Collapse risk of pre-Northridge tall steel moment-resisting frames in the Seattle basin during large-magnitude subduction earthquakes

Abstract

The Pacific Northwest has the potential to experience large-magnitude earthquakes generated by the Cascadia Subduction Zone, which is located approximately 100 km from the city of Seattle. Tall buildings in Seattle are particularly vulnerable to these earthquakes because the city lies above a deep sedimentary basin, which can amplify the intensity of earthquake ground motions at long periods. This paper evaluates the response of an archetype 1970s 50-story steel moment-resisting frame office building in Seattle under 30 simulated scenarios of a magnitude-9 (M9) Cascadia Subduction Zone earthquake with explicit consideration of basin effects. The archetype is assumed to have fracture-prone welded connections consistent with those identified following the 1994 Northridge earthquake. The probability of collapse conditioned on the occurrence of the M9 scenarios, which have a return period of approximately 500 years, is 30%. The annualized collapse risk of the archetype building is also assessed considering all sources (i.e., crustral, intraslab and interface earthquakes) that contribute to the seismic hazard through a multiple stripe analysis procedure. The results indicate a 50-year collapse risk of 6.9% when basin effects are neglected and 10.5% when basin effects are considered. These results exceed the 1% in 50-year target implicit in modern seismic design standards by a factor of 10. These high collapse risks are largely driven by (i) deep sedimentary basin effects and (ii) the expected brittle behavior of fracture-prone welded beam-to-column connections. The simulations of the performance of the building under the M9 scenarios outside of the basin or with ductile beam-to-column connections result in a negligible probability of collapse.