PSI - Issue 78

Giada Frappa et al. / Procedia Structural Integrity 78 (2026) 17–24

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that would be adopted for a new building with the same configuration. To determine its value, verifications of both ductile and brittle failure mechanisms of columns and walls were carried out based on the results obtained from the Fast Nonlinear Analysis. As regards columns, all of them were verified for ductile failure mechanisms based on their rotational capacity. However, all 24 columns (100%) failed the brittle shear verification in the X direction, and 4 in the Y direction (17%), The latter were the columns located on the perimeter frame of the west side of the building, farthest from the center of rigidity (Fig. 1b), Among the seven structural walls and the elevator core, only the elevator core failed to meet the requirements for ductile failure mechanisms, as assessed through the combined axial force and bending moment verification. All walls and the elevator core satisfied the brittle failure verifications related to shear-compression interaction. However, none of them met the verification criteria at the base for both shear-tension and sliding shear. Based on all previous results, a seismic performance index ζ E = 0.34 was obtained. According to the NTC 2018 provisions, the seismic rehabilitation of the building must ensure a final value of ζ V = 1.0 and ζ E ≥ 0.6, as the structure is intended for school use. 4. Seismic retrofitting interventions Based on the specific structural deficiencies identified and the targeted performance levels, a retrofit strategy was developed combining a global intervention, based on the use of supplemental energy dissipation systems, with localized strengthening measures. The objective of the interventions was to improve the overall seismic performance of the structure by regularizing its dynamic behavior through the reduction of the eccentricity between the center of mass and the center of rigidity, and by increasing the capacity against brittle failure mechanisms. Furthermore, another key objective was to minimize the invasiveness of the interventions as much as possible, in order to limit disruptions to the building functionality during construction. 4.1. Intervention with external steel bracing frames and effects The global intervention is implemented through the installation, along the perimeter of the building, of external steel braced frames connected to the existing structure at the level of the floor beams (Frappa and Pauletta, 2022). Installing the bracings within external frames, rather than within the internal structural bays, is advantageous because the work can be carried out from the outside, without interrupting the building's use. In any case, the placement of the bracing systems must take into account architectural constraints, as well as the need to preserve the usability of interior spaces and the functionality of circulation paths. For the building considered, particular attention must be paid to the presence of the gymnasium block, which is directly connected to the east side of the building, and a covered walkway extending from the north side (Fig. 1b). Based on the building configuration, it is proposed to install two steel frames with rigid bracings along Y direction on the entire east façade (Fig. 6b). The choice of using rigid bracings stems from the need to introduce stiffness capable of counteracting translational displacements and torsional effects by shifting the center of rigidity toward the left. This intervention is intended to improve the shear response of the columns located along the west façade, which currently fail verification due to the torsional effects induced by the eccentricity between the center of rigidity and the center of mass. In the X direction, the proposed intervention involves the installation of three steel frames equipped with dissipative bracings, two on the south façade and one on the north façade. This measure is intended to improve the shear behavior of the columns in the X direction. The reduction in shear demand is achieved through both energy dissipation and the decrease in eccentricity between the center of rigidity and the center of mass, due to the position of the bracings. For the vertical elements of the steel frames, HEM 260 sections are adopted, while HEB 260 profiles are used for the horizontal members. The diagonal elements consist of circular hollow sections with different properties: for the rigid bracings, profiles with a diameter of 219.1 mm and a thickness of 20 mm are selected, whereas for the dissipative bracings, sections with a diameter of 168.3 mm and a thickness of 12.5 mm are used. All steel profiles are made of S275 grade steel. The proposed dissipative devices are Buckling-Restrained Axial Dampers (BRADs), installed along the members of the inverted V-bracing frames. These devices dissipate seismic energy by exploiting the stable hysteretic behavior