PSI - Issue 78

Alessandro Fulco et al. / Procedia Structural Integrity 78 (2026) 2046–2053

2050

Fig. 3 Building A: design scheme of the building-exoskeleton connection system

The coupling of the existing structure with external bracing increased the stiffness of the composite system, resulting in a reduction of the fundamental periods and in a three-times increase in overall elastic stiffness. Therefore, the increase in capacity is due to the increased stiffness provided to the structure by the external bracing, which mitigated the seismic actions acting on the existing structure by delaying the activation of collapse mechanisms and also improving the torsional response. Building B The construction is a r/c school building designed in the early 1970s and completed in 1975. It consists of a ground floor, a first floor, and a basement, with a maximum height of 10 m above ground at the highest point of the roof. Its overall footprint comprises a square surface with sides measuring 24,85 m and two additional surfaces measuring approximately 6,75 m and 11,90 m each, one facing north and the other facing east. Analyses carried out for the pre existing situation gove a seismic risk coefficient of 0,30. In this case, the seismic adaptation of the structure, which has to be classified in class of use IV in accordance with the NTC 2018, is expected to be achieved . The intervention strategy is similar to that adopted for building A and therefore involves the use of steel bracing parallel to the seismic-resistant alignments composed of HEB400 profiles and equipped with independent foundations on piles. The connection system is similar to that adopted for building A. In Fig. 4 the plan and elevation of the building are shown. Also in this case, the building's comprehensive bracing interventions significantly increased the system stiffness, mitigating the seismic actions acting on the existing structure and delaying its collapse mechanisms. The structure natural periods were reduced from 0,40 s (pre-existing building) to 0,15 s (strengthened building), resulting in an overall stiffness increase approximately sevenfold compared to the existing conditions. The intervention therefore allowed the achievement of full seismic retrofitting conditions.

Fig. 4 Building B: Plan and elevation

Building C An orthogonal endoskeleton was designed for Building C. The building, used as a preschool and nursery school, was designed and constructed between 1973 and 1974 using prefabricated r/c panel structure cast on-site. The structure has an irregular L-shaped plan consisting of two main blocks and a central connecting block. The building is generally single-story, with the exception of a few double-story portions on the main blocks, where turrets for lighting the internal classrooms stand out. The design interventions were based on the results of seismic vulnerability assessments of the structure, which gave a risk index of 0,25. The seismic improvement project includes the installation of external bracing systems for the structure, achieved through the insertion of new steel structures, resting on new foundations