Application of MR Dampers in Seismic Control of Structures Using Vertical Mass Isolation Method

Vertical Mass Isolation (VMI) method is used for the seismic control of structures. In this method, the entire structure is a combination of two mass and stiffness subsystems with an isolator layer located in-between. This method mainly uses the principle of periodic shift as a solution to reduce the seismic responses of the structure. In this method, the majority part of the structure mass is concentrated in the mass subsystem. However, this subsystem constitutes a small percentage of the stiffness of the main structure. Unlike the mass subsystem, the stiffness subsystem constitutes a small percentage of structural mass and a large percentage is structural stiffness. This isolation method makes the mass subsystem behave like a soft structure and move away from the resonance zone. The important point is the seismic control of the mass subsystem, which will be supplied by its connection with the stiffness subsystem provided by the interface dampers. The main difference between the VMI method and coupled structures methods is that the main structure is divided into two isolated structures with two completely different behaviors in this method. One of them has a soft behavior and the other has a hard behavior. However in the coupled structure methods, the control of the two main structures is done by each other and observing soft or stiffness behavior in these two structures is not required. A magnetorheological (MR) damper is used as the isolator layer to control the structure. The model used for these dampers is based on the modified Bouc-Wen model proposed by Spencer et al. in 1997. The clipped-optimal procedure that is one of the most beneficial algorithms is applied to control the MR damper. Passive-off and semi-active control techniques are applied to control the MR damper based on the applied controlling voltage. The maximum voltage that is applied to the MR damper, is equal to 9 V. A third control technique named Copt, which is based on the optimal damping, is used to compare with MR damper results. To evaluate the performance of the proposed control system, a wide range of structures with low to high floors are considered. To do this, structures with heights equal to 27, 54, 108 and 162 meters were selected. Using approximate fundamental period based on ASCE-7, the periods of structures are 0.5, 1.0, 2.0 and 3.0 seconds, respectively. Since all models are SDOF and linear elastic analysis is applied, the structural mass is considered equal for all models (M=100 ton), but the stiffness of models is calculated based on their mass and periods. The damping ratio for non-isolated structure is considered to be 0.05. The results indicate the proper performance of the semi-active method in reducing the responses of the isolated structures compared to the non-isolated structures. This method decreases, on average, the top floor displacement of mass-subsystem by 6%. However, the Copt method decreases it by 21% and the passive-off method increases it by 8%. The semi-active method decreases, on average, the top floor acceleration of mass-subsystem by 13% and 50%, less than the Copt and the passive-off methods, respectively. The findings demonstrated that the semi-active control method based on a maximum voltage of 9 V will reduce, on average, the maximum base shear of isolated structures by 30% compared with non-isolated structures. The parametric approach based on the Nekooei relation (Copt) that is applied as a passive control device had acceptable results for controlling the acceleration and base shear of the structure. This method led to the absolute acceleration of top floor and the base shear of structures to decrease by 53~68% compared to the uncontrolled structure. However, the efficiency of Copt method to reduce the top floor displacement is less than the maximum base shear of the structure.