PSI - Issue 84

Stefano Bozza et al. / Procedia Structural Integrity 84 (2026) 686–693

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1. Introduction A large part of current in-service bridges in Italy was built between 1955 and 1975 (Pinto and Franchin, 2010), thus according to outdated standards which proposed less severe design loads than current codes (Bozza et al, 2023). After the collapse of the Morandi Bridge (Calvi et al, 2019) and the promulgation of the Italian Guidelines of existing bridges (MIMS, 2022), the necessity to assess the infrastructure system is growing, and a significant effort have been done in recent years. A multilevel approach was developed and applied to different knowledge levels, starting from a fast risk classification (Salvatore et al, 2024), to the accurate analysis of structures (Galassi Sconocchia et al, 2024). Since a large part of the Italian territory was not seismically classified until 2003 (OPCM 2003), most of existing bridges were designed neglecting the effects of earthquake action. Therefore, several in-service bridges could present components, static scheme or construction details that were not evaluated to sustain dynamic excitation. In this study, the seismic behaviour of a case-study post-tensioned concrete bridge with prestressed piers built in the eighties was numerically analysed, focusing on the effect of the prestressing action in the vertical elements on the dynamic behaviour of the structure. The Finite Element (FE) model of the structure was developed in SAP2000 (CSI, 2016), and nonlinear dynamic time-history analyses were performed, both considering and neglecting the prestressing forces in the piers, and taking into account different damping values. More in detail, in Section 2 a brief overview of the case-study bridge was proposed, while in Section 3 the features of the numerical FE model were described as well as the characteristics of the nonlinear dynamic analyses performed in the study. The results were then finally reported in Section 4, enriched by discussion and conclusions to highlight the influence of the modelling assumptions. 2. Case study bridge The case study is a nine-spans post-tensioned concrete bridge, with end spans 27.5 m long and inner spans 55.0 m long, for a total length of 440.0 m. The balance cantilever construction method was used to assemble precast segments, in order to build a continuous curved box girder on multiple supports. Furthermore, the deck is divided into two parts, presenting a hinged joint in the middle of the central span. The hinge device is formed by three shear keys, two placed in the box webs and one in the bottom slab. The girder section is 17.9 m wide, to account a four-lane carriageway, with an average height of 2.50 m. Since the bridge is curved, a not-constant transversal inclination of the top slab is present. The piers are in prestressed concrete and have a rectangular hollow section with stiffeners at the edges, with outer dimension of 2.3 m x 3.4 m, and a slightly bigger section at the base (from 0.0 m to 3.0 m); the pier caps are made in solid concrete, and their width increase from 3.4 m to 7.0 m in the transverse direction of the bridge in order to accommodate two load bearing devices on each pier. Mechanical properties of concrete, reinforcement and prestressing steel were taken from design drawings and test certificates made during the construction phase. Piers were built using a concrete with a cubic compressive strength of 40 N/mm 2 , while for the box girder a stronger concrete was chosen, with a cubic compressive strength of 50 N/mm 2 . Reinforcement bars are in FeB44K steel, while prestressing tendons were made of steel with a tensile strength of 1800N/mm 2 . Both abutments present multidirectional load bearings devices, allowing longitudinal and transversal displacements. Load bearings on the piers prevent all translations, except for devices next to deck joints, which allow only longitudinal displacements (unidirectional devices). Moreover, all devices are spherical bearing that allow small rotations, bolted on top of the piers and to the bottom slab of the girder. Shear keys at midspan joint are made of steel pins that restrain differential displacement along only one direction. The pins are rigidly connected to one side by means of Dywidag bars, while they are inserted in cavities on the other side. Both horizontal and vertical differential displacement are restrained thanks to the appropriate rotation of the three shear keys, while longitudinal displacements and rotation along vertical and horizontal axes are free.