# Seismic Study of An Isolated Cable-Stayed Bridge under Near-Fault Ground Motions
## 5 Influence of Various Factors
### Influence of Ground Motion Type  
In addition to the aforementioned non-uniform excitation, the impact of ground motion type on the bridge's seismic response should not be overlooked. To study the impact of different seismic wave types on the bounding effects of seismic isolation cable-stayed bridges, the responses of the bridge under the influence of near-fault seismic waves in Table 1 and No. 1-10 far-fault seismic waves in reference [33] are calculated separately. The average values of the calculated results are plotted in Figure 10. It is important to note that the seismic wave velocity is assumed to be 800 m/s.  
Figure 10a compares the bounding forces at two expansion joints under the influence of near-fault and far-fault seismic waves. On the left side is the bounding force at expansion joint 1, and on the right side is the bounding force at expansion joint 2. The vertical axis of the bar chart represents the value of the bounding force. It can be observed that, even considering the traveling wave effect, there is no bounding between the main and approach bridges under the action of far-fault seismic waves. However, poundings are significant under near-fault seismic waves. It is worth noting that, in the isolation system considered in this study, the ratio of the fundamental periods of the main bridge to the approach bridge is 0.82, exceeding the threshold of 0.7 specified by AASHTO code. Nevertheless, poundings still occur under near-fault seismic excitations, indicating that the design recommendations for \"conventional\" bridges in AASHTO are ineffective for the \"unconventional\" bridges considered as isolation prototypes in this study. The amplifying effect of near-fault seismic motions on responses should be considered in the design of long-span bridges.  
Figure 10b compares the longitudinal deformations of the main bridge under the influence of near-fault and far-fault seismic waves. Due to the assumed seismic wave velocity of 800 m/s, the bearing deformations exhibit asymmetry along the bridge axis direction under both seismic wave scenarios, with more pronounced asymmetry under near-fault seismic waves. Furthermore, there is a significant difference in the absolute values of bearing deformations under different seismic wave velocities. Under far-fault seismic waves, the maximum longitudinal deformation of the main bridge's bearings is 20.3 cm, while it increases to 64.8 cm under near-fault seismic waves. Figure 10c compares the bending moments at the bridge towers. Under far-fault seismic waves, the bending moment at the base of the tower is 8.12 \(\times\) 10\({}^{5}\) kN\(\cdot\)m, increasing to 1.46 \(\times\) 10\({}^{6}\) kN\(\cdot\)m under near-fault seismic waves.  
The analysis above indicates that, compared to far-fault seismic motions, near-fault seismic motions not only lead to poundings but also significantly increase the longitudinal bearing deformations and tower bending moments in the design of long-span bridges, emphasizing the importance of considering near-fault seismic motions.  
Figure 10: Average maximum responses considering different ground motion type: (**a**) pounding force at each expansion joint; (**b**) longitudinal bearing displacement; (**c**) longitudinal bending moment of tower.