## 3 Ambient Vibration Testing  
On 7 and 8 July 2022, the authors conducted an ambient vibration test on the bell tower. Single-axis piezoelectric accelerometers (Figure 7a), manufactured by PCB Piezotronics, Inc., were used to measure the vibrations (accelerations) of the structure. The tests were performed using two Hottinger Baldwin Messtechnik (HBM) QuantumX 8-channel data acquisition systems (Figure 7b).  
To determine the modal parameters of the bell tower, horizontal accelerations were measured at 16 measurement points on the tower walls at four different levels corresponding to the height of the bell tower: h\({}_{1}\) = 9.45 m; h\({}_{2}\) = 11.69 m; h\({}_{3}\) = 14.76 m; and h\({}_{4}\) = 17.52 m (Figure 8). Given the limited measurement resources and the author's desire to measure the acceleration of the bell tower at as many points as possible, the measurement is divided  
Figure 6: Crack pattern on the southwest wall of the tower.  
Figure 7: Equipment used for testing: (**a**) accelerometer; (**b**) data acquisition system [36].  
into two sets: TEST SETUP 1 and TEST SETUP 2. The arrangement of six accelerometers remained unchanged for both setups (so-called reference accelerometers). For the first set of measurements, most accelerometers were placed on the southeast side of the bell tower, and for the second set of measurements, most accelerometers were placed on the northwest side of the bell tower. The location of the measurement points and the arrangement of the reference accelerometers were carefully selected according to the expected modal shapes of the bell tower.  
At each measurement point, each accelerometer was attached to a steel cube (20 \(\times\) 20 \(\times\) 20 mm). Each cube was welded to steel plates (80 \(\times\) 80 mm), which were attached to the inside of the tower walls with screws (Figure 9). This type of sensor attachment ensures measurement in two orthogonal directions at the measurement points where the two accelerometers are attached.  
Figure 8: Illustration of the arrangement of the measurement points in both tests: TEST SETUP 1 and TEST SETUP 2 (Note: the reference accelerometers are marked in red).  
In both test setups, the accelerations of the bell tower structure were measured and recorded for 960 s (16 min). The duration of the measurement was chosen to comply with a widely accepted recommendation: the length of the time windows should be at least 2000-T\({}_{1}\), where T\({}_{1}\) is the period of the assumed fundamental vibration of the structure [37]. During the measurement, the structure was excited by the action of the wind, by micro-vibrations caused by the movement of people, and by the strikes of the small bells of the bell tower. Examples of the acceleration time histories recorded during the tests can be found in Figure 10. It is noteworthy that only a very small amount of ambient excitation was present during the tests. The maximum recorded acceleration is less than 0.6 m/s\({}^{2}\); it is recorded by the sensor at measurement point seven during TEST SETUP 1 and is probably caused by vibrating bells (Figure 11). The sampling rate was 200 Hz, which is much higher than required because the significant frequency content of the signals for the studied structure is below 20 Hz.  
Figure 10: _Cont._  
Figure 9: Example of accelerometers placed on the tower walls: (a) southeast wall at a height of 14.76 m (TEST SETUP 1); (b) northwest wall at a height of 9.45 m (TEST SETUP 2).