# Seismic Performance Evaluation of Highway Bridges under Scour and Chloride Ion Corrosion
## 2 Earthquake and Scour Combination in the Failure Probability of Bridge
### Application for a Practical Project
#### 2.2.2 Determination of the Critical Section of Members and Structural Failure Mode  
In comparison with the fragility analysis of the No. 1 pile foundation, the No. 2 pile foundation, and pier components under combined action of scour and earthquake, pile foundation components were identified as risk analysis objects by extracting the average response of the component section along different directions at different scour depths when the basic PGA was equal to 0.2 g.  
Although pile group foundations have the same section and material in general, which leads to the similar section resistance, the response under the combined action of earthquake and scour is quite different. Hence, the No. 1 pile foundation and No. 2 pile foundations were selected as the analysis objects to determine the one suffering the most unfavorable forces.  
It can be observed from Figures 4-9 that the seismic effect of the No. 1 pile foundation under combined action along longitudinal and transverse directions was both higher than that of the No. 2 pile foundation, which renders the No. 1 pile foundation the component for failure probability analysis due to its more disadvantageous stress state under the combined action of earthquake and scour. The response of the No. 1 pile foundation along transverse and longitudinal directions of the bridge at different scour depths was calculated under the corresponding time history of seismic peak acceleration which took 0.2 g, 0.4 g, and 0.6 g as the representative values. The maximum value of the response was extracted at the same time. The specific data are shown in Figures 10-13.  
Figure 8: Average value of combined effects of seismic load and constant load on the No. 2 pile foundation in the longitudinal direction.  
Figure 6: Average value of combined effects of seismic load and constant load on No. 1 pile foundation in longitudinal direction.  
Figure 7: Average value of the combined effects of seismic load and constant load on the No. 1 pile foundation in the transverse direction.  
Figure 9: Average value of the combined effects of seismic load and constant load on the No. 2 pile foundation in the transverse direction.  
Figure 11: Capacity-demand ratio in the bending moment of the No. 1 pile foundation in the longitudinal direction.  
Figure 12: Bending moment of the No. 1 pile foundation in the transverse direction at different scour depths.  
Figure 10: Bending moment of the No. 1 pile foundation in the longitudinal direction at different scour depths.  
Figure 13: Capacity-demand ratio in the bending moment of the No. 1 pile foundation in the transverse direction.  
Through the comparison of the capacity-demand ratio in the bending moment of pile foundation along the above two directions, it can be found that the pile foundation was in a more unfavorable stress state under the combination of seismic load input transversely and dead load, which caused the flexural failure of the pile foundation along the transverse direction as the determination of failure mode.