PSI - Issue 84

M.P. Petrangeli et al. / Procedia Structural Integrity 84 (2026) 135–142

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Fig. 10 (a). Modern HRC 10.9 bolted joints compared to the out dated original riveted joints (before any restoration)

Fig. 10 (b). 10.9 cl. M24 slip resistant HRC bolts with tension control system.

6. The seismic conditions After the year 2002, with the revision of the Italian seismic hazard map, the city of Roma has been upgraded to 2nd or 3rd (depending on the area) grade seismic zone while, before that, the whole area of the Italian capital was non seismic; this means that all the structures designed before the year 2002, including the “Ponte dell’Industria”, did not consider seismic forces. In this context, it was imperative to improve the seismic behavior of the bridge operating on several aspects, as follows: • even if the overall width of the new bridge is almost doubled compared with the original one, the seismic mass of the deck has not been increased thanks to (i) the substitution of the original RC slab deck with a significantly lighter steel orthotropic plate (ii) the reduction of the pavement thickness from the original 150 mm to just 60 mm strictly required for orthotropic plates; • the new structural deck is a three span continuous beam connected to the substructure with high dumping rubber seismic isolators while the original bridge was a simple spans scheme with fixed or guided metal bearings. A comparison under ULS conditions between the original structure and the retrofitted one shows that, thanks mainly to the isolators, that at the same time dissipate the seismic energy and increase the modal vibrating period of the structure, for the first modal frequency the spectral acceleration decreases from 0.52 g of the original bridge to just 0.11 g after the seismic retrofitting. It should be noted that this important reduction in seismic acceleration would affect only the isolated mass of the bridge, that is the entire deck above the isolator level, but not the substructures; in any case the comparison before and after the retrofit shows a significant reduction of more than 40% for the seismic horizontal ULS forces at the base of the piers with a consequent huge reduction of vulnerability of the original substructure, to be preserved as much as possible by seismic effects but, as per mandatory design bases, to be kept unchanged as they are. 7. Increasing of foundation capacity As mentioned above the pier-foundations are 2.6 m pipes that have been originally driven 14-:-15 meters below the river bed level and could not be modified in cross-sectional shape because of mandatory design bases and also in order to avoid possible adverse changes in scour conditions on the river bed. In any case, meanwhile the restoration works of the down- stream check dam will be carried out, a temporary traffic load limitation of 26 tons, as defined in the “Linee guida per la classificazione e gestione del rischio, la valutazione della sicurezza ed il monitoraggio dei ponti esistenti” (MIT, 2018), has been introduced.