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 loadin gs. They include: (1)Active variable stiffness, where the stiffness of the structure is adjusted to establish a nonresonant condition betwee n the structure and excitation; and (2) active variable damper, where the damping coefficient of the device is varied to achieve the most re duction in the response. This study is concerned with examining the ef fectiveness of variable dampers for seismic applications. Three algori thms for selecting the damping coefficient of variable dampers are pre sented and compared. They include a linear quadratic regulator algorit hm; 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 examini ng the response on the displacement-acceleration plane and assigning d ifferent damping coefficients accordingly. Two-single-degree-of-freedo m 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 dec reases displacement but increases the acceleration response), variable dampers can be effective in reducing both the displacement and accele ration responses. The algorithms are used to compute the seismic respo nse of two structures: (1) An isolated bridge modeled as a single-degr ee-of-freedom system; and (2) a base-isolated six-story frame modeled as a multi-degree-of-freedom system. The results indicate that variabl e dampers significantly reduce the displacement and acceleration respo nses.