# Study on Dynamic Behavior of Bridge Pier by Impact Load Test Considering Scour
## 3 Results
### Full-Scale Pier Test Results  
Through a series of tests, the mode number of the pier was analyzed, and the natural frequency of the pier in each mode was derived. Figure 9a shows the natural frequency of the pier in case-2 where an impact was applied to the top of the pier in the bridge axis direction. The natural frequency was determined to be approximately 15.14 Hz. In addition, the phase was obtained for each height by using the measured values to analyze the behavior of the pier. Figure 9b shows the phase difference results that were obtained by using the phase for each position. It was determined that all of the phase differences where the natural frequency points occurred had a tendency to converge to \(0^{\circ}\). This is because all the measuring instruments that were attached to each height were deformed in the same direction. In other words, the behavior of the pier occurred in the same direction. The behavior had a tendency to be similar to the first mode of the pier eigenvalue analysis.  
Figure 8: Natural frequency analysis method. (**a**) Representative signal; (**b**) representative natural frequency; and (**c**) representative phase.  
Figure 9: Results of the impact vibration load test in the bridge axis direction. (**a**) Natural frequency of the first mode and (**b**) phase difference of the first mode.  
Figure 10a shows the results in case-1 where the natural frequency for each height was derived by applying an impact to the top of the pier in the pier length direction. The natural frequency of the pier was determined to be approximately 22.4 Hz, which is approximately 7 Hz higher in comparison to case-2 (bridge axis direction). This is because the stiffness of the pier was higher in the pier length direction than in the bridge axis direction. This confirms that the difference in the stiffness affected the natural frequency. To examine the behavior of the pier in the pier length direction, the phase difference was analyzed as shown in Figure 10b. In this instance, the phase difference converged to 0\({}^{\circ}\) when the natural frequency occurred along with the behavior in the first mode mentioned above. This appears to be similar to the behavior in the second mode of the pier eigenvalue analysis.  
Figure 11a shows the natural frequency results in case-3 where the acceleration was measured by applying an impact to the upper outer point of the pier. Two clear natural frequencies were observed, and they were 15.14 and 54.19 Hz. The phase difference results in Figure 11b show that the phase difference converged to 0\({}^{\circ}\) at 15.14 Hz. This indicates that the first mode occurred, as the behavior and the natural frequency were the same as those of the first mode. Figure 11b, however, shows that the phase difference was 180\({}^{\circ}\) at 54.19 Hz. These results demonstrated that the outer part of the pier behaved in opposite directions; thus, indicating torsional behavior. This behavior was similar to the third mode, and it appeared that applying an impact to the outer point of the pier in the bridge axis direction led to the first mode and the third mode, which was the torsional behavior of the pier. Table 2 summarizes the natural frequency of the pier by the mode number according to the surcharge load.  
Figure 11: Results of the impact vibration load test in the bridge axis direction (outside). (**a**) Natural frequency of the third mode and (**b**) phase difference of the third mode.  
Figure 10: Results of the impact vibration load test in the pier length direction. (**a**) Natural frequency of the second mode and (**b**) phase difference of the second mode.