地表面上に設置した物体の垂直方向振動性状

抄録

This paper presents the experimental results of vertical vibration tests on small bodies resting on the surface of foundations, and investigates the spring and damping effects of the foundations comparing the experimental results with the approximate theoretical expressions.First, an approximation method of analyzing a soil-body system of vertical direction as a lumped-parameter system consisting of a mass, a spring and a dashpot is described for the cases of both a rigid base and a uniform contact pressure distribution using the Ground Compliance given by Tajimi.Next, the experimental method and the results are presented. The experiments were conducted on two foundations made of Kanto loam sand. The experimental results and their comparison with the approximate expressions are summarized as follows. For the case of Kanto loam foundation, the spring constant and the damping coefficient depend on the average contact pressure and increase as the contact pressure increases (Figs.7 and 11). The spring constant is proportional to the radius of the contact area, and the damping coefficient to the contact area for given value of the contact pressure (Figs.6 and 10). For small values of contact pressure, the measured spring constant is nearly identical to the static spring constant assuming a uniform contact pressure distribution and as the contact pressure is increase the measured value approaches the static spring constant assuming a rigid base contact pressure distribution(Fig.6). The measured value of the dimensionless natural frequency is nearly identical to the value calculated from the approximate expression assuming a uniform contact pressure distribution (Fig.8). The spring team is a function of the frequency and decreases as the frequency increase (Fig.9). Though the measured value of the damping ratio is larger than the calculated value its tendency seems to agree with the approximate expression (Fig.12). The viscosity of the soil seems to have a damping effect on the vibration of a body, though this effect is small compared with radiation damping. For the case of the sand foundation, the effect of contact pressure is considerably large compared with that for the Kanto loam foundation (Figs.14 and 16). This phenomenon is explained as follows: the rigidity of the sand foundation increase locally when a body is set on the surface of the foundation. Because of this inhomogeneity of the sand foundation, sufficient care needs to be taken when a theoretical result assuming the homogeneity of an elastic medium is applied for a sand foundation. However, the size-effect of the contact area on the spring constant and the damping coefficient agree qualitatively with the approximate expressions (Figs. 13 and 15). The dimensionless natural frequency and the damping ratio of the system seem to vary approximately as the -0.4 power of the mass ratio (Figs. 17 and 18).