The paper illustrates different aspects inherent to the multifaceted task of semi-actively controlling the threedimensional dynamics of a frame structure through interstorey chevron braces embedding smart dampers. The control strategy is finalized to mitigate the latero-torsional motion of a large-scaled laboratory prototype, which experimentally simulates the seismic response of a two-storey building. A synthetic discrete model is purposely formulated in the reduced space of a few significant dynamical variables, and consistently updated to match the modal properties identified from the experimental response of the uncontrolled prototype. The potential occurrence of a significant eccentricity in the storey mass distribution, breaking the global structural symmetry, is considered. The control action is introduced by two magneto-rheological dampers, whose dissipative properties can be modified in real-time, supplying an opportune magnetic field. The suited adjustment of the damper characteristic is governed by a clipped-optimal control law, which entails the instantaneous comparison of the actual force with the active force evaluated through a non-collocated acceleration feedback, designed according to the LQG criterion. Since the dampers are positioned purposely to deliver two eccentric and independent forces, acting on the first-storey displacements, the set-up allows the mitigation of the three-dimensional motion arising when monodirectional ground motion is imposed on the non-symmetric structure. Numerical investigations on the model controlled response to different natural accelerograms are presented. The effectiveness of the control strategy is discussed through synthetic performance indexes. Comments on the real effectiveness of the control strategy in mitigating the experimental response of the prototype are drawn, based on the promising results of a test campaign. Qualitative considerations are finally pointed out for future developments regarding the need of further experimental tests and optimization analyses
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