PSI - Issue 78

Alessandro Fulco et al. / Procedia Structural Integrity 78 (2026) 2054–2061

2056

In the above-ground stories 400x400 mm square columns are mainly present, while at the basement 400x500 and 400x600 columns can be found.. The beams are both lowered and slab-thick with different sections as the various decks. Finally, it should be noted that the structures at the basement story are effectively connected to the r/c perimeter walls, that therefore determines a significantly higher level of rigidity than the rest of the structure in elevation. The connections between the retaining walls and the r/c frames are of two types: the first one is made of 400x500 beams connecting the frames to the wall (shown in blue in Fig. 1 b), while the second type consists of the columns incorporated directly into the r/c wall (shown in red in Fig. 1 b). A typological carpentry of the structure is shown in Fig. 1 a), while longitudinal and cross sections are shown in Fig 1 c) d).

Fig. 1 a) typological carpentry of the structure; b) schematisation of the interferences in the basement floor; c)-d) Structural sections: longitudinal and transversal

2.1. Preliminary criticalities and global vulnerability Preliminarily, it can be observed that the structure presents the shortcomings related to a structural concept that has not yet been updated to the latest seismic standards that are subsequent to the design date (1992). Therefore, the evaluations of horizontal seismic actions and the definition of construction details are not fully adequate. With regard to the global structural configuration, on the other hand, a number of criticalities are evident: (i) irregularities in plan, mainly, and in elevation with significant variations in masses and stiffnesses at the upper levels; (ii) eccentricity of r/c stair cores and other high-stiffness elements that determines a significant distance between centre of stiffnesses and centre of masses in both directions at all the decks. This configuration implies: significant torsional effects in the structure response; inadequate amount of reinforcement in the r/c walls of the cores; inadequate amount of reinforcement in the columns subjected to extra-forces induced by the torsional behaviour of the building. Seismic vulnerability analyses were carried out on the structure in accordance with NTC (2018) by means of linear analyses with response spectrum. The analyses show how the structure is strongly conditioned by a torsional response due to the presence of the r/c stairwell in a position that reduces the overall torsional stiffness of the structure. Furthermore, the configuration of the structure denotes a significant distance between the centre of gravity of the masses and the centre of gravity of the stiffnesses. These aspects determine parasitic stresses on the structural elements that are more centrifugal with respect to the centre of gravity of the stiffnesses and a concentration of stresses on the elements of the stairwell, since the latter have greater stiffness. The response of the structure is therefore characterised by critical aspects such as to prejudice its redundancy and dissipative capacity. Indeed, the concentration of the forces on the stairwell and the parasitic effects due to the torsional effects can trigger fragile and/or local mechanisms determining an early achievement of the global ultimate capacity. By carrying out iterative analyses, reducing the design spectrum until the conditions for satisfying the verifications are obtained, the safety assessment shows inadequate results with values of the seismic vulnerability coefficient ζ E below 0,30. Local criticalities were also found with respect to the overturning mechanisms of claddings with Capacity/Demand ratios < 0,10. It should be noted that the dynamic behaviour of the structure is characterised by oscillation modes having periods lower than 0,50 s and therefore characterised by spectral response accelerations corresponding to the maximum values of the plateau, in this case close to 0.65 g. These values determine a seismic base shear approximately equal to 6500 kN in both directions, not compatible with the shear and flexure capacities of the seismic-resistant elements. Together with the vulnerabilities with respect to the SLV, similar criticalities were found with respect to the SLO with story drift greater than 5‰. Such high stor ey drift values, that at SLV are even higher (close to 10 ‰), also highlight an extremely high level of expected damage to both structural and non-structural elements, such as not to guarantee operability and/or functionality of the building during and after the emergency phase.