Influence of frequency-dependent soil-structure interaction on the fragility of R/C bridges

Bridge performance under earthquake loading can be significantly influenced by the interaction between the structure and the supporting soil. Even though the frequency dependence of the above interaction has long been documented, the simplifying assumption that the dynamic stiffness is dominated by the mean or predominant excitation frequency is still commonly made, primarily due to the associated numerical difficulties when the analysis has to be performed in
the time domain. This study makes use of the advanced Lumped Parameter (LP) models recently developed [1] in order to quantify the impact of the above assumption on the predicted fragility of bridges. This is achieved by comparing the predicted vulnerability for the case of a reference, well studied, actual bridge using both conventional, frequency-independent, Kelvin Voigt Models and the aforementioned LP formulation. Analyses results demonstrate that the more refined consideration of frequency dependence of soil-structure interaction at the piers and the abutments of a bridge, not only leads to different probabilities of failure for given intensity measures but also to different hierarchy and distribution of damage within the structure for the same set of earthquake ground motions even if the overall probability of exceeding a given damage state is the same. The paper concludes with the comparative assessment of the
above effect for different soil conditions, foundation configurations and ground motion characteristics along with the relevant analysis and design recommendations.