Computation of time-dependent prestress losses in prestressed concrete members
The accurate prediction of time-dependent prestress losses due to concrete shrinkage and creep and tendon relaxation in prestressed concrete structures is important for controlling cracking and deflection,over-or under-estimation of prestress losses affects the prestressing-tendon design and the structural performance in service.The presented formulae including those provided in the codes for time-dependent prestress losses are approximate which hold significant differences in expressions and in whether to consider the influence of parameters such as non-prestressed steels.In addition,the values of the introduced simplified coefficients and the simplified terms in each formula are also different,and they cannot rigorously reflect the coupling effect of shrinkage,creep,and relaxation,making it difficult to ensure the prediction accuracy of time-dependent prestress losses.Based on the equilibrium and deformation coordination conditions for internal force change in cross-section at any time,a unified calculation method for time-dependent prestress losses in prestressed concrete flexural members was presented by using the age-adjusted effective modulus method.Considering the characteristic of time-dependent strain variation in prestressed tendons along with the analytical model for relaxation reduction coefficient,the corresponding equation was derived.A practical formula for predicting time-dependent prestress losses was obtained.All the influence factors,such as prestressed and non-prestressed steel ratio and their locations in the section were taken into account.The coupling effect of concrete shrinkage and creep and tendon relaxation was certainty illustrated.The comparison between the values by the formulae calculation proposed in this article and the values in the specifications,and the experimental values in the long-term prestress loss tests were carried out.The theoretical values of the AASHTO and EN 1992-1-1 formulae are 17%to 24%higher than the tested values,while the theoretical values of the TB 10092-2017 and JTG 3362-2018 formulae are 9%to 21%lower than the tested values.The differences between the calculated values of the proposed formulae and the tested values are within 6%,indicating that the new established formulae have good calculation accuracy.
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