Modal matrix, identifiability

The diagonal elements of the excitation spectral intensity matrix describes the energy of the corresponding modes. The variations of their identified values are shown in Figures 3.27 and 3.28 for the Kammuri and Nuri, respectively. Note that comparing the spectral intensities of the modal forces for the different modes is meaningless since their values depend on the normalization of the mode shapes as discussed in Section 3.3.1.2. However, the time variation of the spectral intensity of a mode indicates the variation of the excitation energy with respect... 

Natural vibration modes have been identified from the low-level white noise tests, via the squared matrix frequency-dependent of cross PSD of the wall, represented by a 6-Degree-of-Ereedom (DOE) structure, each DOF corresponding to the location of one acceleration measurement point. Details of the identification procedure are given in (Mordant 2012). The objective of this post-processing is to identify the shape of the vibration mode of the wall, the vibration of the added mass with respect to the wall and the influence of the rubber layers on modal shape. Note that, for such a low level of ground acceleration, no significant rocking is induced in the structure and the system behaves as fixed to the foundation. The analysis is focused on the first two vibration modes of the structure. 

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