Seismic Performance And Design Method Of Self-centering Concrete Frames

To reduce the residual deformation and damage of reinforced concrete frames, a novel self-centering prestressed concrete (SCPC) moment resisting frame (MRF) with web friction devices (WFDs) is developed. An SCPC frame uses horizontally-oriented unbonded post-tensioned (PT) tendons to compress the precast beam and column together. The behavior of an SCPC frame is characterized by the gap opening/closing at the beam-to-column interfaces under earthquakes. The PT tendons contribute to the moment capacity of the connections and provide an elastic restoring force that returns the frame to its initial position (i.e., exhibits a self-centering capability). Steel jackets and steel plates are shop fabricated at the beam ends and columns to avoid concrete damage at the beam-column interfaces. Web friction devices are used at the beam ends and dissipate earthquake energy when the beam rotates relative to the column.In this paper, the SCPC moment resisting frames are systematically investigated through the low cyclic tests on beam-column connections and overall frames, the development of a performance based design procedure and the analytical evaluation of seismic performance of structures. In detail, this study includes contents as follows:(1) An improved SCPC beam-column connection configuration is proposed. The bolted web friction devices instead of welding are used to facilitate the fastening and replacement of the friction devices and further reduce the damage to the connection. Ten low cyclic loading tests were performed to investigate the seismic performance of the improved SCPC beam-column connection. The influence of different parameters (i.e., initial PT forces, friction forces, friction materials, steel jacket at the beam end and arrangements of PT tendons, etc.) on the performance of the SCPC connection was analyzed. Numerical simulation of the proposed connection was conducted using the Open System for Earthquake Engineering Simulation (OpenSees) to replicate the experimental results. Good agreement was observed between the numerical simulation and the test results.(2) In order to further study the performance of the MRFs with the proposed SCPC beam-column connection, large-scale low cyclic loading test was conducted on a self-centering (SC) concrete frame with SCPC beam-column connections and conventional Reinforced column Bases (RB). A conventional Reinforced Concrete (RC) frame was also tested as a reference specimen. The failure pattern, hystereric behavior, energy dissipation, residual deformation and self-centering capability of the SCRB and RC specimen frames were compared and analyzed. In addtion, a post-tensioned column-foundation connection was proposed and applied to the self-centering (SC) frame with SCPC beam-column connections and Post-tensioned column Bases (PB). A series of low cyclic loading tests were performed on two SCPB specimen frames to study the effect of different design parameters (i.e., initial PT and friction forces, etc.) on the SCPB frame system.(3) A seismic design method for the SCPC frame with conventional reinforced column bases is investigated. The seismic performance levels, the intensities of ground motions and ultimate limit states are defined, and the objectives of seismic design of SCPC frames are determined. Thereafter, the design method is proposed, including the selection of beam/column cross-sections, reinforcement arrangements, and the determination of PT and friction forces. Taking a 6-story 4-bay SCPC frame for example, the performance-based seismic design is made, and ten earthquake ground motions are selected for nonlinear time-history analyses. Good agreement is observed between the calculated responses (i.e., displacements, strains and moments, etc.) and the design demands, demonstrating the effectiveness of the proposed seismic design method.(4) To evaluate the seismic performance of the SCPC frames, the numerical model of SCPC beam-column connection is extended to the modeling of multi-story moment frames. Nonlinear static analyses and a series of dynamic time history analyses using the frequent-level and survival-level ground motion records are performed for the SCPC frames as well as the traditional RC frame. Analyses results show that the seismic performance of the SCPC frames are similar to that of the traditional RC frame under the frequent earthquakes. Subjected to the severe earthquakes, the traditional RC frame has significant residual drifts, while the SCPC frames show desirable energy dissipation, self-centering, and moment-resisting capabilities and thus have superior performance when compared to the traditional RC frame.(5) The performance of the SCPC frame was studied using the probabilistic evaluation approach. Incremental dynamic analysis was perfromed on the SCPC frame to obtain the structural demands, and regression analyses were conducted to determine the relationship between earthquake intensity and structural responses. Using the fragility function and results of incremental dynamic analysis, the seismic fragility curves of various performance levels for the SCPC frame were derived. The seismic fragility analyses, combined with the seismic hazard analysis, are used to evaluate the seismic risks for the SCPC prototype frame structure and deterine the annual (50-year) probability of exceedance for various performance levels.