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SUMMARYA base isolation system composed of low-damping isolation bearings and magnetorheological (MR) uid dampers is described. The MR uid changes its properties under the in uence of a magnetic ÿeld resulting in a damper with characteristics that may be modiÿed in real time. This feature enables optimal control under changing excitations in a stable and cost-e ective manner. The voltage is applied according to a selective control strategy. According to the proposed approach the dampers are activated only within a given range of the base displacements. The selective control improves the e ciency of the system and signiÿcantly reduces the control forces required for an optimal structural behaviour. Models of ÿve-and eight-storey buildings are used to study the e ciency of the proposed system.

SUMMARYA method for design of an active control system for multistorey structures using Electrorheological (ER) dampers is presented. Incorporated at various levels of a structural frame, ER dampers are used to improve the response of the structure during earthquakes. Optimal control theory was used to design the ER devices. The aim of the design is to find the most suitable combination of the minimum required forces produced by the ER dampers to obtain the optimal structural response. The mechanical response of ER fluid dampers is regulated by an electric field, depending on the displacements and velocities of the frame. Numerical analysis of an ER damped seven-storey structure is represented as an example. Significant improvement of the structural response was obtained using optimal active controlled ER dampers compared to passive controlled and uncontrolled structures.

SUMMARYThis paper presents a theoretical study of a predictive active control system used to improve the response of multi-degree-of-freedom (MDOF) structures to earthquakes. As an example a building frame equipped with electrorheological (ER) dampers is considered. The aim of the design is to "nd a combination of forces that are produced by the ER dampers in order to obtain an optimal structural response. The mechanical response of ER #uid dampers is regulated by an electric "eld. Linear auto-regressive model with exogenous input (ARX) is used to predict the displacements and the velocities of the frame in order to overcome the time-delay problem in the control system. The control forces in the ER devices are calculated at every time step by the optimal control theory (OCT) according to the values of the displacements and of the velocities that are predicted at the next time step at each storey of the structure.A numerical analysis of a seven-storey ER damped structure is presented as an example. It shows a signi"cant improvement of the structural response when the predictive active control system is applied compared to that of an uncontrolled structure or that of a structure with controlled damping forces with time delay. The structure's displacements and velocities that were used to obtain the optimal control forces were predicted according to an &occurring' earthquake by the ARX model (predictive control). The response was similar to that of the structure with control forces that were calculated from a &known' complete history of the earthquake's displacement and velocity values, and were applied without delay (instantaneous control).

The development and applications of a semi-active friction damping system with amplifying braces is studied. A system of dampers and braces, defined as the friction damping system with amplifying braces (FDSAB) is considered. Active control theory with velocity and acceleration feedback is used to obtain the control forces in the proposed system. The system can be used efficiently to enhance the damping of a structure and improving its response. The efficiency of the proposed system is demonstrated by the numerical simulation of a seven storey building subjected to earthquakes. The simulation shows that the behaviour of the damped structure with the FDSAB is significantly improved. The required control forces are much less compared with a control system with semi-active friction dampers connected either to chevron or diagonal braces.

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