## 4 Results and Validation
### Full-Scale Testing
#### 4.2.2 Railway Bridge Monitoring  
The modal identification of the dominant natural frequencies and mode shapes of the railway bridge in vertical and transverse directions considered seventeen ambient vibration records. Analyzing the information collected, as mentioned in the flexible structure monitoring, every ambient vibration dataset was examined to select the best time windows for accurate modal identification and MAC evaluation. Due to the length of wired sensors and exposure to environmental conditions, the amplitude of the acceleration signal from the PCB L74 accelerometer was disturbed by noise. Thus, it was subjected to a high-pass Butterworth filter to remove the noise component from the data transmission system. Figure 26 presents a time window of the ambient transverse vibration signal from the structural response corresponding to the measurement channel 3. The ambient vibration amplitude for both PCB L74 and ADXL355 RBP2 accelerometers reached approximate values up to 0.0015 g.  
The vertical and transverse dynamic responses of the steel structure in the frequency domain from both SHM systems are shown in Figure 27 and Figure 28, respectively. For detecting the natural frequencies and mode shapes for each direction of analysis separately, the structural response was defined by the S1 values' frequency line of the SVD spectrum. In the vertical dynamic response of the railway bridge, the first singular value peak identified by both monitoring systems reached a frequency of 0.98 Hz. However, the mode shape vector associated with this frequency did not represent the dynamic response of the structure well. Next, for this first singular value peak, two singular value peaks were detected at 6.83 and 15.62 Hz frequencies for both monitoring systems. The mode shape vector related to each frequency remarkably represented the dynamic system response. Therefore, the 6.83 Hz natural frequency was suggested as the first dominant vibration mode, and the 15.62 Hz natural frequency was assumed as the second dominant vibration mode. In the transverse dynamic response of the railway bridge, the first and third singular value peaks identified by both monitoring systems indicated frequencies of 2.93 and 13.67 Hz, respectively. The mode shape vector related to each frequency significantly represented the dynamic system response. Therefore, the 2.93 Hz natural frequency was adopted as the first dominant vibration mode, and the 13.67 Hz natural frequency was suggested as the second dominant vibration mode. The second singular value peak detected was a frequency of 7.81 Hz, but its mode shape vector did not depict the dynamic response of the structure importantly. Finally, the fourth singular value peak dist  
Figure 26: Transverse ambient vibration record of the railway bridge on measurement channel 3.  
the low-cost SHM system were exactly equal considering the approach of the reference SHM system. For the mode shapes' validation from the WSN monitoring system, the MAC values were calculated for the vertical and transverse vibration mode correlation (Figure 31). The diagonal values of the vertical modes MAC matrix were relatively close to unity. The consistency of the vertical vibration mode 1 was really close to the MAC acceptance level, and the correlation of the vertical vibration mode 2 demonstrated a reliable outcome concerning the state-of-the-art acceptance criterion considered in the present study. On the other hand, the diagonal values from the transverse modes MAC matrix were close to unity. The consistency of the transverse vibration modes 1 and 2 was high. The limited vibration mode correlation with the vertical mode shapes of the structure arose because of the vibration dataset asynchrony detected in the low-cost SHM system performance.