# Study on Seismic Isolation of Long Span Double Deck Steel Truss Continuous Grider Bridge
## 1 Introduction  
Earthquakes are a kind of natural disaster with sudden and strong damages. Under the actions of rare earthquakes, even a building's structure can collapse, which threatens the safety of human beings [1; 2; 3]. The emergence of double deck bridges in recent years has eased urban traffic to a certain extent, but consequent seismic problems that follow are thought-provoking. During the Loma Prieta earthquake in 1989 [4; 5; 6], the double deck viuctuct structure in San Francisco, USA, the upper supporting column of the bridge broke and fell onto the lower deck, and the lower deck had excessive lateral displacement under the earthquake action, which caused it to collide with the supporting column and it suffered shear failure. Therefore, in the process of double-layer bridge design, the following problems need to kept in mind: (1) the vertical support columns of upper and lower deck should be able to cope with large deformation, which means to design using ductile components; (2) paying attention to the relative displacement of the upper and lower deck joints to avoid shear failure; and (3) the upper and lower deck beams and joints should be designed according to the capacity protection.  
It was found that, when the damping of a structure is stable and the natural vibration period of the structure exceeds a certain limit, the seismic response of a structure will decrease with an extension in the natural vibration period. When a natural vibration period is stable, the seismic response decreases with the increase in damping. Therefore, in the design of structural seismic dissipation and isolation, we can design according to these two basic rules: (1) by prolonging the natural vibration period and (2) structural dampingincreases. Taking a long-span cable-stayed bridge as the engineering background, [PERSON] et al. established two seismic isolation systems: friction pendulum bearing (FPB) of fixed pier and damping spherical bearing of fixed pier [7]. [PERSON] et al. analyzed the change in the axial force of a bridge structure after using friction pendulum bearing (FPB) compared to seismic structure [8]. [PERSON] et al. applied friction pendulum bearing (FPB) to structural seismic isolation design, analyzed the vertical seismic action of the structure, and deduced the motion equation of the bearing using the Newmark-\(\upbeta\) method [9]. In addition, there are many scholars and experts who have explored and discovered facts about the performance of viscous dampers. [PERSON] et al. introduced the characteristics of viscous damper [10]. Through theoretical analysis, it has been pointed out that the main factors affecting the performance of dampers are the damping coefficient and damping index. [PERSON] studied the optimization of the layout position of a damper, and the influence of the damping coefficient and damping index [11]. [PERSON] and [PERSON] studied a nonlinear damper and optimized the parameters of the damper [12].  
Seismic control of engineering structure is to change and adjust the dynamic characteristics of the structure by installing some isolation devices (such as isolation bearings), some energy dissipation mechanisms (such as dampers), some additional substructure (such as tuned mass dampers [13; 14; 15; 16; 17]), or to exert external forces, so as to limit the dynamic response of an engineering structure under the action of an earthquake within an allowable range, in order to ensure the safety, serviceability, and durability of the engineering structure [18; 19; 20; 21; 22].  
Based on the characteristics of a double deck steel truss continuous girder bridge, to ensure that the structure is in a safe state under the action of an earthquake, this paper studies the scheme of friction pendulum bearing (FPB) and the scheme of a combined use of friction pendulum bearing and viscous dampers, which are a kind of combined seismic dissipation and isolation (CSDI) bearing.