Semi-active control of sliding isolated buildings and bridges with variable stiffness and damping systems
Sahasrabudhe, Sanjay Suhas
Doctor of Philosophy thesis
Sliding isolation systems are effective in strong earthquakes; however, base displacements can be excessive in near source ground motions. Although passive nonlinear dampers can limit base displacements, the isolation forces, superstructure interstory drifts, and superstructure accelerations will be higher. Use of smart devices such as Magneto-rheological (MR) dampers, and semi active independently variable stiffness (SAIVS) devices in the isolation system may provide significant advantages. Independently varying damping and stiffness systems, used separately and together in sliding isolated buildings and bridges is investigated, analytically and experimentally, in this study. New analytical models of the sliding isolated structure with MR dampers and SAIVS devices are developed. New nonlinear control algorithms are developed. Extensive numerical simulations are performed with several near fault ground motions. Structural models and devices are tested on a shaking table in real time and the performance is evaluated analytically and experimentally. It is shown that (1) the newly developed SAIVS device is capable of varying the stiffness continuously and smoothly and the proposed analytical model captures the behavior of the device satisfactorily, (2) the newly developed analytical model for the MR damper predicts the behavior of the damper satisfactorily, (3) the MR damper along with the newly developed Luapunov controller, when introduced at the isolation level, reduces the isolation displacements further than the passive cases, while maintaining the forces, drifts, and accelerations within bounds in both buildings and bridges, (4) the SAIVS incorporated at the isolation level, in the controlled mode switching based on the newly developed control algorithm, reduces the sliding bearing displacements further than the passive open and closed cases, while the total forces, drifts, and accelerations are comparable to the least of the passive cases in both buildings and bridges, and (5) the SAIVS and MR damper in the controlled mode, when incorporated at the isolation level, reduce the displacements further than the passive, variable stiffness, and variable damping cases, while maintaining forces at the isolation level, drifts, and accelerations within bounds. The analytical and experimental study prove that the independently varying stiffness and damping systems and the developed controllers reduce the response of sliding isolated buildings and bridges significantly in near fault earthquakes.