Seismic performance analysis of steel frame structures with separated three-dimensional isolation

Three-dimensional (3D) base isolation can significantly improve the seismic performance of structures. However, significant 3D movement coupling and rocking behavior of the superstructure due to the high gravity center of the structure can be observed, which will lead to a complex design of the isolation system and the isolators. In addition, as the 3D isolators are installed beneath the structures, they need to support the heavy weight of the superstructure with low vertical stiffness. It is difficult to meet both the vertical load and stiffness design demands of the isolators using traditional isolation materials and mechanisms, especially for large-size structures. A separated 3D isolation scheme is proposed by installing the horizontal isolation at the base of the superstructure and the vertical isolation under the floor slabs. Simulations are carried out to examine the dynamic responses including the acceleration, inter-storey drift, and stress of structural members of a typical steel frame structure with the separated 3D isolation scheme. The isolation effect was also studied by shaking table tests of a two-storey steel frame structure. The simulation and test results show that the proposed separated 3D isolation scheme has high performance in mitigating the structural accelerations in both horizontal and vertical directions, and the vertical load on isolators. The rocking behavior of the structure is effectively restrained for the separated 3D isolation scheme due to a low mass center configuration of the vertically isolated slabs. Although the isolation of the floor slabs from the beams in the structure with separated 3D isolation weakens the constraint of the floor slabs to the beam and column members as in a traditional frame structure, the horizontal isolation at the base of the superstructure reduces the adverse effect and the stress of structural members is similar to that in a horizontal base-isolated structure and significantly lower than that in a base-fixed structure. In addition, as the vertical isolation is installed under the slabs, the vertical load on the vertical isolators is small and it is easy to design the isolators. A separated three-dimensional isolation scheme for steel frame structures is proposed by installing horizontal isolation at the base and vertical isolation beneath the floor slabs. It can achieve a high isolation effect, mitigate the motion coupling, and restrain the rocking behavior significantly without any rocking constraint devices, thus protecting the structure and nonstructural components. Besides, the vertical load on vertical isolators is significantly reduced, making it easy to design and manufacture. image