Modal analysis in operational conditions is popular way for obtaining natural modes of civil structures such as bridges or dams nowadays. This approach called Operational Modal Analysis (OMA) overcomes difficulties with input forces used for excitation of the structure in Experimental Modal Analysis (EMA). In OMA the input forces are assumed to be stochastic. The paper presents the experimental measurement and analysis of the mode shapes and frequencies of the laboratory steel truss structure using output-only method based on Frequency Domain Decomposition (FDD). FDD technique for a decomposition of the spectral density function matrix in combination with a method for merging measured data from multiple measurement set-ups are both implemented into a data processing tool designed in LabVIEW. Modal exciter and random impulses in random points are used for the structure excitation. The results demonstrate the influence of different types of operational loading on identified modes.
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