# Seismic Effectiveness of Multiple Seismic Measures on a Continuous Gider Bridge
## 3 Establishment of a Finite Element Model of a Bridge
### Simulation of the Bridge and Seismic Measures
#### 3.2.3 Selection and Simulation of Unseating Prevention Devices  
When the seismic load exceeds the designed strength, and the restrainer is insufficient to control the excessive displacement of the bridge, the unseating prevention device is an effective control measure to prevent the bounding between girders caused by bearing failure and the damage caused by falling.  
There are various unseating prevention measures in bridges, such as steel plates, connecting bars, cables, and pre-stress wires. Compared to the other devices, cables have better applicability, and their initial space is controllable. Thus, a cable unseating prevention device was selected for this study [39], as shown in Figure 4.  
The hoop element in the finite element program was used to simulate the unseating prevention devices; the finite element model is shown in Figure 4c. The standard tensile strength of the cable unseating prevention devices is 1200-1580 MPa, the elastic modulus is 206 GPa, the relaxation rate is 2.5%, the nominal diameter is 5 mm, and the number of wires is 61 [14].  
When the relative displacement between the superstructure and substructures \(d^{\prime}\) exceeds the initial space of the unseating prevention device s, the unseating prevention device will start to work. The design parameters, such as the initial space and stiffness, can be considered in the model:  
s is the initial space of the unseating prevention device,  
\(d^{\prime}\) is the relative displacement of superstructure and substructures of the bridge,  
\(k^{\prime}\) is the stiffness of the unseating prevention device,  
\(f^{\prime}\) is the force of the unseating prevention device, whose value is calculated according to Equation (10):  
\[f^{\prime}=\left\{\begin{array}{c}k^{\prime}(d^{\prime}-s)\\ 0\end{array}\left(\begin{array}{c}d^{\prime}-s\geq 0\\ d^{\prime}-s<0\end{array}\right).\right. \tag{10}\]  
The simulation of the unseating prevention device is similar to that of the restrainer. The initial clearance of the unseating prevention device should be able to prevent the bearing from failure; in addition, the prevention device should be triggered when the restrainer is no longer able to limit the excessive displacement of the bridge.  
It has found that unseating prevention devices with a smaller initial clearance and a more substantial cable length (with a lower cable stiffness) were more efficacious [23; 26]. Therefore, according to the actual conditions of the bridge, an unseating prevention device with an initial  
Figure 4: Different forms of cable unseating prevention devices and finite element model: (**a**) girder-to-girder form, (**b**) pier-to-girder form, (**c**) finite element model of cable unseating prevention devices.  
clearance of 8 cm (slightly less than the allowable deformation of the bearing, which is 8.12 cm), and a length of 2 m was selected as the ultimate seismic control measure to coordinate with the limit device. The stiffness of unseating prevention devices \(k^{\prime}\) was calculated to be 227,620 kN/m, and it has a fracture force of 3340 kN.