# Study on Seismic Isolation of Long Span Double Deck Steel Truss Continuous Grider Bridge
## 4 Conclusions  
Taking a double-deck steel truss continuous girder bridge as the research object, the nonlinear time history analysis method is used to analyze the seismic performance, seismic dissipation, and isolation performance of the bridge structure. The parameters of the FPBs and viscous dampers are analyzed. We can draw the following conclusions:  
(1) When isolation is not carried out, the seismic response of the longitudinal bridge direction is mainly borne by the 1# pier (fixed pier). The superstructure of the bridge is a double-deck steel truss structure, and the continuous beam is not conducive to the safety of the bridge structure.  
(2) When FPBs are used in the seismic isolation design of the whole bridge and R and \(\mu\) are optimized, the results show that, with the increase in R, the seismic response of the pier bottom decreases, but the displacement of the main beam increases; with an increase of \(\mu\), the seismic response of the pier bottom increases, but the displacement of the main beam  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Seismic Structure**} & \multicolumn{2}{c}{**Isolation Structure**} & \multicolumn{2}{c}{**CSDI Structure**} \\ \cline{2-5}
**Node** & **Relative** & **Relative** & **Isolation Rate** & **Relative** \\
**Number** & **Displacement (mm)** & **Displacement (mm)** & **Displacement (mm)** & **Isolation Rate** \\ \hline
1 & 63.13 & 51.44 & \(-\)18.52\% & 48.63 & \(-\)22.97\% \\
2 & 67.25 & 55.56 & \(-\)17.38\% & 50.36 & \(-\)25.12\% \\
3 & 73.16 & 60.53 & \(-\)17.26\% & 55.21 & \(-\)24.54\% \\
4 & 83.94 & 68.09 & \(-\)18.88\% & 60.33 & \(-\)28.13\% \\
5 & 99.65 & 77.78 & \(-\)21.95\% & 68.33 & \(-\)31.43\% \\
6 & 121.41 & 91.11 & \(-\)24.96\% & 70.36 & \(-\)42.05\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Seismic response comparison of superstructure.  
Figure 15: Hysteresis curve of piers adopting the CSDI scheme. (**a**) Hysteresis curve of the 1# pier using FPB. (**b**) Hysteresis curve of the 2# pier using dampers.  
decreases. The weight of the superstructure is relatively large, while the layout requires that the R of FPBs be as close as possible, so R can be used at 7-10 m, and \(\upmu\) is taken as 0.03, which can be used as the FPB parameters of the bridge.  
(3) In the design of CSDI, FPB is used for the 1# pier and ordinary bearings and viscous dampers are used for other piers. The results show that with the increase of C, the seismic response of the pier bottom increases, but the displacement of the main beam decreases; with an increase in \(\upalpha\), the seismic response of pier bottom decreases, but the displacement of main beam increases. When C is 4000-6000 and \(\upalpha\) is 0.3, the effect of CSDI can be brought into play.  
(4) It is found that the time history curves of the shear force and the bending moment at the bottom of the 1# pier are significantly reduced when FPBs are used for isolation research. When the CSDI design is adopted, the hysteresis loop of the FPBs and dampers is regular and the area is full, which can give full play to the effect of the CSDI device. From the point of view of bearing additional stress, the effect of CSDI is better, which can effectively control the displacement of the bridge superstructure and reduce the relative displacement between piers and beams.  
Y.C.: Conceptualization, Formal analysis, Methodology, Resources, Software, Funding acquisition, Project administration; H.S.: Conceptualization, Data curation, Writing--original draft, Methodology, Visualization; Z.F.: Investigation, Validation, Supervision. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Project of Natural Science Foundation of Fujian Province, grant number 2016)05122 and Project of Science and Technology Research of Department of Education of Fujian Province, grant number JA14039, JK2013001.  
The authors declare no conflict of interest.