Collapse of Existing Bridges: From the Lesson of la Reale Viaduct to the Definition of a Partial Safety Coefficient of Variable Traffic Loads

From the post-war period to present day, economic development and urbanization have led to various demands for infrastructure, including bridges and viaducts. However, especially in recent years, there have been reports of sudden and unexpected collapses of bridges and viaducts, associated with large economic losses and, unfortunately, loss of human lives. The study of the causes that have led to these disasters is of strategic importance because it allows for the deepening of the specific technical knowledge. For this reason, this paper presents the study of the causes that led to the collapse of one of the spans of the "La Reale" viaduct in the Piedmont region in Italy. The main reason for the collapse was to be found in the incorrect injection of cables, which led to the absence of grout inside the sheaths for a length equal to one third of the deck. In this region, cables were subjected to a highly aggressive environment with severe corrosion. This resulted in fragile failure of the deck, with no signs that could suggest such important structural problems during normal inspection operations. The only evidence was some white efflorescence. However, in light of the knowledge prior to the present study, this evidence could not be easily connected to the type of phenomenon in progress that led to the collapse. Based on the experimental tests carried out on the collapsed portion, this study highlights new symptoms not associated with any structural pathology previously studied. The main concern relative to the structural safety assessment in cases such as the one presented in this study is related to the lack of evidence on the current state of cable degradation. Therefore, objective data for numerical evaluations is coming from bridge load tests. In this regard, the evaluation of a partial safety coefficient of variable traffic loads according to two different approaches based on bridge load tests and safety factors suggested by structural codes is presented.