Estimating Time-Dependent Deformations of Prestressed Elements: Accuracy and Variability

A common structural system for bridges and highway overpasses often consists of precast, prestressed concrete girders made continuous through a cast-in-place deck and diaphragms. To assess the risk of cracking under service conditions the designer needs to estimate the internal stresses in the concrete. The paper will first compare, for a particular case study, differences in the time dependent response predictions resulting from changes in the structural analysis methodology and/or the material models used to describe concrete behavior. Second, the paper will assess the variability of the material models used in making predictions. A straightforward statistical approach is used to determine the 95% confidence intervals on predictions of the material properties critical to time dependent deformation. The study instrumented and monitored two precast, prestressed girders for strains and deflections for a five month period from fabrication to erection.To describe the behavior of the concrete used in the girders, laboratory tests were conducted to measure comparative strength, modeled of elasticity, creep, and shrinkage.