In this paper a new frequency domain technique is introduced for the modal identification of output-only systems, i.e. in the case where the modal parameters must be estimated without knowing the input exciting the system. By its user friendliness the technique is closely related to the classical approach where the modal parameters are estimated by simple peak picking. However, by introducing a decomposition of the spectral density function matrix, the response spectra can be separated into a set of single degree of freedom systems, each corresponding to an individual mode. By using this decomposition technique close modes can be identified with high accuracy even in the case of strong noise contamination of the signals. Also, the technique clearly indicates harmonic components in the response signals.

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This paper aims the study of the accuracy provided by the identification of modal damping ratios based on ambient and free vibration tests. For that purpose, numerical simulations were developed to generate artificial experimental data concerning both types of tests. This simulated data allowed the illustration of the influence of factors like non-proportional damping or the proximity of natural frequencies on the quality of the estimates. The accuracy of two output-only identification algorithms (Enhanced Frequency Domain Decomposition and Covariance driven Stochastic Subspace Identification methods) and of two alternative procedures to process the free decays was also analyzed.

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The mass change method is used to estimate the scaling factors, the uncertainty is reduced when, for each mode, the frequency shift is maximized and the changes in the mode shapes are minimized, which in turn, depends on the mass change strategy chosen to modify the dynamic behavior of the structure. On the other hand, the aforementioned objectives are difficult to achieve for all modes simultaneously. Thus, a study of the number, magnitude and location of the masses must be performed previously to the modal tests. In this paper, the mass change method was applied to estimate the scaling factors of a steel cantilever beam. The effect of the mass change strategy was experimentally studied by performing several modal tests in which the magnitude, the location and the number of the attached masses were changed.

When operational modal analysis (OMA) is used to estimate modal parameters, mode shapes cannot be mass normalized. In the past few years, some equations have been proposed to scale mode shapes using the mass-change method, which consists of repeating modal testing after changing the mass at different points of the structure where the mode shapes are known. In this paper, the structural dynamic modification theory is used to derive a set of equations, from which all the existing formulations can be derived. It is shown that the known equations can be divided into two classes, the exact and the approximated equations, where the former class does in fact fulfill the equations derived from the theory of structural modification, whereas the remaining equations do not, mainly because the change of the mode shapes of the modified structure is not taken properly into account. The paper illustrates by simulations the large difference in accuracy that exists between the approximate and the exact formulations. The paper provides two new exact formulations for the scaling factors, one for the non-modified structure and -as the first time in the literature -one for the modified structure. The paper illustrates by simulation the influence of errors on the measured natural frequencies and mode shapes on the estimation of the scaling factors using the two exact formulations from the literature and the new exact formulation proposed in this paper. Further, the paper illustrates statistics of the errors on mode shape scaling. All simulations were carried out using a plate with closely spaced modes.

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