## 4 Vibration Measurements
### Long-Term Monitoring Results  
Continuous monitoring under ambient environmental conditions was conducted twice without altering the prestressing forces. The bridge was prestressed at 360 kN and 200 kN, representing normal and low levels of prestressing conditions, respectively.  
To retrieve the impulse responses from the vibration measurement data, the procedure introduced in Section 4.2 CCFs was applied to the recordings at the stations. By correlating 15-s segments in traces at two stations as a pair, the total CCFs in an hour were stacked and normalized. The causal parts represent the impulse responses at hourly intervals for over 60 h. The impulse response in the first hour was chosen as the reference wave signal for CWI analysis, allowing for the estimation of velocity variations over the 60-h period.  
To identify velocity changes in low- and high-frequency wave propagations, only the wavefield between the vibration Station pair 1-4 was compared with the ultrasonic wavefields, as the velocity variations observed between vibration Station pairs 1-2 and 3-4 were similar to those between vibration Station pair 1-4. The results, shown in Figures 11 and 12, reveal a significant discrepancy in the relationship between velocity variation and bridge temperature. The \"bridge temperature difference\" in Figures 11-14 refers to the comparison with a temperature reference, which is part of the reference data record chosen for CWI analysis. This representation allows measurement data from different days to be combined in a single plot for comparison.  
Two different scales of wavelength traveling in their corresponding wave propagation distances demonstrate consistent velocity variation. To quantify the impact of bridge temperature differences, linear regression analysis was applied to show their correlation by ratios (Figures 13 and 14).  
The difference between the two ratios under normal and low prestressing conditions in the vibration signals is smaller than that in the ultrasonic signals. Overall, the slopes at 200 kN are steeper than those at 360 kN (Figures 13 and 14). This is because heat can cause existing cracks to open, leading to a greater decrease in velocity. It is straightforward to infer that the bridge has more cracks under 200 kN than under 360 kN, which are influenced by high bridge temperatures, causing more significant velocity decreases. The influence of  
Figure 10: The causal parts of eight CCFs with a cut-window length of 7.5 s.  
Figure 9: The vibration records at Stations 1 and 2 (south span).  
damage and cracks on velocity/temperature dependence has also been demonstrated in other research [38].  
Based on this observation in Figures 12 and 13, the north mid-span is identified as having a higher damage level than the south mid-span. These continuous monitoring results provide a basic assessment for identifying prestress loss in the experiments.  
Figure 11: Continuous monitoring of velocity variations in vibration signals under the two prestressing levels: 360 kN and 200 kN.  
Figure 12: Continuous monitoring of velocity variations in ultrasonic signals under the two prestressing levels: 360 kN and 200 kN.  
Figure 13: Correlation between velocity variation of ultrasonic signals and bridge temperature in continuous monitoring.