Modal identification of a time-invariant 6-storey model test RC-frame from free decay tests using multi-variate models

The scope of the paper is to apply multi-variate time-domain models for identification of eigen-frequencies and mode shapes of a time-invariant model test Reinforced Concrete (RC) frame from measured free decays. The frequencies and mode shapes of interest are the two lowest since they are normally the only ones activated in ground motion shaking of structures. For purely frequency identification, FFT, ARV, ERA and ARMAV models are applied and for mode shape identification, multi-variate ARV and ARMAV models and the ERA are used. Furthermore, the results of a finite element analysis is included in the comparison. The data investigated are sampled from a laboratory model of a plane 6-storey, 2-bay RC-frame. The laboratory model is excited at the top storey where two different types of excitation were considered. In the first case the structure was excited in the first mode and in the second case in the second mode. It is found that the estimates of the frequency, damping ratio and mode shape for the first mode estimated by the multivariate ARV, ARMAV and the ERA give nearly identical results for both types of excitation. Also the estimates of the frequency, damping ratio and mode shape of the second mode are nearly of the same magnitude. Compared with the FEM results the estimates are comparable for the first mode while there is a deviation between the FEM and estimated mode shapes for the second mode.

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