Noise Analysis of Strong Motion and Microtremor Observation Records for Soil-Building Structure Dynamic Interaction

Abstract eng:
Dynamic soil-structure interaction largely affects the seismic performance of building structures in many cases. It is effective to measure real behaviors of a building and the surrounding ground with seismographs to explain this effect. Some previous studies detected the SSI effects to system parameters such as natural frequencies and damping coefficients by using Fourier spectrum ratios to evaluate earthquake response characteristics of building structures. The Fourier spectrum ratios calculated from observation records correspond to transfer functions from input motions to responses. However, they have different features from the general transfer functions in actuality. They are not smooth and include some noises. In this study, we investigate characteristics of these noises included in microtremor and strong motion observation records and generation factors of them. The target building in this study is a five-story apartment with a reinforced concrete wall structural system. The dimension of its plan is 65.7m by 6.96m. The height is 26.7m. We arranged sensors for microtremor and strong motion observation at multi-points in this building. For the microtremor observation, we also set sensors at the multi-points on the surrounding ground. On the other hand, we set only one sensor at the single-point on it for the strong motion observation. We acquired records of three directional motions. However, this paper deals with records in only a long-span direction of the building. Analysis results of microtremor observation records showed that coherence between ground motions observed near both edges of the building decreased in a higher frequency range and became almost zero in the range higher than 2Hz. It means that there were significant phase differences between ground motions input to the both edges of the building structure especially in the range of high frequency. In contrast, the coherences among records observed in the building became higher than that of ground motions. Based on these results, we can presume that the rigidity of the floors in the building probably has a particular effect decreasing noises included in the observation records. Using observed ground motions, we execute numerical analyses and examine effects of floor rigidities to noises appeared in the Fourier response spectral ratios. Also, we analyze noises included in microtremor and strong motion observation records using the signal power ratio proposed by Ikeura. From the results of this study, we concluded that flexible floors of the building with a long span tend to generate larger noises on the Fourier spectral ratios, and we can evaluate more appropriate transfer functions from observation records with considering the signal power ratio.

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