Collapse risk of tall steel moment-resisting frames in deep sedimentary basins during large magnitude subduction earthquakes (2020)
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. Steel moment-resisting frames are important, because they are one of the most common structural system types in the existing tall building inventory of western US cities, and due to concerns regarding the potential for fracture-prone welded connections, which came to light following the 1994 Northridge earthquake. This thesis 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, which has a return period of approximately 500 years. The resulting probability of collapse, conditioned on the occurrence of the M9 scenarios considered, is 30%. The annualized collapse risk of the archetype building is also assessed considering all earthquake sources that contribute to the seismic hazard through a multiple stripe analysis. 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 by a factor of 10 the 1% in 50-year target implicit in modern seismic design standards. These high collapse risks are largely driven by: (i) deep sedimentary basin effects, which amplify long period shaking; 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. In terms of economic impacts, the earthquake-induced repair costs of the archetype building conditioned on the occurrence of the simulated Seattle M9 ground motions are estimated at 44% of building replacement cost, and the annualized losses are 0.19% of building replacement cost when basin effects are neglected versus 0.29% when basin effects are considered.