# Study on Impact-Echo Response of Concrete Column near the Edge
## 3 Impact Response of Concrete Column
### Edge Simulation Results  
In order to study the transient response of concrete frame columns under point impact, a three-dimensional finite element simulation was carried out. During the analysis, a dynamic load was applied at the edge of the column and the predominant frequency obtained by the analysis at the receiving point was compared with the predominant frequency in the central region of the column. The time history curve and predominant frequency of the central region of the column obtained were via simulation. The results are shown in Figure 5:  
As can be seen from Figure 4, the simulated predominant frequency of the middle part of the concrete is 2.563 kHz. To further study the influence of the edge on the dominant frequency, the acquisition points were set up every 5 cm from the column to the edge for the application of the load and the acquisition of the signal, and the results are shown in Figure 6: \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Edge Distance (cm)** & **1** & **10** & **19** & **20** & **The Middle of the Column** \\ \hline Impact-echo measured value (kHz) & - & 2.563 & - & 2.686 & 2.686 \\ \hline SPC-MATS measured value (kHz) & 2.441 & 2.441 & 2.686 & 2.686 & 2.686 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of typical dominant frequencies of the edge test results. Figure 5: Test in the middle of the column under the edge condition: (**a**) time\(-\)displacement curve; (**b**) amplitude spectra. Figure 6: The edge acquires a spectrum every 5 cm: (**a**) spectrogram; (**b**) frequency change diagram for each point. The simulation position is impacted at different positions from the edge, and the spectrum analysis results are as follows:  
Table 2 shows that the simulation results are divided into two parts after analysis. In the range of 1-15 cm from the edge, the predominant frequency of the signal acquisition point was 2.319 kHz, which was lower than the predominant frequency of the central region of the column. When the distance from the edge is 20-35 cm, the predominant frequency of the signal acquisition point changed from 2.319 kHz to 2.563 kHz and then remained unchanged. Interestingly, the amplitude corresponding to the dominant frequency also changed. This was because the column was subjected to torsional deformation at the edge. At this point, the impact energy was only absorbed by the column angle, the bending deformation was large, and the amplitude was at its highest level. If the tapping point is located in the middle of the column, the mass of the column absorbs the impact energy and the bending deformation remains small, so the amplitude is low. Therefore, when the predominant frequency is at a low frequency, the amplitude decreases with the increase in distance. When the striking point is located in the middle of the column, the amplitude increases with the distance. Preliminary simulation showed that the frequency mutation was located 15-20 cm from the edge. Combined with Table 3, the variation of predominant frequency and amplitude in this range was analyzed. When the distance from the edge was 14-18 cm, the predominant frequency of the signal acquisition point was still low. When the distance from the edge was 19-21 cm, the frequency of the signal acquisition point became the dominant frequency in the middle of the column. At the same time, the amplitude corresponding to the main frequency also changed. Based on the above two tables, it can be seen that for 700 \(\times\) 700 mm concrete columns, when the impact-echo method is used to detect at 19 cm from the edge, the predominant frequency changes, and the amplitude carried by the low frequency gradually decreases with the distance from the edge. The amplitude accepted by the high frequency increases with the distance from the edge.