Design issues for smart isolation of structures: past and recent research

The paper focuses on a number of original researches developed by the authors concerned with the development of new design approaches for smart base isolation systems for structures. Base Isolation (BI) systems represent the first kind of control devices applied to civil structures. In the paper, advancement in technology is exploited in this field, allowing to conceive new BI typologies possibly based on the adoption of special smart materials or on the coupling of the basic passive device with additional corrective devices, in such a way to minimize the disadvantages deriving from the simply passive system. Illustrated procedures also embed in the design pattern of base-isolation systems the interaction effects between structure and soil in order to provide the best tuning of the isolation parameters and to get the maximum performance of the devices, finally summarizing a number of original approaches to design under passive, semi-active and hybrid modes.

APPROPRIATE BASE STIFFNESS OF SMART BASE ISOLATION SYSTEM

In this paper, the effect of base stiffness on the performance of hybrid control system of base isolation system and magnetorheological (MR) damper has been studied and its appropriate base stiffness has been determined. Many researches have been proposed that in the structure controlled by the single base isolation system without MR damper, the base stiffness should be designed such that the fundamental period of isolated structure is almost triple the fundamental period of fixed-base structure. To determine the appropriate base stiffness of hybrid control system, different values have been considered as base stiffness and MR damper has been also employed in two cases of passive form that voltage and dynamical behaviour of MR damper is constant (hybrid base isolation) and semi-active form that MR damper voltage is applied by H2/linear quadratic Gaussian (LQG) and clipped-optimal control algorithms (smart base isolation). For numerical simulation, a three-story shear frame has been subjected to El Centro, Northridge and Tabas earthquakes. Results show that in the structure controlled by the single base isolation system, the peak responses of structure strongly depend on the base stiffness while the sensitivity of peak responses to the base stiffness is lower when the structure is controlled by hybrid base isolation system. According to results, it can be concluded that the peak base drift of hybrid base isolation system reduces with the increase of the base stiffness while this reduction trend is less considerable in the stiffness that are more than the proposed stiffness for the single base isolation system. Hence the proposed stiffness for single base isolation system is the appropriate stiffness for hybrid base isolation system, too. Results also show that under earthquakes considered in this paper, the smart base isolation system is mostly more effective than hybrid base isolation system in mitigating and controlling both root mean square and maximum of structure responses such as base drift, inter-story drift and acceleration.

A genetic algorithm–based design approach for smart base isolation systems

This article presents a new approach for designing effective smart base isolation systems composed of a low-damping linear base isolation and a semi-active magneto-rheological damper. The method is based on transforming the design procedure of the hybrid base isolation system into a constrained optimization problem. The magneto-rheological damper command voltages have been determined using H2/linear quadratic Gaussian and clipped-optimal control algorithms. Through a sensitivity analysis to identify the effective design parameters, base isolation and control algorithm parameters have been taken as design variables and optimally determined using genetic algorithm. To restrict increases in floor accelerations, the objective function of the optimization problem has been defined as minimizing the maximum base drift while putting specific constraint on the acceleration response. For illustration, the proposed method has been applied to design a semi-active hybrid isolation system for a four-story shear building under earthquake excitation. The results of numerical simulations show the effectiveness, simplicity, and capability of the proposed method. Furthermore, it has been shown that using the proposed method, the acceleration of the isolated structure can also be incorporated into design process and practically controlled with a slight sacrifice of control effectiveness in reducing the base drift.