This study deals with a shake table test on a three-story base-isolated steel frame. The frame rests on four roller bearings for isolation and is equipped with four laminated rubbers as shear spring. An MR damper is used in the test to perform semi-active seismic response control. The basic control algorithm applied in the study is to simulate the load-deflection of an origin-restoring friction damper (ORFD) which is a sort of friction damper that looses its resistance when it moves toward the origin, making sure for the base-isolated system to minimize residual displacement even after an extremely strong ground motion. Also attempted is a hybrid type control that superposes viscous damping on the ORFD when the damper moves from the peak displacement toward the origin.
This paper presents a comprehensive study on the application of the MR damper to base-isolated building structures. It first proposes a simple semi-active control algorithm for a base-isolated structure with an MR damper. The algorithm, in which the MR damper's hysteresis shape is controlled, aims to reduce the isolator's displacement without increasing the acceleration responses of the upper structures. The second part of this paper covers the properties of an MR fluid and an MR damper developed for a base-isolated model structure. The damper has a nominal capacity of 40kN, which can be adjusted in accordance with the applied magnetic fields. In the test, the damper is subjected to cyclic sinusoidal displacements with different amplitudes, velocities and magnetic field intensities. The last part describes shaking table tests carried out using the MR damper and the base-isolated model structure. It is confirmed that the proposed semi-active control method is effective in reducing the isolator's displacement without increasing the acceleration responses.
First part of this paper covers experimental studies on mechanical properties of two types of magneto-rheological fluid (MRF) dampers. One is a commercial built-in-pass type damper and the other an original by-pass type damper. In the test, they are subject to cyclic sinusoidal displacements with different amplitudes, velocities and intensities of magnetic field. Not only their hysteretic properties but also their quickness to respond to the applied magnetic field are examined. In the second part, two analytical methods to represent the mechanical properties of the dampers are presented. One is a semi-empirical method making use of a Bingham Model to simulate the hysteretic properties of the damper. The other one, an analytical method based on the theory of non-Newtonian fluid. A design formula to predict the resistance of the damper is so obtained as to take into consideration the damper's dimensions, the properties of the fluid and the intensity of the magnet field applied.
Two kinds of Magneto-rheological fluid damper (MRF damper) have been designed and manufactured. One has a nominal capacity of 2kN and the other 20kN. A bypass flow system is adopted for both dampers and each has the same capacity of electromagnet attached to the bypass portion. The effective fluid orifice is the rectangular space and the magnetic field is applied from the outside. A test was performed by applying different magnetic fields to the orifice portion of the rectangular space. The damping force and the force- displacement loop were evaluated. The test results yielded the following: (1) Two type's of dampers functioned by using one unit of the electromagnet under an appropriate electrical current control. (2) The magnitude of the damping force depends on the input magnetic field, but it has an upper limit. (3) Without an applied magnetic field, the MRF damper exhibits viscous-like behavior, while with a magnetic field it shows friction-like behavior. A mechanical model of the damper is estimated by taking account of the force-displacement loop. It is clarified that MRF dampers provide a technology that enables effective semi-active control in real building structures.
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