PSI - Issue 44

Giuseppe Brandonisio et al. / Procedia Structural Integrity 44 (2023) 1316–1323 Giuseppe Brandonisio et al. / Structural Integrity Procedia 00 (2022) 000–000

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used: B-type frames, which consist of three pilasters, spaced 8.60 and 5.60 m respectively, and two continuous beams, standing at (+) 3.40 m and (+)7.20m from the first floor slab; the B’-type frames, that are similar to the previous ones, except for the absence of the central pillar that leads to having a continuous beam of 14.20 m. Along the southern short side, other two type of transversal frames are used: C-type frames, which consist of three pilasters, spaced 10.90 and 6.80 m respectively, and two continuous beams, standing at (+) 3.40 m and (+)7.20m form the first floor slab; C’-type frames, which are similar to the previous ones, consists only of three pilasters and a continuous beam at the top (+7.20 m), without intermediate connecting horizontal elements. The whole structure stands upon deep foundations, made of plinths on piles connected by foundation beams. The structure has been interested in the same renovation works in the recent years. In 2001some internal partitions were replaced by thinner plasterboard elements, to redefine interior spaces. In 2010, upon the original plan roof, it had been introduced a light structure made of steel frames, connected at the top by metal sheets. Considering the overall building configuration, the “floating plane ensures the structural continuity between portions having different stiffness, i.e., a wide upper steel part more deformable than the small lower R.C. Resulting from a gravity-load design approach, without taking into account seismic actions, this monumental building is highly irregular at each level, with a noticeable asymmetry due to the shear walls distribution. They are the only seismic-resistant elements at the upper levels, guaranteeing a certain horizontal stiffness. In this case, the deformable steel frames are partially braced by slender steel elements, not placed in correspondence with the lower pillars.

Fig. 1. Plan view of the building: (a) distribution of R.C. pillars and shear walls at the first level and (b) detail of floating plane.

3. Seismic vulnerability of the structure at the current state

Seismic assessment of the structure at the current state has been evaluated through accurate 3D models closed to the actual building configuration. The analyses have been performed by using the software CDS-WIN (Computer Design of Structure s, S.T.S. S.r.l., 2021) as in Fig. 2 (a) and SAP2000 (vers.23) as in Fig.2 (b) Linear static and dynamic analyses have been done to evaluate seismic force distribution between different structural elements in x direction and y-direction. Fig. 3, Fig. 4, and Fig. 5 synthesize the seismic force distribution between different the vertical structural elements. In particular, Fig. 3 refers to the third level. When the seismic force acts in x-direction, Fig. 3 (a), the two shear walls bear 62% of the shear force, while the remaining one is absorbed by the steel frames. Similarly, for seismic force in y-direction, Fig. 3 (b), the shear walls bear 73% of the seismic action, while the remaining one is absorbed by the steel frames. Fig. 4 refers to the second and third levels, Fig.4 (a) associated to horizontal force in x-direction and Fig. 4 (b) related to horizontal force in y-direction. In this case, 92% of the seismic action in x and y directions is carried by shear walls, reducing to 8% the contribution of steel frames. Fig.5 shows shear distribution at the base. When the seismic force acts in x-direction, Fig. 5 (a), the 55% is carried by the shear walls, the remaining 45% by the R.C. pillars. Considering the seismic force in y-direction, Fig. 5 (b), the