THE REHABILITATION THROUGH EXTERNAL PRESTRESSING OF HISTORICAL REINFORCED CONCRETE BRIDGES WITH REDUCED PERFORMANCE: A CASE STUDY

Many existing reinforced concrete bridges exhibit behaviour at the Service Limit State and the Ultimate Limit State which can be considered unsatisfactory with respect to the current provisions of Codes, but which are actually deficiencies deriving from obsolete calculation methods, structure age, material degradation, diffuse or localized corrosion and increased loads. Among these, cantilever bridges with half-joints may present a decrease in global safety coefficients for the most stressed current sections of the deck or in the local ones, that affect the performance of elements sensitive to degradation, such as Gerber saddles. In these cases, simple strengthening interventions through external prestressing that reduce the tensile stress in concrete areas subject to cracking, as well as the deformability of the points that have accumulated displacements over time (due to creep or degradation), can often be implemented without the need to recourse to interventions of greater impact, obtaining an improvement in the structural behaviour or a variation of the static scheme that improves the overall performance. This is the case of a concrete Niagara-type cantilever bridge with half-joints, designed and built in the 60s in southern Italy by Riccardo Morandi, presented here as a case-study, which does not show a very high or widespread state of decay but rather a slight insufficiency of some elements, in particular related to the cantilever bridge behaviour. A methodology is proposed for the safety assessment of these bridges and a solution for improving performance through minimal rehabilitation interventions with the introduction of external prestressing. The results of the analysis in the present state, the deficiencies found as well as the possible level of increase in the safety coefficients at ULS with the proposed intervention are presented.