In recent years, civil engineering applied to infrastructure, bridges in particular, has been facing new challenges arising from the evolution of regulatory frameworks, the increase in traffic loads, and the progressive degradation of structural materials. Many existing bridges, designed before the development of modern seismic knowledge, are now required to meet performance demands significantly higher than those originally expected.

The introduction of recent Italian Guidelines for the classification and management of seismic risk in bridges and viaducts has provided more sophisticated tools for vulnerability assessment. However, important issues remain open, especially the accurate modelling of the structural system, degradation mechanisms, and their influence on the dynamic behaviour.

This study presents the results of a numerical analysis conducted on an existing reinforced concrete bridge with Gerber saddles, aimed at evaluating the impact of degradation in terms of cross-sectional loss, reinforcement corrosion, and concrete cracking, on the global seismic response of the structure.

The analysis reveals that degradation, often underestimated during preliminary assessment phases, can significantly reduce structural stiffness and energy dissipation capacity, increase deformations, and alter the natural frequencies of the system. As the degradation level varies, corresponding changes in structural response are observed. Specifically, under the current degradation conditions, the structure exhibits a strength reduction of approximately 10-15%, along with deformation increases of up to 30%. Moreover, the results demonstrate that the combined effects of material degradation and increased contemporary traffic loads act synergistically, further worsening service conditions and compromising structural performance under seismic loading.