## 3 Results
### Processing the Signals and Modal Parameters
Two types of vibration were identified, a so called rotational and flexural mode. By rotational vibrations we will denote rotations of the upper horizontal triangle around the vertical z axis, i.e., three upper nodes are moving along the circle inscribed in the equilateral triangle of these upper nodes (Figure 5a). By flexural vibrations we will denote the rotations of the upper base triangles around the horizontal y axis (Figure 5b). The signals were recorded with a relatively high, for the test purposes, sampling frequency of 1500 Hz. The acceleration signals did not need such high frequency, yet to record the detail of the short-time transient inducing signal generated by the impact hammer such a frequency was entered. The signals lasted nearly 16 s. The post-processing of the recorded signals involved clearing the offset and decimation by 10, which caused, due to the Nyquist frequency and aliasing phenomena, the maximum value of frequency being equal to 75 Hz. Figure 6 presents as an example the signals from the 4 th node accelerometer after the FFT for directions x (upper), y (central) and z (lower) after the strike at azimuth 120 degrees (left) and 330 degrees (right). Comparing two different strike azimuths, it is seen that different vibration frequencies were induced. The strike of the direction perpendicular to the circle of the upper triangle at 330 degrees is inducing the dominant, well visible, single flexural mode of vibrations at approximately 16 Hz. This very strong visible frequency is presenting the mode, or as it is shown in stability diagrams, two very closely situated modes. The strike at 120 degrees induces the rotational and flexural modes of vibrations and can be used to find all modes. For this strike all directions of vibrations are showing the same frequencies, although with the different intensity. The frequencies are most clearly seen and they are of the flexural mode at approximately 16 Hz and less than those for the two rotational modes at approximately 5 and 62 Hz. Nevertheless, only the 4y acceleration signal (in the middle of Figure 6) shows clearly that the modes near 16 Hz are actually two different ones. This is not clearly visible when looking only on the signals 4x and 4z. Therefore, measuring three orthogonal directions is helping to find all modes that reveal themselves with different intensity. Figure 7 presents the acceleration signals in time domain (upper), in the frequency domain (middle) and the power spectrum density of the acceleration in the frequency domain (lower) for the 4 th (left) and 13 th (right) prestress level. The comparison of the two presented signals shows, that the first natural frequency has increased significantly after the increase of the pre-stress level. For the 4 th pre-stress level the concentration of natural frequencies gathers around the value of 6, 12 and 16 [Hz]. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Frequency [Hz] of Mode No:**} & \multicolumn{4}{c}{**Frequency [Hz] of Mode No:**} \\ \hline
**no** & **1\({}^{\text{ r}}\)** & **2\({}^{\text{ f}}\)** & **3\({}^{\text{ f}}\)** & **4\({}^{\text{ f}}\)** & **5\({}^{\text{ r}}\)** & **no** & **1\({}^{\text{ r}}\)** & **2\({}^{\text{ f}}\)** & **3\({}^{\text{ f}}\)** & **4\({}^{\text{ f}}\)** & **5\({}^{\text{ r}}\)** \\ \hline
1 & 3.36 & 11.3 & 15.8 & - & - & 8 & 7.57 & 11.6 & 16.1 & 57.7 & 62.2 \\ \hline
2 & 4.32 & 12.1 & 15.5 & - & - & 9 & 8.2 & 11.5 & 15.9 & 57.6 & 62.7 \\ \hline
3 & 5.29 & 11.9 & 15.8 & 58.4 & - & 10 & 8.7 & 11.6 & 15.8 & 57.8 & - \\ \hline
4 & 5.87 & 12.0 & 16.3 & 58.4 & - & 11 & 8.98 & 11.9 & 16.3 & 58.5 & 62.8 \\ \hline
5 & 6.4 & 12.1 & 16.4 & 58.5 & 62.5 & 12 & 9.19 & 12.0 & 15.9 & 58.2 & 62.8 \\ \hline
6 & 6.78 & 11.4 & 15.6 & 58.0 & - & 13 & 9.54 & 11.7 & 16.3 & - & 63.2 \\ \hline
7 & 7.12 & 11.6 & 16.0 & 58.3 & 62.5 & & & & & \\ \hline \hline \end{tabular} \({}^{\text{r}}\) Mode of rotational type and \({}^{\text{f}}\) flexural type.  
\end{table} 
Table 4: First five natural frequencies for 13 prestress levels.