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
This paper describes the experimental study on heat-mechanics interaction behavior of laminated rubber bearings, such as lead rubber bearings (LRB) and high damping rubber bearings (HDB) under larger and more cyclic lateral deformation. For the rubber bearings installed to the base-isolated structures, the seismic energy they absorb is transformed into the thermal energy, so heat is generated causing high temperatures in the lead plug and high damping rubber. There are few experimental data so far, especially using full-scale specimen for heat-mechanics interaction behavior. Dynamic loading tests were conducted using full-scale and reduced-scale rubber bearing specimens under the sinusoidal and the earthquake response displacement inputs to confirm the effects of damping characteristics in line with the rising temperatures. The results of the tests show that the damping characteristics of the rubber bearings were deteriorated under the influence of the temperature rise and the similarity low is approved for the different size of rubber bearings.
Recently, some weaknesses of a lead rubber bearing (LRB) have been identified such as excessive tensile stress at some basemat-edge isolators and degradation of lead plug damping performance by temperature rising due to responses during the large and long duration earthquakes. The degradation of lead plug damping performance has become a design factor and a concern with response evaluation method to consider the thermal effect to lead plug has been growing. In this paper, to study the degradation of lead plug damping performance, firstly loading tests (a multi-cyclic loading tests to get thermal conditions of the lead plug, and a horizontal/vertical directional loading tests to get mechanical behaviors of the LRB under multiple degree of freedom loading) for LRB specimens were performed, then an analysis program to simulate the tests are newly developed that are validated by comparing to the test results. By these tests and analyses, some new findings are given on heat generation characteristics of the lead plug along with its thermal boundary conditions. By checking the LRB performance after the loading tests, another finding is given that mechanical characteristics are almost recovered when the lead plug temperature is enough cooled even that was once heated up to 300 degrees Celsius.
Cyclic loading experiments on Lead Rubber Bearing (LRB) with large diameter lead plug were performed. Four-size LRB specimens which had almost identical shape factors were used. Effects of temperature and scale on LRB properties were examined based on one-direction loading tests. In addition, behavior under two-direction loading was compared with one under one-direction loading. The relationship between temperature and yield stress of lead plug was derived from temperature measured by using thermocouples installed in the center of lead plug. Heat conduction analysis using the relationship can predict force and temperature of lead plug with high accuracy.
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