Carlos Molina Hutt

Fragility functions are integral to modelling building vulnerability in regional seismic risk models. Regional seismic risk assessments are often used to inform seismic policy development, even if there is often relatively low confidence in the suitability of the underlying fragility functions. This work investigates the defensibility of using generalized fragility functions when using regional seismic risk models to inform seismic policy development. This thesis presents a methodology to objectively quantify the impact that uncertainty in the fragility function damage state medians has on the variance the resulting regional seismic risk outputs, namely, casualties, building recovery time, and number of buildings with significant damage. This is achieved by means of a variance-based sensitivity analysis. The methodology is demonstrated by implementing this framework for the City of Vancouver, using a deterministic, active crustal, M7.2 earthquake scenario to represent ground shaking hazard and evaluating 15 fragility functions selected based on their contribution to the exposure and baseline risk. The findings indicate that variance in casualties is driven by uncertainty in the fragility function medians of pre-code era buildings, constructed prior to 1973, with light frame wood, unreinforced masonry, and concrete shear wall structural systems. This is attributed to their relatively low capacity and considerable presence within the broader exposure dataset. Similarly, the variance in building recovery time and significantly damaged buildings is driven by uncertainty in the medians of low-rise unreinforced masonry structures constructed prior to 1973 and light frame wood structures constructed from between 1973 and 2004. This sensitivity is driven by the underlying probabilities of damage that occur as a result of the shifts in the median damage states. Ultimately, this methodology provides a tool to better understand which input fragility functions can reduce uncertainty in regional seismic risk outputs, which is an important consideration when leveraging regional seismic risk models to inform policy decision making.

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The Metro Vancouver region of southwestern British Columbia could experience Magnitude 9 (M9) Cascadia Subduction Zone (CSZ) earthquakes. The seismic hazard of the region is further exacerbated because sits above Georgia sedimentary basin, which can amplify earthquake ground motions. Given the critical role of bridges as essential nodes within the transportation network, it is important to assess the potential impacts of such seismic events on these structures. This thesis evaluates the seismic performance of bridges within the Metro Vancouver region under plausible M9 CSZ earthquakes, with explicit consideration of the amplification effects of the Georgia basin. An inventory of more than 200 bridges was assembled by collecting as-built drawings. The detailed properties of approximately 80 bridges were extracted, with the focus on bridges supported by reinforced concrete (RC) circular columns and rectangular walls. Detailed bridge data was used to develop Single-Degree-of-Freedom (SDOF) models to characterize bridge response by following five key steps: (1) determination of bridge weight, (2) estimation of bridge stiffness, (3) evaluation of lateral strength, (4) definition of a material model to characterize nonlinear response, and (5) SDOF model assembly with the corresponding weight, stiffness, strength and calibrated material model to simulate the force-deformation behavior of the bridge. Nonlinear time history analyses were carried out to evaluate the response of each bridge under 30 physics-based ground motion simulations of M9 CSZ earthquakes adjusted for the corresponding soil conditions at each site. The results indicate that bridge damage correlates well with basin depth, with 33%, 48% and 92% of probability of complete damage, on average, for bridges outside the basin, in the basin edge and in deep-basin sites, respectively. While newer bridges perform considerably better in outside-basin sites, their performance is comparable to older bridges at basin-edge and deep-basin locations.

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As seismic design in Canada continues to evolve, many buildings designed to previous buildingcode editions are left to their own devices, creating a vulnerability in communities exposed to largeearthquakes. This thesis will examine the preparedness of communities in the lower mainland ofBritish Columbia in the event of a M9 megathrust earthquake. It originates from the CascadiaSubduction Zone, a seismic source off the west coast of Canada and the United States. The focusof this thesis will be on the seismic performance of tall, pre-1980s, reinforced concrete shear wallbuildings, which represent a large proportion of Vancouver’s tall buildings. A nonlineardynamic analysis of an archetype building fitting the mentioned criteria was carried out using shellelements in LS-Dyna. Furthermore, using the FEMA P-2018 rating system, a rapid screening tool that assigns a rating to indicate relative collapse risk for each building considered, a general picture of the risk these building pose to the community could be formed. In recognition that collapse safety is not enough to create a more resilient community, as part of this study, a recently developed framework called TREADS (Tool for Recovery Estimation and Downtime Simulation) is used to describe the performance of the building by following resilience-based performance measures. Results for the nonlinear assessment showed that the archetype building has a large shear capacity due to the large number of walls. However, it is very brittle, and fails abruptly at a low drift ratio. Nevertheless, the FEMA P-2018 showed that this case study building has a rating indicating a relatively low seismic risk. By contrast, the loss and downtime assessment of the building showed that performance would fall short of the target recovery state of shelter-in-place after a 475yr return period earthquake and the target downtime to functional recovery of 4 months. Since the analysis using FEMA P-2018 suggests that the archetype building would perform better relative to other similar buildings, detailed analyses are recommended for any buildings of this type.

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Past earthquakes have illustrated the impacts of reduced hospital functionality due to physical damage resulting in a health service deficit immediately after a major seismic event. In this paper, a methodology was developed to quantify the deficit in health care anticipated due to a loss of functionality of a hospital emergency department (ED) and a surge in demand due to regional damage in an earthquake scenario. Earthquake-induced patient arrivals were calculated using multi-severity casualty estimation for the catchment area of the hospital. The surge in patients was then compared to the ability of the hospital to treat patients (capacity) based on anticipated functionality. Nonlinear response history analysis of the hospital building was performed using simplified structural models, and the structural and nonstructural component damage was estimated based on FEMA P-58. Expected damage was linked to the post-earthquake functionality of the ED services areas on each floor by incorporating the fault tree analysis method. Lastly, Discrete event simulation was utilized to evaluate the ED surge capacity, providing hospital performance metrics such as wait times (WT) and length of stay (LOS) for patients of ranging acuity. A case study of a hospital in the City of Vancouver subjected to an Mw9.0 Cascadia Subduction Zone scenario earthquake was presented. Emergency rooms were identified as the ED bottleneck during the emergency response. The mean ER WT exceeded its limit of two hours and reached up to 17 hours in the most unfavorable simulation. Likewise, the mean LOS nearly doubled from 6.5 to 12 hours, also exceeding the established target of 10 hours. The deployment of field hospitals for less severe patients as an emergency plan to mitigate the ED overcrowding was also analyzed to demonstrate that the methodology can be used as a decision support tool to improve healthcare disaster planning.

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Modern tall residential Reinforced Concrete Shear Wall (RCSW) buildings in Metro Vancouver are exposed to a considerable hazard due to the proximity of various seismic sources, such as the Cascadia Subduction Zone (CSZ), and the presence of Georgia sedimentary basin, which can amplify the intensity of ground motions at medium-to-long periods. Current building codes do not account for basin amplification effects, they intend to ensure life-safety in extreme earthquakes and do not explicitly minimize damage to building components that preserve building functionality. This study aims to provide insights into the expected loss and functional recovery time of tall RCSW buildings in Metro Vancouver under a variety of earthquake intensities. To this end, nonlinear models of archetype RCSW buildings are developed for eight different locations in Metro Vancouver. These models are subjected to ground motions representative of a range of hazard levels as per Canada’s 2015 National Seismic Hazard Model, which neglects basin effects, as well as a suite of simulated ground motions of M9 CSZ earthquake scenarios, which explicitly accounts for basin amplification. The structural responses are employed to conduct a loss assessment using a well-established methodology and a downtime assessment using a recently developed framework. Loss estimates show that the mean loss ratios under the M9 motions vary between 1.4% and 32% across Metro Vancouver and range from 0.7% to 14% for the range of hazard levels considered in this study. Downtime estimates show that the functional recovery time of buildings subjected to the M9 motions can range from 175 to 543 days and vary between 164 to 491 days for the range of hazard levels considered. The archetype buildings do not meet the robustness criteria of ensuring that there is a probability of less than 10% of not achieving sheltering capacity under the functional level earthquake (~ 475 year return period). Similarly, the archetype buildings do not meet the rapidity criteria of observing less than a 10% probability of not achieving functional recovery within four months after the functional level earthquake. Downtime deaggregation shows that the main contributor to functional recovery time is attributed to slab-column connection damage.

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High-rise residential buildings are considered as one of the best solutions to the current lack of space in urban areas. In high-density cities in the Canadian Pacific Northwest, reinforced concrete shear wall structures are one of the main typologies used in tall buildings design. This type of building is composed of a seismic-force resisting system and a gravity-force resisting system. While the former system is designed to resist lateral loads, failure of the gravity-force system is recognized as one of the main causes of building collapse under earthquake demands. Accurate estimation of seismic demands in this system is critical to provide a safe design. The goal of this study is to obtain gravity system flexural stiffness modifiers to safely estimate their seismic demands following a linear-elastic analysis. The proposed flexural stiffness modifiers were derived from the moment-curvature analysis of members within a nonlinear 3D reinforced concrete shear wall structural analysis building model (with both seismic-force and gravity-force resisting systems modelled as nonlinear). These quantitative results for individual members are used to perform regression analyses to develop generalized equations to estimate the flexural stiffness modifiers in gravity-frame columns and slabs. Typical flexural stiffness modifiers range from 3-100% and 18-85%, for columns and slabs, respectively. In most of the cases, the results show that the gravity system bending moment demands of a linear-elastic analysis model with the proposed effective stiffness modifiers are consistent with the moment demands in an equivalent nonlinear model. The proposed recommendations provide appropriate estimates of seismic demands in the gravity-force system by means of realistic stiffness factors. Moreover, they support the implementation of the Simplified Analysis procedure for the gravity-system design of reinforced concrete shear wall buildings as outlined in the Canadian concrete standard (CSA A23.3-19 § 21.11.2.1) by practicing engineers.

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While modern seismic design codes intend to ensure life-safety in extreme earthquakes, policy-makers are moving towards performance objectives stated in terms of acceptable recovery times. This thesis describes a framework to estimate downtime and model the post-earthquake recovery of buildings. Downtime estimates include the time for mobilizing resources after an earthquake and conduct necessary repairs. The proposed framework advances the well-established FEMA P-58 and REDi methodologies by modeling temporal building recovery trajectories to target recovery states such as stability, shelter-in-place, reoccupancy, and functional recovery, as well as by providing probabilistic seismic performance measures that are useful for decision-making. The proposed framework is implemented to evaluate a range of modern 8- to 24-story residential reinforced concrete shear wall buildings located in Seattle, WA. The assessment results indicate that under a functional-level earthquake (roughly equivalent to ground shaking with a return period of 475-years), the average probability across all building heights of not achieving a target shelter-in-place recovery state immediately after the earthquake is 16%, and the probability of downtime to functional recovery exceeding four months is 91.5%. These probabilities exceed the 10% threshold suggested for similar performance measures in the 2015 NEHRP guidelines and FEMA P-2090, respectively. Furthermore, the framework is used to quantify the impact of design strategies on the building’s downtime performance. The results illustrate that certain structural design interventions are effective in ensuring a small probability (
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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.

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