PSI - Issue 44

Silvia Caprili et al. / Procedia Structural Integrity 44 (2023) 1030–1037 Sivia Caprili et al. / Structural Integrity Procedia 00 (2022) 000–000

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gravitational loads but a poor performance when it comes to lateral loads induced by earthquakes, as widely demonstrated by recent seismic events in European seismic-prone countries (e.g. L’Aquila 2009, Lorca 2011, Emilia 012 and Central Italy 2016, e.g. Di Ludovico, 2017). This results in relevant economic and human losses. During seismic events, local in- and out-of-plane failure mechanisms can be activated. Approaching them requires the development of ad-hoc solutions: several strategies emerged over time, with the aim to improve the behavior of walls towards in-plane shear forces and/or out-of-plane bending. Typically, the possible strengthening measures can be subdivided into three conceptual categories (Chuang and Zhuge, 2005; Calvi, 2013): 1) the introduction of base isolation systems, 2) the reduction of seismic-induced vibrations through damping devices (Morelli et al., 2019a, 2019b; Panzera et al., 2020), and 3) the modification of damage and collapse modes. This latter category covers both traditional retrofitting techniques (e.g. cracks grouting, external or internal post-tensioning with steel ties, shotcrete jacketing etc.) and innovative techniques such as the employment of fiber-reinforced polymers or fabric-reinforced cementitious matrix (FRCM). The use of modern and organic materials is gaining importance also in the perspective of finding environmentally sustainable and cost-effective solutions, coupling them with energetic considerations. Indeed, from an energy perspective, an estimated 75% of EU's building stock is not efficient, with energy consumption levels so significant as to place the sector among the most meaningful sources of CO 2 in Europe. Improving the energy performance has become essential to match the EU Commission’s aim to reach a highly energy efficient and decarbonised building stock by 2050 (“Directive EU.,” 2018). In this context, it becomes paramount to enhance the safety, energy efficiency and durability of the existing building stock with integrated solutions. Confirming this, in recent years, the topic of combined seismic and energy retrofitting is on the front burner and new retrofit methodologies are under investigation: exoskeletons made of insulated concrete form panels (Pertile et al., 2018, 2021), inner coat done with cross-laminated timber (CLT) panels (Valluzzi et al., 2021), plastering of building through FRCM (Longo et al., 2021) and GFRP-reinforced jacketing (Borri et al., 2016) are just some example of integrated renovation approaches proposed in literature. The present research work details an alternative suggestion: the introduction of RC shear walls made of expanded clay blocks for a combined enhanced seismic/energy retrofit, realizing around the building a ‘double skin’ responsible for the behaviour improvement. The system, conceived and developed by PAVER S.p.A., is essentially composed by a thin reinforced concreate layer cast on-site within a permanent formwork made of expanded-clay blocks. A high density Neopor insulation is integrated within the blocks, allowing to improve both seismic and energy aspects in a single intervention. In the following sections, the elements composing the system and the installation process are detailed; the feasibility and applicability of the system is evaluated through the application to a case-study, representative of the Italian building scenario of the 20 th century, analyzed from both a structural/seismic and an energy The proposed system (Fig. 1a) is an external envelope composed by vertical walls (expanded-clay block walls with a RC layer), connected to the existing structure at floor level using L-bent bars. This configuration allows of the transfer the seismic action from the floor to the external concrete structural layer and then to the foundation, while vertical gravitational load still burden on existing masonry walls. The presence of a high-density insulation layer contributes to a better overall insulation of the envelope, improving energy performance and indoor thermal comfort. As a result, the proposed system decreases both the energy and seismic deficiencies of buildings through a single combined intervention. The inner septum of structural concrete is reinforced with steel bars arranged in a single layer along the longitudinal direction and in a double layer along the transverse one. In correspondence of existing curbs, the thickness of the concreate layer is increased to obtain perimetral horizontal ribs, used to connect the retrofit system to the existing structure through the steel L-bent connectors, suitably spaced and embedded in the curb with injections of resins. The whole system has to be fixed to the foundation to transfer the seismic load to the ground: if the existing foundation is large enough and can resist the seismic load, the system can be directly connected to it, otherwise, a new foundation curb, adjacent and anchored to the existing one, shall be realized. The expanded-clay blocks are produced off-site and suitably shaped for the insertion of vertical, horizontal and even diagonal reinforcing bars and for concrete casting. Individual elements are first dry-assembled by using ‘male’ and ‘female’ joints, then a 10 cm thick casting of structural concrete, previously reinforced during the assembly phase, is executed within the blocks. The wall thus realized has the typical feature of being not a continuous RC wall due to efficiency point of view before and after the retrofit intervention. 2. The proposed combined seismic/energy retrofit system