Seismic fragility and risk assessment of isolated bridges subjected to pre-existing ground displacements

This study focuses on the seismic fragility of isolated continuous-superstructure bridges affected by pre-seismic differential ground displacements that affect the deformation capacity of the isolators. The research methodology is designed based on observations of a case-study bridge in Southern Italy that was subjected to a slow-moving landslide leading to shear strains of the isolators. In this study, fragility analyses are carried out for ten bridge realisations, with varying number of spans and isolator types subjected to ground displacements at the base of the substructure components. The study presents a methodology to compute the probability of exceedance of a damage state conditioned to a given seismic intensity and pre-seismic isolation strains. In addition, vertical ground displacements (e.g. subsidence, settlements) on the substructure components lead to a negligible effect on seismic fragility. Findings reveal that localised horizontal ground displacements involve significant pre-seismic isolator strains, especially in shorter bridges with isolators having low rubber heights. Conversely, vertical displacements and variation of axial loads have a reduced effect on seismic fragility. In presence of pre-seismic isolator strains, the mean annual frequency of suffering slight damage increases significantly, while the mean annual frequency of reaching a severe damage is less affected. In most of the analysed cases, the effect of the presence of ground displacements has minimal influence on the expected annual loss, provided that pre-seismic shear strains are lower than 0.40.