## 4 Overview of Experimental Research in the Literature
### Effect of Strength of Components and Loading Orientation
\begin{table}
\begin{tabular}{l l l} \hline \hline \multicolumn{1}{c}{**Reference**} & \multicolumn{1}{c}{**Masony Type**} & \multicolumn{1}{c}{**Model**} \\ \hline Eurocode 6 [24] & Clay and calcium silicate units & \(f_{\text{k}}=0.55f_{\text{b}}\,^{0.70}f_{\text{m}}\,^{0.3}\) \\ [PERSON] et al. [15] & Calibrated for table moulded brick masonry & \(f_{\text{m}}=0.317f_{\text{b}}\,^{0.866}f_{\text{m}}\,^{0.134}\) \\ [PERSON] et al. [69] & Calibrated for clay brick walletite & \(f_{\text{m}}\)= \(f_{\text{b}}\,\left(\frac{4+0.1/\pi}{151/\pi+587f}\right)\) \\ [PERSON] and [PERSON] [70] & Calibrated for clay brick masonry & \(f_{\text{m}}=0.317f_{\text{b}}\,^{0.531}f_{\text{m}}\,^{0.208}\) \\ [PERSON] et al. [76] & Calibrated for fired clay brick walletite & \(f_{\text{m}}=0.56f_{\text{b}}\,^{0.53}f_{\text{m}}\,^{0.5}\) \\ [PERSON] et al. [80] & Calibrated for HCB walletite & \(f_{\text{m}}=0.886f_{\text{b}}\,^{0.25}f_{\text{m}}\,^{0.18}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A summary of selected equations proposed in the literature for predicting either the characteristic compressive strength of the masonry,\(f_{\text{k}}\), or the mean value,\(f\,\text{m}\), using the normalized compressive strength of the unit,\(f_{\text{b}}\), and the compressive strength of the mortar,\(f\,\text{m}\). Figure 5: Plot of compressive strength of URM wall against masonry elements reviewed in this article (**a**) unit strength (**b**) mortar strength. Such observations are also noted in [19], where HCB and brick masonry walls were tested in compression. Results from the study gave evidence--and in accordance with previous findings--that the HCB wall panels exhibit higher compressive strength in comparison to wall panels built with bricks. [PERSON] et al. [13] also reported a 100.2% increase in the compressive strength of full-scale URM walls built with HCB units (\(f_{\text{b}}\) = 15.76 MPa) in comparison to masonry constructed with lower strength units (\(f_{\text{b}}\) = 8.64 MPa). Likewise, experimental investigations by [PERSON] et al. [72] on clay brick masonry wallettes constructed with a mortar strength of 5.84 MPa demonstrated that the compressive strength of masonry wallettes built with a unit strength of 19.61 MPa was 175.3% higher than those constructed with lower unit strength (\(f_{\text{b}}\) = 14.23 MPa). However, the difference in the masonry strength was mainly attributed to the scale effect of the unit. Traditionally, lower-strength mortar and stiffer units are used in the construction of URM walls. However, in recent years, engineers worldwide have encountered unique scenarios where mortar needs to be stronger than units. This can be attributed to various factors, including advancements in mortar cement properties and the necessity of using locally available soft units for affordable housing construction in developing nations [15; 81; 82; 83; 84]. A better understanding of how the relative strength of mortars and units affects the performance of URM walls under axial compression loads has been made possible by recent research that has been published in the last few years. According to research findings, when low-strength units are used, increases in mortar compressive strength have only a little impact on the compressive strength of the wall. Figure 5b graphically displays the compressive strength values of mortars relative to the compressive strength of URM walls, as reported in Table 1. The dataset is presented based on M1 (\(f_{\text{m}}\) ranging between 0.86 and 2.6 MPa), M2 (\(f_{\text{m}}\) = 5 and 11.9 MPa), and M3 (\(f_{\text{m}}\) = 12.2 and 26.9 MPa) mortars used in the different experimental campaigns. The dataset in Figure 5b indicates that the ratio between the compressive strength of masonry and mortar compressive strength (\(f_{\text{m}}/f_{\text{m}}\)) in M1 mortars ranges between 0.67 and 2.79 MPa. While M2 and M3 mortars presented \(f_{\text{m}}/f_{\text{m}}\) values of 0.13 to 2.49 MPa and 0.13 to 1.27 MPa, respectively. For example, [PERSON] et al.