PSI - Issue 44

P. Morandi et al. / Procedia Structural Integrity 44 (2023) 1060–1067 Author name / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction In recent years, the increased attention towards energy efficiency of buildings, together with the awareness of the importance of seismic performance of structures, have led to the development of innovative combined reinforcement systems for existing masonry buildings, with the aim to ensure an improvement of both structural and energetic behavior (for example, see Guerrini et al. 2021, Miglietta et al. 2021). For this reason, Progetto Sisma s.r.l. designed “Resisto 5.9”, an external steel reinforcement system integrated with a thermal insulation coating. The reinforcement system consists of modular steel frames, connected to each other by means of steel bolts and to the masonry through chemical anchoring with threaded bars. The steel frames consist of cold formed L-shaped sections and plate elements as horizontal and diagonal bracings, obtained from galvanized steel thin sheets. The modules must be positioned on the external surface of the wall and connected to each other and to the masonry with regular pitch. More details on the reinforcement system can be found in Manzini et al. 2022. Concerning structural aspects, the purpose of the proposed system is to guarantee an improvement in the connection between orthogonal walls and among walls and horizontal elements, a better redistribution of the seismic actions among the different structural elements, a limitation of the out-of-plane overturning of walls, together with an improvement of the in-plane performance of each strengthened masonry pier. This last aspect is the main goal of the experimental and numerical research currently underway at the EUCENTRE Foundation in Pavia, actually aimed at evaluating the in-plane seismic behaviour of the “Resisto 5.9” reinforcement system. The lateral in-plane performance of existing masonry walls has been evaluated through cyclic in-plane pseudo-static tests on full-scale specimens, comparing specimens strengthened with the proposed system, with the related unreinforced case. Two different masonry typologies, representing common solutions in Italian existing buildings, have been adopted so far in this campaign: the one considered in this paper is made up with solid clay bricks and lime mortar, with “header bond” pattern. The performed experimental tests included first the complete mechanical characterization of units, mortars, masonry and of the reinforcement system components (i.e. steel elements and chemical anchors). Cyclic in-plane pseudo-static tests were then performed on different sets of full-scale specimens, in order to investigate the effects of the proposed reinforcement system on the lateral response of the walls, compared to their unreinforced situations. The cyclic behaviour of the masonry piers was analysed in terms of elastic stiffness, lateral strength, displacement capacity, energy dissipation and associated failure mechanism. The experimental testing was supported by an extensive numerical study which consisted in the development of advanced discontinuum models based on the Distinct Element Method (DEM). Although several literature works have demonstrated how such models are able to satisfactorily predict the response of unreinforced masonry structures (Lemos 2007, Lemos 2019, Malomo et al. 2019, Pulatsu et al. 2020, Malomo et al. 2021), the inclusion in DEM framework of possible retrofit solutions represents a topic that has not yet been fully explored. In this paper, a numerical procedure to explicitly model the considered retrofit system is outlined. Experimental and numerical results are then compared to validate the proposed modelling strategy. 2. Experimental in-plane cyclic tests The cyclic in-plane pseudo-static shear-compression tests on full-scale masonry specimens were carried out at the experimental laboratory (ShakeLab) of the EUCENTRE Foundation, which provides a three-dimensional configuration of strong floor and two orthogonal strong walls for these type of tests. Concerning the set-up, the specimens were built on reinforced concrete (RC) footings, clamped to the strong floor by means of post-tensioned steel bars. A horizontal actuator, fixed to the strong wall perpendicular to the specimen, applied the horizontal force to a steel spreader beam connected with the RC beam at the top of the wall, while two vertical actuators, reacting on a steel frame fixed on the strong wall parallel to the specimen, controlled the applied vertical load and allow to provide different boundary conditions. The horizontal force was measured by a load cell in the horizontal actuator, while the horizontal displacement ( δ ) at the top of the wall was controlled by an external linear potentiometer. Additional displacement transducers were installed on each wall, in order to evaluate the deformations of the masonry pier and, in the case of the strengthened specimens, of the reinforcement system and the relative displacements between them.