PROGRAM | Civil Engineering

By: Shaymaa Obayes Chair: Monique Head

ABSTRACT

Previous studies have been conducted on the vulnerability of bridges, but there is a notable research gap on the evaluation of existing bridges, specifically multi-span precast concrete girder (MSPCG) and slab bridges, subjected to long-duration earthquakes in high seismic regions. Through the development of a vulnerability assessment framework presented herein, this research uniquely quantifies the impacts of an anticipated moment magnitude (MW) 9.0 earthquake event characterized by its long-duration for incipient collapse risk of a typical MSPCG and slab bridge in the Pacific Northwest (PNW) region. The PNW region is within the Cascadia Subduction Zone (CSZ), where MW 9.0 and megathrust earthquakes are anticipated and could cause structural damage and fatalities if structures are not retrofitted accordingly. The framework presents a comprehensive methodology to evaluate damage states and assess bridge vulnerability using updated seismic maps and two different bridge design specifications. Results from these different methods can be systematically compared as part of the vulnerability framework. The assessment includes both flexural and shear failures MSPCG and slab bridge columns. MSPCG and slab bridges are modeled in OpenSees as case studies to quantify the risk of incipient collapse using nonlinear time history (NLTH) and fragility analyses. The bridges studied were designed prior to the 1990s and thus were not equipped to consider risks in the same manner as current (2023) risk-targeted approaches. Additionally, parametric studies assess the impact of bridge skew under comparable loading conditions. From the NLTH analyses, the flexural failure assessment results showed that MSPCG can endure higher seismic intensities before suffering notable damage than slab bridges. While MSPCG bridges exhibit higher resilience in flexural failure scenarios, they still face challenges in terms of shear failure due to inadequate shear reinforcement and deficiencies in design detailing.  Slab bridge columns, however, experience larger drift ratios than MSPCG bridge columns when subjected to the same loading conditions, DS1 and DS2. The shear failure assessment results show that the drift capacity ratio at the first damage state, i.e., DS1, is approximately 50% less than the maximum drift demand ratio for both bridges due to simulated MW 9.0 ground motions. In other words, the capacity as predicted by the model would be less than the demand, revealing an exceeding probability of shear failure per ASCE/SEI 41 specifications.  Moreover, the skewed angle for the slab bridge increases the probability of shear failure for the bridge column due to increasing lateral displacement as captured by the drift ratio compared to the straight slab bridge responses. From the fragility analyses, the risk-targeted analysis results show that the estimated collapse risk increases when the OpenSees bridge simulation model results under the effect of MW 9.0 earthquakes are considered alone (termed RiskMw9.0). Comparisons with updated USGS spectral acceleration maps (AASHTO-2023 Web Services) demonstrate that the spectral characteristics of MW 9.0 events are more damaging. This study highlights the consequences of the lack of strict seismic design standards in older design codes, especially for MSPCG and slab bridges built before the 1990s. In addition, it has shown the importance of incorporating MW 9.0 simulated ground motions, particularly due to their potential to drastically increase collapse risks, thus revealing bridge vulnerability of bridges designed before the 1990s. The outcomes from this research show the structural vulnerabilities of aging bridges and how other bridges can be analyzed to determine their probability of failure when subjected to long-duration earthquake effects.

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