PSI - Issue 84

Manuel Capogna et al. / Procedia Structural Integrity 84 (2026) 717–724

720

achieved, a Confidence Factor (FC) of 1.20 was adopted for the subsequent vulnerability analyses. This in-depth study confirmed that the main criticalities reside not only in the quality of the materials but especially in the absence of effective stirrups in the joints and the excessive spacing of transverse reinforcement, necessitating local strengthening interventions through steel jacketing techniques to increase shear resistance and overall ductility (Gkournelos et al., 2021; Riyad S. Aboutaha, 1999). 3. Structural modeling and capacity analysis Based on the geometric and structural surveys, the Minervini-Sisti complex is configured as a reinforced concrete framed structure characterized by a marked volumetric articulation. The building consists of different blocks separated by structural joints, with direct foundations consisting of reverse beams. The load-bearing skeleton is made of rectangular columns and beams which, in many cases, are embedded within the floor thickness (shallow beams), limiting the overall torsional stiffness of the floor diaphragm. The floors are made of hollow clay tiles and concrete with a collaborative upper topping, originally designed to ensure the distribution of vertical loads; however, their capacity to act as a rigid diaphragm must be carefully evaluated during numerical modeling for the correct distribution of horizontal seismic actions. The reinforcement validation phase, was conducted by comparing the construction details extracted from the original 1974 technical drawings with the results of direct inspections and pacometric surveys carried out on site. This process confirmed the layout of longitudinal bars and stirrups in the columns and main beams. However, the comparison highlighted the typical deficiencies of buildings from that era: transverse reinforcement is often insufficient to ensure the confinement of the concrete core, especially at the ends of the elements and within the beam column joints. As reported in technical literature, accuracy during this validation phase is crucial, as the mechanical properties of steels from that period exhibit variability that directly affects chord rotation capacity and, consequently, the overall ductility of the structure (Gerardo Verderame et al., 2001; Verderame et al., 2011). The information gathered for the structural assessment, formed the basis for the construction of the Finite Element Model (FEM) using professional calculation software. In this phase, structural dead loads, non-structural permanent loads, and live loads were defined according to the school's functional requirements. The modeling accounts for the cracked stiffness of the elements and the presence of infill walls which, although non-structural, influence the dynamic response of the system. The preliminary structural assessment identified several potential brittle failure mechanisms, with particular reference to the lack of shear strength in short columns and the vulnerability of unconfined joints, confirming the need to proceed with non-linear analyses to quantify the actual seismic risk index of the school (Gkournelos et al., 2021; Riyad S. Aboutaha, 1999). 4. Modal analysis and dynamic behavior Modal analysis was performed on the finite element model to determine the natural frequencies and main vibration modes of the school complex, the graphic details of which are reported Fig.2. The results highlight how the dynamic response is influenced by planimetric irregularity and the stiffness distribution of the frames. The first vibration mode is predominantly translational in the direction of least stiffness, while subsequent modes show a significant torsional component. This behavior is typical of buildings designed in the 1970s for gravity loads only, where the lack of a rigid beam network in both directions promotes coupled responses that can amplify stresses on the perimeter elements. To ensure the significance of the seismic analysis, the output data confirm that the number of modes considered allows for the activation of a participating mass exceeding 85% in the two main directions of analysis, surpassing the threshold prescribed by current regulations. The identified natural frequencies reflect the flexibility of the reinforced concrete structure; however, the interaction with masonry infills contributes to an initial stiffening of the system. The correct identification of the fundamental period of vibration is a crucial step for the subsequent application of the N2 Method, as it allows for the definition of the control point for the capacity curve (Fajfar and Eeri, 2001).