## 6 Discussion of the Control Effects of the Collapse Control System
### Fluid Viscous Damper Control
#### 6.2.2 Analysis of Isolation and Collapse Control Effect of the FVD  
(1) Isolation effect  
After the BC system is replaced with the control system, the seismic response of the bridge specimen is weakened. In order to compare the collapse control effect of different control systems in the later stage, the damping effect of the CFVD system and CLUC system under conventional ground motion is needed to be close to that of the CHDR system. Taking the El Centro wave case with longitudinal PGA of 0.4 g as an example, the peak relative displacement between the side pier and the girder in the CHDR system was reduced from 12.3 mm of the BC system to 10 mm, which showed an isolation rate of 18.7%. In the CFVD system, the peak relative displacement between the side pier and the girder was reduced from the initial 12.3 mm to 9.1 mm, and the isolation rate is 26%, as shown in Figure 17 and Table 5. The displacement control of the damping scheme was slightly better than that of the HDR isolation scheme, which was due to the fact that the actual energy consumption of the FVD is larger than that of the HDR, as shown in Figure 18. In addition, the damping force influence of the FVD on the middle pier is almost the same  
Figure 16: Time history curve of vertical displacement of the end of the main girder. as that of the HDR. Therefore, the response of the CFVD system is comparable to that of the CHDR system. In summary, the design parameters of the FVD are quite suitable for control effect analysis. (2) Collapse Control effect  
The maximum damping force \(F_{\text{max}}\) and the maximum stroke \(D_{\text{max}}\) of the FVD are determined to be 10 kN and \(\pm\)190 mm respectively by trial calculation, which ensures that the output force and displacement of the dampers do not exceed the capacity range before the bridge collapse. Under the longitudinal action of the El Centro seismic wave, the CDVD system finally collapsed when the PGA reached 1.4 g, as shown in Figure 19. The specific process is as follows: when \(t\) was 0.82 s, the damage begins to appear at the bottom of the middle pier; when \(t\) was 0.83 s, the middle bearings failed and the main girder began to slide; when \(t\) was 0.85 s, the longitudinal displacement response of the top of the middle pier reached 9.7 mm(see Figure 20a), resulting in the leakage of tendons at the bottom of the middle pier; when \(t\) was 0.86 s, the damage position of the bottom of the middle pier extended upward; when \(t\) was 0.9 s, the damage of the main girder resulted in the redistribution of the stress of the girder and the damage of the bottom of the girder near the middle pier; when  
Figure 17: Relative displacement time history curve between side pier and girder of the CFVD system. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Value1 (CHDR System)** & **Value2 (CFVD System)** & **(Value1 – Value2)/Value1** \\ \hline Relative displacement between the side pier and the girder (mm) & 10 & 9.1 & 9\% \\ Peak bending moment at the bottom of middle pier (kN\(\cdot\)m) & 38.4 & 38.1 & 1\% \\ Peak shear force at the bottom of middle pier(kN) & 179.6 & 176.8 & 2\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of seismic responses of two systems. Figure 18: Comparison of hysteretic curves between the FVD and HDR. was 1 s, the longitudinal displacement of the pier top reached 9 cm under the action of earthquake, the bottom of the middle pier had been completely destroyed, and the vertical displacement reaches 12 cm (see Figure 20b). Therefore, it could be concluded that the middle pier collapsed. The bottom of the girder near the original middle fulcrum of the main beam lost the support of the substructure, thus the bending cracking was serious, and the bridge deck function was lost, so the bridge specimen collapsed as a whole.