Abstract: The free-standing equipment could be damaged because of their sliding, rocking and toppling during a seismic event. In this paper, the free-standing equipment is modeled as a rigid body, and the base on which the equipment is placed is regarded to be rigid. The response of the rigid body on a harmonically motivated base is studied, and it is discovered that in the overturning movement there exist three types of response, the periodic, quasi-periodic and chaotic. The reason the response of this system is sensitive to initial condition is uncovered.