Prestress loss in unbonded prestressed concrete beams based on the strain reduction coefficient

Based on the strain reduction coefficient methodology, which enables an analysis of a beam with external or internal unbonded tendons as a beam with bonded tendons, and using the age-adjusted effective modulus method for time analyses of prestressed concrete members under long-term loads, this paper proposes an equation for the calculation of prestress loss in simply supported beams with internal unbonded tendons. The proposed equation takes into account the effects of concrete creep, concrete shrinkage, relaxation of the prestressing steel, and the presence of a bonded, non-prestressed reinforcement. The main goal of this study is to reduce a relatively complex problem, in which it is necessary to analyse the member as a whole, to one of sectional analysis. As our main conclusion, we find that the loss of prestress obtained by applying the proposed equation has an accuracy that is comparable with the results of previous studies and with the application of existing formulations.