## 4 A Progressive Chronicle of Masonry Provisions in Turkish Earthquake Codes (1940-2018)
### Mortar Classes, Joint Thickness, and Masonry Unit Testing  
In the mid-twentieth century, Turkish seismic regulations provided merely a qualitative recognition that \"sufficient\" mortar should be utilized, generally referring to lime or cement-lime compositions without delineating minimum compressive strengths or joint dimensions. Indeed, within the codes from 1940 to the early 1960s, creating earthen mortars was deemed an acceptable practice for single-story structures, whereas multi-story masonry remained sufficiently infrequent that numerical benchmarks for mortar strength or joint dimensions were overlooked. By 1975 [51], the regulatory language became more precise in advocating for \"well-made\" mortar and \"consistent joint thickness\", although it had yet to mandate formal compressive testing or numerical classifications (e.g., M2.5, M5). Often, discretionary site assessments were deemed adequate, with expansions by occupants, particularly in rural southeastern Turkiye, continuing to evade official documentation, thereby escaping scrutiny regarding mortar quality. The 1997 [52] modification was the first instance in which the code included a quasi-quantitative standard for minimum mortar strength in load-bearing walls, establishing an insufficient relationship to national or local testing standards. The 2007 code [53] formalized \begin{table}
\begin{tabular}{c c c c c} \hline
**Code/** & **Ring Beams** & **Mortar** & **URM Retrofit Guidance** & **Occupant Expansions** \\
**Guideline** & & **Classification** & & \\ \hline \multirow{2}{*}{TBEC (2018) [13]} & Mandatory in multi-story URM and & & M5 minimum in & \\  & moderate-high seismic-risk zones; & moderate–high-risk zones & No formal retrofit chapter & Not directly addressed \\  & ring beam spacing \(\leq\) 4 m & & & \\ \hline \multirow{2}{*}{Eurocode 8 [56]} & Mandatory in seismic & Specific strength & Detailed provisions for & \\  & zones; tied into floor & classes (e.g., M2.5, M5, & URM entrengthening, & Not explicitly covered \\  & diaphragps & M10) required & including FRB, ties, anchors & \\ \hline \multirow{2}{*}{ASCE 41 [57]} & Required if needed to improve & Based on ASTM & Extensive (URM retrofit & \\  & out-of-plane behavior & & standards & diaphragps and & \\ \hline \multirow{2}{*}{NZS 1170.5 [58]} & Required in URM; often retrofitted & & Mortar performance & Comprehensive retrofit & \\  & using steel ties or RC bands & verified by lab or field & guide for URM based on & Recognized as a risk, but not \\ \hline \end{tabular}
\end{table} 
Table 5: Comparison of TBEC (2018) [13] with international seismic codes for URM design and retrofit. this shift by specifying defined mortar \"classes\", including M2.5, M5, and M10, and stating that multi-story masonry or masonry in moderate-/high-seismic-risk areas must at least be of class M5. This code established guidelines for joint thicknesses, often necessitating bed joints of around 10-15 mm and mandating consistent bed and head joints to mitigate shear-plane weaknesses. Nevertheless, occupant expansions continued to disregard or minimize these needs, since unlicensed projects seldom passed laboratory or site-based compressive testing. By the TBEC 2018 [13], mortar categorization transitioned to a definitive numeric standard, requiring class M5 or above in zones of moderate-to-high seismic activity, with compressive strengths of about 5 MPa. The references to standardized tests, often using TS EN 1015-11 [59] techniques, indicated that the code now regarded systematic laboratory or field testing as essential for validating consistent mortar performance, particularly in multi-story URM or RM. Simultaneously, the maximum joint thickness (\(\leq\)15 mm) and guidelines for adequately compacted head joints were established. Notably, while tenant expansions are required to comply with these regulations, the legal classifications for \"partial additions\" sometimes enable them to bypass formal building permits.