Square seismic isolation bearings are economical to manufacture, offer the advantage of simple connection configurations and have compact geometry requiring a minimum of space for installation. To be able to more effectively utilize square bearings for seismic isolation systems, a new mechanical model for predicting the large shear deformation behavior of square elastomeric isolation bearings is presented in this paper. The new model is developed by extending to three dimensions an existing model for elastomeric isolation bearings under severe axial loads and shear deformations. The model comprises multiple shear springs at the mid-height and a series of axial springs at the top and bottom boundaries. Static loading tests of square lead-rubber isolation bearings were performed to investigate the influence of horizontal loading direction and axial load magnitude on bearing behavior. The test results showed that the ultimate behavior is strongly influenced by loading direction, especially under large shear deformation and high axial load. To confirm the validity of the model, analyses are performed of the loading tests of the square lead-rubber isolation bearings. The results of analyses using the new model show very good agreement with the experimental results.

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.

The paper describes a new analytical model for predicting the large displacement behavior of square lead-rubber isolation bearings. The present model is developed by extending to three dimensions an existing model for elastomeric isolation bearings under severe axial loads and shear deformations. Static loading tests of square lead-rubber bearings are performed to investigate the effect of loading direction on bearing behavior. The test results showed that the ultimate behavior is strongly influenced by loading direction, especially for large shear deformation and high axial load. The results of analyses using the new model show very good agreement with the experimental results. Keywords : Seismic isolation, Lead-rubber bearing, Large deformation, Axial force, Non-linear hysteresis model

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