Nonlinear negative stiffness effect for enhanced isolated structures vibration Control: Mechanism and real-time hybrid simulation testing

Nonlinear negative stiffness devices (NNSD) have been demonstrated to be effective in controlling structural vibration responses. In this study, the influence of the nonlinear negative stiffness effect of NNSD with varying compression ratios on the performance of base-isolated structures (BIS) is investigated. Based on the force-displacement hysteresis curves of NNSD, analytical models are proposed, and their feasibility is subsequently evaluated. The third-order Taylor expansion model is revisited not to challenge its validity, but to show that a fifth-order model is necessary for accurately capturing the nonlinear behavior. Higher-order expansions are not needed, and the required expansion order should match the level of geometric nonlinearity. Then, a series of real-time hybrid simulation (RTHS) confirmed that NNSD with fixed compression ratios can effectively control structural vibration. These findings underscore the importance of collaboratively optimizing the compression ratio in conjunction with the stiffness and displacement responses of the isolation story in BIS. When the hybrid system approaches a quasi-zero stiffness state, further reduction of the compression ratio does not yield additional improvements in control performance. Selecting an appropriate compression ratio is therefore essential for achieving optimal vibration control. To further enhance the control effect, the incorporation of additional damping or energy dissipation devices may be considered. Lastly, the compression ratio of the NNSD determined based on the expected displacement of isolation story under various seismic design standards is illustrated, and their performance is evaluated.