Fragility and vulnerability analysis of deteriorating ordinary bridges using simulated ground-motion sequences

Highway bridges are critical components of the lifeline transport infrastructure in urban areas. They are designed with the expectation of not sustaining significant structural/non-structural damage after major earthquake-induced ground shaking. However, in the current structural performance-based assessment practice, the effects of a pre-damaged state during ground-motion sequences are often neglected. Additionally, environmentally induced deterioration mechanisms (e.g., steel rebar corrosion) may exacerbate the consequences of such ground-motion sequences on the seismic structural performance during the bridge design lifetime; yet such effects are commonly overlooked. This study proposes a computational methodology to derive state-dependent fragility and vulnerability relationships (i.e., explicitly depending on the damage state achieved by the bridge structure during a first shock) for bridge structures subjected to chloride-induced corrosion deterioration and ground-motion sequences. The methodology is demonstrated for a case-study ordinary bridge structure (representing a typical bridge vulnerability class in southern California) under seismic sequences assembled from the CyberShake 15.12 (hybrid) simulated ground-motion database. In the proposed approach, parameterised (i.e., dependent on the corrosion deterioration level) vector-valued probabilistic seismic demand models are developed for the bridge components (i.e., columns and shear keys). These models, calibrated through sequential cloud-based nonlinear time-history analyses, relate the dissipated hysteretic energy in the ground-motion sequence to a deformation-based engineering demand parameter induced by the first shock and a ground-motion intensity measure of the second shock for a given corrosion deterioration level. Fragility relationships are first derived for a single ground motion at the component-level; state-dependent fragility relationships are then derived by considering the additional damage induced by a second ground motion within the simulated sequence (structure-specific damage states are considered). Furthermore, a component-based simulation approach accounting for the correlations across the components’ response is utilised to derive fragility relationships at the system level (i.e., for the bridge structure). Finally, appropriate damage-to-loss models for ordinary bridges are used to derive state-dependent vulnerability relationships. The results demonstrate the significant impact of earthquake-induced ground-motion sequences and environmentally-induced corrosion deterioration, emphasising the necessity of accounting for this multi-hazard threat in the structural performance-based assessment of bridges. Relative variations up to 120% can be found in the system-level state-dependent fragility median values comparing the results for the bridge in pristine and deteriorated conditions.