Seismic behavior of rocking base-isolated structures

It has been known that base-isolation structures may have long-period response under earthquakes. Many researchers have pointed out that long-period resonant-like response of isolated structure may be induced by a ground motion containing strong long-period frequency spectra. To mitigate this problem, isolation systems with variable mechanical properties such as different isolating stiffness or various frictional damping with respect to isolator displacement have been proposed recently. In this paper, a new type of isolator called variable-frequency rocking bearing is proposed. The system allows the bearings to remain still under a moderate seismic load but start to rock back-and-forth during severe earthquakes. The vibration is damped throughout each impact of the bearings on structural footing and foundation surface. Based on the force-displacement relations and analytical damping of the system, the seismic response of the structure may be estimated through a modified elastic response spectrum. To validate the proposed idea, a one-bay-one-story spaced structure base-isolated with rocking bearings was constructed. In total, 261 shaking table tests were conducted with various investigated parameters including aspect ratio of the bearing, shapes of rocking plate (plane, spherical or polynomial surface), spring stiffness and waveform of ground motion etc. Test results showed that the vibration behavior was nonlinear with respect to the rocking amplitudes. It is also found that damping ratio of rocking base-isolated structures can be approximately estimated by the proposed theory.