Introducing the inverted pendulum as a negative stiffness mechanism and a novel structural system to improve seismic performance using a quasi-zero stiffness approach

Earthquakes are significant factors contributing to the damage and failure of structures. Seismic isolators are one of the effective solutions for mitigating the impact of earthquakes. This study investigates a hybrid seismic isolator system that combines a pendulum-suspended isolator with an inverted pendulum and incorporates negative stiffness. The governing equations of the system are derived, and its response is analyzed under multiple earthquake scenarios. The results show that negative stiffness increases the natural period. A comparison of energy input to the structure, with and without the seismic isolator (both with and without negative stiffness), indicates that the inclusion of negative stiffness reduces the energy input to the system. FFT analysis further supports the positive effect of negative stiffness on system performance. To demonstrate the capability of the proposed system, a 4-story frame was modeled in Abaqus software. The results reveal, the hybrid isolator achieves quasi-zero stiffness and significantly reduces response acceleration, reaching near-zero levels. Unlike traditional isolation systems that typically increase displacement, the proposed system reduces displacement, offering a significant advantage. Finally, the study explores important factors such as system stability, the type of connection between isolator components, period control, and the system's performance under both free and locked states.