Modal parameter estimation from output-only measurements in presence of transient phenomena

A traditional modal analysis from output-only data in the frequency domain starts from the power- and cross-spectral functions of the output measurements. To estimate this power- and cross-spectral functions accurately, sufficient data has to be available. The part of the time domain data without transient effects is divided in different parts (periodogram. method) and Hanning windows are used in order to reduce the leakage effects. The frequency domain estimators that are used to process these power- and cross-spectral functions do not take transients effects into account. Recently it has been demonstrated that it is possible to estimate transient effects simultaneously with the SISO model parameters from an output-only Fourier spectrum. In the present contribution, it is shown that it is also possible to extract modal parameters from the output only spectra. Therefore the technique is extended for MIMO systems and the extraction of the modal parameters from the estimated rational transfer function model is investigated. The developed technique starts from a small outputonly data set with transient effects to estimate the modal parameters. Instead of using the power- and crossspectral functions the developed method processes the output Fourier spectra directly. These methods can be used in practice for in-operation tests, where only short time domain records with transient effects are available. This technique is compared to the traditional techniques and the advantages and disadvantages are discussed.