## 4 A Progressive Chronicle of Masonry Provisions in Turkish Earthquake Codes (1940-2018)
### Wall Slenderness, Story Height, and Openings  
The progressive amendments concerning wall slenderness, permissible floor heights, and allowable openings within Turkish seismic regulations demonstrate a gradual evolution from ambiguous, experience-based standards to comparatively precise numerical limitations. As illustrated by the codes of 1944 [47], 1949 [48], and 1953 [49], references  
Figure 16: Mandatory requirements related to ring beams in TBEC (2018) [3].  
to maximum wall heights or thicknesses were predominantly articulated in qualitative terms, such as advocating for \"adequate thickness\" to ensure the stability of stone or adobe constructions, while formal floor height regulations were seldom more detailed than suggesting single-story structures for earthen mortar. The 1961 [50] revision marked a subtle recognition of multi-story masonry, albeit reiterating broad recommendations that walls must not be \"unduly slender\" and granting limited authority to local administrations to evaluate occupant additions on an individual basis. A significant transformation occurred with the 1975 [51] code; it introduced preliminary numerical criteria for multi-story masonry, recommending specific height-to-thickness ratios, typically not exceeding 12:1 or similar, especially in moderate-to-high-seismic-risk zones, although expansions involving additional upper floors remained largely outside rigorous municipal oversight.  
By 1997 [52], the code stated more explicit constraints, assigning maximum H/t (height-to-thickness) ratios for unreinforced stone or brick walls (e.g., 10:1 or 12:1 in some official or local guidelines) and demanding that multi-story URM carefully respects these limits to reduce diagonal shear cracking and out-of-plane bending. At the same time, the 1997 [52] text began referencing recommended story heights (generally 3.0 m or less for typical URM) and limiting window/door openings by specifying minimum piers to mitigate plane instability, although occupant expansions often introduced large new openings for interior remodeling without abiding by these constraints. The 2007 [54] code built upon these numeric guidelines, tying them more concretely to seismic zones: moderate-to-high-seismic-risk areas might require an even stricter H/t ratio (e.g., 8:1 or 10:1), capping story heights at nearly 3.0 m and mandating that each opening remain within specified bounds (such as not exceeding 50% of the wall length). Multi-story occupant expansions in moderate/high-risk zones thus theoretically had to conform to these numeric slenderness and story height rules, but without a distinct \"occupant expansions\" category, application remained uneven.  
Finally, the TBEC (2018) [13] refined slenderness restrictions further, linking them to the performance-level objectives (Limited Damage or Collapse Prevention) for multi-story or confined masonry. This code also strictly limited openings, usually demanding a robust wall segment around doors and windows, and recommended corner piers of at least 0.80 m. By insisting that occupant expansions meet these same guidelines, the TBEC (2018) [13] attempted to curb expansions that historically produced extremely slender walls, extensive openings, or additional stories without proportionate strengthening. In practice, occupant expansions still frequently occur under unlicensed conditions, thus undermining the code's numerical, capacity-based approach to slenderness, story height, and opening limitations. The allowed unsupported wall lengths according to the TBEC (2018) [13] are illustrated in Figure 17.