Reliability of U‐shaped steel dampers used in base‐isolated structures subjected to biaxial excitation

Summary A reliable performance of anti-seismic devices when the upper-structure is subjected to strong biaxial seismic excitation is of vital importance to ensure the latter doesn't reach critical behavior. U-shaped steel dampers are hysteretic devices used to dissipate the earthquake-induced energy of base-isolated structures. In the framework of performance-based design, which is gaining more and more recognition, it is of particular importance to assess the performance of base-isolated structures with such dampers under different intensity levels of bidirectional ground motion. To achieve this goal, an analytical model able to simulate the bidirectional displacement response of an isolation system is adopted. Incremental dynamic analysis (IDA) is used to obtain the relation between the earthquake-induced bidirectional damage of U-shaped steel dampers and different intensity levels of the considered records. The performance of the dampers is categorized into 5 levels delimited by 4 limit states for which fragility curves are derived. The results obtained using the bidirectional approach are quantitatively compared to those given by employing an in-plane model (widely used in current design practices in Japan) with the purpose of assessing whether the latter provides unconservative estimates of the performance of the dampers. The main conclusion is that, for large seismic intensities, the safety margin against fracture of the dampers is significantly overestimated when an in-plane model is adopted. Copyright © 2016 John Wiley & Sons, Ltd.