PSI - Issue 44

Gabriele Guerrini et al. / Procedia Structural Integrity 44 (2023) 1877–1884 Gabriele Guerrini et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction Unreinforced masonry (URM) structures usually represent a significant portion of the built environment and constitute historical and cultural heritage assets in several regions. They are generally associated with a high seismic vulnerability. Many authors proposed in literature retrofit solutions to enhance the in-plane and out-of-plane capacities of URM walls (e.g., Babaeidarabad et al. 2014; Giarreton et al. 2018; Tomaževič et al. 2015, Darbhanzi et al. 2014, Ma et al. 2012). Several studies were conducted on the influence of masonry-to-floors connections on the seismic behavior of URM buildings and how to improve them to guarantee a global response, preventing local failure mechanisms (e.g., Guerrini et al. 2019; Podestà and Scandolo 2019). Within the proposal of a new structural retrofit system, its effectiveness and mechanical contribution need to be experimentally validated. After reaching a satisfactory understanding of the experimental results, numerical studies can support the seismic vulnerability assessment of specific building typologies, characterized by geometrical and material properties that can be typically found in situ. To this end, it is fundamental to be able to produce numerical models that are capable to simulate the experimental tests. The availability of reliable models allows the extension of laboratory findings to large scale considerations, i.e., analyzing the variability of the structural responses under different ground-motions records, investigating different geometries, material properties or retrofit configurations and details (Damiani et al. 2022). This paper discusses the numerical simulation of two experimental tests performed in the framework of a comprehensive in-situ and laboratory testing program supporting seismic risk analysis of URM buildings subjected to induced seismicity (Graziotti et al. 2019): a cyclic quasi-static test performed on a timber retrofitted masonry pier, and a shake-table test performed on a full-scale URM building retrofitted with the same intervention. The strengthening solution relied on timber frames mechanically connected to masonry elements, floors, and foundation systems, on which Oriented Strand Boards (OSB) are nailed. The analyses were performed employing the software TREMURI (Lagomarsino et al. 2013), based on an equivalent-frame formulation using nonlinear macroelements (Penna et al. 2014) and neglecting the local out-of-plane (OOP) responses. A modelling strategy to simulate the contribution of the new retrofit system was devised and validated against the experimental results. 2. Retrofit system and experimental campaign The new timber-based retrofit solution stemmed from the use of vertical timber posts connected to the masonry to improve the OOP capacity of piers, which was proposed and investigated by Giaretton et al. (2016), Dizhur et al. (2017) and Cassol et al. (2021). Their system was modified to improve also the in-plane capacity of masonry piers creating a timber frame mechanically connected to the masonry and floor and foundation systems, on which an OSB boards layer is nailed. From the in-plane point of view, the pier resisting moment is enhanced by the tie-down connectors that anchor the timber frame to foundation and floors, while the shear resistance is improved by the nailed OSB. From the OOP point of view, the vertical posts act as double pinned beams, which add OOP shear resistance and allow the transfer of inertia forces to floor and foundation systems. Fig. 1 shows an overview of the system applied to a single pier and to an entire building. The application of the system to an entire building ensures not only the improvement of pier capacities, but also the enhancement of the masonry-to-floor system connections, which are well known to represent one of the most impacting weaknesses of URM buildings (Magenes et al. 2010; Mendes, Lourenço, and Campos-Costa 2014; Tomassetti et al. 2019; Senaldi et al. 2020). The experimental investigation on the effectiveness of the timber system was performed at the facilities of the EUCENTRE Foundation and of the University of Pavia, Italy. Two full-scale single wythe calcium silicate (CS) masonry pier, one in bare and one in retrofitted configuration, were subjected to quasi-static in-plane shear compression tests, as deeply described by Guerrini et al. (2021). The specimens represented the first-story longest pier of the end-unit of a cavity-wall terraced house that was lately tested by the same research group (Miglietta et al. 2021). Both piers were tested up to their ultimate conditions under the same vertical overburden stress and boundary conditions. Experimental evidence demonstrated that the retrofit system is effective in improving the in-plane capacity of masonry piers: in particular, the shear capacity was increased of about 35% and displacement capacity of 167%.