Non-linear Finite Element Analyses for the Assessment of the Structural Performance of Existing Bridges

Bridges are fundamental components of transportation networks, ensuring safe passage across natural and artificial obstacles such as rivers, valleys, and roadways. In Italy, a large portion of the existing infrastructure network consists of reinforced and precast concrete bridges and viaducts, many of which were constructed before the introduction of modern seismic and durability design standards. Reinforced concrete (RC) bridges can be classified according to their structural typology, including arch, tied-arch, integral (frame), beam/girder, cable-stayed, suspension, cantilever, and truss systems. Each typology exhibits distinct structural behaviour, load-transfer mechanisms, and vulnerability characteristics. This thesis focuses on RC girder bridges, as they represent the most widespread configuration within the Italian infrastructure stock. Bridges are subjected to multiple actions and hazards, both natural (e.g., earthquakes, floods, fluvial erosion, landslides) and technological (e.g., traffic loads, fire, explosions, vehicle impacts). These can occur independently or in combination. The present research investigates the seismic performance of RC girder bridges under combined multi-hazard scenarios, specifically considering scour-induced foundation weakening, hydrodynamic actions due to water, and corrosion-induced material degradation. The influence of corrosion on the mechanical behaviour of bridge components such as cross-sectional loss, reduced confinement, and reinforcement buckling is explicitly accounted for. Despite recent advancements in the Eurocodes and other design frameworks, many existing bridges were designed without explicit consideration of time-dependent deterioration or multi-hazard interaction. This limitation restricts the accuracy of structural assessments and the effectiveness of retrofitting and maintenance strategies. Furthermore, Italian guidelines lack comprehensive provisions addressing the influence of pier height irregularity and deck-abutment boundary conditions on the global seismic response of bridges. To address these gaps, the study employs nonlinear finite element (FE) modelling, pushover analyses, and fragility curve development to assess the performance of representative girder bridge case studies with varying static schemes and pier configurations. The analyses incorporate the time-dependent effects of scour and corrosion, allowing evaluation of the evolution of structural capacity and seismic vulnerability throughout the bridge’s service life. The results provide a robust framework for performance-based seismic assessment in accordance with Eurocodes and Italian codes, supporting the calibration of limit state criteria for aging infrastructure. Moreover, the findings can contribute to improving current assessment methodologies, offering recommendations for updating national guidelines and enhancing the resilience and safety of existing RC bridge systems.