SUMMARY Horizontal bidirectional loading tests are conducted for real‐sized high‐damping rubber (HDR) bearings with diameters of 700 mm (HDR700) and 1300 mm (HDR1300). The hysteresis loops of these bearings under bidirectional horizontal loadings are compared with those under unidirectional loadings. The results show that the bearing force measurement in the primary direction of loading increases when there is displacement in the orthogonal direction. Unusually, the maximum restoring force in the orthogonal direction to the primary loading direction occurs near zero displacement. On the basis of the observations of the restoring forces, a rate‐independent model is proposed. This model simulates well the test results under both bidirectional loading and unidirectional loading. It can reproduce the irregular restoring forces characteristics around zero displacement as described above. Bidirectional loading induced twist deformation in the HDR bearings that increased local shear strains. This phenomenon results in an early failure as observed in HDR700. The additional shear strain is estimated based on the twist deformation measured by video image analysis. The comparison of the nominal total shear stress demonstrates that the increase of shear stress because of bidirectional loading occurs when the average shear strain is larger than about 200%. The larger the shear strain, the greater the bidirectional effect. It is shown that the nominal total shear stress of average strain of 350% under bidirectional circular loading pattern is approximately the same as the average shear strain of 400% under unidirectional loading. This means that the average shear strain of 350% under a bidirectional circular loading corresponds to a local shear strain of 400%. Copyright © 2012 John Wiley & Sons, Ltd.

Horizontal biaxial loading tests were conducted on full-scale high-damping rubber bearings (HDR) to observe its performance. Two HDRs of diameter 700mm and 1300mm were used. Hysteresis loops by biaxial loadings became more swollen than those by uniaxial loadings. Hysteresis loops in the sub axis were so singular that the maximum restoring force was given around zero displacement. In strong nonlinear state, restoring force act in circumferential direction that cause torsional deformations of HDRs. This effect resulted in an earlier failure of HDR700. A technique to identify HDR's biaxial force deformation relationship was presented and demonstrated to have good performance.�

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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.

Ultimate properties of seismic isolation rubber bearings under horizontal biaxial loading are investigated with scaled-model testing. It was observed that the high-damping rubber bearings was twisted by torque generated by damping factor under biaxial deformation, and showed shear-break in smaller shear strain range than in uniaxial loading. Additionally, finite element analysis (FEM) was conducted to simulate the twisting deformation and local stress and strain. The results showed good agreement with test results, and indicated the possibility of FEM as an effective tool for prediction of failure strain of isolators in biaxial loading.