Fragility analysis of prestressed concrete girder bridges affected by corrosion under traffic loads

Italian and European transportation networks include a considerable number of existing bridges, built since the early ‘60s, characterised by simply supported prestressed-concrete (PC) girders with post-tensioned steel tendons. Corrosion of tendons, which may lead to significant loss of structural capacity, cannot be detected by simple visual inspections and requires advanced and expensive testing by bridge owner companies. Therefore, procedures aimed at risk-informed prioritisation for network-scale assessment are needed, to provide fundamental support for drawing up priority lists to be followed for an optimal allocation of resources to be employed for advanced inspections and possible retrofit. The thesis presents a study on the fragility of existing PC girder bridges considering traffic loads, accounting for corrosion-induced effects. An automated framework is proposed, aiming at the efficient probabilistic structural assessment of the investigated bridge class accounting for 1) the influence of knowledge-based uncertainty related to geometric and mechanical properties and 2) different scenarios including diverse critical corrosion levels and code-compliant traffic load models. A simplified analytical method and a refined finite element method are adopted as structure modelling strategies. To simulate corrosion effects, geometric and mechanical characteristics of prestressing steel tendons are modified by reducing the steel area and stiffness, ductility and strength properties. For analytic calculations, such modifications are integrated into a specific algorithm able to estimate variations in flexural and shear bearing capacity of critical cross-sections. In the thesis, the framework is tested with reference to a dataset of case-study superstructures with different span lengths and numbers of beams. The obtained fragility curves are deeply discussed, focusing on the corrosion effect on structural fragility and highlighting the variation of the corrosion-induced increase in fragility among the selected bridge archetypes. Although the proposed methodology presents some simplifications, it could improve the current practices of risk prioritisation, by supporting transportation authorities in ensuring the safety of the existing bridge stock.