Sung Gook Cho

MODELING OF NONLINEAR CYCLIC LOAD BEHAVIOR OF I-SHAPED COMPOSITE STEEL-CONCRETE SHEAR WALLS OF NUCLEAR POWER PLANTS

In recent years steel-concrete composite shear walls have been widely used in enormous high-rise buildings. Due to high strength and ductility, enhanced stiffness, stable cycle characteristics and large energy absorption, such walls can be adopted in the auxiliary building; surrounding the reactor containment structure of nuclear power plants to resist lateral forces induced by heavy winds and severe earthquakes. This paper demonstrates a set of nonlinear numerical studies on I-shaped composite steel-concrete shear walls of the nuclear power plants subjected to reverse cyclic loading. A three-dimensional finite element model is developed using ABAQUS by emphasizing on constitutive material modeling and element type to represent the real physical behavior of complex shear wall structures. The analysis escalates with parametric variation in steel thickness sandwiching the stipulated amount of concrete panels. Modeling details of structural components, contact conditions between steel and concrete, associated boundary conditions and constitutive relationships for the cyclic loading are explained. Later, the load versus displacement curves, peak load and ultimate strength values, hysteretic characteristics and deflection profiles are verified with experimental data. The convergence of the numerical outcomes has been discussed to conclude the remarks.

Investigation of Damping Ratio of Steel Plate Concrete (SC) Shear Wall by Lateral Loading Test & Impact Test

Steel plate concrete (SC) composite structure is now being recognized as a promising technology applicable to nuclear power plants as it is faster and suitable for modular construction. It is required to identify its dynamic characteristics prior to perform the seismic design of the SC structure. Particularly, the damping ratio of the structure is one of the critical design factors to control the dynamic response of structure. This paper compares the criteria for the damping ratios of each type of structures which are prescribed in the regulatory guide for the nuclear power plant. In order to identify the damping ratio of SC shear wall, this study made SC wall specimens and conducted experiments by cyclic lateral load tests and vibration tests with impact hammer. During the lateral loading test, SC wall specimens exhibited large ductile capacities with increasing amplitude of loading due to the confinement effects by the steel plate and the damping ratios increased until failure. The experimental results show that the damping ratios increased from about 6% to about 20% by increasing the load from the safe shutdown earthquake level to the ultimate strength level.

Experimental Investigation of the Lateral Load Capacity and Strength Characteristics of a Steel Plate Concrete (SC) Shear Wall

Research on steel plate concrete (SC) structures for the modularization of nuclear power plants have been performed recently in Korea. In this study, the seismic capacity and stiffness characteristics of unstiffened SC shear walls under the effects of earthquakes were investigated through static pushover tests. Failure modes, sectional strength, and stiffness characteristics of SC structures under lateral loads were inspected by analyzing the experimental results. The strengths obtained by the experiments were also compared with those derived by the design code of the SC structures. One of the main failures of unstiffened SC shear walls was found to be the type of bending shear failure due to the debonding of the steel plate at the concrete interface. The ductility capacity of SC structures was also confirmed to be improved, which is considered to be a confining effect on steel plates in the longitudinal behavior of SC structures.

PROBABILISTIC SEISMIC ASSESSMENT OF BASE-ISOLATED NPPS SUBJECTED TO STRONG GROUND MOTIONS OF TOHOKU EARTHQUAKE

The probabilistic seismic performance of a standard Korean nuclear power plant (NPP) with an idealized isolation is investigated in the present work. A probabilistic seismic hazard analysis (PSHA) of the Wolsong site on the Korean peninsula is performed by considering peak ground acceleration (PGA) as an earthquake intensity measure. A procedure is reported on the categorization and selection of two sets of ground motions of the Tohoku earthquake, i.e. long-period and common as Set A and Set B respectively, for the nonlinear time history response analysis of the base-isolated NPP. Limit state values as multiples of the displacement responses of the NPP base isolation are considered for the fragility estimation. The seismic risk of the NPP is further assessed by incorporation of the rate of frequency exceedance and conditional failure probability curves. Furthermore, this framework attempts to show the unacceptable performance of the isolated NPP in terms of the probabilistic distribution and annual probability of limit states. The comparative results for long and common ground motions are discussed to contribute to the future safety of nuclear facilities against drastic events like Tohoku.

Seismic Response Prediction Method of Cabinet Structures in a Nuclear Power Plant Using Vibration Tests

This paper presents a seismic response prediction method using vibration tests of cabinet-type electrical equipment installed in a nuclear power plant. The proposed method consists of three steps: 1) identification of earthquake-equivalent forces based on lumped-mass system idealization, 2) identification of a state-space-equation model relating input-output measurements obtained from the vibration tests, 3) seismic prediction using the identified earthquake-equivalent forces and the identified state-space-equation. The proposed method is advantageous compared to other methods based on FEM (finite element method) model update, since the proposed method is not influenced by FEM modeling errors. Through a series of numerical verifications on a frame model and 3-dimensional shell model, it was found that the proposed method could be used to accurately predict the seismic responses, even under considerable measurement noise conditions. Experimental validation is needed for further study.

Vibration Control of Nuclear Power Plant Piping System Using Stockbridge Damper under Earthquakes

Generally the piping system of a nuclear power plant (NPP) has to be designed for normal loads such as dead weight, internal pressure, temperature, and accidental loads such as earthquake. In the proposed paper, effect of Stockbridge damper to mitigate the response of piping system of NPP subjected to earthquake is studied. Finite element analysis of piping system with and without Stockbridge damper using commercial software SAP2000 is performed. Vertical and horizontal components of earthquakes such as El Centro, California, and Northridge are used in the piping analysis. A sine sweep wave is also used to investigate the control effects on the piping system under wide frequency range. It is found that the proposed Stockbridge damper can reduce the seismic response of piping system subjected to earthquake loading.

Effects of the Excitation Level on the Dynamic Characteristics of Electrical Cabinets of Nuclear Power Plants

국문요약 >>원자력발전소에 설치되는 안전관련 캐비닛형 전기기기는 설치 전에 내진검증이 요구된다 . 전기기기의 동특성분석은 내진검증에 포함된 중요한 과정이며 , 기기의 정확한 해석모델을 작성하기 위해서도 필수적으로 요구되는 업무이다 . 이 연구에서는 입력진동수준에 따른 기기의 동특성 변화를 분석하기 위하여 원전 지진감시시스템 캐비닛을 대상으로 진동대시험을 수행하고 , 입력진동운동의 수준별로 계측된 진동응답신호를 진동수영역분해법으로 분석하였다 . 분석결과, 대상기기는 입력진동수준의 크기에 따라 동특성이 비선형적으로 변화하고, 국내 원전의 안전정지지진 수준 이하의 진동에서도 동특성이 비선형적 거동을 보이고 있음을 확인하였다 . 이러한 입력진동수준에 따라 전기기기의 동특성이 비선형적으로 변하는 원인은 대상기기의 특성과 입력진동수준을 고려할 때 일반적인 재 료 비선형보다는 각 부품들의 마찰력과 기하학적인 비선형성에 기인하는 것으로 판단된다 . 따라서 전기 캐비닛의 입력진동수준에 따른 동특성의 비선형적 변화는 향후 안전관련 기기의 내진검증 업무에서 중요하게 검토되어야 할 것으로 판단된다 .주요어 내진검증, 전기기기, 동특성분석, 동특성, 진동대시험, 비선형 거동ABSTRACT>>Seismic qualification (SQ) is required prior to the installation of safety related electrical cabinets in nuclear power plants (NPPs). Modal identification of the electrical equipment is one of the most significant steps to perform SQ, and is an essential process to construct a realistic analytical model. In this study, shaking table tests were conducted to identify a variation of the dynamic characteristics of a seismic monitoring system cabinet installed in NPPs according to the excitation level. Modal identification of the cabinet has been performed by a frequency domain decomposition method. The results of this study show that the dynamic properties of the cabinet are nonlinearly varied according to the excitation level and the specimen behaves significantly in a nonlinear manner under safe shutdown earthquake motion in Korea. The main sources of the nonlinear behavior of the specimen have been judged by friction forces and geometrical nonlinearity rather than material nonlinearity. The nonlinear variation of the dynamic characteristics of the electrical cabinet might be accepted as an important fact that should be considered during the SQ of safety related equipment.Key words Seismic qualification, Electrical cabinet, Modal identification, Dynamic characteristics, Shaking table test, Nonlinear behavior한국지진공학회 논문집 제14권 제3호 (통권 제73호) 2010년 6월, pp. 23~30DOI 10.5000/EESK.2010.14.3.023

Updating of Analytical Model to Consider Aging Effects of Lead-Rubber Bearings for the Seismic Design of Base-Isolated Nuclear Power Plants

Abstract The application of base isolation to nuclear systems has been limited to date for some important reasons, including the deprivation of sufficient data for the long-term operation of such isolation devices and the lack of specific standards. Moreover, it is difficult to provide seismic protection in the vertical direction and to qualify the large-scale isolators up to experimentally large deformations in real dynamic conditions. The effect of aging on isolators is therefore one of the issues to be considered for the safety and reliability of base-isolated nuclear power plants (NPPs). Accounting for the variations of the post-aging parameters of the isolators on the structural performance of the plant, this study proposes a simplified and efficient method to update the analytical model of base-isolated structures based on mean-iterative neural networks (MINNs). A bilinear model with a zero length element was built to represent the characteristics of lead-rubber bearings for their numerical analysis. Analytical model updating by MINNs has been successfully performed, and the observed results are found to be in good agreement with those obtained from experiments. Additionally, it is observed that the stiffening or hardening with time in the shear properties of isolation devices affects the seismic performance of the base-isolated structure. Seismic design over the service life span of NPP structures should take these aging effects of the isolators into account.

A PROCEDURE FOR GENERATING IN-CABINET RESPONSE SPECTRA BASED ON STATE-SPACE MODEL IDENTIFICATION BY IMPACT TESTING

The in-cabinet response spectrum is used to define the input motion in the seismic qualification of instruments and devices mounted inside an electrical cabinet. This paper presents a procedure for generating the in-cabinet response spectrum for electrical equipment based on in-situ testing by an impact hammer. The proposed procedure includes an algorithm to build the relationship between the impact forces and the measured acceleration responses of cabinet structures by estimating the state-space model. This model is used to predict seismic responses to the equivalent earthquake forces. Three types of structural model are analyzed for numerical verification of the proposed method. A comparison of predicted and simulated response spectra shows good convergence, demonstrating the potential of the proposed method to predict the response spectra for real cabinet structures using vibration tests. The presented procedure eliminates the uncertainty associated with constructing an analytical model of the electrical cabinet, which has complex mass distribution and stiffness.

Comparative Evaluation of Formulas of Strength Reduction Factors for the Generation of an Inelastic Demand Spectrum

국문요약 >>비탄성요구스펙트럼의 형상은 구조물의 내진성능평가 결과에 많은 영향을 미친다 . 비탄성요구스펙트럼은 강도감소계수를 적용하여 탄성응답스펙트럼을 비례축소시킴으로써 얻어질 수 있다 . 이 연구는 기존에 많은 연구자들이 제안한 강도감소계수의 공식을 조사하였다. 이 논문에는 과거에 제안된 공식에 따라 작성된 강도감소계수 곡선과 비탄성요구스펙트럼 곡선의 형상 및 특성을 비교하 고 있다. 조사된 공식으로 작성된 강도감소계수 곡선의 평균곡선을 구하고 , 회귀분석을 통하여 평균 곡선의 공식을 유도하였다 . 비교 연구를 통하여, 새롭게 제안된 강도감소계수 공식에 따라 작성된 비탄성요구스펙트럼의 형상은 기존에 제안된 공식으로 생성한 비탄성요구 스펙트럼의 평균곡선과 일치함을 확인하였다 .주요어 비탄성요구스펙트럼, 내진성능평가, 강도감소계수, 평균곡선ABSTRACT>>The shape of an inelastic demand spectrum may have a major impact on the seismic evaluation results of a structure. The inelastic demand spectrum could be obtained by scaling down from the elastic response spectrum by applying the strength reduction factor (SRF). This study has investigated formulas for SRFs that were suggested by numerous previous studies. This paper compares their characteristics, including the shapes of the curves of the SRFs and the inelastic demand spectra that were produced by applying the various formulas for SRFs. The mean curve was computed from the SRF curves generated by the various formulas. This study derives a new formula for the SRF curve through regression analysis. From the comparative study, it is shown that the proposed formula for the SRF can generate the mean curve of the inelastic demand spectra which have been previously suggested by others.Key words Inelastic Demand Spectrum, Seismic evaluation, Strength Reduction Factor, Mean Curve