## 5 Case Study: Progressive Damage of a Masonry Arch Bridge Model  
To assess the applicability and adaptability of the proposed methodology to real engineering structures, data collected from an experimental study conducted at the Materials Lab of the Civil Engineering Department at Istanbul Technical University is presented. The test specimen comprises a 1/4-scale model of a specific span of the Uzunkopru bridge, a historical Ottoman bridge, dating back to the 15 th Century, located over the Ergene River in Edirne, Turkey. The model features an arch ring with a span of 1.73 m and a rise of 0.94 m. It has an overall width of 1.4 m, encompassing both the spandrel walls and the fill. A reinforced concrete slab, 10 cm thick, with plan dimensions of 3.20 \(\times\) 1.55 m, served as the foundation for the bridge model. Figure 5a,b display the photo of the bridge and the sketch of the test specimen. Figure 4: Localization of Damage for the Damage Cases 1–7. As part of an extensive experimental campaign investigating the collapse of this arch bridge, vibration testing was conducted to monitor its structural health. Impact tests were performed, and acceleration data were collected at both the initial baseline and various damage stages of the specimen. Progressive damage states were simulated by incrementally increasing the vertical load applied at the quarter length of the specimen in three stages, up to a collapse load of 70 kN. The first stage (DS 1) involved loading up to 30 kN, the second stage (DS 2) up to 60 kN and the final stage reached the ultimate load of 70 kN. Each stage included three repetitions of loading-unloading cycles, with impact tests conducted after the third cycle of unloading to monitor the structural response. A total of six accelerometers were used to collect acceleration measurements: four triaxial units (A\({}_{2}\), A\({}_{4}\), A\({}_{6}\) mounted on the structure; A\({}_{5}\) mounted on the foundation to capture baseline motion) and two unidirectional units (A\({}_{1}\), A\({}_{3}\)) mounted on the structure. The experimental setup is illustrated in Figure 5c,d. The arrangement of the accelerometers and the impact locations are indicated on the sketch, along with the geometric features of the specimen. Acceleration data were recorded with a sampling frequency of 1000 Hz (sampling interval of 0.001 s) to capture the dynamic response of the structure over a wide frequency range. While the beam model served as a simplified, controlled validation platform for demonstrating the fundamental capabilities of the proposed damage detection methodology under idealized conditions, the masonry arch bridge test provided a more demanding proof-of-concept in a complex, real-world structure. This setup introduced additional challenges such as discontinuities, material heterogeneity, and nonlinear boundary effects that are absent in the beam model. The intent was not to replicate the mechanical behavior of the beam but to assess how well the transmissibility-based approach generalizes to practical applications under realistic conditions. Figure 5: (**a**) Uzunkopru Bridge, Edirne, Turkey, (**b**) sketch of the test specimen with accelerometer arrangement and impact locations, (**c**) front view, (**d**) back view of the test specimen. Figure 6 shows the load-displacement curves recorded during the static testing and the observed cracks when loading the specimen to the first 30 kN (DS 1) and then to 60 kN (DS 2). The damage observed during the first loading step of 30 kN included a vertical crack in the spandrel wall joint originating from the keystone extending to the upper edge as well as initiation of longitudinal cracking between the extrados and the wall. The longitudinal cracking that occurred between the voussoirs was measured to be around 1 mm, as shown in Figure 6c. Upon unloading the specimen, there was no significant permanent deformation observed at this load level. Damage observed at the end of the second load-unload cycles, which reached 60 kN, is shown in Figure 6d,e.