Evaluation of the long-term residual safety of a reinforced concrete bridge: a case study

The present work proposes a comprehensive study of the safety evaluation of a reinforced concrete bridge in terms of the actual and expected degradation status. A bridge constructed in the early 1930s over the Cassibile River in Sicily, Italy, is selected for consideration. The study starts with a safety assessment based on the original design codes in use at the time of construction and then follows, by an exegetical approach, the evolution over time of the prescribed loading and design rules, both under the hypothesis of the undamaged and degraded structure. Therefore, the study examines the effects of increased traffic loads with the evolution of the code rules and, specifically, as defined by the Italian Technical Standards (NTC18), on a theoretically undamaged bridge. The results show a reduction in the safety factor across all critical structural components, apart from the lateral beams. These latter beams, in fact, benefit from a more comprehensive consideration of the overall resistance of the section, rather than just the localized stress values. Notably, the transition from the stress-based approach (Allowable Stresses Method, ASM) to a capacity-based evaluation of the full cross-section, as prescribed by NTC18, has minimal impact on elements subjected to purely axial loads, such as the hangers. Overall, the study aims to contribute to the understanding of the expected behaviour of an old structure facing the evolution of acting loads and allows the authors to restate, among other facts, the need to consider the expected degradation phenomena beyond the simple visual findings from surveys.