PSI - Issue 78

Ingrid Boem et al. / Procedia Structural Integrity 78 (2026) 457–464

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To calculate the ground acceleration associated to the different DL, the well-known Capacity Spectrum Method (MIT, 2019) based on the equivalent viscous damping is applied to the SDOF capacity curve. The shape of the response spectrum is that indicated in the Italian technical standards (CSLLPP, 2018) with subsoil category A; the damping coefficient law suggested by Lagomarsino and Cattari (2014),  = 5 + 15 (1- 1/  ), being μ the ratio between target and yield displacement (ductility), is considered. 3.3. Fragility curves To get the fragility curves, the ground accelerations, a g , obtained from the different buildings are grouped consistently with the sub-typologies identified in the taxonomy of §2 and with the different DLs. It is specified that the values of both X and Y directions are included, assuming that the direction of the seismic action is unknown a prior. Then, assuming a lognormal format for the fragility curves, the median value of a g and the dispersion β are calculated. It is observed that, in this study, any intrinsic variability in the building materials or in the seismic demand (i.e., the spectral shape) is not investigated, focusing instead on the extrinsic inter-building variability of the schools under the same sub-typology. 4. Masonry strengthened with CRM 4.1. Strengthening technique The Composite Reinforced Mortar - CRM technique (CSLLPP, 2019) consists in the application, on the masonry surface (one or both sides) of a mortar coating (thickness range T=30-50 mm) with preformed Fiber-Reinforced Polymer - FRP composite meshes embedded (pitch range 30mm-4T). Transversal connectors, injected in drilled holes throughout the masonry, foster the collaboration between CRM and the support and provide masonry confinement. CRM, properly designed, can improve the resistance, displacement and dissipative capacities of existing masonry against both in-plane and out-of-plane actions, since the composite mesh embedded in the mortar coating, due to excellent tensile performance, stiches cracks in the masonry, opposing to their widening and delaying the masonry collapse. Clearly, an effective connection with the existing substrate is necessary to ensure the composite action between the masonry and the strengthening system. In this study, reference is made to a specific CRM configuration (Fig. 1), combining a mixed lime-cement mortar (M10 to 15) with Alkali-Resistant Glass FRP meshes (square grid 66x66mm 2 , tensile strength  77 kN/m, limit strain  2%). The reinforcement is herein considered applied extensively to the whole building and at both wall faces; couples of AR-GFRP L-shape preformed transversal connectors, with a FRP mesh device hooked at the ends to distribute stresses within the coating, are introduced; angular GFRP mesh element ensure the continuity of the reinforcement at corners. The choice is due given the extensive research available in the literature on its effectiveness (Gattesco et al., 2024). Among the several experimental tests carried out in the recent past, it is particularly relevant here to highlight the benefits against in-plane collapse of masonry piers and spandrels. As an example, in Fig. 2 it is plot the comparison obtained by testing under a quasi-static cyclic loading a 350 mm thick double-leaf rubble stone masonry (350 mm thick), with (RM) and without (URM) the CRM reinforcement.

(a) (b) Fig. 1. The CRM strengthening technique considered in this study: details in (a) vertical and (b) horizontal sections.