Seismic vulnerability assessment of retrofitted bridges using probabilistic methods

While the 1971 San Fernando earthquake heightened the awareness of the seismic vulnerabilities of highway bridges and motivated seismic retrofit programs in California, seismic design considerations were not made in regions of moderate seismicity such as the Central and Southeastern United States (CSUS) until after 1990. This has resulted in an inventory populated with highly vulnerable bridges for which states in the CSUS are now beginning to evaluate deficiencies and undertake seismic retrofit activities. There has been limited research to date to evaluate viable retrofit strategies for the bridges common in the region or support decision making on seismic upgrade. However, an emerging tool in seismic risk assessment, bridge fragility curves, provides valuable support for risk mitigation activities.; There is an urgent need for the development of fragility curves for retrofitted bridges, particularly for the CSUS. These fragility curves are conditional probability statements of potential levels of damage over a range of earthquake intensities. The development of reliable retrofitted bridge fragility curves would allow for assessment of the effects of various retrofit measures on the performance of different CSUS bridge types. Therefore, a primary objective of this work is to develop a methodology for fragility assessment of bridge retrofit, in order to support seismic risk mitigation efforts in the region.; A range of potential retrofit measures for typical classes of bridges in the CSUS are evaluated ranging from response modification to capacity enhancement or partial replacement. This is accomplished by developing detailed analytical models of the retrofitted bridges and subjecting them to suites of synthetic ground motions appropriate for the region of interest. The impact of retrofit on the demand placed on various critical components in the bridge is captured through the development of joint probabilistic seismic demand models (PSDMs). The potential for meeting different measures of performance is assessed incorporating both the uncertainty in the seismic demand and capacity in order to develop the fragility curves. Appropriate levels of uncertainty are evaluated and propagated through the analysis. The fragility curves reveal the impact of retrofit not only on component vulnerability, but on bridge system performance. The results indicate that different retrofits become more effective depending upon the bridge type and damage level of interest. Frameworks for the use of the fragility curves in retrofit selection are highlighted including performance-based retrofit and cost-benefit analyses. These types of assessments illustrate the vital role the retrofitted bridge fragility curves developed as a part of this work play in supporting risk mitigation efforts for transportation networks.