Operational modal analysis (OMA), as a branch of the system identification, plays a very important and practical role in determining the dynamic characteristics of the structures. In the operational approach that is implemented based on the ambient vibration test, the ambient and operation loads are considered as the excitation source of the structure. In the present research, an integrative method composed of frequency domain decomposition (FDD) and wavelet transform (WT) called FDD-WT is proposed in order to identify the natural frequencies and damping ratios of an arch concrete dam. Furthermore, the wavelet transform of the seismic responses is also calculated in order to validate and compare the results. For this purpose, the time and frequency position of the system modes during the different earthquakes is evaluated using the time-frequency representation obtained from wavelet transform. In this paper, Pacoima arch concrete dam located in California, US is selected as the case study and the seismic records related to 1994 Northridge, 2001 San Fernando and 2008 Chino Hills earthquakes are also used to evaluate the dynamic characteristics and structural health monitoring during the period between 1994 to 2008. Investigation of changes in the natural frequencies of the structure indicates that the dam had taken serious damage during 1994 Northridge earthquake (about the fourth second), while the vibrations of the concrete structure has been almost linear during the first 4 seconds of the earthquake and also in 2001 and 2008 earthquakes.

Keywords: Operational modal analysis, Ambient vibration test, Frequency domain decomposition, Wavelet transform, Structural health monitoring

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