PSI - Issue 84

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

689

The masses considered in the analyses are calculated from self-weight of structural elements and permanent weight of pavements and barriers, estimated equal to 43 kN/m. A normal traffic on the bridge was assumed, thus no quasi permanent additional loads were considered, since 2 =0 for normal traffic loads. In order to investigate the influence of the prestressing action in the piers on the dynamic behaviour of the structure, two different models were developed. In each model, tendons were modelled as elements. In the reference model, an initial prestress of 1250 N/mm 2 and stress losses of 375 N/mm 2 were assigned to the tendons. The stress losses account for creep effects, concrete shrinkage and steel relaxation, for a combined effect of 30% the initial prestress action. In the second model, no prestressing action was assigned to the tendons, representing either a total loss of the prestressing action or no application of prestressing forces in the piers. In each time-history analysis, the ground motion was uniformly applied at the base of the piers in all three directions. A first set of seven analyses were performed applying the east component of a recorded ground motion in the X direction, the north component in the Y direction and the vertical component in the Z direction, then a second set of seven analyses were performed rotating by 90° the horizontal components, for a total of 14 time-history analyses for each model considered. In the reference model, a 2% damping was assumed (value suggested by EN 1998-2:2005 for the equivalent viscous damping ratio for prestressed structures), while a 5% was used in the second model (value suggested for reinforce concrete elements). To distinguish the effects on the dynamic behaviour of the structure due to the pre-stressing action of the piers and those related to the different damping adopted, further analyses were performed assuming a 5% damping in the reference model, and a 2% damping in the second model. In all the analyses, the damping was modelled as viscous proportional damping between the first period (1.2 s) and the last significant period (0.2 s). Spatial variability of the ground motion was neglected, since preliminary studies have suggested that it has little influence on the global response of the structure for the case study considered. 4. Results and discussion Maximum forces and displacements were evaluated via nonlinear time-history analyses, focusing on load bearings, shear keys and deck joints. In the next paragraphs the results are reported for each model considered, and discussed in terms of mean maximum displacements both along restrained and free degrees of freedom. 4.1. Load bearings and shear keys restrained displacements For unidirectional bearings (placed in PI01, PI04, PI05, PI08), the restrained displacement is the transverse deformation of the bearing, while for fixed bearings (PI02, PI03, PI06, PI07) the restrained displacement was considered as the maximum between longitudinal and transversal deformation. For shear keys (labelled SK 1, SK 2, SK 3), the displacement along the restrained direction was considered. The mean maximum restrained displacements in load bearings and shear keys, as well as their elastic threshold dy, are reported in Fig. 1.