# Bridge Health Monitoring via Displacement Reconstruction-Based NB-IoT Technology
## 3 Bridge Health Monitoring Experiment
### Full-Scale Bridge Experiment  
A full-scale bridge is used to demonstrate the implementation of the proposed Level 1 damage detecting method. Wenzhou Bridge, located in Taipei city, is selected as the test bridge because it is an important bridge, and the riverside park under the bridge provides test personnel to facilitate the assembly of test equipment. Wenzhou bridge is constructed in the Year 2000. It is a prestressed girder bridge with four lanes. No damage is visually detected before our testing. The experiments are initiated at the beginning of April 2020, and field data are continuously collected until now. Due to lack of information and based on the visual inspection, the status of the bridge in April 2020 is assumed to be in a healthy condition. Wenzhou Bridge is an RC prestress girder bridge with dimensions shown in Figure 26. Accelerometers integrated with NB-IoT are attached along the bridge span at the first, second, and third quarters. The power of the equipment is supported by several detached batteries that need to be refilled regularly. The sampling rate is 125 Hz, and acceleration data are recorded every 12 min. Each record consists of 4096 points. That is, each record is approximately 33 s-long. The collected acceleration data are transformed into displacements based on the proposed algorithm. Because the proposed reconstructed displacement algorithm requires the bridge's fundamental frequency, FFT analysis is first performed to obtain the frequency, as shown in Figure 27. It is seen that a technical problem occurred from 6/15 to 6/30 (month/date), causing a gap in collecting data. However, such a gap does not affect the following analysis. The frequency of the bridge is assumed to be 4 Hz that is calculated based on the mean of the calculated FFTs. As explained in Section 3.1.2, different frequencies are observed, as shown in Figure 27, where the frequency is ranging from 3.5 Hz to 4.5 Hz.  
When applying the proposed Level 1 damage detection to the Wenzhou Bridge, the condition in April is assumed to be the original condition, which is considered as a reference point for calculating the Level 1 damage indicator. Damage indicator is calculated for each month except June. Figures 28-30 show the Level 1 damage indicator for the Wenzhou Bridge. It is shown that the bridge still remains in  
Figure 24: Level 2 damage indicator for Case 5.  
Figure 25: Level 2 damage indicator for Case 6.  
the same condition as that in April. Figures 28-31 depict the proposed Level 1 indicator by comparing the normalized _RMSD_ in April. If the difference is large, indicating that there is damage occurrence. As shown, the largest _RMSD_ difference is \(8\times 10^{-3}\), which is considered a relatively small value compared to those of the small-scale experiments. It is, therefore, concluded that the bridge condition from May to September does not significantly change from April.  
Figure 26: (**a**) Sensor positions and bridge dimension; (**b**) power supply for accelerometer and NB-IoT; (**c**) side view of Wenzhou bridge.  
Figure 28: Level 1 damage indicator of Wenzhou bridge in May.  
Figure 27: Calculated fundamental frequencies for each set of data.  
Figure 29: Level 1 damage Indicator of Wenzhou Bridge in July.