Building Damage Evaluation Based on Changes of Story Shear-Wave Velocities Extracted From a 1D Vertical Ambient Noise Observation System

Abstract eng:
The response of a building structure can be seemed as, in frequency domain, the super summation of shaking modes and alternatively, in time domain, the wave propagation within the structure and multiply reflected by the base and the top of the building. Using the frequency domain method, i.e. the Fourier transform, the frequencies of the building system including soil-structure interaction can be known from the predominant frequencies of the spectrum of the response of the top. Besides, the apparent frequencies of the response including the vibrations due to the base rocking and the upper structure itself can be extracted from the transfer function, which is the ratio of Fourier spectrum of the response of the top and that of the base in frequency domain. It is the so-called impulse response due to the input impulse (visual source) from the base. Because of the effect of the vibration due to the base rocking, the true shear-wave travel time (the phase velocity) within the upper structure is difficult to be extracted from the impulse response. However, for the virtual source on the top of the building, the response of the building, which is also a kind of impulse response (in this study we call it deconvolved waves), only including one acausal up-going wave (one impulse) and one causal down-going (the other impulse) wave, from which the shear-wave travel time from the top to the inter stories can be extracted from the according time of the impulses. And there is little effect of the soil-structure interaction, i.e. the base rocking, to the deconvolved waves if the higher modes are included when the deconvolved waves are calculated. In this study, we first using a multiple-degree-offreedom model with rocking and horizontal springs at the base to prove the frequency and time domain method can work well to extract the frequency and time domain properties, respectively. Then, we try to apply the frequency and time domain n method to a 1D vertical ambient noise observation system to extract the shear-wave velocities traveling within stories. Based on the deterioration of shear-wave velocity of each story, the story-by-story damage evaluation of a 9-story steel reinforced concrete (SRC) building, which was severely damaged during the Great East Japan Earthquake, is performed. The extracted shear-wave velocities from ambient noise are compared with those calculated ones based on the relationship by inputting the design shear rigidities and masses of a similar building to the damaged one, because the design statements of the damaged building is unavailable. Based on the comparison between extracted shear-wave velocities and those calculated ones, we found that shear-wave velocities in the longitudinal directions reduced more dramatically than in the transverse direction. The shear-wave velocities in the longitudinal direction are almost reduced to less than 300m/s, especially at the first, 4th, 5th, and 8th story, which are reduced to less than 200m/s. In the transverse direction, shear-wave velocities generally become slower with the increase of height. Shear-wave velocities of 5th and 6th story reduced to less than 200m/s in the transverse direction.

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