A novel gas spring based negative stiffness mechanism for seismic protection of structures

The integration of negative stiffness (NS) elements in the concept of seismic base isolation constitutes an innovative and highly effective approach for ensuring the safety of structures against the damaging effects of earthquakes. With these isolators, the system's natural frequency is significantly decreased and thus the trans-missibility of seismic forces for all the frequencies above the natural one is reduced. The KDamper is such a representative device that has been implemented in many types of structures improving their dynamic perfor-mance due to its superior damping and isolation properties. This study investigates the performance of a variant of the KDamper, referred to as SBA (Seismic Base Absorber), which is equipped with a novel NS element. The SBA combines an extended version of the KDamper with a parallel-connected inerter used to further decrease the structure's natural frequency without reducing its static stiffness. The NS element of this device is selected to be realized with a modified Scott-Russell mechanism with an extension link that is connected through a revolute joint with a vertical pre-compressed gas spring. This configuration enables the NS element to keep the pre-tension of the spring internally without transmitting it to the superstructure. Additionally, the gas springs pro-vide high and undiminished values of NS in the entirety of motion and are capable of supporting heavy oscil-lating structural masses without running into issues like buckling or yielding, unlike the conventional compression springs. The performance of the SBA equipped with this NS element is evaluated through numerical simulations both in frequency and time domains, considering a single degree of freedom (SDOF) system and having as excitation input 30 artificial and 20 real accelerograms. To draw the necessary conclusions these simulations are also conducted for a conventional and a highly damped base isolation system as well as a linear SBA system. In addition, a comparative analysis is performed between the proposed NS element and an equiv-alent element that incorporates linear springs. The results highlight the effectiveness of the SBA in reducing the maximum dynamic responses and the superiority of gas springs over the linear ones in terms of stability and feasibility of the system.