Computational elastoplastic Limit Analysis of the Paderno d'Adda bridge (Italy, 1889)

The Paderno d'Adda bridge is a historical iron arch bridge built in 1889 for connecting the provinces of Lecco and Bergamo across the Adda river, in northern Italy. The bridge was designed through the so-called Theory of the Ellipse of Elasticity. Its use is two-fold: a railway track lays within the inner deck of its upper continuous beam; automotive traffic runs on top. Today, after 128 years of continuous duty, the viaduct keeps in service, with trains crossing at slow speed and alternated one-way road traffic with no heavy-weight vehicles. Starting from a computational formulation for the elastoplastic Limit Analysis of 3D truss-frame systems, apt to provide the exact limit load multiplier and attached collapse mechanism, the full evolutive piece-wise-linear response of the bridge is derived, for different try-out loading configurations. The structural analysis, wholly original in its computational implementation, aims at outlining a survey on the potential plastic collapse characteristics of the bridge. Results reveal possible structural deficiencies for the upper continuous beam, where main plasticizations appeared. Instead, elements of the piers plasticized only for tests with loading much un-symmetrically located to the crown of the arch and the slender doubly built-in parabolic arch rarely showed some plasticization. This evidences the arch as a robust characteristic beautiful feature of the bridge. Moreover, analyses pointed out that the effect of the normal forces on the elastoplastic response of the bridge shall not be considered as negligible, as instead it may be often considered in the Limit Analysis of frames.