Brad D. Weldon

Experimental Evaluation of Posttensioned Precast Concrete Coupling Beams

This paper describes the results from eight half-scale experiments of unbonded posttensioned precast concrete coupling beams under reversed-cyclic lateral loading. Each test specimen includes a coupling beam and the adjacent concrete wall pier regions at a floor level. Under lateral loads, the nonlinear displacements of unbonded posttensioned coupling beams are governed by the opening of gaps at the beam-to-wall joints. Steel top and seat angles are used at the beam ends to yield and provide energy dissipation. The test parameters include the beam posttensioning tendon area and initial stress, initial beam concrete axial stress, angle strength, and beam depth. The results demonstrate the lateral stiffness, strength, and ductility of the coupling beams under cyclic loading, with considerable energy dissipation concentrated in the angles. It is shown that the residual displacements of the structure upon unloading are small due to the restoring effect of the posttensioning force. The sustained chord rotation capacities of the test specimens are compared with those from previous tests of monolithic coupling beams.

Analytical Modeling and Design Validation of Posttensioned Precast Concrete Coupling Beams for Seismic Regions

This paper provides an experimental validation on the analytical modeling and design of unbonded posttensioned precast concrete coupling beams. Previous test results from a series of floor-level coupling beam subassembly specimens are compared with the predicted results from a fiber-element analytical model. The test results are also used to validate an idealized coupling beam end moment versus chord rotation relationship that was developed as a design tool following basic principles of equilibrium, compatibility, and constitutive relationships. Based on the experimental and analytical comparisons, the paper presents a seismic design approach to achieve the desired lateral strength, stiffness, ductility, and energy dissipation capacity in unbonded posttensioned precast coupling beams.

Unbonded Post-Tensioned Precast Concrete Coupling Beams: An Experimental Evaluation

This paper describes the observed nonlinear reversed cyclic behavior of a half-scale precast concrete coupling beam subassembly for seismic regions. In this new system, coupling of concrete walls is achieved using unbonded post-tensioning. The test specimen includes a coupling beam and the adjacent concrete wall regions at a floor level. Under lateral loads, the nonlinear deformations of the coupling beam occur primarily due to the opening of gaps at the beam-to-wall interfaces. Steel top and seat angles are used at the beam ends to yield and provide energy dissipation. Prior to the testing of the specimen, the concrete at one end of the beam had to be patched due to poor consolidation during casting. The beam was able to sustain three cycles of displacement at 6.4% chord rotation and failed during the second cycle to 8% rotation due to the low cycle fatigue fracture of one of the top and seat angles. Most of the damage in the beam was concentrated in the angles (which can be replaced after an earthquake) and the patched concrete. The unpatched end of the beam received almost no damage other than a small amount of cover concrete crushing at the corners. The mild steel reinforcement in the beam performed well throughout the entire displacement history; however, premature fractures of post-tensioning strand wires occurred inside the anchors.

Coupling of Concrete Walls Using Post-Tensioned Precast Concrete Beams

This paper describes an analytical investigation on the nonlinear behavior of post-tensioned precast concrete beams to couple reinforced concrete walls. Different from conventional systems that use monolithic cast-in-place reinforced concrete coupling beams or embedded steel coupling beams, the nonlinear behavior of post-tensioned precast concrete coupling beams is governed by the opening of gaps at the beam ends. Steel top and seat angles are used at the beam-to-wall connections to yield and dissipate energy in the event of a large earthquake. Two types of analytical models are developed using the DRAIN-2DX and ABAQUS programs for verification of the results and more detailed evaluation of the expected behavior. Nonlinear reversed cyclic lateral load analyses of four coupling beam subassemblies are conducted. The main parameters investigated are the beam depth, the area of the post-tensioning steel, and the size of the top and seat angles. The results indicate that post-tensioned precast concrete coupling beams possess stable behavior through large rotations, significant self-centering capability due to the post-tensioning force, and significant energy dissipation provided by the yielding of the top and seat angles.