Seismic analysis and performance of semi-active spring for base-isolated structures

The present study investigates the performance of a semi-active spring (SAS) in the mitigation of the seismic response of base-isolated structures. Initially, under stationary filtered white-noise earthquake excitation, the response of the multi-floor flexible base-isolated structure with SAS is examined to observe the response control effects. The equivalent linearization technique is used to obtain the stochastic response, as the force-deformation behaviour of SAS is non-linear. The performance of SAS in terms of added stiffness, damping, and response mitigation to the isolated structure is also investigated. It is found that the SAS controls the isolator displacement effectively. Furthermore, it was noted that there is an optimum stiffness value for the SAS devices for a particular system and excitation, at which point the RMS top floor acceleration reaches a minimum value. Next, approximate formulas are proposed for the RMS isolator displacement, the top floor absolute acceleration, and the optimum stiffness of the SAS. It is observed that these formulas accurately predict the expected response and can be applied to the initial design of base-isolated structures using SAS. Finally, using the non-linear model of the SAS, the seismic response of flexible base-isolated structures is determined for actual near-fault earthquakes, considering different values of the isolation periods and stiffness of the SAS device. The SAS was effective in controlling the isolator displacement under near-fault motions. The trends of the results of isolated structures with SAS devices under near-fault earthquake motions were also in good agreement with those under stochastic excitation.