Italy is characterized by a complex and highly articulated morphological and orographic landscape, which has necessitated the widespread construction of bridges, viaducts, and tunnels to ensure territorial connectivity. The country is also exposed to multiple natural hazards, including floods, seismic events, landslides, volcanic activity, and the increasingly evident impacts of climate change.

A significant portion of the national infrastructure was constructed in the post-World War II era, particularly during the economic expansion of the 1960s. These structures were often designed according to obsolete codes that did not account for current traffic demands or modern seismic performance requirements.

The collapse of the Polcevera Bridge in 2018 catalyzed a critical reassessment of infrastructure condition and management practices across Italy. In response, the Italian government established the National Archive of Public Infrastructure (AINOP) and issued the "Guidelines for Risk Classification and Management, Safety Assessment, and Monitoring of Existing Bridges" (LG20). These measures support a more comprehensive, risk-informed, and data-driven approach to the evaluation and maintenance of existing infrastructure.

In this context, optimizing the allocation of available resources while ensuring structural safety necessitates the implementation of robust methodologies for assessing the effectiveness of retrofit and maintenance interventions.

This study investigates the seismic performance of simply supported bridges - a prevalent typology within the national bridge inventory - retrofitted with different seismic isolation devices. Drawing on preliminary data obtained from the application of LG20, the research evaluates the conditional probability of failure associated with two different isolation systems (friction pendulum and elastomeric devices) and develops fragility curves for a range of retrofit and bridge structural configurations. The study considers the relative displacements at the supports and the shear in the piers as the seismic demand increases to be amongst the primary verification parameters.

The results indicate that friction pendulum isolators exhibit superior performance relative to other isolation systems, particularly in terms of reducing the probability of exceedance of predefined damage states under increasing levels of seismic intensity.