Seismic Response Assessment and Improvement of Highway Bridges Using Fragility Function Method.

摘要

Highway bridges, the critical components in transportation networks, have experienced various levels of damage during past earthquakes. To accurately assess their seismic responses and improve the performance, this dissertation characterizes the failure mechanisms and influencing factors of bridges under both dynamic shaking and liquefaction-induced lateral spreading using the fragility function method. Under this probabilistic framework, the variability and uncertainties in structural details, foundation/soil properties and ground motions can be appropriately incorporated to render objective assessment, which will lead to logical choices for retrofitting, design and performance improvement through seismic isolation.;First, the numerical modeling approaches are established to simulate nonlinear responses of bridges under seismic shaking with consideration of soil-structure interaction (SSI). Built upon the macro-spring modeling approach for SSI, the study develops the p-y modeling approach based on beam on nonlinear Winkler foundation (BNWF) framework. Distributed nonlinear p-y springs are developed for embankments using nonlinear finite-element analysis of 3D continuum embankment model. Both modeling approaches are validated by the recorded responses of a real bridge.;Second, the fragility functions relating the damage probability with intensity measure of earthquakes are derived and compared for typical California bridges with different structural characterizations. Various methods of generating fragility functions are evaluated and important structural parameters affecting the damage probability are identified. The interactive effects of pounding and skewed geometry are also evaluated for multi-span highway bridges using the fragility function methods.;Third, the numerical modeling approaches are investigated to model the responses of bridges under liquefaction-induced lateral spreading scenario. A simplified static analysis procedure and a detailed 3D global dynamic simulation approach are established and demonstrated through analysis examples. Subsequently, the fragility functions are also derived for quantifying the vulnerability of highway bridges under liquefaction-induced lateral spreading using both static and dynamic approaches. The loading mechanisms due to lateral spreading are clarified by comparing to bridge responses in non-liquefaction cases. The dominant structural, soil and motion properties are identified. A simplified response coefficient (CEDP) method is proposed to estimate the responses of bridges under liquefaction-induced lateral spreading on the basis of corresponding non-liquefaction responses.;Finally, recognizing that seismic isolation can be used to mitigate the damages of bridges under both seismic shaking and lateral spreading cases, the study adopts the performance-based evaluation approach to investigate the effectiveness and optimum design parameters of isolation devices so as to minimize the overall damaging potential of seismically-isolated bridges. The findings can serve as a practical guide for isolation device designs.