Forensic estimation of the residual capacity and imposed demand on a ruptured concrete bridge stay at the time of collapse

Subsequent to a given structural collapse, forensic efforts aimed at identifying the root cause behind the observed failure are typically deployed, not only within the framework of responsibility seeking endeavours, but also, and perhaps more importantly, with the objective of drawing lessons that will hopefully prevent a repetition of a similar ruinous event in the future. Such forensic analyses typically involve the combined employment of both analytical/numerical, as well as empirical approaches, whereby results derived from numerical models are compared against measurements or observations not just during the collapse, but also in the days or weeks prior to such event. This type of forensic back-analysis approach is herein applied to the case-study of the Morandi bridge, which, on 14th August 2018, collapsed suddenly into the Polcevera river due to the rupture of one of its stays. The residual axial-flexural (N-M) capacity of this failed stay is first estimated considering two different hypotheses on the level of cable corrosion (in the location where such defect, unknown prior to the collapse, was subsequently detected), through the use of fibre sectional analyses that are subsequently verified through the employment of advanced 3D discrete element modelling. Then, comparisons with pre-collapse loading scenarios are used to identify the one of the two alternative N-M capacity estimations that is most likely to characterise the residual capacity of the stay, which is then finally juxtaposed against the N-M demands induced by the different loading scenarios observed or hypothesised as having potentially occurred at the time of the bridge collapse.