This paper analyzes the soil-structure interaction (SSI) effect on vibration control effectiveness of active tendon systems for an irregular building, modeled as a torsionally-coupled (TC) structure, subjected to base excitations such as those induced by earthquakes. The SSI effect is governed by the slenderness ratio of superstructure and by the stiffness ratio of soil to superstructure. An H_∞ direct output feedback control algorithm that uses minimization of the entropy is implemented to reduce the seismic responses of TC structures. The control forces are calculated directly by multiplying output measurements by a pre-calculated frequency independent and time-invariant feedback gain matrix which is obtained based on a fixed-base model. Numerical simulations show that the required number of sensors and actuators and their locations highly depend on the degree of floor eccentricity. For a large two-way eccentric building, an one-way active tendon system placed at the opposite side of center of resistance (C.R.) can reduce both translational and torsional responses. If the SSI effect is significant, the proposed control system can still reduce the structural responses, but its performance is much worse than that of the corresponding fixed base model. Therefore, the TC and SSI effects should be considered in the design of active control devices, in particular, for a high-rise building founded on soft site. In this paper, an optimal, practical, and cost-effective design procedure for an active tendon system is proposed for the vibration control of irregular buildings under earthquake excitations.
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