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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1. Historical notes The construction of the bridge over the river Po along the A7 motorway was the subject of a tender announced by the Società per l’Autostrada Serravalle -Milano-Ponte Chiasso (hereinafter only “Company” or “Contracting Station”) in October 1956, with the form of a competition for design & construction, inviting 20 leading construction companies. The award committee concluded its evaluations on May 7, 1957, identifying as best project the one drawn up by prof. Giulio Krall on behalf of the company Ferrocemento , which was judged to be of greater value, such as to justify a higher price than those offered by the other competitors. The documents of the final design that are still available are dated between October 1957 and November 1958; It is reasonable to imagine that the construction was carried out between the end of 1958 and the first months of 1960. The selected project was organized as a “Gerber beam” scheme, isostatic, on 11 spans 69.32 m long, realized using reinforced concrete. The cross section (fig. 2) was a box consisting of 4 main beams with variable section (tri-cell box) of sufficient length to overcome a span and form 2 cantilevers, towards the adjacent spans, each 17.33 m long. The gaps between the cantilevers, with length of 34.66 m, were closed with grid deck, simply resting on Gerber saddles (buffer beams).

Fig. 1. The bridge under construction in 1959

1.1. The appearance of the first cracks

Since the very first years of life, both the box girders and the buffer beams showed the presence of capillary fissures, whose orientation suggested a shear origin. In fact, the main shear reinforcement was highly concentrated being made up of smooth bars F = 40 mm, among other things with inefficient anchors. Other particularly delicate areas showing the appearance of cracks, were constituted by the support saddles of the buffer beams, both for the size of the support base, limited to 38 cm, insufficient also for the anchoring of the reinforcing bars, always with a diameter of 40 mm, and for the positioning of the rebars themselves. Even in correspondence of the upper slab, which at the time had a minimum thickness of 0.16 m, at the supports on the piles, and 0.24 m in the span axis, since 1970, transverse cracks had been observed in the areas corresponding to the piles. Despite the presence of the framework described above, the checks carried out on the structure allowed to affirm that: "the most vital parts of the construction are not compromised", while prescribing the execution of a series of maintenance interventions consisting mainly of the restoration of the slab through the removal of the damaged parts, the integration of the reinforcements and the reconstitution of the concrete cover also increasing its thickness.