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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1. Introduction

Seismic isolation is a widely used and effective strategy to mitigate the strong-motion impact on new and existing structures. As argued by the authors (De Luca et al., 2001), in the last decades, a significant contribution to the improvement of this technology has derived from the application of IS-systems in the retrofit of existing buildings, originally designed to bear vertical loads. As occurred in the U.S., the devastating effects of the Loma Prieta (1989) and Northridge (1994) earthquakes encouraged this approach also to safeguard cultural heritage structures. Different solutions were proposed to protect prestigious historical monumental buildings, as in the cases of Oakland City Hall in 1995, San Francisco City Hall in 1998, and Los Angeles City Hall in 2001. As described below, these interventions show an improving design approach in using rubber bearing solution (Brandonisio et al., 2017) (Brandonisio et al., 2019) (De Luca et al., 2022) if compared to previous BIS applications. Referring to the technical evolution of base isolation systems (De Luca et al., 2019), to counteract the unexpected spectral values of acceleration and displacement (Miyazaki, 2008), a conscious effort in BIS design looks for the use of larger (rubber) devices to get longer vibration periods. Adopting a reduced number of bearing points, such wider devices (up to 1300 mm diameter) can accommodate the expected large horizontal deformations. Progress in this direction can be seen in the following examples. In the case of Oakland City Hall, the retrofit solution was designed by Forell and Elsesser (2003) in 1995. A hybrid strategy was adopted, opting for a base isolation system (T FB = 1.30 s; T ISO = 3.20 s; d H = 43 cm; f 940mm). It was combined to strengthen actions on the super-structure, by introducing new shear walls and a new system of horizontal steel braces forming a “ diaphragm ” below the first floor to transfer lateral loads to a system of 113 elastomeric bearings. Also, for San Francisco City Hall base isolation appeared to be the most suitable solution, with the minimum intrusion to ensure maximum seismic protection: a system of 530 lead plug rubber bearings was installed ( f 700 mm), guaranteeing a design displacement d H = 65 cm for T ISO = 2.80 s. Los Angeles City Hall is another paradigmatic example of seismic retrofit, being a 32-storey building with a total height of 140 m. A base isolation system was used to reach a T ISO = 2.50 s for a maximum d H = 50cm, as described by Youssef and Hata (2005). Located below the basement level and above the foundation, the BIS consists of 416 HDRBs from 700 to 1300mm diameter and 90 flat sliding bearings, supplemented by 52 viscous dampers. A hybrid strategy was also adopted in this case: the structure was strengthened by introducing reinforced concrete shear walls that have to add strength to the existing building, re-distribute seismic over-turning forces, stiffen the super-structure to increase the BIS effectiveness, and improve the lateral force load path. These paradigmatic examples show the effectiveness of hybrid strategies for the seismic retrofit of existing buildings, proposing technically feasible solutions that improve their seismic response, avoiding any kinds of functional or structural limitations on the existing. In line with this approach, this paper describes a hybrid strategy for seismic retrofit of an existing monumental building. This kind of solution has already been presented by authors for other applications (De Luca et al., 2015; De Luca and Guidi, 2020). 2. The case study The building, dating back to the middle of the sixties, stands in Rome (seismic zone of the third category). It has a rectangular plan of 42.65 m x62.00 m, with a central open court of 22.65 x 28.75 m, partially decentralized in the whole plan scheme, as visible in Fig.1(a). This is a mixed steel-concrete structure, having three floors above the ground, with a total eave height of 10.85 m. On the first level, the main structure is characterized by R.C. pillars having different sizes; there are also two R.C. shear walls in correspondence with the staircases, not symmetrically placed in the plan. SAP-type slabs are used, characterized by precast reinforced-tile elements, assembled on site. The vertical R.C. elements at the first level support the “floating plan”, characterized by primary (h = 1050 mm) and secondary (h= 550 mm) steel beams. This steel transferring system supports the upper two levels, made of steel frames placed in series and connected to each other by transversal steel beams. These frames, having different geometric characteristics, vary in span lengths depending on their location in the whole structure, as visible in Fig. 1 (b). In particular, on the long side, two types of transversal steel frames are used: A-type frames, which consist of three pilasters, spaced 5.50 and 3.85 m, respectively, and two continuous beams, standing at (+) 3.40 m and (+)7.20m from the first floor slab; A’-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 9.35 m. Along the short northern side, two types of transversal frames are