Behavior, Design, and Analysis of Unbonded Post-Tensioned Precast Concrete Coupling Beams

This dissertation describes an experimental, analytical, and design investigation on the nonlinear behavior of precast concrete coupling beams, where coupling of reinforced concrete shear walls is achieved by post-tensioning the beams and the walls together at the floor and roof levels. The new coupling system offers important advantages over conventional systems with monolithic cast-in-place beams, such as simpler detailing, reduced damage to the structure, and reduced residual lateral displacements. Steel top and seat angles are used at the beam-to-wall joints to yield and provide energy dissipation. The results from eight half-scale experiments of unbonded post-tensioned precast coupling beams under reversed-cyclic lateral loading are presented. Each test specimen includes a coupling beam and the adjacent wall pier regions at a floor level. The test parameters include the post-tensioning tendon area and initial stress, initial beam concrete axial stress, angle strength, and beam depth. The results demonstrate excellent stiffness, strength, and ductility of the specimens under cyclic loading, with considerable energy dissipation concentrated in the angles. Compliance of the beams to established acceptance criteria is demonstrated, validating the use of these structures in seismic regions. The critical components of the structure that can limit the desired performance include the post-tensioning anchors as well as the top and seat angles and their connections. The experimental results are also used to validate the analysis and design of the new coupling system. Two different analytical models, one using fiber elements and the other using finite elements, are investigated. In addition, an idealized coupling beam end moment versus chord rotation relationship is developed as a design tool following basic principles of equilibrium, compatibility, and constitutive relationships. The comparisons demonstrate that the analytical and design models are able to capture the nonlinear behavior of the structure, including global parameters such as the beam lateral force versus chord rotation behavior as well as local parameters such as the neutral axis depth at the beam ends. Using these models, the effects of several structural properties (such as beam length) on the behavior of unbonded post-tensioned precast coupling beams is analytically investigated to expand the results from the experiments.