Seismic performance assessment of quasi-isolated highway bridges with seat-type abutments

Seismic isolation is one of the most popular strategies to protect civil engineering structures against earthquake hazards. For highway bridges, isolation physically decouples a bridge superstructure from its substructures resting on a shaking ground, leading to a significant reduction in the seismic forces transmitted from the superstructure to the substructures and foundations. The isolation technique has conventionally been employed in protecting highway bridges in high-seismic zones and the decoupling is typically realized by interposing specially designed isolators between bridge superstructures and substructures. In recent years, bridge engineers of the Illinois Department of Transportation developed an innovative "quasi-isolation" strategy to improve bridge seismic resilience in geographical regions with low-to-moderate seismicity, such as the Midwestern United States. Different from conventionally isolated bridges, non-seismically designed commonplace bearing components are employed as sacrificial connections between superstructures and substructures of quasi-isolated bridges. During a major earthquake, fusing actions of the sacrificial connections as well as subsequent bearing deformation and sliding are intended to reduce seismic demands on bridge substructures and foundations. In conjunction with the sacrificial connections, conservatively designed bearing seat widths at substructures are relied upon to accommodate displacement demands of bridge superstructures and eventually prevent span loss. The objectives of this study are to assess the seismic performance of prototype quasi-isolated highway bridges with seat-type abutments, validate the current design strategy, and provide recommendations for improving the bridge seismic performance. To encompass common configurations of quasi-isolated highway bridges, a suite of prototype bridges with variations in the span arrangement, girder type, skew angle, pier column height, and foundation soil condition were computationally studied. Detailed yet efficient three-dimensional nonlinear finite-element models were developed for the bridges, incorporating various critical structural components and geotechnical mechanisms. Multi-mode adaptive pushover analyses were conducted to investigate bridge response characteristics in terms of force distribution among substructures, sequence of limit state occurrences, fusing of sacrificial connections, and vulnerability of critical bridge components. Additionally, eigenvalue modal analyses were performed in the elastic and inelastic bridge deformation states to reveal modal response characteristics of the bridges. The study culminated in a comprehensive and extensive seismic performance assessment of prototype quasi-isolated bridges, for which thousands of nonlinear dynamic time-history analyses were carried out using a supercomputer. The bridges were subjected to a suite of site-specific earthquake ground motions, taking into account the site condition and regional seismicity of Cairo, Illinois. The assessment results validated that the current quasi-isolation bridge design strategy is generally effective and the majority of the studied prototype bridges are unlikely to fail in global collapse when subjected to horizontal earthquake ground motions with a 1,000-year return period in the Midwestern United States. Although many of the prototype bridges exhibited satisfactory seismic performance, the response of a small number of bridges demonstrated a high risk of bearing unseating and severe pier column damage. Aiming at improving the seismic performance of these bridges, preliminary recommendations for calibrating the current design strategy were proposed and their efficacy was demonstrated by comparative studies