Two horizontal components and a vertical component are usually employed in seismic observations. Therefore, only those components are used as the input ground motions in seismic design code. However, there must be rotational or rocking component together with basic three components in ground motion, because of the existence of Rayleigh wave, phase difference between vertical components at grounds with different stiffness, and no mechanism preventing ground motion from being horizontal for excitations from various directions. Seismographs are generally installed more than several hundred meters apart from each other even in arrayed observations. This distance is too long to identify the existence of rotational or rocking component. And there is no seismograph which can measure this component. Since there has been no information on this component, such component cannot be taken into account in seismic design code. However such component may have an impact on some rigid structures as well as flexible structures. It is indispensable to examine the effects of this component on structural response. In this study, experiments using the model ground excited by a shaking table and simulations of ground motion using FEM are carried out, in which existence of rocking components are examined in earthquake motion generated at various grounds such as the ground with irregular form foundation. Consequently, the experiment and analysis verified existence of rocking motion component. It is shown that the vibration of horizontal direction is the generating factor of rocking motion components in the experiment and that the vibration of vertical direction is the amplifying factor of rocking motion components. Moreover, rocking motion component is large near the border between two different grounds, and depends on the vertical amplitude of vibration and on excitation frequency.
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