PSI - Issue 78

Elisabetta Bonaguro et al. / Procedia Structural Integrity 78 (2026) 1016–1023

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3. Partial retrofit on SUs “Partial” intervention refers to floor strengthening of a single SU in one or more specific floors. This approach, related to the vertical subdivision of a building into storeys, is further classified based on its “position” and “number”. In the position-based approach, theintervention is selectivelyapplied to anyfloor in the building. On the other hand, anumber-based intervention consists in progressively retrofittingmultiple storeys, starting from the ground floor up, until the entire SU is involved, thus resulting in a full- unit (“unitary”) intervention. The best performances areachieved bythislatter type of approach, as demonstrated bythecapacitycurves (Fig. 3a) showing a maximum base shear of 2500 kN, approximately 20% greater than that attained by the best-performing position-basedintervention (Fig. 3b):specifically, when thestrengtheningisconfined solelytotheroof, thestructural performance deteriorates, as evidenced by the corresponding curve, which exhibits a steeper slope and much more frequent fluctuations in shear capacity. a b

Fig. 3. Capacity curves of a single SU related to a) number-based application; b) position-based application.

Moreover, when considering thenumber-based approach (Fig. 3a),thereplacement of all diaphragms(twofloorings and theroof) gives an increase of +66%in shear strength compared to the unreinforced model, while in the position based method it is suggested that replacing the second floor leads to better results in terms of base shear and numerical stability in the plastic range. At the same time, an increase in the number of replaced floors corresponds to almost constant increases in shear capacity. In conclusion, it seems that reinforcing a single floor predisposes toexcessively localized stiffening without leading to a real increase in capacity. The tensile strains patterns predicted by DIANA, considered at the elastic limit of each analysis, suggest a damage configuration typically observed in post-earthquake scenarios: in particular, when considering flexible diaphragms, the prevailing damage mechanism is characterized by out-of-plane horizontal bending of the Y-walls (Fig. 4a), which is progressively inhibited by stiffer floor diaphragms (Figg. 4b, c, d). In case of horizontal restraint at both the base and thetop of a wall, due to a stiff slab, the structure exhibits a bi-directional out-of-plane bending mechanism, with enhanced deformations in the mid portion of the wall (Fig. 4d). With roof-strengthening only, the out-of-plane bending mechanism is even more pronounced (Fig. 4e) although concentrated at mid-height of a wall, due to the absence of intermediate rigid diaphragms capable of restraining the overturning of transverse walls.