Issue 48

M. L. Puppio et alii, Frattura ed Integrità Strutturale, 48 (2019) 706-739; DOI: 10.3221/IGF-ESIS.48.66 707 Also, in the context of historical masonry buildings the improvement of seismic behaviour can be evaluated by different techniques, especially related to the fact that connections between vertical elements and between vertical and horizontal diaphragms are weak. For these issues, new approaches have recently been proposed, related to energy dissipation [10] and to rocking analysis of walls with horizontal restraints [11,12]. Indeed, steel tie-rods are generally the simplest and most effective tools to impede out-of-plane modes for both masonry and precast concrete structures [13,14] or to reduce relative displacements in case of flexible diaphragms [15]. There are various solutions to improve structural efficiency. One of the most interesting is the introduction of external bracing elements. In this case, the anti-seismic core can be realized with concrete and steel trusses. The trusses solution is characterised by the possible configurations (i.e. concentric X, inverted V, with links, etc.), the plan disposition and the adopted material. In this paper, different application to real cases are showed. Furthermore, the creation of a new structure designed to absorb horizontal action is very relevant because (1) it is in line with the original static conception of the building; (2) it decreases the seismic action on the existing structure; (3) it presents less uncertainty compared to a traditional intervention on an existing structure. For these reasons the intervention with external bracing system finds a natural application in this case. The design of a new anti-seismic braced structure is usually performed through dissipative diagonals. In this case it is natural to concentrate dissipation at the interface between the bracing system and the existing building by means of the installation of a dissipative interface. Buildings similar in term of constructive typology, but different in term of architectural and structural demands, can be consolidated in different ways. This is the case with two R.C. low rise buildings, the “XXV April” primary school in Arcola (Italy) [16] and a social housing complex in Livorno (Italy) [17]. Arcola The building of the “XXV April” primary school of Arcola (La Spezia, Italy) is composed of two floors with a R.C. frame. The seismic vulnerability is relevant due to plan asymmetry, ribbon windows and squat reduced section columns between the basement and the ground floor. The insertion of four external steel bracings (Fig. 1) significantly reduces the transverse loads on the R.C. frames. Figure 1: Steel frameworks used for the "XXV April" school in Arcola (Italy) [16]. The connection between the external retrofitting system and R.C. structure is ensured by horizontal short steel profiles. This makes it possible, in case of high-intensity earthquakes, to dissipate energy by bending. Livorno The case study in Livorno (Italy), is a three-storey pilotis R.C. social housing complex. Two different solutions are presented (Fig. 2). Solution A consists of the insertion of steel - glass frames only at the shorter length sides of the building. Solution B introduces multi-bracing frames on both directions. Both solutions are characterized by a traditional steel multi-braced frame in the centre of the building, but solution A has lower cost and impact. Because of the glass plates, the stiffness of the multi-bracing hybrid system can be used by inserting dissipative links in connection with the building.