PSI - Issue 62

Giuseppe Colombo et al. / Procedia Structural Integrity 62 (2024) 377–384 Colombo, Mariani, Lambrugo, Giardina/ Structural Integrity Procedia 00 (2023) 000 – 000

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• Interlocking between the box girders and the buffer structures, with elimination of residual excursions in of the expansion joint. • Severe localized damages to the concrete.

• Oxidation and corrosion of unprotected reinforcement. • Damage to the elastomeric bearings on the saddles.

To cope with the reported damage, to restore the functionality of the elements and ensure an adequate degree of protection, a radical intervention was planned on all the saddles of the viaduct that required, to ensure full accessibility to all the surfaces of the saddle and allow the necessary repair work to be carried out, the lifting of the buffer spans to a height of about 180 cm. The intervention, in fact, consisted in the removal of the concrete cover from all surfaces, vertical and horizontal, both of the box girders and of the buffer beams, the replacement and integration of corroded rebars, the restoration of surfaces with special mortars, and the installation of new reinforced elastomeric bearings. The lifting operation was carried out with an equipment specifically developed for the viaduct over the Po, which allowed lifting "from above" using 8 strand jacks mounted on a metal structure which was fixed to the ends of the cantilever of box girders. Once a buffer beam, with a weight of 1400 tons, was repositioned on the saddles, the system was able to move along the deck to reach the lifting position of the next buffer beam. 2.4. Seismic retrofitting The natural plano-altimetric configuration does not determine significant differences in stiffness between the piers that support the deck, however, the problems due to the reduced shear strength of the same piers, amplified by the large mass of the deck, suggested the adoption of a seismic isolation solution. The solution was achieved by replacing the metal pot bearings with new seismic isolators in reinforced rubber with energy dissipation of more than 10% for isolated period and fundamental period around 2 seconds. The dissipative elastomeric isolators, inserted at the top of the piers, at the level of disconnection of the structure, allow the relative horizontal motion between the part of the structure above the devices (superstructure) and the part below fixed to the ground (substructure). From a technological point of view, the decoupling between the horizontal motion of the superstructure and that of the substructure takes place thanks to the shear deformation of the rubber. The longitudinal axial continuity was achieved through the installation of dynamic-hydraulic restraint devices, called shock transmitters , interposed between buffer beams and box girders. Such devices can create a very rigid constraint in the face of a dynamic action, while allowing the slow movements of the structures, for example those produced by thermal variations, without offering appreciable resistance (fig. 6). On the other hand, the regularization of the transversal behaviour of the deck was obtained with the insertion of transversal shear keys, mounted at the buffer beam-box girder interface. Finally, the counterweights at the ends of the deck were freed from the surrounding ground to make them free to move in the event of an earthquake. To contain the ground, it was necessary to build a new wall with reinforced concrete diaphragms 25 m deep, 1 m thick and with a T-section, that would isolate, together with the existing wing walls, the excavation volume.

Fig. 6. Shock transmitter