Evaluation method for earthquake resistance capacity of small-scale buildings using microtremors

Abstract

We propose an effective evaluation method of earthquake resistant capacity of small-scale buildings using microtremors. The dynamic properties of buildings are crucial information for evaluation of earthquake resistance, because the defect of building directly influences to its dynamic motion. In this study, we mainly recorded vibration of second-floor of building as well as its ground surface (i.e. microtremors). By calculating spectra of these motions, we then calculated the transfer function of the buildings. The response of building due to earthquake depends on the natural period and resonance of the building. From the transfer function, therefore, we calculated (1) natural period, (2) quality factor (Qvalue), and (3) maximum amplification ratio. To evaluate the capacity for earthquake resistance from these dynamic responses, the earthquake resistant capacity index (C index) has been proposed. The validity of this index has been examined through the investigation of the dynamic properties of small-scale buildings (over 100 buildings). When we evaluated a house using our proposed methods before and after retrofitting, the C index is significantly improved, suggesting that this index reflects earthquake resistance of the small-scale buildings. When we compared dynamic responses (i.e. C-index from transfer function) for orthogonal directions, furthermore, we could evaluate the weaker direction of the building.