# Dynamic Identification on an Irregular Structure
## 4 Identification Results and Up-Dating Procedure  
### Structural Dynamic Identification  
The acquired vibrations have been analyzed by means of ARTeMIS software [23], and thus a geometrical model has been defined introducing the directions of acquisition of the 28 installed sensors, which are reported as arrows in Figure 9 where the xyz reference system, the same of the FE model, is also indicated. The vibrations have been analyzed by Stochastic Subspace Identification (SSI-UPC), and, considering the particularly complex shape of the building, a high number of projection channels has been considered. Figure 9 shows the channels used for the identification of the two sub-structures; in particular, the 12 reference channels are colored in magenta. Moreover, to have a confirmation of the experimental identification, also the CFD identification method, which operates in the domain frequency, has been used for checking the detection of the frequencies identified by SSI-UPC. Figure 10 shows the results of the SSI-UPC analysis performed on the data acquired during Test 1. The dynamic identification was carried out considering all the acquired data, with a sampling frequency of 1024 Hz. The final analysis, which led to the identification here presented, was carried out in the interval [0-12.8] Hz. The robustness of the identification was ascertained both by verifying the results obtained by means of various identification techniques and modifying some parameters which must be chosen during the numerical elaboration of the recoded data. For the sake of simplicity, four channels were considered in the identification shown in Figure 10, but the same results were obtained considering  
Figure 9: Model of the “Municipal Headquarters” of Palagianello defined in Artemis software. The arrows show the position and direction of each installed accelerometer. a greater number of channels. Furthermore, a resolution of 4096 was considered and the order was set to its maximum value equal to 100 for the SSI decomposition in the diagram shown in Figure 10. However, numerous tests were carried out even for these parameters by varying them in reasonable intervals and obtaining the same results. Finally, further proof of the goodness of the identification is ensured by the extreme repeatability of the results in different tests conducted on the structure. In addition to the three tests (Test 1, Test 2 and Test 3) reported in the present work, other experimental tests were carried out giving the same results and, for sake of brevity, not reported in this article. The identification results of the first three frequencies identified with both the identification methods and for all the three different tests, named Test 1, Test 2 and Test 3, are reported in Table 2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Mode**} & \multirow{2}{*}{**Test 1**} & \multicolumn{2}{c}{**Test 2**} & \multicolumn{2}{c}{**Test 3**} & \multicolumn{2}{c}{**Mean**} \\  & & **Training** & **Ratio [\%]** & **Frequency** & **Damping** & **Ratio [\%]** & **Frequency** & **Damping** & **Ratio [\%]** & **[Hz]** \\ \hline
1 & 5.17 & 1.25 & 5.14 & 1.75 & 5.35 & 1.30 & 5.20 \\
2 & 7.68 & 2.75 & 7.65 & 2.58 & 7.66 & 2.98 & 7.66 \\
3 & 8.86 & 2.09 & 8.63 & 1.93 & 8.79 & 2.06 & 8.76 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Identified frequency values. Figure 10: Results of the SSI-UPC identification analysis performed on the data acquired during Test 1. To complete the estimation of the modal parameters, also the damping corresponding to the identified frequencies have been inserted in Table 2.  
The mode shapes have been evaluated: the experimental identification results principally show that the first detected modes do not, in any case, have a purely flexural behavior (first or second flexural along x or y).