# Health Monitoring of Civil Infrastructures by Subspace System Identification Method:
## 4 Comparison between SSI and Other System Identification Algorithms
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Identification Method** \\ \hline [PERSON] et al. [55] & SSI & * Generally, there was a good agreement between the obtained modal frequencies and mode shape in all algorithms, however the damping ratio shows the most significant scatter. * The PP method is an appropriate technique for modal identification of closely spaced modes. * SS and PP is a more appropriate choice for modal identification where closely spaced frequencies are not of the same type (e.g., bending or torsion). * PP and FDD are not suitable tools for identification of the modal parameters in cubic-stayed bridges. Further analysis must be undertaken to successfully discriminate the vibrational influence of the cables from the global bridge frequencies. * FDD & * The FDD and SSI yielded unreliable results for the second pair of closely spaced frequencies due to the coincidence of the modes and coupling effect in ambient excitation. * Estimation of damping ratios with wind-induced excitation has shown a significant level of scatter. Further research on ambient vibration testing is needed to approach a reliable identification of damping coefficients. * All the above-mentioned algorithms performing human judgment during implementation. The user defined parameters for the SSI, FDD and PP are system order, singular values, and global modes, respectively. \\ [PERSON] et al. [120] & SSI-COV & * Performance of both SSI/COV and H-SCF was found to be good in the identification of natural frequencies and mode shapes. * I is demonstrated that the p-SCF can provide better estimation for the daily variations of damping coefficients compared to SSI SSI & * All of the modal identification methods perform well when the responses are exposed to noise. * Model order is an important parameter in extracting correct modal parameters in all methods. * ARMA & * Significant errors are observed in the damping coefficient when system order varies. * Damping coefficient is sensitive to noise pollution, specifically for the first mode. * More stable results for modal identification were obtained by the SSI method in numerical simulations. However, the ERA method outperforms other algorithms for field testing. * ARMA shows the worst results for both field and numerical experiments. \\ [PERSON] et al. [122] & MNEAT-ERA & Comparing output-only methods of SSI-DATA, MNEAT-ERA and EFDD, it can be inferred that: * The identified natural frequencies using these methods display good agreement in each damage scenario. * The identified damping coefficient shows larger variability but still in a reasonable range. * Lower damping ratios are obtained through FDD than the results identified using SSI-DATA and MNEAT-ERA. * The identified mode shapes using SSI-DATA were confirmed to be the most reliable among all methods. * Comparing input-output methods of DSI, OKID-ERA, and GRA, the following were observed: * The identified mode shapes in all of the above-mentioned algorithms are close to the actual mode shape of the structure. * In general, the calculated mode shapes by the input-output methods are more accurate than those obtained from output-only methods. * The mode-shape parameters obtained from the GRA appear to be in good agreement with the actual mode shapes of the structure. * The same result in terms of damage detection was obtained using both the SSI-COV and FDD procedures. * In terms of modal parameters, perfect results were extracted through using both methods. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison between subspace system identification (SSI) method and other system identification algorithms.