Superstructure response of sliding-isolated multi-story frame structure

A cost-effective solution for the prevention of earthquake damage in low to medium rise buildings can be achieved by seismic isolation. This paper deals with the seismic response of the superstructure (story shears and interstory drifts) in multi-story buildings using a sliding isolation system. Quarter scale braced and unbraced seven story building models were subjected to a variety or simulated earthquakes on the shake table. Spherical sliding isolators known as the Friction Pendulum System (FPS) were placed directly at the base of individual columns instead of beneath a rigid base. Overturning effects caused by very strong motions resulted in large variations in the column loads reducing it to zero in some instances. A simple modal decomposition of the experimental response was done to assess the relative contribution of higher modes. Experimental results can be predicted using a time history dynamic analysis incorporating variable column loads and a bilinear force-displacement model for the bottom story containing the isolators. It was also found that available software for analysis of base-isolated structures such as 3D-BASIS that considers constant bearing loads can be used.