A method for rational estimating the local soil effects on the seismic force of a structure is investigated based on reinforced concrete (RC) buildings in Fukuoka city. The array microtremor measurements both on ground surface and structure have been carried out about 180 RC buildings in Fukuoka From the separation of predominant period of ambient vibrations into soil effect or building effect, both natural fundamental vibration period of the building (T_b) and that of surface ground (T_g) can be estimated exactly. There are three cases in vibration mode of building and surface ground; (1)T_b>T_g, (2)T_b≒T_g, and (3)T_b<T_g. The transfer functions and the coherence functions, which are calculated statistically between ambient vibrations of building and that of surface ground, are used for evaluating vibration types. In order to evaluate the magnification of surface ground, two dimensional analyses have been carried out using the boundary element method After estimating the magnification of the structure including the local soil effects, the buildings of T_b≒T_g are subject to the largest response.
