Collapse risk of tall steel moment-resisting frames in deep sedimentary basins during large magnitude subduction earthquakes

Buildings in Seattle, WA have the potential to experience large-magnitude earthquakes generated by the Cascadia Subduction Zone, which is located approximately 100 km from the city. Furthermore, the city lies above a deep sedimentary basin which can amplify the intensity of earthquake ground motions at long periods and the resulting damage in tall structures. Steel moment-resisting frames are of importance because of their prominence as one of the most common structural system types in the existing tall building inventory in Seattle, and due to concerns regarding the potential for fracture-prone welded connections, which came to light following the 1994 Northridge earthquake. This paper evaluates the response of a representative 1970s 50-story steel moment-resisting frame office building in Seattle under 30 simulated scenarios of a magnitude-9 Cascadia Subduction Zone earthquake. The resulting probability of collapse, conditioned on the M9 scenarios considered, is 42%. This collapse risk is greater than the 25% probability of collapse for the 975-year return period probabilistic estimate of the hazard, and it is below the 85% probability of collapse for the risk-targeted Maximum Considered Earthquake (MCER), when basin effects are considered. The estimated collapse risk exceeds by a factor of 8.5 the 10% or less probability of collapse under MCER ground motions targeted by modern codes for new design. 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 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.Applied Science, Faculty ofNon UBCCivil Engineering, Department ofUnreviewedFacultyPostdoctoralGraduat