Abstract

Conventionally, empirical modal analysis requires a known external excitation for a correct identification of the structure´s modal response. Due to this, most field testing must be done under controlled conditions in order to minimize the influence of unknown ambient excitations. Nevertheless, on public structures or service installations, as highway or railway bridges, it is not possible to stops the structure standard operation and they must be tested using ambient vibrations responses as output only measured data, known as Operational Modal Analysis (OMA). This essay investigates on experimental methods for operational modal frequency identification of large structures, based on discrete Wavelet packet decompositions of ambient vibrations measurements and post analysis on time and frequency domain through Hilbert spectrum. Comparison of results obtained with other methods will also be done. FEM model as a theoretical reference, LSCE and 4SID modal identification as empiric method references, and Hilbert-Huang Transform as experimental spectral reference method.