# Dynamic Identification on an Irregular Structure
## 6 Conclusions  
This research has the goal of identifying the structural behavior of the \"Municipal Headquarters\" of Palagianello Town, a building with a strategic importance. The case study gives us the opportunity to discuss the importance of a correct design of the test setup in the presence of in-plan and in-elevation structural irregularities. The dynamic identification was made considering the structure subjected only to environmental actions and through the installation with \"classical\" distributions of highly sensitive sensors; so, the latter collected accelerations data during ordinary service conditions. The advantages of the OMA technique are the precision of the results and the possibility to collect the vibration data when the structure is under its operating conditions; so, there is no activity interruption during the monitoring phases in the structure. The latter highly facilitate the monitoring and the determination of the dynamic characteristics and the vulnerability of the \"strategic\" buildings. The present paper investigates and analyses two questions:  
* The variation of the identified frequency values by assuming two different configurations of the installed sensors;
* The efficacy of the FE model to match the experimental results by including diagonal strut connected at the intersection points of the r.c. frame. The experimental acquisitions have been analyzed by means of OMA method by changing the number of considered sensors, with the final aim of verify how the number and location of sensors may influence the identification process. Thus, three different setups of sensors have been examined, and the results show that a reduction in sensors does not influence significantly on the frequency values, which manifests a very low percentage variation; however, the more significant modification is recorded on the modal shapes. In fact, the identified shapes change substantially, losing (second and third identified \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Axis** & \multicolumn{2}{c}{**FE Model without Infill Walls**} & \multicolumn{2}{c}{**FE Model with Infill Walls**} \\ \hline  & **U1** & **U2** & **U3** & **U1** & **U2** & **U3** \\ \hline  & \multicolumn{4}{c}{Raised floor} \\ \hline x & 10.35 & 10.35 & 6.02 & 10.36 & 10.36 & 6.02 \\ y & 8.88 & 8.88 & 12.32 & 8.88 & 8.88 & 12.32 \\ z & 0 & 0 & 0 & 0 & 0 & 0 \\ \hline  & \multicolumn{4}{c}{First floor} \\ \hline x & 9.36 & 9.36 & 7.47 & 9.36 & 9.36 & 7.47 \\ y & 9.71 & 9.71 & 10.01 & 9.71 & 9.71 & 10.01 \\ z & 3.60 & 3.60 & 3.60 & 3.60 & 3.60 & 3.60 \\ \hline  & \multicolumn{4}{c}{Second floor} \\ \hline x & 7.29 & 7.29 & 7.13 & 7.31 & 7.31 & 7.13 \\ y & 10.21 & 10.21 & 10.93 & 10.21 & 10.21 & 10.93 \\ z & 6.90 & 6.90 & 6.90 & 6.90 & 6.90 & 6.90 \\ \hline  & \multicolumn{4}{c}{Roof floor} \\ \hline x & 7.07 & 7.07 & 7.03 & 7.10 & 7.10 & 7.03 \\ y & 9.80 & 9.80 & 10.82 & 9.80 & 9.80 & 10.82 \\ z & 10.20 & 10.20 & 10.20 & 10.20 & 10.20 & 10.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Center of Mass coordinates of the two FE Model for each floor. mode) the torsional characteristic that is very important for a correct comparison with the FE model. The paper shows that the adoption of ad hoc chosen locations of the sensors could modify the experimental results, and the influence of the masonry infills on the frequency values may be significant. Furthermore, the paper discusses the influence of the stiffness contribution on the dynamic properties of an irregular building which is usually neglected into the updating procedures, but as demonstrated, it is important for a correct model improvement. Conceptualization, M.D. and M.F.S. ; methodology, M.D., N.I.G. and M.F.S. ; validation, M.D. and N.I.G. ; data curation, N.I.G. ; writing--original draft preparation, M.D. and M.F.S. ; writing--review and editing, M.D., N.I.G. and M.F.S. ; supervision, D.F. ; project administration, D.F. ; funding acquisition, D.F. All authors have read and agreed to the published version of the manuscript.