# Study on Seismic Isolation of Long Span Double Deck Steel Truss Continuous Grider Bridge
## 3 Seismic Response of Structures
### Comparative Analysis of Seismic Response  
It is found that the seismic responses of the pier bottom are within the resistance range of the pier after the isolation scheme is adopted. Figure 14 shows the seismic response time history curve of the pier bottom of the seismic and isolation structure.  
The isolation response time history curve of the 1# pier bottom changes obviously when compared with the seismic structure. After using FPB, the time history curve of the pier bottom of the structure is apparently enveloped in the seismic response of the structure. The shape of the time history curve is basically consistent, and the peak point of the isolation structure is evidently smaller than the seismic structure.  
It can be seen from Table 3 that the bending moment and the shear force of the 1# pier are reduced by 65.33% and 62.67% compared with the seismic structure after using the isolation scheme, and by 66.22% and 63.76% for the CSDI scheme. At the same time, the bending moment and shear force at the bottom of the 2# pier increased by 174.54% and 67.88% compared with the seismic structure when FPB was used. However, the increase in amplitude was small, which is 56.02% and 15.87% when CSDI was adopted; this is more conducive to the safety of the pier.  
It can be seen from Table 4 that the relative displacement between the upper and lower joints decreases after the seismic isolation schemes are adopted, which is conducive to protecting the upper steel truss structure. In the seismic structure, the relative displacement  
Figure 14: Seismic and isolation structure of time history curve of the 1# pier bottom. (**a**) Shear time history curve of the 1# pier bottom. (**b**) Moment time history curve of the 1# pier bottom.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Seismic Structure**} & \multicolumn{2}{c}{**Isolation Structure**} & \multicolumn{2}{c}{**CSDI Structure**} \\ \cline{2-7}
**Pier** & \begin{tabular}{c} **Bending Moment** \\ **in Pier Bottom** \\ **(kN-m)** \\ \end{tabular} & \begin{tabular}{c} **Shear in Pier** \\ **Bottom (kN)** \\ \end{tabular} & \begin{tabular}{c} **Moment** \\ **Isolation Ratio** \\ \end{tabular} & \begin{tabular}{c} **Shear Isolation** \\ **Ratio** \\ \end{tabular} & \
\begin{tabular}{c} **Moment** \\ **Isolation Ratio** \\ \end{tabular} &
\begin{tabular}{c} **Shear Isolation** \\ **Ratio** \\ \end{tabular} \\ \hline
0\# & \(4.86\times 10^{5}\) & \(2.33\times 10^{4}\) & \(-21.40\%\) & \(-16.65\%\) & \(-15.23\%\) & \(-20.32\%\) \\
1\# & \(1.35\times 10^{6}\) & \(7.55\times 10^{4}\) & \(-65.33\%\) & \(-62.67\%\) & \(-66.22\%\) & \(-63.76\%\) \\
2\# & \(2.16\times 10^{5}\) & \(1.92\times 10^{4}\) & \(174.54\%\) & \(67.88\%\) & \(56.02\%\) & \(15.87\%\) \\
3\# & \(5.33\times 10^{5}\) & \(2.15\times 10^{4}\) & \(-20.45\%\) & \(-16.97\%\) & \(-23.64\%\) & \(-30.58\%\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Seismic response comparison of substructure.  
reaches 121.41 mm. After using FPB, the relative displacement is reduced to 91.11 mm, which is 24.96%; when CSDI is adopted, the relative displacement is reduced by 42.05%.  
It can be seen from Figure 15 that the hysteresis curve of the 1# pier using FPB is close to that of a parallelogram with a large area of hysteresis loop, which effectively exerts its isolation energy dissipation performance. For the hysteresis curve of the 2# pier using dampers, it is found that the hysteresis loop area is also full. This indicates that the CSDI scheme can effectively exert the isolation performance of FPB and the characteristics of the dampers.