## 4 Overview of Experimental Research in the Literature
### Effect of Strength of Components and Loading Orientation
Essentially, stronger mortar contributes to the overall strength of the masonry panels, particularly under vertical compressive loading, where this effect is more pronounced than under horizontal compressive loading. Recent findings by [PERSON] et al. [75] gave further evidence that the strength of URM walls can be lower when subjected to horizontal compressive loads. Likewise, recent experimental investigations by [PERSON] et al. [73] on multi-perforated clay bricks masonry demonstrated higher compressive strength of wallette specimens loaded perpendicular to the bed joint compared to specimens loaded parallel to the bed joint, a fact mainly attributed to the internal configuration of the bricks (layout of webs and shells). This internal configuration impacts the resistant areas in each direction, consequently affecting the compressive performance of the walls. Although several studies have reported that the performance of URM walls is affected by the loading orientation; however, all of these studies were conducted using clay bricks with essentially stronger units and weaker mortar strength. More studies focusing on URM walls--possibly in full scale--built with concrete block units, taking into account stiff mortar--weak unit combinations, should be considered to verify the axial compression performance based on the loading orientation. Figure 7 plots the compressive strength of masonry (\(f^{\prime}_{\text{m}}\)) derived from the experimental campaigns discussed in this paper against the compressive strengths of units used in each campaign, while Figure 8 plots the normalized compressive strength of the masonry (i.e., \(f_{\text{m}}/f_{\text{b}}\) vs. \(f_{\text{b}}/f_{\text{m}}\)) reviewed in this present article. Observations from the literature suggest that the strength of URM walls is largely influenced by the strength of the unit, i.e., the  
Figure 6: Tests on masonry wall: (**a**) loading perpendicular to the bed joints; (**b**) loading parallel to the bed joints. greater the unit strength, the greater the compressive strength of the wall, and this feature holds true for most of the experimental studies considered. However, as depicted in Figure 7, a significant degree of scatter on the compressive strength of URM walls can be observed from the experimental literature. A similar trend could also be observed in the normalized compressive strength of the masonry when considering all masonry types, as shown in Figure 8. The high degree of variation observed in the figures is mostly due to the different wall geometries and material units employed in the various campaigns. As discussed in Section 2, existing design standards such as the Eurocode 6 [24] provide empirical expressions used to predict the compressive strength of masonry structures. On the contrary, masonry standards such as the CSA S304 [25] and MSJC [40] provide tabulated values to estimate the masonry's compressive strength, accounting for various strength of unit and mortar combinations. In this present review article, the compressive strength values of URM walls were compared to the strength values predicted by the empirical  
Figure 8: Normalized compressive values of masonry reviewed in this article [13; 14; 15; 16; 17; 69; 72; 73; 74; 80]. Figure 7: Compressive strength of unreinforced masonry walls derived from recent experimental literature [13; 14; 15; 16; 17; 69; 72; 73; 75; 80]. equations proposed in the Eurocode 6 [24] and the tabulated values from the MSJC [40] and CSA S304 [25]. Figure 9 illustrates the comparison between the compressive strengths of URM walls derived from the experimental campaigns discussed in this paper and the compressive strengths predicted by the various design standards. The values tabulated in CSA S304 [25] were not applied to masonry constructed with mortars that could not be classified as type S or N according to CAN/CSA-A179 [90].