274 S. Yousefianmoghadam et al. Fig. 33.8 Mode shapes of the structure at DS0 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 DS0 DS1 DS2 DS3 Frequency (Hz) Mode1-Ambient vibration Mode1-Demolition Mode1-Forced vibration Mode2-Ambient vibration Mode2-Demolition Fig. 33.9 Identified natural frequencies at different damage states the 40-component mode shape vectors between the average mode shapes estimated using different sources of excitation described above. It can be seen from the figure that the mode shapes are in excellent agreement with the MAC value of 0.98 or higher. 33.5 Conclusions and Remarks A comparative study between the system identification methods, ERA, NExT-ERA, and peak picking using the free vibration, ambient vibration, and forced vibration data respectively is presented here. The measurements were obtained from dynamic tests of a 10-story RC building which was damaged through the demolition of selected perimeter walls using a jack hammer. The identification orders used in the ERA method applied on the free vibration measurements are significantly lower than those of NExT-ERA method applied on the ambient vibration recordings. This difference may occur because of the additional processing of the ambient vibration signals to obtain the free vibration signal (NExT) which may add computational errors to the identification process.
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