PSI - Issue 44

Carpanese Pietro et al. / Procedia Structural Integrity 44 (2023) 1752–1759 Carpanese Pietro et al./ Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction In the last decades, Italy has experienced seismic events that have caused thousands of victims and about 180 billion euros in economic losses (DPC 2018). These losses are no longer sustainable, and it is therefore necessary to pursue large-scale seismic mitigation policies. In this context, the Italian Department of Civil Protection (DPC) is working in synergy with the scientific community to evaluate the possible strategies that can be adopted to mitigate seismic risk (Dolce et al. 2021, da Porto et al. 2021). In particular, the work presented in this article is part of a project involving ReLUIS (Network of University Laboratories of Seismic Engineering), whose purpose is to develop large-scale vulnerability models for buildings in their original state and after consolidation, and then perform cost-benefit analyses in order to identify the effectiveness and convenience of possible seismic mitigation interventions. To do this, it is firstly necessary to evaluate the variation of vulnerability between the as-built configuration and the case in which retrofit interventions have been applied. This can be done through the development and the comparison of fragility curves. In this contribution, fragility curves are obtained through a simplified mechanical method for Italian residential buildings, both for their as-built state and also considering different types of anti-seismic retrofit interventions. The study focuses on masonry buildings, divided into macro-typologies according to their construction period (Pre-1919, 1919-1945, 1946-1960, 1961-1980) and their number of storeys (Low-Rise, with 1 or 2 storeys, and Mid-Rise, with 3 storeys or more). The vulnerability models are then implemented in a platform called IRMA (Italian Risk MAps), developed by the European Centre for Training and Research in Seismic Engineering Eucentre (Borzi et al. 2021), to produce as-built and mitigated seismic risk maps, and therefore highlight the benefits that the proposed interventions can bring to different areas of the Italian territory. 2. Interventions and mitigated fragility curves 2.1. Selection of representative and effective retrofit interventions To develop the models of mitigated fragility, retrofit interventions were selected according to the specific macro typology of buildings on which they shall be implemented, based on specific literature and also direct field observations (Saretta et al. 2021). Specifically, the interventions involving masonry buildings have been grouped into three categories: a) interventions to increase the strength and compactness of the walls; b) interventions to improve wall-to-wall and wall-to-floor connections; c) interventions to increase the stiffness of the horizontal diaphragms. The interventions of masonry strengthening depend on the type of masonry (da Porto et al. 2018). The load-bearing structure of historical buildings is usually composed of stone (random stone or ashlar masonry) or solid bricks. On the other hand, solid or hollow bricks are usually found in more recent buildings. In the case of inconsistent masonry, interventions of grout or mortar injections were simulated (Silva et al. 2014a; 2014b), while for masonry composed of regular elements, reinforced concrete plasters (Modena et al. 2009) or FRCM-TRM (Fiber/Fabric Reinforced Cementitious Matrix/Mortar - Textile Reinforced Mortars) (CNR-DT 215/2018; Giaretton et al. 2018) were adopted. The intervention of masonry strengthening has been proposed in two steps (MSN1, MSN2) in the case of historic buildings (pre-1945), in which the first step considers the application of single or local interventions with a lighter impact on the structure, while the second step involves the application of several extensive interventions. In the case of modern buildings (1946-1980) a single intervention (MSN) was assumed because of the limited achievable improvement given by a good as-built state. With regard to interventions aimed at improving the connections between the structural elements of a masonry building, the most effective and widespread solution is the insertion of tie-rods (TR) or confining rings (CR) (Modena et al. 2009). The tie-rods were selected for pre-1945 buildings as they are more compatible with historical structures. On the other head, confining rings have been adopted for modern buildings. Confining rings have the same function of tie-rods but are placed on the external face of the walls. For this reason, the effect of confining rings on recent buildings has been considered as the equivalent of the one given by tie-rods to historical ones, since the goal of confining rings is to counteract the out-of-plane mechanisms, exactly as for the tie-rods.