Assessing the fragility of existing bridges is crucial for a reliable evaluation of seismic risk in transportation networks, for estimating potential losses under seismic scenarios, and for supporting effective infrastructure management and preservation strategies. In the Italian context, a significant number of bridges currently in service, particularly in key road networks, were designed either without consideration for seismic actions or according to outdated standards from the 1970s and 1980s, which often lacked adequate seismic provisions. The seismic performance of such structures is frequently governed by the response of the piers, as the supports were typically overdesigned and, in many cases, have been replaced with modern code-compliant devices without accompanying pier retrofitting interventions.

This study presents a parametric investigation into the seismic fragility of circular bridge piers, accounting for key variables such as the pier slenderness ratio, the span length-to-pier diameter ratio, and both longitudinal and transverse reinforcement ratios. The piers are modeled as cantilevers subjected to the tributary deck mass, and are designed according to historical Italian standards. The selected case studies are representative of both the longitudinal and transverse response of simply supported bridge decks, as well as of the transverse response of intermediate piers in continuous multi-span bridges, which are usually characterized by a dual load path mechanism. The analysis addresses fragility associated with both brittle and ductile failure mechanisms, using pier top displacement as the engineering demand parameter. A series of Multiple Stripe Analyses is carried out to gain insights into the seismic response of piers and to develop fragility curves corresponding to various performance levels, including damageability, life safety, and collapse. Finally, closed-form expressions are derived to estimate the parameters of the fragility probability density functions as a function of the governing design variables, offering practical tools for the seismic assessment of existing bridge infrastructure.