Seismic performance of special concentrically braced frames in deep basins during subduction-zone earthquakes

Abstract

Deep sedimentary basins are known to increase the intensity of ground motions, but this effect is not explicitly considered in the seismic design values used in the U.S. building codes. In the Pacific Northwest, the basin amplification of ground motions from subduction zone interface earthquakes is particularly important, because such earthquakes contribute significantly to the seismic hazard at natural periods for which basin amplification is high. Additionally, interface earthquakes have long durations, the effects of which are also not considered in building codes. Research into seismicity of these regions has been plentiful but few studies have evaluated the impact on the built environment. Using advanced nonlinear modeling approaches, a commonly used system, special concentrically braced frames, was studied with an eye towards comparing the performance of crustal and subduction-zone earthquakes. Using standard performance and collapse-assessment procedures, 3-, 9-, and 20-story SCBF archetypes were subjected to large-magnitude interface earthquakes recorded in basins in Japan; these motions were selected because they were recorded in locations with similar depths as the Seattle basin. The results demonstrate that deep basins in combination with the long ground-motion duration results in brace fracture at lower spectral accelerations, Sa. Similarly, collapse is associated with lower values of Sa. The results suggest that the strength of the archetypes inside deep basins and subjected to interface earthquakes would need to be increased by a factor of over 2.0 to ensure similar collapse probabilities as those located outside basins and/or subjected to crustal earthquake hazards. The results are similar to those obtained for other seismic resisting systems previously studied.