Modal parameters of a nine-story steel-reinforced concrete building damaged heavily by the 2011 off the Pacific coast of Tohoku Earthquake were identified from ambient vibration data using frequency domain decomposition (FDD). The characteristics of the identified three-dimensional mode shapes, torsional component of the mode and deformation ratio of the mode were discussed with respect to the seismic damages. In addition, we identified the mode shapes by two kinds of FDD: one for a general viscous damping system, and the other for a proportional damping system. The influence of a viscous damping condition on the identified mode shapes was discussed.
We discussed stochastic properties of the spatial auto-correlation(SPAC) coefficients for microtremor data which are observed simultaneously at two sites. Usually, the SPAC method can provide the phase velocities using the data observed simultaneously on circular array with four or seven sites. The method, however, suggests analytically the possibility that we can estimate the phase velocities using only the data observed simultaneously at two sites. To clarify the limitation of this idea, some mathematical analyses are performed and stochastic properties of the SPAC coefficients are derived. Furthermore, some real data of microtremors are applied to the above analytical results and the validity is discussed.
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