Demand fragility surfaces for bridges in liquefied laterally spreading ground.

摘要

The purpose of this research has been to evaluate the fragility of bridges in liquefied laterally spreading ground and generate demand fragility surfaces for different engineering demand parameters (EDP) at a range of values for each of the EDPs. The analyses have been performed numerically using the finite element program OpenSees. The method used in performing the analyses is the "global equivalent static analysis method", in which the entire structural components of the bridge are modeled and the lateral spreading displacement demands of the soil are modeled using nonlinear soil-structure (p-y, t-z, and q-z) springs with considerations for liquefaction.;To capture the variability in the bridge and site properties, distributions were assigned to all of the input parameters of the models. The distributions were selected based on the available information and the bridge models represented realistic scenarios. The ground displacement patterns applied on the bridge were also varied to represent the natural variability of the ground displacement patterns.;Bridge models for different classes of bridges were created. Roughly 1000 analyses were performed for bridges of every class. During the analyses the maximum free-field ground displacements were recorded at different values of several EDPs. Demand fragility function which is the probability of exceeding a certain EDP value for a given EDP at a given value of ground displacement, was generated for different values of different EDPs. Fragility functions for different EDP values were fitted by a single "Demand Fragility Surface" per EDP. Each surface was characterized by only 6 fitting parameters.;The results of the demand fragility surfaces showed that the pre-1971 bridges were more fragile that the newer bridges, due to lack of proper transverse reinforcement in the piers and poor quality piles. Also it was found that simply-supported bridges mobilize less demand on the piers (due to rotational flexibility at the top), while they often have more demand at foundation and abutment level (i.e. pile cap and abutment displacement and rotation). The damages in different components of the continuous bridges were better correlated, since this type of bridges is more effective in transferring demands.