The purpose of this thesis is to design a proper LRB (laminated rubber bearing) which can meet the required criteria of seismic responses for an isolated test model structure under the given conditions of an input motion and a superstructure. The required criteria are to limit as low as possible seismic responses: peak acceleration, floor response spectra and relative displacement of the isolated structure, because the accelerations and relative displacement are mutually contrary in a seismic base isolation design.
The five different sets of the LRB such as NRB (natural rubber LRB), NLRB (high damping rubber LRB), LLRB1-3 (lead plug LRB with 3 different lead plug diameters) were designed and fabricated to have the stiffness and the damping ratio to be lower than upper bound (524-2368Kgf/cm, 1.9229.3%). The mechanical properties of the LRB obtained by the shear-compression test were characterized to consider the effects of variation of shear strain, loading rate and axial load.
Among linear, bilinear and modified bilinear models of the isolation system, a linear model of isolation system was selected. Because the linear model representing multi-linear stiffness and constant viscous damping coefficient is closer to the mechanical properties of the LRB except the stiffness difference in lower shear strain range for LLRB3 and HLRB. The simplified beam structure model combining the linear model of various LRB isolation system was used to predict the responses of the isolated test model structure. The structure is assumed to be subjected to the artificial time history of PGA (peak ground acceleration) of 0.412g.
This approach was verified by a series of shaking table test. The predicted responses for the isolated test model structure with, each 1 /8 scale LRB were in a good agreement in isolation frequencies, shear displacements and floor response spectra. However, there are some discrepancies in lower peak acceleration of floor response spectra and in lower shear displacement in LLRB2 and LLRB3. Considering the measured responses as well as the predicted seismic responses of the KALIMER reactor structure obtained by the linear model of isolation system, the required criteria for the isolated test model structure are assumed as 1) ZPA (Zero Period Acceleration) to be less than 80% of PGA, 2) peak acceleration of floor response spectrum in frequency range of 4-25Hz to be less than 2 times of PGA, and 3) LRB shear displacement to be less than 70% of LRB total rubber height. The measured responses of the test model structure with NLRB and LLRB1 meet the required criteria, where HLRB has low stiffness with large damping ratio (563Kgf/cm, 12.2% and isolation frequency of 1.61 Hz), while LLRB1 has large stiffness with low damping ratio (670Kgf/cm, 6.3% and isolation frequency of 1.75Hz). The verified linear model was used to determine an isolation system to further reduce floor response spectrum but not to exceed the other criteria. With the isolation system, an application of LLRB design which met the required criteria was made using the experimental relation of lead diameter and mechanical properties obtained by the shear-compression tests of the LLRB with 3 different lead plug diameters.
