# Effect of Environmental Parameters on Structural Health Status Assessment Using OMA Techniques
## 9 Conclusions  
The present work aims to highlight the limits of structural health monitoring techniques based on the analysis of output-only data from an accelerometer network, when the variation of the frequencies of the monitored system is used as the only indicator of damage. The main cause found to limit the effectiveness of OMA are thermal disturbances. Three of the most common OMA methods are applied to the same experimental data to obtain a comparative assessment and to study the effect of ambient parameters (temperature) on the output of the methods. Longer sampling (10 min instead of 5 min) provides slightly better results because of the higher dynamic information content, but the difference is almost negligible for all the three methods used. Therefore, using 5-min signals as input could be useful to shorten the calculation time. The application of the peak-picking method is successful only for the first mode of vibration which is easier to excite with environmental vibrations. However, the peak-picking method is useful for preliminary analysis of sampled signals. It allows an initial assessment of the sampled data, a qualitative evaluation of the system's own frequencies and an initial understanding of the system's behavior. For high-order modes, the excitation due to environmental vibrations is lower and the system's own modes begin to be confused within the ambient noise. The limitations shown by the peak-picking method can also be related to the fact that no optimization for PSD evaluation was performed. The focus was on obtaining their maximum frequency resolution. In addition, optimization would require the evaluation of PSDs on discretization intervals different from those chosen and working well with the other methods. Therefore, a comparison between the peak-picking method and the other two methods would not have been possible. The other two methods, Cov-SSI and PolyMAX, can provide good results for modes higher than the first. Three different simulated damages on one beam are identified with different degrees of accuracy, depending on the cleaning procedure of the signal from temperature drift. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **PP** & \multicolumn{2}{c}{**Cov-SSI**} & \multicolumn{2}{c}{**PolyMAX**} \\ \cline{2-7}  & **Mode 1** & **Mode 1** & **Mode 2** & **Mode 3** & **Mode 1** & **Mode 2** & **Mode 3** \\ \hline
**Mass 1\%** & \(-3.5\%\) & \(-3.4\%\) & \(-1.7\%\) & \(-1.8\%\) & \(-3.4\%\) & \(-1.2\%\) & \(-1.9\%\) \\
**Mass 3\%a** & \(-3.4\%\) & \(-3.4\%\) & [-] & \(-2.1\%\) & \(-3.3\%\) & [-] & \(-2.1\%\) \\
**Mass 3\%b** & \(-4.2\%\) & \(-4.4\%\) & \(-0.4\%\) & \(-2.6\%\) & \(-4.2\%\) & \(-0.2\%\) & \(-2.9\%\) \\
**Mass 5\%** & \(-2.4\%\) & \(-2.4\%\) & \(-2.5\%\) & \(-3.1\%\) & \(-2.3\%\) & \(-1.9\%\) & \(-3.3\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Beam 1: frequency variation, 5-min intervals discretization, correction with temperature. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **PP** & \multicolumn{2}{c}{**Cov-SSI**} & \multicolumn{2}{c}{**PolyMAX**} \\ \cline{2-7}  & **Mode 1** & **Mode 1** & **Mode 2** & **Mode 3** & **Mode 1** & **Mode 2** & **Mode 3** \\ \hline
**Mass 1\%** & \(-3.3\%\) & \(-3.5\%\) & \(-1.7\%\) & \(-1.8\%\) & \(-3.4\%\) & \(-1.2\%\) & \(-1.9\%\) \\
**Mass 3\%a** & \(-3.2\%\) & \(-3.4\%\) & [-] & \(-2.1\%\) & \(-3.3\%\) & [-] & \(-2.0\%\) \\
**Mass 3\%b** & \(-4.1\%\) & \(-4.4\%\) & \(-0.4\%\) & \(-2.6\%\) & \(-4.2\%\) & \(-0.1\%\) & \(-2.9\%\) \\
**Mass 5\%** & \(-2.3\%\) & \(-2.4\%\) & \(-2.5\%\) & \(-3.2\%\) & \(-2.3\%\) & \(-1.8\%\) & \(-3.2\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Beam 1: frequency variation, 10-min intervals discretization, correction with temperature. A result of considerable interest is the difference between the results obtained performing correction of the raw frequency data using a mechanical parameter (the tensile force in the rods) and the results obtained using ambient temperature correction.