On the flexural behavior of concrete beams prestressed with unbonded internal and external tendons.

Abstract

The analysis of beams prestressed with unbonded tendons, whether internal or external, offers several computational difficulties and challenges for proper modeling. These are addressed in this thesis which comprises three major parts. The first part describes a nonlinear analysis model developed for the prediction of the complete moment vs. deflection relationship of beams containing any combination of bonded prestressing tendons, unbonded internal or external prestressing tendons (metallic or otherwise), and nonprestressed tensile and/or compressive conventional reinforcing bars. The proposed model uses an extensive iterative procedure which involves performing an overall nonlinear analysis at various locations along the beam. The analysis is performed several times during any loading stage until the stress in the unbonded tendons is calculated within a reasonable tolerance to satisfy force and moment equilibrium. Two important issues not addressed in previous investigations are carefully examined, namely: (1) the effect of member span-to-depth ratio on the stress increase in unbonded tendons using the Variable Angle Truss Model; and (2) the change in eccentricity under load (i.e., second order effects) in beams prestressed with external tendons using geometrical considerations. In the second part of this investigation, the model is used to run an extensive parametric evaluation to study the effect of varying several parameters on the stress in unbonded tendons throughout loading. The third part involves developing a simplified equation for the prediction of the stress, f$\sb{\rm ps},$ in unbonded tendons at ultimate nominal resistance. The equation, derived analytically from force equilibrium and strain compatibility, is shown to account for most of the variables found important in the analysis and to predict experimental results with much better accuracy than any of the prediction equations reviewed. Experimental verification of all models was carried out using 143 beam tests taken from 15 different investigations. Excellent correlation was obtained between predicted vs. experimental results. The present investigation suggests that shear deformations may have some effect on the stress increase in unbonded tendons for beams having low span-to-depth ratios, while eccentricity variations in external tendons significantly affect the overall behavior for beams having high span-to-depth ratios.