## 3 Observed Damage Mechanisms in Eastern T\(\ddot{\text{u}}\)rkiye's Masonry Buildings
### Damage Modes Attributable to Poor-Quality Units and Mortar
#### 3.1.3. Hollow Clay Tiles and Composite Configurations  
HCT modules are often used in regions seeking to partially modernize conventional masonry structures. The anticipated benefits often focus on decreased weight, enhanced insulation, and superior homogeneity in comparison to entirely sun-dried bricks or uneven stone blocks [4]. Field observations after major earthquakes in eastern Turkiye, including the 2011 Van, 2020 Elazig-Sivrice and 2023 Kahramanmaras earthquakes, reveal that the advantages of HCTs are often compromised by inferior manufacture, insufficient mortar,  
Figure 7: Observed damage due to use of variable-quality stones.  
and irregular installation. Consequently, HCT-based masonry walls may demonstrate brittleness, reduced shear capability, and a propensity for sudden failures under moderate-to-severe seismic forces.  
Nonetheless, this design reduces the effective cross-sectional area that counters lateral and in-plane forces, especially when the tile webs are inadequately filled or poorly bonded with mortar. If mortar application is careless or insufficient, the tile voids may remain partly unfilled, preventing a secure integration between units. Field observations verified several occurrences in which whole HCT-based infill or partial load-bearing walls experienced out-of-plane ejection of tile segments. Minor fissures emerged in the corners of the tile webs and rapidly expanded, compromising the wall's weight transfer system.  
Local construction methods sometimes endeavor to integrate HCTs with other masonry materials, such natural stone, adobe bricks, or partly baked clay bricks, inside a singular building [45]. This method may arise from gradual enhancements or extensions executed by homeowners, whereby the bottom level retains a stone foundation, but the subsequently added higher level is constructed using HCTs to save weight. Although these mixed-material configurations theoretically address certain mass-related challenges, they practically result in significant stiffness discontinuities and inconsistent bonding surfaces.  
The diverse mechanical and thermal qualities of various masonry materials hinder their smooth integration. A stone foundation with an uneven top surface might obstruct appropriate seating for the HCTs above, resulting in substantial mortar joints susceptible to breaking. The interaction with inferior mortar exacerbates these interface zones, promoting delamination when cyclical seismic stresses induce out-of-phase displacements between the two segments. In several residences analyzed after the 2023 Kahramanmaras earthquake, the top HCT section was noted to have largely collapsed independently of the bottom stone or adobe section, highlighting how inadequate interface quality essentially negated any composite action. Common damage models in HCTs and composite walls can be attributed as out-of-plane ejection of tile segments, diagonal shear with rapid crack propagation, and interface failure in hybrid walls. Example photos are shown in Figure 8.  
Figure 8: Damage due to use of HCT units.