Development of Nonlinear Geometric Seismic Isolation with a Duffing Spring

Seismic isolation is widely used in several countries, and the number of seismically isolated buildings has increased rapidly in recent decades. Seismic isolation extends the natural period of a building to decrease the absolute acceleration and seismic force. As there is a trade-off between the absolute acceleration and displacement, a soft layer results in a large displacement for a large seismic wave, but the hard one causes the large absolute acceleration even for a small seismic wave. The restoring force of a Duffing spring is given by the third and first orders of the displacement. This spring has been applied to protect a building from large earthquake waves. However, the influence of the coefficient of the Duffing spring that determines the dynamic characteristics of the system has not been clarified. Thus, a used Duffing spring may not be appropriate for seismic resistance. Moreover, most studies are based on analytical methods, and the advantages of the Duffing isolation have not been verified in an actual system. To address these problems, this paper reveals the influence of the coefficient of the Duffing spring on structural responses to seismic waves. Moreover, this paper devised a way to implement a Duffing spring for seismic isolation and carried out experiments to verify the validity in actual systems. The experimental results presented that the Duffing spring was effective in protecting a building in actual systems.