Multiaxial active isolation for seismic protection of buildings

Passive isolation has been widely accepted as an effective means for the protection of structures against seismic hazards. The isolation bearings, typically placed at the base of the structure, increase the flexibility of the structure and shift its fundamental frequency away from the dominant frequency of seismic excitations, resulting in significantly reduced interstory drifts and floor accelerations. During severe earthquakes, the performance of passive isolation systems is usually achieved at the expense of having large base displacements. Alternatively, active isolation combines isolation bearings with adaptive actuators to effectively mitigate the base displacements, while maintaining reasonable interstory drifts and floor accelerations. Despite successfully theoretical proof documented in previous studies, most experimental implementations only verified active isolation with unit-axial actuators under unidirectional excitations. Earthquakes are intrinsically multidimensional, resulting in out-of-plane responses such as torsional responses. Therefore, the focus of this paper is the development and experimental verification of active isolation strategies for multistory buildings subjected to bidirectional earthquake loadings. First, a model building is designed to be dynamically similar to a representative full-scale structure. The selected isolation bearings feature low friction and high vertical stiffness, providing stable behavior. In the context of the multidimensional response control, three custom-manufactured and appropriately scaled actuators are employed to mitigate both in-plane and out-of-plane responses. In addition, the structure is subjected to multi-directional earthquake ground motion. To obtain a high-fidelity model of the active isolation systems, the authors propose a hybrid identification approach, which combines the advantages of the lumped mass model and nonparametric methods. Control-structure interaction is also included in the identified model to further enhance the control authority. By employing the H2/LQG control algorithm, the controllers for the hydraulic actuators are shown to offer high performance and good robustness. Active isolation is found to possess the ability to reduce base displacements and produce comparable accelerations and interstory drifts to passive isolation. The proposed active isolation strategies are validated experimentally for a six-story building tested on the six-degree-of-freedom shake table in the Smart Structures Technology Laboratory at the University of Illinois at Urbana-Champaign. Copyright © 2013 John Wiley & Sons, Ltd.