PSI - Issue 84

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

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These curves highlight an initial elastic phase followed by a progressive loss of stiffness due to the formation of plastic hinges in the most stressed elements, until the collapse limit state is reached (see Fig.3). The transformation of the capacity curve of the Multi-Degree-of-Freedom (MDOF) system into that of an equivalent Single-Degree-of-Freedom (SDOF) system was carried out following the N2 Method (Fajfar and Eeri, 2001). The comparison between the displacement capacity of the structure and the seismic demand required by the response spectrum for the Rieti site allowed for the determination of the seismic risk indicators. The analysis shows that the seismic demand exceeds the structural capacity in several directions, confirming the vulnerability of the school complex. The mapping of plastic hinges shows a concentration of damage in the columns of the lower levels, with the activation of brittle mechanisms before the theoretical global ductility is reached. The risk indicators calculated, expressed as the ratio between the maximum sustainable acceleration and the design acceleration (PGA), are significantly lower than unity for the Life Safety Limit State (SLV). This lack of safety is attributable to the low shear capacity of the columns and the absence of adequate construction details in critical zones, typical factors for reinforced concrete structures from the 1970s (Verderame et al., 2011). The results of the pushover analysis validate the need for a seismic retrofitting intervention based on increasing local deformation capacity, confirming the potential effectiveness of reinforcements using steel jacketing to prevent premature brittle failures

(Gkournelos et al., 2021; Riyad S. Aboutaha, 1999). 6. Intervention strategy and final considerations

Based on the results of the pushover analysis (see Fig.4), which highlighted widespread deficiencies in shear capacity and limited global ductility, the intervention strategy focuses on the local reinforcement of the most stressed vertical elements. The adopted technical solution involves the use of steel jacketing for the columns on the ground and first floors. a c

b

d

Fig. 4. Worst-case collapse mechanisms analyzed: (a) Simultaneous plastic hinge formation in the stairwell columns (35-40); (b) Plastic hinge formation in column 38 belonging to Frame 5; (c) Capacity curve for inverse triangular load distribution in the negative Y direction; (d) Results for inverse triangular load distribution in the negative Y direction.