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A simplified method for prediction of long-term prestress loss in post-tensioned concrete bridges.
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2006-07-01
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Abstract:Creep and shrinkage of concrete and relaxation of prestressing steel cause time-dependent changes in
the stresses and strains of concrete structures. These changes result in continuous reduction in the
concrete compression stresses and in the tension in prestressing steel. Reasonably accurate estimate
of the long-term prestress losses are needed to avoid any serviceability problems of the structure (due
to cracking and/or excessive deflection). An analytical method is presented to predict the long-term
prestress losses in continuous cast-in-place post-tensioned bridges. The method is based on the basic
principles of solid mechanics and satisfies the requirements of equilibrium and compatibility of the
bridge cross section. The proposed method for a section in concrete girder reduces to a single
equation with three coefficients, which are functions of the modulus of elasticity and creep
coefficient of concrete, location and amount of prestressing and non-prestressed steel, and geometry
of the cross section. To expedite the use of the method and to make it more appealing to practicing
engineers, design aids are provided to estimate these three coefficients. The method is further
extended to continuous bridge girders by using the force method to calculate the change in
connecting moments at intermediate supports and hence the increase or decrease in prestressing
losses. The predictions of the proposed method are compared with the current provisions of design
standards and codes of practice. It is shown that the present empirical equations of the bridge
standards can overly underestimate or overestimate the long-term prestress losses, depending on the
concrete creep and shrinkage properties as well as prestressing and non-prestressed steel ratios.
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