Analysis of Shaking Table Tests on a Rocking Foundation Model

Isolated footings are a commonly used foundation types on stiff ground. When lateral loads are applied to a superstructure with footings, the induced moment will make the footing produce rotations. Further, during an earthquake, the footing may have rocking response due to seismically induced lateral loading. Previous studies have shown that the foundation rocking could reduce the acceleration response of the structure, which is referred to rocking isolation. However, although foundation rocking could reduce earthquake loading and further the designed size of the footing, it is also necessary to investigate the influence due to the accompanying lager footing rotation and soil plasticity. Rocking behavior could reduce the rotational stiffness of the footing and further influence the response of the structure. In order to investigate the change of dynamic characteristics of rocking system during rocking, this study utilize system identification methods to analyze experimental data of shaking table tests of a rocking-dominant column-footing model which had been conducted at National Center for Research on Earthquake Engineering (NCREE). The short time system identification method as ARMA model is applied to analyze the change in dynamic properties of the structure and soil system. During the process of shaking, the predominant vibration frequency decreased with increasing shaking intensity, and it would go up again as the shaking intensity decreased. Besides, the evolution of the identified damping ratio is depending on the degradation behavior of the vibration frequency. The damping ratio increased as the vibration frequency decreased. Based on the comparisons between the system identification results and hysteretic moment rotation curves, the identified rotational stiffness and damping ratio are consistent with those obtained from the hysteresis curve for the cases with small excitation; the difference would be significant under large excitation due to the assumption that the ARMA model regards the nonlinear spring behavior as an equivalent linear spring model with viscous damping.