Seismic analysis, behavior, and design of unbonded post-tensioned precast concrete frames

In unbonded post-tensioned frames, the beam-column connection is constructed by post-tensioning the beams to the columns using strands or bars that are not bonded to the concrete. The clamping force between beam and column provides vertical shear force transfer at the interface between the beam and column, so corbels are not needed. This type of construction provides ductile connections, where under seismic loading, nonlinear behavior occurs in the connections, and the beams and columns remain elastic.;The dissertation investigates the seismic behavior of unbonded post-tensioned frames analytically. The behavior of these frames is controlled by gap opening along the interface between the beams and columns. This behavior results in a frame with significant self-centering capability, but low inelastic energy dissipation under seismic loading. An analytical model for unbonded post-tensioned beam-column subassemblages and frames, based on fiber beam-column elements, is developed and correolated with test results.;The dissertation proposes a seismic design approach for unbonded post-tensioned beam-column connections and frames. Under a "design" level ground motion, the following behavior is expected: (1) nonlinear flexural behavior in the beam-column connections; (2) cover spalling in the connections; (3) no yielding of the post-tensioning steel in the beams; and (4) yielding of the columns at the base. Under a "survivability" level ground motion, the following behavior is expected: (1) yielding of the post-tensioning steel in the beams, but no fracture of the steel; (2) no crushing of concrete in the beam-column connections; and (3) no shear slip at the beam-column interface. Using the proposed design approach, four prototype frames have been designed.;The dissertation provides an in-depth discussion of the seismic response of the prototype frames, based on the results of a large number of push-over static analyses and nonlinear dynamic time-history analyses. The results show that the response of the prototype frames is essentially nonlinear elastic under "design " level ground motions with little damage. The prototype frames survive the "survivability" level ground motions with damage, but without failure. Nonlinear dynamic roof displacement amplification factors and beam-column connection ductility demand factors are recommended for different U.S. seismic regions and soil conditions.