• Earthquakes and Structures
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• v.3 no.3_4
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• pp.249-270
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• 2012

This paper investigates the effectiveness of widely used identification methods to identify the response of seismically isolated structures supported on bearings with bilinear behavior. The paper shows that while both time domain and frequency domain methods predict with high accuracy the modal characteristics of structures isolated by linear isolation system, their performance degrades appreciably when the isolation system exhibits bilinear behavior even when its strength assumes moderate values (say 5% of the weight). The paper also shows that the natural period of isolated structure that results from bilinear isolation systems can be satisfactorily predicted with wavelet analysis.

• Journal of the Earthquake Engineering Society of Korea
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• v.16 no.6
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• pp.29-36
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• 2012

Involved in a research for the application of seismic isolation to the nuclear industry, this study evaluates firstly the responses of seismic isolation system considering general ranges of structural period and damping ratio by using preliminary design formula. Secondly, coupling effects of input motions were evaluated to find out appropriate conditions of excitations and effect of the iteration for calculating yield displacement of lead core was also assessed in terms of response of a seismically isolated structure. Finally, the results of preliminary design calculation were compared with those of dynamic analysis and the propriety of the formula was evaluated and appropriate ranges of reduction factor were also suggested from the results.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2001.04a
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• pp.387-393
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• 2001

Installing vibration control devices in the structure rises as a solution instead of increasing structural strength considering construction cost. Especially, viscoelastic dampers show excellent vibration control performance at low cost and are easy to install in existing structures compared with other control devices. Therefore, cost-effectiveness of structure with viscoelastic dampers needs to be evaluated. Previous cost-effectiveness evaluation method for the seismically isolated structure(Koh et al., 1999;2000)is applied on the building structure with viscoelastic dampers, which combines optimal design and cost-effectiveness evaluation for seismically isolated structures based on minimum life-cycle cost concept. Input ground motion is modeled in the form of spectral density function to take into account acceleration and site coefficients. Damping of the viscoelastic damper is considered by modal strain energy method. Stiffness of shear building and shear area of viscoelastic damper are adopted as design variables for optimization. For the estimation of failure probability, transfer function of the structure with viscoelastic damper for spectral analysis is derived from the equation of motion. Results reveal that cost-effectiveness of the structure with viscoelastic dampers is relatively high in how seismic region and stiff soil condition.

• Journal of the Earthquake Engineering Society of Korea
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• v.21 no.6
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• pp.265-275
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• 2017

In order to improve the seismic performance of structures, friction pendulum system (FPS) is the most commonly used seismic isolation device in addition to lead rubber bearing (LRB) in high seismicity area. In a nuclear power plant (NPP) with a large self weight, it is necessary to install a large number of seismic isolation devices, and the position of the center of rigidity varies depending on the arrangement of the seismic isolation devices. Due to the increase in the eccentricity, which is the difference between the center of gravity of the nuclear structure and the center of stiffness of the seismic isolators, an excessive seismic response may occur which could not be considered at the design stage. Three different types of eccentricity models (CASE 1, CASE 2, and CASE 3) were used for seismic response evaluation of seismically isolated NPP due to the increase of eccentricity (0%, 5%, 10%, 15%). The analytical model of the seismic isolation system was compared using the equivalent linear model and the bilinear model. From the results of the seismic response of the seismically isolated NPP with increasing eccentricity, it can be observed that the effect of eccentricity on the seismic response for the equivalent linear model is larger than that for the bilinear model.

• Journal of the Earthquake Engineering Society of Korea
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• v.19 no.1
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• pp.1-11
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• 2015

In order to perform a soil-isolation-structure interaction analysis of seismically isolated nuclear power plant (NPP) structures, the nonlinear behavior of a seismic isolation system may be converted to an equivalent linear model used in frequency domain analysis. Seismic responses for seismically isolated NPP containment structures subjected to a simple artificial acceleration history and different site class earthquakes are evaluated for the equivalent-linear and nonlinear models that have been applied to lead-rubber bearing (LRB) modeling. It can be observed that the maximum displacements of the equivalent linear model are larger than that of the nonlinear model. From the floor response spectrum analysis for the top of NPP containment structures, it can be observed that the spectral acceleration of an equivalent linear model at about 0.5 Hz frequency is about 2~3 times larger than that of a nonlinear model.

• Journal of the Earthquake Engineering Society of Korea
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• v.19 no.1
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• pp.37-43
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• 2015

This paper presents a detailed procedure for a nonlinear soil-structure interaction of a seismically isolated NPP(Nuclear Power Plant) structure using the boundary reaction method (BRM). The BRM offers a two-step method as follows: (1) the calculation of boundary reaction forces in the frequency domain on an interface of linear and nonlinear regions, (2) solving the wave radiation problem subjected to the boundary reaction forces in the time domain. For the purpose of calculating the boundary reaction forces at the base of the isolator, the KIESSI-3D program is employed in this study to solve soil-foundation interaction problem subjected to vertically incident seismic waves. Wave radiation analysis is also employed, in which the nonlinear structure and the linear soil region are modeled by finite elements and energy absorbing elements on the outer model boundary using a general purpose nonlinear FE program. In this study, the MIDAS/Civil program is employed for modeling the wave radiation problem. In order to absorb the outgoing elastic waves to the unbounded soil region, spring and viscous-damper elements are used at the outer FE boundary. The BRM technique utilizing KIESSI-3D and MIDAS/Civil programs is verified using a linear soil-structure analysis problem. Finally the method is applied to nonlinear seismic analysis of a base-isolated NPP structure. The results show that BRM can effectively be applied to nonlinear soil-structure interaction problems.

• Earthquakes and Structures
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• v.1 no.3
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• pp.231-252
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• 2010

Previous research on pounding between seismically isolated buildings during earthquakes has been focused on impacts at the bases of structures and the effect of simultaneous interactions at the bases and at the superstructures has not been studied in details. In this paper, the seismic responses of adjacent buildings supported on different or similar base systems considering impacts between bases and superstructures are numerically investigated. The study is carried out in three parts for the two types of adjacent buildings: (i) both structures have fixed bases; (ii) one structure has fixed base and the other is seismically isolated and (iii) both structures have base isolation systems. The results of the study indicate that the pounding-involved responses of the buildings depend mainly on the type of structural base systems and on the structural parameters of both buildings. For the base-isolated building, the variation of the peak accelerations and displacements of the storeys have been found to be relatively low. On the other hand, significant differences have been observed for the fixed base building. The results of the parametric study conducted for different values of the gap size between colliding structures show the reduction in the peak base displacements as the gap distance decreases.

• Journal of the Earthquake Engineering Society of Korea
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• v.18 no.6
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• pp.261-269
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• 2014

In order to increase the seismic safety of nuclear power plant (NPP) structures in high seismicity regions, seismic isolation techniques can be adapted to NPP structures. In this paper, the applicability of multi-step analysis of seismically isolated NPP containment structures with lead-rubber bearings (LRB) is evaluated. The floor response spectrum of NPP containment structures with equivalent linear LRB and nonlinear LRB are compared. In addition, the force-displacement relationships for equivalent linear LRB and nonlinear LRB are compared.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2001.10a
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• pp.553-560
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• 2001

To substantiate the application of LRB(Laminated Rubber Bearing) to the seismic isolation system, it is necessary to develop a seismic analysis method considering the non-linear behavior of LRBs, which may significantly affect the seismic responses. In this paper, seismic analyses and shaking table tests are carried out for a seismically isolated structure using four LRBs. The parameter equations of seismic isolation frequency are obtained from the shaking table tests and the quasi-static tests of LRB itself to investigate the effects of the LRB characteristics in the prediction of maximum peak acceleration responses by analysis. From the comparison of the maximum peak acceleration responses obtained from numerical analyses and experiments, it is verified that the horizontal stiffness variations of LRB should be carefully considered in seismic analysis to obtain more accurate results.

• Journal of the Earthquake Engineering Society of Korea
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• v.20 no.5
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• pp.319-329
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• 2016

The Eradi Quake System (EQS) is a seismic isolation bearing system designed to minimize forces and displacements experienced by structures subjected to ground motion. The EQS dissipates seismic energy through friction of Poly Tetra Fluoro Ethylene (PTFE) disk pad. In general, a force-displacement relationship of EQS has post yield stiffness hardening during large inelastic displacement. In this study, seismic responses of seismically isolated nuclear power plant (NPP) subjected to design basis earthquake (DBE) and beyond design basis earthquakes (150% DBE and 167% DBE) are compared considering the post yield stiffness hardening effect of EQS. From the results, it can be observed that if the post-yield stiffness hardening effect of EQS is increased, the displacement response of EQS is reduced, and the acceleration and shear responses of containment structures of NPP is increased.