Keywords: base isolation, magneto-rheological damper, shaking table test, numerical analysis, variable-oval-control rule.

Recently, base isolation systems have been widely applied to many buildings in Japan. A base-isolation layer is made up of bearings, springs and damping elements. As the damping elements, hysteretic dampers such as lead dampers or steel dampers are widely used, because they are relatively inexpensive. Velocity-dependent dampers such as oil dampers are also employed as damping elements. In the case of an extremely large seismic event, however, the oil damper reaction force becomes large independent to the base-isolation layer displacement. This is because the damper force of the velocity-dependent damper is relative to the response velocity not to the displacement. This means that large acceleration and shear force of the super-structure occur to reduce the base-isolation layer displacement. This is the disadvantage of the velocity-dependent damper in terms of the displacement control design. On the other hand, a hysteretic damper which has a constant maximum damper force, cannot provide enough damping in an extreme seismic event, because the equivalent damping ratio decreases at a large deformation. This is also a disadvantageous characteristic of the hysteretic damper.

Sagami et al. [1,2] presented a displacement-dependent control rule for the magneto-rheological (MR) damper. They showed that the maximum damper force can be reduced by the proposed control rule compared with a conventional oil damper. The proposed control rule, however, requires the measurement of not only the response displacement but also the response velocity.

In this study, a new oval-shaped hysteretic loop with displacement dependency, variable-oval-control rule is proposed to reduce the maximum damper force. The present control rule generates the maximum force relative to the maximum displacement of previous loop, and only the measurement of the displacement is required.

From a series of numerical analyses for a reinforced concrete building with a base isolation layer, it is shown that the variable-oval-controlled damper can reduce the maximum damper force and isolation layer acceleration compared to the oil damper. Moreover, the variable-oval-controlled damper can reduce the relative displacement, story shear force, and acceleration at the super-structure compared to the oil damper.

Shaking table tests are also conducted to focuses on the possibility of creating the variable-oval-control rule by a MR damper. The results show that the variable-oval-control rule can be realized with a MR damper. The shaking test results agree well with the simulation analysis results.

1

Y. Sagami, N. Hori, N, Inoue, "A study on control method by magneto rheological fluid damper in base isolated structure", J. Struct. Constr. Eng., AIJ, 627, 709-716, 2008. doi:10.3130/aijs.73.709

2

Y. Sagami, N. Hori, K. Ikago, N. Inoue, "A study on the displacement control design with complex model of the dampers in base isolation structures", Journal of Structural Engineering, AIJ, 56B, 163-170, 2010.

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