Friction-Damped Unbonded Post-Tensioned Precast Concrete Moment Frame Structures for Seismic Regions

This dissertation describes the development of a new type of friction damper for unbonded post-tensioned precast concrete building moment frame structures in seismic regions. Previous research has shown that these structures have desirable seismic characteristics such as a self-centering capability and an ability to undergo large nonlinear lateral displacements without significant damage; however, the displacements during an earthquake may be larger than acceptable as a result of small energy dissipation. To reduce the seismic displacement demands, the proposed friction dampers are placed at selected beam ends of a frame, and dissipate energy through the displacements that occur as a result of gap opening between the beam and column members. Large scale beam-column subassemblies are tested under pseudo-static cyclic lateral loading with the following varied parameters: damper normal force, type of friction interface, area and initial stress of beam post-tensioning steel, and beam depth. In addition, dynamic loading experiments are conducted on isolated dampers to investigate the effects of displacement rate and slip amplitude on damper behavior. The experiments show that the dampers can provide a significant amount of supplemental energy dissipation at the beam ends, while the self-centering capability of the structure is preserved. The results from the experimental program are used to validate analytical models for the dampers as well as for beam-column subassemblies without and with dampers. Then, the subassembly models are extended to the modeling of multi-story moment frames.A seismic design procedure is developed to determine the damper forces and post-tensioning steel areas needed to satisfy prescribed lateral strength, energy dissipation, and lateral drift requirements for a frame under earthquake loading. The proposed design procedure is critically evaluated based on nonlinear static lateral load analyses and dynamic time-history analyses of prototype friction-damped precast concrete frame structures. Comparisons of the prototype structures with structures that use only mild steel reinforcement crossing the beam-column joints as well as structures that use only post-tensioning steel and structures that use combinations of mild steel and post-tensioning steel are also provided. The results demonstrate that friction-damped unbonded post-tensioned precast concrete frame structures constitute a viable and competitive system for seismic regions.