Effects of Basins during Subduction Earthquakes on the Collapse Fragility of Existing Tall Steel Buildings Molina Hutt, C; Marafi, N.; Berman, J.; Eberhard, M.
Sedimentary basins tend to increase the intensity of earthquake ground motions at long periods and the resulting damage in tall structures. However, the effects of basin amplification on spectral acceleration are not directly included in the seismic hazard maps used in US building codes. This issue is particularly important in the Pacific Northwest, which has several deep basins and the potential for large-magnitude subduction earthquakes that can dominate the seismic hazard at long periods. This study aims to evaluate the impact of deep basins and long-duration shaking on the response of existing tall steel moment-frames by means of a 50-story archetype office building designed per the 1973 Uniform Building Code. Suites of ground motions, developed to investigate the impacts of magnitude 9 Cascadia subduction zone earthquakes on the Pacific Northwest, are used to evaluate the impact of basin effects by comparing seismic demands for records inside versus outside a basin. The study also isolates the effect of duration by using spectrally equivalent motions to the inside and outside basin sets from shorter duration ground motions recorded during crustal earthquakes. A suite of ground motions consistent with the 2475 year return period UHS in Seattle is also considered in order to study the impact of ground motions whose spectra match the UHS versus ground motions consistent with large-magnitude subduction earthquakes. An incremental dynamic analysis approach is carried out for each ground motion set in order to evaluate impacts on the expected collapse fragility of the archetype building. The results indicate that for the archetype 50-story 1970s steel moment frame building: (i) inside basin motions are more likely to cause collapse than the outside basin motion, (ii) duration effects have negligible impact on the relative collapse capacity, and (iii) UHS motions are the least likely to cause collapse.
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