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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Keywords: seismic retrofit, R.C: building, hybrid strategy, base isolation system, “all rubber” solution, structural strenghtening.

1. Introduction Within the European countries, 80% of the buildings is over 30 years old, 40% was built before the 60’s, with a third of buildings over 50 years old (Gkournelos et al., 2021) (Economidou et al., 2011). Many of these existing structures are the results of a gravity-load design approach, prior to the introduction of seismic codes. In particular, in Italy, a whole seismic country, many the existing R.C buildings date back to 1950-60s: their potential deficits in seismic capacity can be related to common design practice of their time. This makes necessary a preliminary evaluation of their seismic vulnerability (Calvi et al., 2006) (De Luca and Verderame, 2021), later looking for the appropriate strategy for seismic upgrading, or better yet, retrofitting. For the seismic retrofit of existing structures, base isolation systems (BIS) have often been used, as confirmed also by their application to historical and strategic buildings (De Luca et al., 2001), (De Luca, Guidi, 2019) (De Luca, Guidi, 2020), (Terenzi et al., 2020). Referring to seismic retrofit of R.C. buildings, this paper argues about a hybrid procedure involving the use of base isolation system. The case study is a high school building in Naples, built at the end of 1970s, without anti-seismic design criteria. The structural complex consists of three jointed portions, all R.C. framed structures. The peculiar aspect of these buildings, common to other existing structures dating back to the same construction age, is the absence of resisting frames in longitudinal direction. This leads this school building to have a low seismic capacity longitudinally and a high lateral deformability (fundamental vibration period of T AS-IS =1.37s). In this case, seismic retrofit by using base isolation system is necessary to improve its seismic response. In fact, at the current state, the existing details cannot guarantee the structural components to face the effects of plastic range. On the other side, the main aspects of modern capacity design approach are not complied with between dissipative and no-dissipative components. For these reasons, it is assumed that the current structural system has no ductility reserves, on local and global scale. For seismic retrofit of the existing structure an “all rubber” base isolation system has been proposed, later described. To maximize the effectiveness of the BIS, the three bodies have been connected at each floor level and the upper structure has been strengthened previously. In this way, the use of the new shear walls guarantees to reduce the fundamental vibration period from T AS-IS =1.37s to T FB =0.30s, while the application of BIS allows to reach a T ISO = 4.0s. This is a typical example of hybrid strategy for seismic retrofitting. It has been obtained by merging two conceptually opposite design strategies, i.e. a base isolation system applied to an existing structure, previously stiffened ad hoc by using shear walls. 2. The case study The building complex houses the High School “R. Caccioppoli” in Naples (NA). Built at the end of 1970s, it has four floors above the ground, with a maximum height of 14.00 m. It is characterized by three jointed portions: “Building 1” and “Building 2” have a rectangular plan of 50m x 20m; the exception is given by the second floor, whose longest side is about 70m. “Building 3” corresponds to the gym, it has a square plan (25m x 25m), with a height of 6.70m, as visible in Fig. 1. All the buildings are made of R.C. framed structures. “Building 1” and “Building 2” are characterized by resisting frames only in transversal direction. These ones support the actions transferred by the floors, whose load-bearing elements are warped longitudinally. In transversal direction, it seems plausible the presence of edge-beams included in the slab thickness, in correspondence of concentrated loads due to the internal partitions. In these structures there are also some R.C. shear walls. “Building 1” has two transversal shear walls in correspondence of the inner staircase and a longitudinal one for the outside staircase. “Building 2” is characterized by a single transversal shear walls in correspondence of the outside staircase. “Building 3” is made of resisting frame in x direction, 5m-spaced from each other. They are characterized by two different span lengths, 21 m-long the widest, 3 m-long the shortest. The roof slab has joists in y-direction along the widest spans and in x-direction along the smallest ones. The inner frames are characterized by extrados beams on the long side (21 m), while the roof slab is inclined for a short stretch, giving the possibility to create large windows. An interesting aspect of the “Building 3” is represented by the bleachers, that consist of R.C inclined slabs, standing at two staggered levels, 2 m and 3.50 m. This inclined slab transfers loads to different structural components: on one side it is linked to shear walls, on the other one it transfers loads to a downstand beam, later connecting to the R.C. floor slab of the shortest frame spans. These three