Semiactive Control Algorithms for Structures with Variable Dampers

Semiactive control systems combine the features of active and passive control to reduce the response of structures to various dynamic loadings. They include: (1) Active variable stiffness, where the stiffness of the structure is adjusted to establish a nonresonant condition between the structure and excitation; and (2) active variable damper, where the damping coefficient of the device is varied to achieve the most reduction in the response. This study is concerned with examining the effectiveness of variable dampers for seismic applications. Three algorithms for selecting the damping coefficient of variable dampers are presented and compared. They include a linear quadratic regulator algorithm; a generalized linear quadratic regulator algorithm with a penalty imposed on the acceleration response; and a displacement-acceleration domain algorithm, where the damping coefficient is selected by examining the response on the displacement-acceleration plane and assigning different damping coefficients accordingly. Two-single-degree-of-freedom structures subjected to 20 ground excitations are analyzed using the three algorithms. The analyses indicate that, unlike passive dampers (where for flexible structures, an increase in damping coefficient decreases displacement but increases the acceleration response), variable dampers can be effective in reducing both the displacement and acceleration responses. The algorithms are used to compute the seismic response of two structures: (1) An isolated bridge modeled as a single-degree-of-freedom system; and (2) a base-isolated six-story frame modeled as a multi-degree-of-freedom system. The results indicate that variable dampers significantly reduce the displacement and acceleration responses.