## 4 Overview of Experimental Research in the Literature  
As previously stated, various construction and testing parameters can influence the compressive performance of URM walls. The primary factors are the strength of masonry units (brick/block) and mortar strength, as well as the slenderness ratio and eccentric load conditions of the walls [17]. Apart from these parameters, the presence of openings is another significant parameter which could affect the performance of the masonry structure and should be properly taken into account. All the above-mentioned parameters can be varied during the construction and testing of masonry walls, leading to a more accurate assessment of the design of load-carrying URM elements. When evaluating the structural response of URM walls, both shear capacity and bending moment should be considered by taking into account its interaction with compression forces due to lateral load and gravity [58; 59; 60]. In the context of masonry structures in general, substantial eccentricity loadings could be precipitated either by in-plane or out-of-plane lateral forces or a combination of both [61]. The latter are typically induced by thrusting components (e.g., vaults and arches), wind and soil pressures and seismic actions transmitted from the masonry floor. In contrast, the former can be seismic-induced shear force ensuing from box-type global seismicity of the building structures. As observed from the literature (e.g., see ref. [62]), axial loading differs from eccentrically loaded walls since the former is a statically determinate problem in which the strength of a given wall panel may be presumed to be maximum compression forces divided by the total cross section (Figure 4). On the contrary, eccentrically loaded wall panels are statically indeterminate problems in which the bending moment could be defined by integrating the normal stress and the corresponding axial strain over the given gross sectional area [63; 64; 65]. As a general recommendation in the design of URM structures, the capacity reduction factor (\(\Phi\))--expressed as a ratio of prism strength to the strength of the wall--can be introduced to accommodate and account for different characteristics and to obtain the permissible compressive strength of the masonry [24; 40; 66]. In evaluating the axial compressive performance of URM walls, experimental tests carried out on brick and concrete block masonry wall specimens available in the literature have been analyzed (e.g., see a selection in Table 1). However, it is important to highlight that  
Figure 4: Axial compressive test of URM wall: (**a**) wall with test frame and (**b**) concentric and eccentric loading. previous research on the compressive behaviour of full-scale masonry walls is still limited. To augment the understanding, tests conducted on vallettes and partially grouted wall panels subjected to axial compressive loading have been used since the actual behaviour of these types of wall panels is identical to full-scale URM wall panels [12]. In addition, URM wall panels subjected to shear-compression loading have also been examined. However, the results from such experiments reported in the literature were not included in the database since these types of tests are closely associated with earthquake loadings. Based on a series of assumptions, scholars have introduced a wide range of predictive models to estimate the compressive strength of masonry. These models vary from simple linear ones with a single variable to complex nonlinear ones that consider the influence of multiple variables. Linear models were proposed in [67; 68], while nonlinear expressions were introduced via standards such as Eurocode 6 [24]. These expressions established correlations between the compressive strength of masonry and both the compressive strength of the unit and the compressive strength of mortar. In many cases, the expressions proposed did not consider any distinctions based on the type of masonry.