PSI - Issue 44

Brandonisio Giuseppe et al. / Procedia Structural Integrity 44 (2023) 1292–1299 Giuseppe Brandonisio et al. / Structural Integrity Procedia 00 (2022) 000–000

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force distributions in transversal direction; in this case, the first collapsing element is the shear wall of the external staircase, suffering for shear failure.

Fig. 3. Collapse mechanism for the building 3 by pushover in X+-direction (left) and seismic vulnerability assessment in ADRS plane (right).

Table 1. Results of pushover analyses for buildings 1, 2, and 3.

Distribution of lateral forces

z E =PGA C /PGA D

Pushover

Building 1

Building 2

Building 3

1 2 3 4 5 6 7 8

Modal: X+ Modal: X- Modal: Y+ Modal: Y- Masses: X+ Masses: X- Masses: Y+ Masses: Y-

0.063 0.063 0.173 0.134 0.071 0.063 0.181 0.134

0.442 0.442

0.423 0.421 0.205 1.464 0.433 0.418 0.205 1.464

0.11 0.11

0.637 0.637 0.126 0.126

For “Building 3” the lowest value of seismic vulnerability index, z E = 0.205, is obtained for modal and mass distributions (considering that it is a single-storey building); in this case the first collapsing element is a beam at the intermediate level, undergoing shear failure. From previous evaluations, it is clear that these three buildings have a high seismic vulnerability. In fact, the vulnerability index is always lower than one ( z E < 1), at least for one of the considered distributions of lateral forces labelled in Tab. 1. This means that it is necessary to look for seismic upgrading, or better yet, retrofitting of the existing school complex. 4. Hybrid solution for seismic retrofitting Base isolation is an effective strategy for seismic retrofitting of existing building (De Luca et al., 2001) It consists in introducing at the base of the structure anti-seismic devices (rubber bearings or sliders), i.e. flexible elements, very stiff vertically and soft in horizontal direction, able to decouple the upper structure form the ground. This system gives the possibility to increase the fundamental vibration period of the structure and to reduce the spectral accelerations estimated for the design earthquake, as function of the site where the building stands. For the school complex, previously described, a “hybrid” strategy for seismic retrofitting has been proposed. Authors have opted for such kind of solution also in other cases (De Luca et al., 2015) (De Luca et al., 2015) (De Luca, Guidi, 2019) (De Luca, Guidi, 2020). This strategy works on a double front: from one side it acts as a filter of seismic forces through base isolation system (BIS) (De Luca et al., 2022); from the other side it neutralizes the low seismic forces transferred to the upper structure by using ad hoc shear walls, isolated at the base. More in detail, the seismic retrofit of the existing complex is obtained through: (a) the connection of three bodies at each floor level; (b) the strengthening of the upper structures by using shear walls, 30 cm- wide; (c) the introduction of an isolation system, made of HDRB 600, at the base of the strengthen whole complex.