Date of Award
Anil K. Agrawal
Eddy Current Damping, Electromagnetism, Friction, Passive control, Semi-active Control, Stick-slip motion
Energy dissipation is critical to limiting damage to civil structures subjected to extreme natural events such as earthquakes. Friction is one of the most reliable mechanisms of energy dissipation that has been utilized extensively in friction dampers to improve seismic performance of civil structures. Friction dampers are well-known for having a highly nonlinear hysteretic behavior caused by stick-slip motion at low velocities, a phenomenon that is inherent in friction and increases the acceleration response of the structure under control unfavorably, in spite of the fact that the displacement is generally reduced because of the energy dissipation. This increase in acceleration can, for example, significantly affect the seismic response of a multi-story base-isolated building as it undermines the seismic isolation system by inserting high-frequency pulses into the floor acceleration. This may pump a considerable portion of the seismic input energy into higher modes, resulting in the increase of the floor inter-story drift. Therefore, a passive friction damper not only decreases the comfort of occupants but also increases the risk of damage to non-structural components during large earthquakes. The focus of this dissertation is on developing novel electromagnetic passive and semi-active friction dampers in which the undesirable effects of stick-slip motion are effectively reduced.
The first part of this research focuses on the development of passive friction dampers for seismic hazard mitigation of civil structure. The first proposed passive friction damper, which is termed as passive electromagnetic eddy current friction damper (PEMECFD), utilizes a solid‐friction mechanism in parallel with an eddy current damping mechanism to maximize the dissipation of input seismic energy through a smooth sliding in the damper. In the proposed PEMECFD, friction force is produced through magnetic repulsive action between two permanent magnets (PMs) magnetized in the direction normal to the friction surface, and the eddy current damping force is generated because of the motion of the PMs in the vicinity of a copper plate. The friction and eddy current damping parts are able to individually produce ideal rectangular and elliptical hysteresis loops, respectively; which, when combined in the proposed device, are able to accomplish a higher input seismic energy dissipation than that only by the friction mechanism. The idea of combining friction with eddy current damping is further investigated by proposing the second passive friction damper in which arrays of cubic PMs have been used to generate attractive magnetic normal force across the sliding surfaces and induce eddy current damping. This damper has a fully solid configuration and, for this reason, is termed as Magneto-Solid Damper (MSD). The influence of eddy current damping on energy dissipation due to friction is further investigated through modeling, design, characterization testing, and model identification and validation of proof-of-concept prototype dampers in laboratory.
In the second part of this research, a smart/semi-active electromagnetic friction damper (SEMFD) is proposed for the control of seismic response of civil structures. The SEMFD consists of a ferromagnetic plate and two similar arrays of thick rectangular ferromagnetic-core coils (FCs) connected in series. The FCs are attached to the two sides of the ferromagnetic plate through two non-magnetic friction pads. The force in the damper is developed because of the friction between the friction pads and the ferromagnetic plate when the FCs moves relative to ferromagnetic plate. The normal force between the friction pad and the ferromagnetic plate is caused by the attractive magnetic interactions between the FCs arrays and the ferromagnetic plate. The magnitude of this force is controlled by a proposed semi-active controller that is capable of varying the current flowing through the FCs in such a way that it is able to avoid stick-slip motion to smooth the nonlinear hysteretic behavior of the SEMFD. The capability of the proposed SEMFD and its semi-active controller to control the seismic responses of base-isolated buildings and horizontally curved bridges is demonstrated. The numerical results show that the proposed SEMFD is capable of limiting the displacement of the base floor in base-isolated buildings without noticeably increasing the inter-story drifts and absolute accelerations of the floors. Further assessment of numerical results indicates that the proposed SEMFD is also effective in limiting the motion of the deck in horizontally curved bridges and thereby preventing it from unseating, which is one of the most common modes of failure in horizontally curved bridges.
Amjadian, Mohsen, "Development of Electromagnetic Friction Dampers for Improving Seismic Performance of Civil Structures" (2019). CUNY Academic Works.