In classical operational modal analysis (OMA), the modal parameters of a structure are determined from its dynamic response to ambient forces. The application of this technique to civil engineering structures is particularly interesting since heavy and expensive artificial excitation devices, required for experimental modal analysis (EMA), are not necessary. However, disadvantages of OMA are the limited frequency content of the ambient excitation and the fact that mass-normalization of the mode shapes is not possible. A possible solution is to use operational modal analysis techniques that allow deterministic or exogenous inputs (OMAX). The difference between OMAX and EMA lies in the fact that with OMAX, the ambient forces are not considered as noise, but as part of the excitation. As a result, OMAX requires only small and relatively cheep artificial excitation devises, but it requires special combined deterministic-stochastic system identification methods. In this paper, the reference-based combined deterministic-stochastic subspace identification method for OMAX is discussed. The theory is illustrated on a benchmark problem: the determination of the modal parameters of the Z24 bridge. With the presented method, the best benchmark results reported so far are obtained.
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