Evidence from historical earthquakes suggests that the vulnerability of highway bridges is significantly affected by large permanent ground deformations caused by liquefaction as well as soil-structure interaction (SSI). The vulnerability of a typical multi-span simply-supported (MSSS) concrete girder bridge found in Charleston, South Carolina, is evaluated with consideration for liquefaction and SSI effects. In general, existing bridges in this region were not originally designed with consideration for seismic events or liquefaction of underlying soils. Fragility curves that represent the probability of exceeding predefined performance levels of damage given an earthquake of a particular intensity are used to evaluate the effects of liquefaction and SSI on the performance of the bridge components and the entire bridge system. Because of the lack of earthquake damage data in this region, obtaining analytical bridge models capable of accounting for realistic nonlinear soil and structure behavior is a requirement for creating accurate representations of bridge fragility.
To better understand the effects of liquefaction and its possible consequences to the reliability framework, a detailed 2D finite element model representing the MSSS concrete bridge and a typical Charleston soil profile is subjected to a simulation of nonlinear time history ground motions to generate probabilistic seismic demand models (PSDMs). The combined soil-structure model captures constitutive soil behavior, SSI, loss of foundation stiffness due to liquefaction, and structural nonlinearities. Liquefiable dynamic nonlinear springs dependent on the excess pore water pressure of adjacent soil elements are utilized to model the lateral SSI interaction in conjunction with free-field soil columns. Soil columns are used to perform a nonlinear seismic response analysis to better understand soil model behavior and the amplification of bedrock ground motions. Simplifying assumptions limited the study of liquefaction to include only the loss of foundation stiffness as a result of increased excess pore pressure buildup.
Component fragility curves were created and combined using a joint probabilistic seismic demand model (JPSDM) to form system fragility curves for the MSSS concrete girder bridge. As a result of the limited liquefaction effects included in this study, a few of the stronger ground motions induced noticeable large variations in seismic demand to the displacement dependent bridge components; however, the sensitivity of the model to frequency content and soil profile compositions limited the appearance of liquefaction effects for this particular study. These results suggest that the consideration of the permanent ground deformations associated with liquefaction should be considered as well as the ground shaking hazard, but that additional liquefaction effects including vertical settlement and lateral spreading of the soil need to be considered in the analytical model for the creation of fragility curves considering liquefaction. The use of the dynamic p-y method with free-field soil columns successfully modeled SSI effects and offers the potential to represent liquefaction effectively in the fragility framework. This is particularly true if lateral spreading and vertical settlement caused by liquefaction can be effectively captured by a soil model with fewer simplifying constraints.