A general equation for estimation of time-dependent prestress losses in prestressed concrete members

Abstract

Due to the inherent nature of concrete shrinkage, creep and relaxation of prestressing strands, the prestress in prestressed concrete structures decreases continuously with time, i.e., there is an apparent time-dependent prestress loss phenomenon for prestressed concrete structures. Therefore, it is an important issue for the estimation of time-dependent prestress losses in the design of prestressed concrete structures. Many current design codes provide relevant equations for the evaluation of prestress losses. For the prestressed concrete structure, in addition to the prestressing strands, a great amount of ordinary reinforcing steel is configured to enhance its load-carrying capacity. It is generally considered that no stress relaxation occurs for ordinary reinforcing steel due to its lower stress. However, the presence of ordinary reinforcing steel affects the stress distribution in the structure and thus has an obvious impact on the prestress losses, which is not considered in the prediction equations in most current codes for the design of concrete structures up to date. In this study, based on the cross-sectional analysis of prestressed concrete members that combines the age-adjusted effective modulus method of concrete creep analysis, a general equation for estimation of the time-dependent prestress losses in prestressed concrete members considering the influence of ordinary reinforcing steel is derived and simplified. When the influence of ordinary reinforcing steel is ignored, i.e., the percentage of ordinary reinforcing steel is taken as zero, the proposed equation reduces to those in Eurocode EN 1992-1-1:2004 and AASHTO LRFD 2020. In the end, an example of a T-shaped prestressed concrete girder is provided to show the application of the proposed equation for the evaluation of time-dependent prestress losses and the error resulting from the simplification of the equation. The results show that ordinary reinforcing steel has a significant effect on the time-dependent prestress losses in prestressed concrete members for the commonly used percentage of ordinary reinforcing steel, and the error arising from the simplification of the first derived equation is quite small.