Seismic Fragility and Reliability of Base-Isolated Structures with Regard to Superstructure Ductility and Isolator Displacement Considering Degrading Behavior

The present study investigates the effects of the degradation of mechanical properties (pinching, stiffness degradation, and strength degradation) in load-deformation relationship of the superstructures equipped with friction pendulum systems on the seismic fragility and reliability results. An equivalent two-degree-of-freedom model is employed, in which the Bouc-Wen hysteretic model characterizes the inelastic behavior of both the superstructure and the isolator. To examine a wide variety of system combinations with different structural properties and degradation severity, a comprehensive parametric study is performed. Incremental dynamic analyses are carried out to develop fragility functions. Subsequently, the seismic reliability of both the superstructure and the isolation level is assessed. Results indicate that the negative effects of degrading behavior are more prominent in stiff superstructures as it significantly decreases structural capacity and reliability indices. It is demonstrated that the system is primarily influenced by pinching behavior rather than degradation in strength and stiffness, especially at low intensity levels that correspond to lower limit-states. In the majority of investigated systems, the seismic demand in the isolator decreases when superstructures undergo degrading behavior. Overall, such effects should be considered in the reliability-based design procedure to attain design consistency in terms of reliability. The results of this paper that account for the degrading effects enable reliable estimations of key parameters essential for the design of the superstructure and the isolator.