Carlos Molina Hutt

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.

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Tall residential RC shear wall buildings (RCSW) are predominant in Metro Vancouver. These buildings have the potential to experience large-magnitude earthquakes generated by the Cascadia Subduction Zone (CSZ). Furthermore, the region lies above the Georgia sedimentary basin, which can amplify the intensity of ground motions at medium-to-long periods and the resulting damage in tall structures. The goal of this thesis is to provide insights into the effects of the Georgia sedimentary basin amplification on: (i) spectral accelerations associated with M9 CSZ earthquakes, (ii) resulting force- and deformation-controlled actions in tall RCSW buildings, and (iii) ensuing earthquake induced repair costs and times. To this end, a suite of physics-based ground motion simulations of a range of M9 CSZ earthquake scenarios, which explicitly consider basin effects are used. These scenarios are benchmarked against a range of seismic hazard intensities, as defined in Canada’s 2015 National Seismic Hazard Model (NSHM), which neglects basin effects. Relevant ground motions are propagated through a suite of archetype RCSW buildings designed to comply with the requirements of the 2015 National Building Code of Canada (NBC) at eight locations throughout Metro Vancouver with distinct basin depths. Nonlinear dynamic analysis results under probabilistic seismic hazard estimates result in negligible collapse. However, collapse risk conditioned on the occurrence of the M9 motions results in probabilities as high as 15% at the deepest basin site. Additionally, seismic demands from the M9 simulations at deep basin sites result in earthquake-induced repair costs and times that exceed the 2475-year hazard level, far exceeding the ~500-year return period associated with large-magnitude CSZ earthquakes. Furthermore, the 2015 NSHM fails to capture the significant variability in seismic demands and resulting building performance observed across the Georgia sedimentary basin. Supplementary materials available at: http://hdl.handle.net/2429/77294.

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