In this paper, a new vibration control device realizing negative stiffness in a passive manner is proposed in order to reduce the absolute response of structures under strong seismic motions. The developed device consists of a sliding plate with a PTFE portion, and they are vertically pressurized by coil springs. The shape of the sliding plate is convex, by which the control force is negatively proportional to the deformation. The prototype of the proposed device was assembled, and its performance was investigated through both sinusoidal and hybrid loading tests. It was confirmed that the proposed device reduced the maximum acceleration of the structure significantly without any significant increase in absolute displacement.

Reduction of the horizontal deformation capacity of elastomeric isolation bearings under bi-directional loading has been one of the major concerns since that such a phenomenon was observed for high-damping rubber bearings. Using a newly constructed loading machine, a series of tests of lead rubber bearings (LRBs) were conducted to investigate their detailed mechanical characteristics under horizontal bi-directional deformation in the paper. The influences of the configurations, such as shape of cross section, diameter of a lead plug and aspect ratio, on the bi-directional behavior were carefully examined. Simulation analyses were also conducted to predict the bi-directional behavior of LRBs.

A series of bidirectional loading tests were conducted on a high friction type sliding rubber bearing. Tests were conducted under horizontal bidirectional loading and constant or fluctuant vertical loading. Regardless of the vertical loading methods, the maximum shear strain under bidirectional loadings increased approximately 40%-50% compared to nominal shear strain. Reflecting mechanical characteristics of the bearing, the analytical model of elastic sliding bearings was proposed. This model accurately represented force-displacement relationships under horizontal bidirectional loading and constant or fluctuant vertical loading.

In surveys on the damage of buildings caused by the 2011 Tohoku-Pacific Ocean earthquake, it was reported that damages such as cracks were detected in a number of lead dampers used in seismically isolated buildings. After investigations of the damaged dampers in detail, it was confirmed that these cracks were generated by accumulation of smallamplitude loading like middle-small earthquakes or winds rather than by large-amplitude loading like large earthquakes or strong winds. From this background, fatigue characteristics of a lead rubber bearing (LRB) under repeated small-amplitude were evaluated by cyclic loading tests of scaled models. The shear-strain amplitudes of the testing were 1%, 10%, 20%, and the maximum number of the cycles for each level of amplitude was 4000000, 59000, and 17000, respectively.

In the first part of this study, theoretical and numerical evaluation of negative stiffness appearing in the skyhook control is conducted. The skyhook control is widely known for the vibration control method in the mechanical engineering field. The skyhook control can also achieve absolute response reduction. In order to realize a negative stiffness, however, the control force that accelerates the deformation should be generated. At present, such a performance is achieved only by using loading actuators or semi-active devices with sophisticated controllers and sensors. In the second part of this research, a new damper realizing a negative stiffness and stable energy dissipation in a passive manner is proposed, and its dynamic performance is investigated through large-scale shaking table tests. It is confirmed that the innovative negative stiffness passive damper reduces both the absolute acceleration and the relative displacement of a bridge model.

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