Development of a long-period vertical base isolation device with variable stiffness for steel frame structures

The current isolation technology is mainly developed for seismic response control of structures in horizontal direction. Seismic records and shaking table test results show that there is a strong need to mitigate the vertical responses of structures. This paper proposes a three-dimensional (3D) isolation device that consists of a traditional horizontal bearing and a newly developed long-period vertical isolation device with variable stiffness (LVIVS). The LVIVS includes three layers of springs and two of them are prestressed. With the aid of displacement constraint components, the non-prestressed springs work independently or in series with the prestressed springs at different loading stages to realize variable stiffness characteristic. As designed, the LVIVS has large vertical stiffness in the normal stage to restrain the displacement under live loads and ground motions with small amplitudes, small vertical stiffness in the isolation stage to prolong the vertical period of the base isolation system, and large vertical stiffness in the limit stage to restrain large vertical displacement at extreme events. The prestress of the springs leads to a self-centering feature of the device. Also, the variable stiffness characteristic shortens the deformation demand of the device under self-weight which enhances the stability of the device significantly and thus it is easier to achieve a long isolation period in the isolation stage. The hysteretic model for the LVIVS is derived, and sinusoidal excitation tests are conducted to verify the proposed hysteretic model of the LVIVS and its variable stiffness mechanism. The experimental results show that the variable stiffness characteristics of the LVIVS can be achieved, and the isolation period reaches 0.95 s. Finite element simulations are also conducted for a steel frame structure to verify the isolation effect of the proposed 3D isolation device.