Modal Parameter Transition of a Low-Lise RC Building During Retrofitting Works

Abstract eng:
Transition of dynamic characteristics, such as natural frequency, damping ratio, and mode vectors, was investigated by using a long-term ambient vibration test on a low-rise existing building. The measured building is a 4-story reinforced concrete (RC) building, which has been seismically retrofitted by adding concrete walls, foundations and piles. The ambient vibration had been continuously measured for nine months, from “before retrofitting” to “after retrofitting” in order to evaluate effect of the seismic retrofitting work, as well as to study on the ambient vibration test based structural health monitoring. The modal parameters are estimated by a cross-spectrum based modal identification scheme, therefore, which can be evaluated as included soil-structure dynamic interaction. By evaluating the ambient vibration records we found that the effect of seismic retrofitting on the modal parameter of the low-rise building can be clearly detected from the mode vectors, even though the natural frequency does not give a clear indication. For instance, with progresses of the retrofitting works, the natural frequency decreased due to mass effect by adding of the concrete walls, on the other hand, the rotation component of foundation and the deformation of the superstructure decreased by adding the piles and the concrete walls, respectively. Thus it is expected that the stiffness degradation of low-rise building can be also evaluated in terms of the mode vectors estimated by the ambient vibration test.

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