# Study on Dynamic Behavior of Bridge Pier by Impact Load Test Considering Scour
## 3 Results
### Field Pier Test Results  
The conditions in the field pier test were mainly divided into three cases: (1) the pier with a girder, (2) the pier without a girder (removed), and (3) the 1 m deep scour on the ground that is adjacent to the pier. The natural frequency of the field pier was analyzed in the same way as the analysis method of the full-scale pier model test. Figure 10 shows the first mode natural frequency under the three field conditions. The natural frequencies under the field conditions were 21, 13, and 9.5 Hz, respectively, as demonstrated in Figure 12a-c. In the first mode, the natural frequency had the most sensitive change according to the field conditions (surcharge load and scour). This indicates that there are limitations in accurately evaluating the stability of the pier using the first mode natural frequency, which is affected by all of the variables. \begin{table}
\begin{tabular}{c c c c} \hline
**Surcharge Load (kN)** & **First Mode (Hz)** & **Second Mode (Hz)** & **Third Mode (Hz)** \\ \hline
0 & 22.40 & 15.14 & 54.19 \\
25 & 23.40 & 14.65 & 55.66 \\
50 & 24.40 & 13.67 & 56.12 \\
75 & 22.95 & 12.70 & 56.61 \\
100 & 22.40 & 12.20 & 57.12 \\
125 & 23.90 & 11.23 & 54.20 \\
150 & 24.40 & 10.74 & 55.18 \\
175 & 23.50 & 10.25 & 55.66 \\
200 & N/A & 9.766 & N/A \\
225 & N/A & 9.765 & N/A \\
250 & N/A & 8.3 & N/A \\ \hline \end{tabular}
\end{table} 
Table 2: Natural frequency results of the full-scale model pier according to the surcharge load. Figure 12: First mode natural frequency for each field condition. (**a**) With a girder; (**b**) Without a girder; and (c) 1 m deep scour on the ground adjacent to the pier. Figure 13 shows the natural frequency of the second mode for each field condition. The natural frequencies under the field conditions were determined to be 20, 20, and 14 Hz, respectively, as displayed in Figure 13a-c. In the second mode, the natural frequency was identical regardless of the presence of a girder, unlike the first mode. This was similar to the result that the natural frequency of the second mode was not affected by the surcharge load in the full-scale pier model test. Due to the occurrence of scour, the natural frequency was reduced by approximately 6 Hz. Therefore, it is determined that the second mode natural frequency can be used as an indicator that can predict the condition of the ground that is adjacent to the pier without being affected by the surcharge load. Figure 14 shows the third mode natural frequency for each field condition when the torsional behavior of the pier occurred. The accelerometers were attached to the left and right of the top of the pier. The side that simulated scour was named left and the side that did not simulate scour was named right. There was no significant difference in the third mode natural frequency depending on the field conditions as in the first and second modes. Figure 14a shows the natural frequencies in the torsional behavior (third mode) before removing the girder. It was observed that very similar natural frequencies occur on both sides. In Figure 14b, however, a difference of approximately 2.5 Hz occurred between the natural frequencies that are measured on both sides after the girder removal. This is because the fixed end effect of the upper girder disappeared with the girder removal; thus, restraining the torsional behavior. Figure 14c shows the natural frequency results for the torsional behavior (third mode) when a 1 m deep scour was simulated on one side of the pier. The natural frequency values were determined to be similar to those in Figure 14b, but the acceleration of the side with a 1 m deep scour exhibited a sharp reduction in the amplitude. This indicates that it is possible to predict the location and degree of the scour. Table 3 summarizes the natural frequency results for each mode number of the pier according to the presence of scour. It was confirmed that the presence of scour decreases the natural [29; 30; 31]. Figure 13: Second mode natural frequency for each field condition.