# Evaluation of High-Speed Railway Bridges Based on a Nondestructive Monitoring System
## 3 Evaluation of the Bridge Condition Using SHM Monitoring Data
### Evaluation of the Bridge Gorder Vibration Behavior  
Figure 4a,b illustrate the typical vertical acceleration time histories of the passage monitoring points of the girder measured from accelerometers 1 to 5 for the 290 km/h and 406 km/h speed, respectively. It can be seen that the \(dt\), (\(dt=t_{2}\) (leave time) \(-t_{1}\) (entrance time)) values are reported as 5.9, 2.92, 1.9 and 1.85 s with the speeds 290, 360, 400 and 406 km/h, respectively. This means that the vibration time effect on the bridge decreased by 68.65% as the train speed increased from 290 to 406 km/h. In addition, it is observed that the vibrations of the monitored points at a speed of 290 km/h are approximately the same, while as the speed increased, the entrance and exit monitoring points are experiencing high vibration response compared to the mid span point. Figure 4: Vertical acceleration time histories of the girder caused by a high-speed train. (**a**) passage response points for 290 km/h; (**b**) passage response points for 406 km/h. Figure 3: Structural health monitoring (SHM) system locations and components. Figure 5 shows that the vertical acceleration of the mid-span point of the girder at 406 km/h is much smaller than that of 290 km/h. In addition, it is noticed that the vibration response for the passage way is higher than that of the opposite side at 400 and 406 km/h, while at 290 and 360 km/h, the acceleration responses for the passage and opposite sides are equal. These indicate that, although the structural layouts of the five monitoring points of the main girder are the same, there is a significant difference between the vertical vibration characteristics of two sides of the bridge due to the rail irregularity in the vertical directions with different speeds. Thus, there is a need to monitor the vertical accelerations of the span in the long term so as to realize anomaly alarms for vibration behavior of the main girder.In addition, the measured vertical acceleration is smaller than the FEM results by 62% according to the design criteria of the bridge (as shown Table 1),which means that the real response is safe. Furthermore, the girder torsional behavior should be studied with new development trains' speeds. Figure 6 illustrates the dynamic torsional behavior of the bridge girder based on vertical acceleration measurements [21].The torsional behavior of the girder due to train passage at 406 km/h is shown in Figure 6a. While the torsional behavior of the mid span point at different speeds is shown in Figure 6b. \[\mathrm{T}=\frac{\mathrm{a}_{\mathrm{pass}}-\mathrm{a}_{\mathrm{opps}}}{l} \tag{1}\]  
where \(\mathrm{T}\), \(\mathrm{a}_{\mathrm{pass}}\), \(\mathrm{a}_{\mathrm{opps}}\) and \(l\) are the torsion, acceleration for the passage and opposite points and distances between sensors' positions. It is noticed that the torsional values increased as the train velocity increased with maximum values at points 1 and 5. As the speed increased from 290 to 406 km/h, the torsional values increased by  
Figure 5: Vertical acceleration time histories of the passage and opposite mid span girder point. (a) passage response of mid span point; (b) opposite response of mid span point. Figure 6: Torsional measurements for the bridge girder. (a) the torsional of the monitoring points at 406 km/h passage; (b) mid span point torsional values. 61.20%, while they increased by 22.33% as the train speed increased from 360 to 406 km/h. This means that the bridge deck torsional response is higher than the vibration response. The girder torsional value at the mid span point, 0.103 rad/s\({}^{2}\), is within the design values, as shown in Table 1.  
The statistical parameters, maximum and root mean square (RMS), for the acceleration measurements and torsional calculations at 360 and 406 km/h are presented in Table 2. The 360 km/h is assumed as the base-speed for the study of the bridge safety because the bridge design speed is 350 km/h.