Near-Fault Earthquake Ground Motion and Seismic Isolation Design

Abstract

Seismic isolation is one of the most reliable passive structural control techniques with adequately established standards for the earthquake protection of structures from earthquakes. However it has been shown that the seismic isolation systems may not function the best for the near-fault ground motions since in the proximity of a capable fault the ground motions are significantly affected by the rupture mechanism and may generate high demands on the isolation system and the structure. In fact several earthquake resistant design codes state that the seismically isolated structures located at near-fault sites should be designed by considering larger seismic demands than the demand on structures at far-field sites. When the fault ruptures in forward direction to the site most of the seismic energy arrives in coherent long-period ground velocity pulses. The ground-motion prediction equations (GMPEs) typically cannot account for such effects with limited distance metrics and lack adequate data at large magnitudes and near distances. For the reliable earthquake design of the isolated structure in near fault conditions that meets the performance objectives the 3D design basis ground motion(s) need to be appropriately assessed. Measures in the design of the isolation system such as modifications in the stiffness and damping characteristics as well as in the limitation of vertical effects are needed. The behavior of the base-isolated buildings under near-fault (NF) ground motions with fling-step and forward-directivity characteristics are investigated with a rational assessment of design-basis near-fault ground motion are investigated in a parametric format. The parametric study includes several variables including the structural system flexibility, number of stories, isolation system characteristic (yield) strength and the isolation periods related to the post-elastic stiffness. Furthermore the effect of additional damping by viscous dampers were tested for some selected cases. Important findings observed from the parametric performance results and the overall conclusions of the study are provided.