## 4 Overview of Experimental Research in the Literature
### Effect of Openings  
The failure mechanism of solid masonry under compression is well-established based on previous research. Failure usually occurs due to the incompatible elastic properties of the masonry units and mortar. This leads to the formation of vertical cracks in either the units or mortar parallel to the direction of the applied compressive load [36; 80; 81; 96; 97]. However, in recent decades, one aspect of structural masonry walls which has piqued the interest of scientists is the design and failure behaviour of URM wall panels with openings subjected to compressive loads. Openings in masonry buildings are required to be provided in the wall for the functional needs of new buildings or functional alterationsto an existing structure. These openings are of vital importance in designing building structures, particularly for spatial optimization and repurposing for long-term conditions. However, the openings constitute a vulnerability and may size-dependently decrease the stiffness and load capacity of the walls relative to the equivalent walls without an opening [3, 79, 98]. In particular, the presence of window or door openings in URM walls may affect the load path, leading to unfavourable stress concentrations at the corners of the opening under the applied compressive loads. These stresses are the main cause of the propagated cracks, which lead to final failure [80]. However, the current American [40] and Canadian [25] masonry design standards do not provide design rules for masonry wall panels with openings, while Eurocode 6 [24] specifies some limitations. Within this constraint, very few experimental works have been carried out in the past on masonry walls using various opening configurations in order to understand the behaviour of such walls under axial compressive loads, as well as to develop or verify analytical failure models. [PERSON] et al. [79, 99] reported axial compression test results on fired clay brick masonry wall panels with small openings (380 \(\times\) 380 mm\({}^{2}\) opening area). As seen in Figure 11, typical vertical cracks were observed in the tested walls, and the location of these cracks was largely influenced by the position of the openings. Moreover, a significant reduction in the cracking load value was also reported for wall panels with openings compared to solid panels without openings. Test observations demonstrated that the reduction in load-bearing capacity and post-cracking stiffness is directly related to the position of the openings [99]. [PERSON] [100] also reported a series of test results on the behaviour of full-scale brick masonry wall panels--5600 mm \(\times\) 2475 mm \(\times\) 50 mm their nominal sizes--with different opening sizes under lateral loads. Four different forms of opening configurations were considered, being representative of doors and windows. Observations from the study, as expected, gave evidence of significant differences in crack patterns and failure modes of wall specimens due to differences in opening size and positions (see Figure 11). Wallet strengths also proved to be conservative relative to the ultimate load capacity of the full-scale wall panels. The strength of the walls with openings (less than 10% of the total wall area) was around 25% lower than that of walls without openings. Similar observations are also noted in [101], where the authors conducted a comparative study on the axial load response of full-scale URM walls with different opening configurations. The wall specimens--1200 mm \(\times\) 1200 mm \(\times\) 110 mm, their nominal sizes--were made with clay brick and an opening size of 400 mm \(\times\) 800 mm (approximately 22.2% of the wall area). While the wall panels failed as a result of vertical cracks, the cracking patterns of the tested walls, as expected, differed significantly due to the presence of openings. The authors also gave evidence of a reduction in the load capacity by about 35% in addition to the increase in maximum displacement by 27% due to the presence of openings.