed steel ratios. Examples show that the presence of nonprestressed steel has significant effects on the long-term deformation and compressive stress remaining in the concrete after prestress losses.