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Received Aug 25, 2017; Accepted Jan 28, 2018
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1. Introduction
Semiactive (SA) structural control systems rely on smart devices able to provide a rapid variation of their stiffness and/or damping properties. Although the probably first implementation of a SA structural control system is based on variable stiffness devices [1], today most of the research efforts are aimed at the adoption of variable damping schemes. The latter idea was first introduced in the early 1970s by Crosby and Karnopp [2], who showed the possibility of exploiting a variable-constant viscous damper in the context of automotive industry. The original work of Crosby and Karnopp envisioned a SA suspension driven by a two-state switching policy that makes the viscous damper behave pretty much like a sky-hook device. One of the advantages of such idea is the corresponding model-free control algorithm, whose implementation does not require a previous knowledge of the system parameters and/or of the external excitation [3].
Although these control algorithms are widely described in the scientific literature, their effectiveness is almost always shown by numerical applications. Notable exceptions are cited in the following. Li and Xu [4] performed shaking table tests on a three-storey one-bay frame model, controlled by a double-ended shear mode combined with valve mode MR fluid device placed between the ground and the first floor. The validity of the SA control system was verified by implementing three different control algorithms: the instantaneous optimal control algorithm, the classical linear optimal control algorithm, and the linear-quadratic Gaussian control algorithm. Lee et al. [5] adopted a full-scale five-storey testing structure to make an experimental comparison of different SA algorithms (Lyapunov algorithm, neurocontrol logic, and maximum energy dissipation algorithm) to control the behavior of the MR damper-based system, under the effect of four historical earthquakes and one artificial seismic input. Basili et al. [6] carried out shaking table tests to verify the effectiveness of a SA MR damper system in reducing seismic vibrations of adjacent structures. The physical model is represented by two 1 : 5 scaled steel structures connected at the second level by a commercial MR damper...