Prediction of Time-Dependent Stresses and Deflections in Prestressed, Concrete Girders: From Start of Fabrication to End of Service Life
Davison, William Leander
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Fabricators and contractors need accurate prediction of the camber in precast, prestressed, concrete girders. Large differences in the camber between adjacent girders lead to significant construction difficulties that often have financial and/or legal ramifications. Many factors affect the time-dependent deflections of these girders including; creep and shrinkage of concrete, prestressing relaxation, temperature variations and numerous fabrication conditions. The current models used to predict the deflection history are largely empirical and, although they account approximately for some of the important effects, they do not explicitly consider the interactions among these factors. The goal of this research was to generate a camber prediction algorithm that links the time-dependent constitutive models and explicitly considers the fabrication conditions. This was done by using classical structural analysis techniques and combining them with explicit, time-dependent material models. The analysis was divided into four time phases that encompass a girder's life-span, from fabrication through it service life. These phases are: * Strand jacking: The phase during which the prestressing strands are brought up to their specified jacking stress, * Pre-bonding: The phase during which the prestressing strands are anchored to the abutments in the casting bed, but the strands have not yet bonded to the surrounding concrete, * Post-bonding: The phase during which the concrete has bonded to the strands, but the girder is still resting in the casting bed, and the strands are still restrained by the abutments. * Post-release: The phase during which the prestressing strands have been released from the abutments and the girder is removed from the casting bed. This phase includes the entire service life. In each of these four phases, the boundary conditions are different and the system is analyzed to determine the stresses, deformations and deflected shape. The calculations are necessarily iterative because the constitutive laws for the strand and concrete are time-dependent. The foundation of this analysis method is the proper time-dependent constitutive models. A time-dependent constitutive model was developed for concrete creep using basic Kelvin-Voigt rheological models, modified to include time-dependent parameters. This new model was then calibrated against currently accepted creep models in order to optimize model parameters for a specific girder concrete. For the time-dependent strand relaxation model, the model proposed by Bazant and Yu (2012) was used. Unlike the commonly used Magura model (1964) this model is capable of addressing variable stress loading. It also accounts for the key factors that affect relaxation, including temperature and variations in strain. This model was calibrated against relaxation data. Using these calibrated material constitutive models, the camber prediction algorithm was used to predict the pre-release material stresses as well as the camber history for a girder. The resulting predictions were compared with measured cambers at release and up to ten hours after release. The resulting predictions were reasonable and resulted in expected trends. The predictions also compared favorably with the AASHTO (2012) model for long-term predictions.
- Civil engineering