## Code Provisions for Shear Strength of In-Plane FRCM Strengthened Masonry Panels
In particular, for the failure mode characterized by diagonal cracking, two approaches are available in literature and are recalled in the Italian code (IBC, 2018 [35]) and commentary (IBCC, 2019 [36]):  
1. models that describe the masonry as an equivalent isotropic material and consider the development of cracks along the main directions of tension, as the one proposed by ([PERSON] and [PERSON], 1970 [41]); these approaches suppose the tensile failure occurs in the masonry units;
2. models that describe the masonry as a composite material as the one proposed by ([PERSON] and [PERSON], 1980 [42]) and suppose separately the development of cracks along its constituent components, i.e., mortar joints and blocks. In particular, when cracks occur in the mortar, the strength formulations depend on equivalent parameters of masonry, which are functions of local parameters (i.e., shear strength for sliding in absence of normal stress and friction coefficient) and of an interlocking masonry factor. The approach proposed by ([PERSON] and [PERSON], 1970 [41]) has the advantage of using a single mechanical parameter \(f_{t}\), i.e., the masonry's tensile strength, completely neglecting the actual anisotropy of the masonry. The Italian code, starting from the one dated back 1996 (D.M. 09/01/1996 [43]), adopts such an approach to estimate the shear strength of masonry walls in case of diagonal cracking of masonry units for existing buildings made of irregular texture masonry or, according to a safe approach, allows to use it also for diagonal cracking in regular masonry walls:  
\[V_{t}=l\cdot t\cdot\frac{f_{td}}{b}\cdot\sqrt{1+\frac{\sigma_{od}}{f_{td}}}, \tag{3}\]  
where \(l\) and \(t\) are the length and the thickness of the wall, respectively (see Figure 1a), \(\sigma_{\text{0d}}\) is the mean normal stress acting on the wall and referred to the gravitational load applied on the total area of the cross section (\(l\cdot t\)), and \(f_{td}\) is the design value of the tensile strength of masonry for diagonal cracking. Alternatively, the parameter \(f_{td}\) can be also expressed as \(1.5\cdot\tau_{\text{odd}}\), with \(\tau_{\text{odd}}\) being the pure shear strength, in agreement with the original formulation of Reference [41]. In addition, the Italian code provides the parameter \(b\), which is related to the stress distribution on the section and depends on the slenderness of the masonry wall: it can be assumed as \(b=l\eta l\), and, in any case, it must not be greater than \(1.5\) and not smaller than \(1\), with \(h\) being the height of the wall. As previously explained, the Italian guidelines for design with FRCM materials (CNR DT 215, 2018 [26]) suggest to calculate the shear strength of masonry panels strengthened with FRCM materials as sum of two contributions: one is the FRCM contribution provided by Equation (1). The other is the masonry contribution in case of \"diagonal cracking failure\", but the document does not specify which formulation has to be used (cracking in the mortar in case of regular texture or cracking in the units in case of irregular texture). Moreover, no reference to the sliding shear failure is done in the CNR document. Since the panels investigated in the numerical analyses are made of low strength masonry blocks, the authors assume that cracks may intersect the masonry unit and the mortar joints as it is typically observed in experimental diagonal compression tests. For this reason and considering that the commentary (IBCC, 2019 36) allows to use Equation (3) for both irregular and regular masonries, it will be used for predict the shear strength of the simulated walls. In addition, Equation (3) depends on the masonry's tensile strength, \(f_{t}\), which, according to the Italian code (IBC, 2018 [35]), can be directly assessed by means of diagonal compression tests.