Issue 46

F. Bazzucchi et alii, Frattura ed Integrità Strutturale, 46 (2018) 400-421; DOI: 10.3221/IGF-ESIS.46.37 412 Figure 17 : Involved structure in the collapse: tower and Gerber decks. Figure 18 : resisting mechanism of the Polcevera Tower: equilibrium in the three planes [www.ingenio-web.it ]. (a) ; planar failure surfaces (b) . Existing road infrastructures vs. robustness concept One of the shocking aspects of the Polcevera Viaduct collapse was its catastrophic nature. Even if the causes are currently under investigation, it is clear that the failure of one component has been followed by the entire structure, composed by the tower (green, Fig. 17) and two simply supported Gerber decks (red, Fig. 17). As anticipated, the equilibrium of the system rests upon the balance between the symmetry axes of the tower (Fig. 18a). A lack of balance in one of the two directions of the equatorial planes induces a flexural moment in the deck plane that can lose in-plane stability and could break due to to unbalanced compressive forces (blue, Fig. 18(a)). Under these circumstances, the in-plane twisting could cause the slip of the Gerber decks, that without a vertical support become free to fall. At the same time, a disequilibrium in the plane of the bridge direction induces torsion and bending moment to the double framed A tower (red, Fig. 18a). This structure could not have the ductility capacity to resist at such asymmetric stress, because of the limited rotational capacity at the base (wider) and the lack of redundancy for force redistribution. Moreover, unbalanced forces in the plane of the stays causes deck uplifting on one side and subside to the other. This mechanism, could be again