Modeling the Effect of Prestressing on the Ultimate Behavior of Deep-to-Slender Concrete Beams

Deep concrete beams with shear-span-to-depth ratios a/d≤2.5 are used to resist large shear forces due to their ability to develop arch action. In high-rise buildings, deep transfer girders are typically loaded by discontinuous columns from multiple floors, while in bridges deep pier caps support the girders of the superstructure. In these situations, prestressing is typically applied to limit the opening of cracks under service loads, but its favorable effect on the shear strength is either neglected or underestimated. This paper examines experimental evidence for the shear behavior of prestressed beams and proposes a kinematics-based model for predicting their strength and deformations at failure. It is shown that prestressing alters the crack patterns of concrete beams, and consequently results in the development of arch action in beams with shear-span-to-depth ratios reaching 6 or 7. Therefore the proposed model, which stems from a two-parameter kinematic theory for deep reinforced concrete beams, is extended and applied to prestressed test specimens with a/d varying from 0.55 to 6.73 producing adequate predictions of strength and deformations.