One of the main challenges civil engineers face today is to find more effective means for protecting structures and their contents from the damaging effects of dynamic hazards such as strong earthquakes. Vibration control of civil engineering structures have been studied for three decades.  Casualties and economic damages in recent earthquakes at different countries show the importance of vibration control of structures. Vibration control of structures evolves from passive control, active control to semi-active control.  Recently, semi-active control systems have attracted much attention because they have good efficiency with best features of both passive and active control. A semiactive control system can produce a large control force simply by dynamically changing parameters such as the damping coefficient and stiffness of the control device. Various control devices that have been tried by different researchers for control applications include stiffness control devices, electrorheological and magnetorheological dampers, friction control devices, fluid viscous dampers, tuned mass dampers, and tuned liquid dampers. From this point of view,  semi-active control systems have reliability of passive control and flexibility of active control.
In this paper the performance of the semi-active control systems used by piezoelectric friction dampers was studied. The damper system was set on 5-story structure model and in order to adjust and assign the voltage on piezoelectric elements; the fuzzy logic controller was used. Some characteristics of fuzzy logic controllers appealing to control engineers are their effectiveness and ease in handling structural non-linarite, uncertainties and heuristic knowledge make it a better choice for control applications. Added to the nice ties present in a fuzzy system, a fuzzy control applied to structural system can handle the hysteretic behavior of the structure under earthquake. The performance of FLC depends on various design parameters related to selection of membership functions and definition of rule base. It is especially important for FLC to have an effective and reliable rule base to perform at the desired level. The structure was modeled in OpenSees finite element software and the fuzzy logic controller was designed in MATLAB software in order to determine the command voltage to apply to piezoelectric layers. Furthermore, in order to make a connection between both OpenSees and MATLAB softwares; the TCP/IP method was used.
Two input variables including relative displacement and velocity of stories were assigned to the fuzzy logic controller; moreover, the command voltage was assigned as output variable. Incremental dynamic analysis of structure with maximum acceleration from 0.2g to 1g was used. For this purpose, to assess the performance of studied control systems; four earthquakes including the 1940 El Centro, 1968 Hachinohe; 1994 Northridge and 1995 Kobe were applied to the structure. Both of the El Centro and Hachinohe earthquakes are far-field while the Northridge and Kobe are near-field earthquakes. The results of both semi-active and passive control were compared. The average results of semi-active control for evaluation criteria for displacement and base shear show11 and 10.5% of improvement copared to the passive control, respectively; for far-field earthquakes. Furthermore, results show 7% and 5% improvement compared to the passive control for displacement and base shear evaluation criteria respectively; for near-field earthquakes. Moreover, the sensitivity of the semi-active control systems shows 16.53% and 11.8% improvement than passive control for far-field and near-field earthquakes respectively.