Abstract
The purpose of this study is to investigate the influences of the foundation types and ground conditions on the natural period of actual buildings. About 200 buildings on different kinds of ground were selected. These buildings have similar plan type and are of reinforced concrete or steel reinforced concrete structure of 2 to 6 stories. In order to obtain the natural period, we have measured microtremors in both principal directions of the buildings. The measured values were compared with the theoretical ones for the rocking vibration of continuous body of shear type on an elastic half-space. Summary is as follows; 1. The natural period depends on the types of foundations. The natural period of the buildings with piled foundation is generally longer than that with the spread foundation. 2. The properties of the soil to the depth of about ten meters determine the dynamic characteristics of the buildings relatively rigid. 3. Strong correlation has been observed between the measured natural period and the average N-value by standard corn penetration test to the depth of ten meters. The natural period increases as the average N-value decreases regardless of the types of foundations. This tendency is clearer as lower the buildings. 4. The measured natural period approximately agrees with the theoretical value except the buildings supported on piles in soft ground. If the effect of increasing of stiffness of soil by existing piles is considered, the "dynamical ground compliance" theory can be applied.
