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A Fail-safe, Bi-Linear Liquid Spring, Controllable Magnetorheological Fluid Damper for a Three-dimensional Earthquake Isolation System
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Building codes governing building design and construction require that loss of human life is not anticipated during a large, infrequently occurring earthquake. However, earthquake-induced damage to the building load carrying components, nonstructural components, including architectural and mechanical systems, and internal equipment or contents, is still expected in code compliant buildings. Recent earthquakes have shown that economic losses are dominated by damage to nonstructural components and contents. Seismic isolation systems, which consist of layers of rubber or friction bearings separating the building from its foundation, are effective in protecting buildings from damage due to horizontal ground shakings. However, recent realistic large-scale earthquake shaking tests have shown that nonstructural components and contents in isolated buildings are susceptible to damage from vertical motions. In this study, a fail-safe, bi-linear liquid spring, controllable magnetorheological (MR) damper is designed, built and tested. The device combines the controllable MR damping in addition to the fail-safe viscous damping and liquid spring features on a single unit serving as the vertical component of the building suspension system itself. The controllable MR damping offers an advantage in the case that the earthquake intensity might be higher than that of the design conditions. The bi-linear liquid spring feature provides two different stiffnesses in compression and rebound modes. The higher stiffness in the rebound mode can prevent a possible overturning of the structure during rocking mode of vibrations.The device can be stacked together along with the traditional elastomeric bearings that are currently used to absorb the horizontal ground motions. In the occasion of an earthquake, it is not only exposed to vertical excitations, but also large residual shear excitations. It has to pass these shear forces between the ground and isolated structure. The theoretical and simulation modeling to overcome this major challenge and achieve other system requirements are presented. In addition, a comprehensive optimization program is developed in ANSYS platform to achieve all design requirements. The fabrication and experimental procedures are discussed. The test results showed that the device performed successfully under the combined axial and shear loadings. To our knowledge, this is the first device that not only can provide large damping and spring forces, but can also operate simultaneously under combined axial and shear loadings. The test results are compared against the theoretical modeling, and the results are discussed.