## 4 Overview of Experimental Research in the Literature
### Effect of Strength of Components and Loading Orientation
Furthermore, the tabulated values in MSJC [40] were not used to predict the compressive strength of masonry built with mortars that could not be classified as S, N, or M, as outlined in ASTM C270-19a [91]. Lastly, the predictions were not applied to URM walls constructed with units, mortar, or grout of strength classes and types not covered by CSA S304 [25] and MSJC [40]. It can be seen in Figure 9 that the empirical expressions recommended by Eurocode 6 [24] and CSA S304 [25] are more conservative than those proposed by MSJC [40]. Considering all masonry types reviewed in this present article, the model introduced by the Eurocode 6 [24] can be considered to be the most reliable one with the data reported in the experimental literature. Nonetheless, a substantial amount of the experimental data showed significant deviations from the equality line depicted in Figure 9. In many instances, the CSA S304 [25] underestimated the compressive strength of URM walls, with the exception of masonry constructed with high-strength concrete block units, whereas the MSJC [40] consistently overestimates the compressive strength of the analyzed cases. These observations align with recent discussions presented by [PERSON] et al. [80], which indicate that existing design standards lack consistency and tend to make non-conservative predictions regarding masonry strength. Similarly, [PERSON] et al. [14] noted that the empirical expressions outlined in the masonry standards fail to adequately consider the significant impact of high-strength mortar on the compressive performance of URM walls built with high-strength concrete block units. This limitation is further supported by the findings of [PERSON] and [PERSON] [81], who pointed out that the existing strength formulations overestimate the compressive strength of the masonry, particularly in cases where low strength units (\(f_{\text{b}}=<5\) MPa) are employed. Furthermore, numerous research efforts have been dedicated to establishing empirical expressions that relate the compressive strength of unreinforced masonry with the compressive strength of mortar and masonry units. These expressions are often developed via statistical analysis of numerous tests conducted on unreinforced masonry walls in various countries around the world and to compare with the expressions provided in masonry design standards. From Table 2, it can be seen that the empirical relationships proposed by the various masonry design standards in predicting the compressive strength of unreinforced masonry show significant deviation from the empirical expressions proposed in the literature for most of the experimental studies considered. The studies examined in this paper emphasize significant shortcomings in the empirical expressions and tabulated  
Figure 9: Comparison between compressive strength of masonry walls derived from the literature and values predicted by the different masonry codes. values proposed in the existing design standards, underscoring the need for their urgent and thorough revision. Moreover, the compressive strength values observed in this present review article are much lower than those observed in the review of masonry prisms conducted by [PERSON] et al. [36]. In their recent review study, the authors reported compressive strength values ranging between 1.1 and 41.8 MPa when considering all types of masonry prisms [36]. In comparison, compressive strength values ranging from 1.1 to 19.8 MPa were observed for wallets and full-scale masonry walls reviewed in this present article. Based on the dataset reviewed in this paper, it appears that estimating masonry compressive strength via full-scale tests may be a more suitable approach than relying on the expressions and tabulated values proposed in existing masonry standards, which are mainly based on tests of small-scale specimens.