Identifying significant earthquake intensity measures for evaluating seismic damage and fragility of nuclear power plant structures

Nguyen, Duy-Duan,Thusa, Bidhek,Han, Tong-Seok,Lee, Tae-Hyung Korean Nuclear Society 2020 Nuclear Engineering and Technology Vol.52 No.1

Seismic design practices and seismic response analyses of civil structures and nuclear power plants (NPPs) have conventionally used the peak ground acceleration (PGA) or spectral acceleration (S_a) as an intensity measure (IM) of an earthquake. However, there are many other earthquake IMs that were proposed by various researchers. The aim of this study is to investigate the correlation between seismic responses of NPP components and 23 earthquake IMs and identify the best IMs for correlating with damage of NPP structures. Particularly, low- and high-frequency ground motion records are separately accounted in correlation analyses. An advanced power reactor NPP in Korea, APR1400, is selected for numerical analyses where containment and auxiliary buildings are modeled using SAP2000. Floor displacements and accelerations are monitored for the non- and base-isolated NPP structures while shear deformations of the base isolator are additionally monitored for the base-isolated NPP. A series of Pearson's correlation coefficients are calculated to recognize the correlation between each of the 23 earthquake IMs and responses of NPP structures. The numerical results demonstrate that there is a significant difference in the correlation between earthquake IMs and seismic responses of non-isolated NPP structures considering low- and high-frequency ground motion groups. Meanwhile, a trivial discrepancy of the correlation is observed in the case of the base-isolated NPP subjected to the two groups of ground motions. Moreover, a selection of PGA or S_a for seismic response analyses of NPP structures in the high-frequency seismic regions may not be the best option. Additionally, a set of fragility curves are thereafter developed for the base-isolated NPP based on the shear deformation of lead rubber bearing (LRB) with respect to the strongly correlated IMs. The results reveal that the probability of damage to the structure is higher for low-frequency earthquakes compared with that of high-frequency ground motions.

근거리지진의 영향을 고려한 면진 원전구조물의 지진취약도

Nguyen Duy-Duan,Thusa Bidhek,이태형 한국방재학회 2018 한국방재학회논문집 Vol.18 No.7

근거리지진은 방향효과(forward-directivity effect)와 플링스텝(fling-step)이라고 부르는 영구지반변형을 유발하는 특징이 있다. 본 논문에서는 근거리지진의 영향을 고려하여 면진된 원자력발전소 구조물의 지진취약도를 평가하였다. 다양한 지진기록을 수집하여 방향효과를 포함한 근거리지진, 플링스텝을 포함한 근거리지진, 원거리지진의 세 그룹으로 분류하였다. 한국형 신형 가압경수로인 APR1400을 수치해석을 위한 대상으로 선정하였다. 지진취약도 곡선은 최우도추정법을 이용하여 도출하였으며, 여기에는 면진장치인 납-고무받침(LRB)의 변형능력을 한계상태로 고려하였다. 해석결과, 원거리지진을 고려했을 때 LRB 최대변형의 중간값이 근거리지진을 고려한 경우보다 작았다. 또한, 동일한 지진강도에 대해서 근거리지진이 원거리지진보다 원전구조물을 손상시킬 확률이 높게 계산되었다. This study investigates the effect of near-fault ground motions on the seismic fragility of base-isolated nuclear power plant (NPP) structures. Because near-fault motion has either directivity or a so-called fling-step characteristic, or both, three groups of ground motions, namely, near-fault forward-directivity, near-fault fling-step, and far-fault ground motions, are selected for fragility analyses. A base-isolated advanced power reactor 1400 (APR1400), a recently developed NPP in Korea, is employed for numerical analysis. A set of fragility curves are derived for various limit states based on the maximum likelihood estimation. The limit states are defined in terms of deformation capacity of lead rubber bearing (LRB). The numerical results reveal that the median maximum deformations of LRBs were smaller for far-fault ground motions than for near-fault ground motions. In addition, the comparison of fragility curves demonstrates that the probability of failure of base-isolated NPP structures is higher for near-fault ground motions than for far-fault motions given the same earthquake intensity.

Optimal earthquake intensity measures for probabilistic seismic demand models of ARP1400 reactor containment building

Duy-Duan Nguyen,Bidhek Thusa,Md Samdani Azad,Viet-Linh Tran,이태형 한국원자력학회 2021 Nuclear Engineering and Technology Vol.53 No.12

This study identifies efficient earthquake intensity measures (IMs) for seismic performances and fragilityevaluations of the reactor containment building (RCB) in the advanced power reactor 1400 (APR1400)nuclear power plant (NPP). The computational model of RCB is constructed using the beam-truss model(BTM) for nonlinear analyses. A total of 90 ground motion records and 20 different IMs are employed fornumerical analyses. A series of nonlinear time-history analyses are performed to monitor maximum floordisplacements and accelerations of RCB. Then, probabilistic seismic demand models of RCB are developedfor each IM. Statistical parameters including coefficient of determination (R2), dispersion (i.e. standarddeviation), practicality, and proficiency are calculated to recognize strongly correlated IMs with theseismic performance of the NPP structure. The numerical results show that the optimal IMs are spectralacceleration, spectral velocity, spectral displacement at the fundamental period, acceleration spectrumintensity, effective peak acceleration, peak ground acceleration, A95, and sustained maximum acceleration. Moreover, weakly related IMs to the seismic performance of RCB are peak ground displacement,root-mean-square of displacement, specific energy density, root-mean-square of velocity, peak groundvelocity, Housner intensity, velocity spectrum intensity, and sustained maximum velocity. Finally, a set offragility curves of RCB are developed for optimal IMs.

Efficiency of various structural modeling schemes on evaluating seismic performance and fragility of APR1400 containment building

Nguyen, Duy-Duan,Thusa, Bidhek,Park, Hyosang,Azad, Md Samdani,Lee, Tae-Hyung Korean Nuclear Society 2021 Nuclear Engineering and Technology Vol.53 No.8

The purpose of this study is to investigate the efficiency of various structural modeling schemes for evaluating seismic performances and fragility of the reactor containment building (RCB) structure in the advanced power reactor 1400 (APR1400) nuclear power plant (NPP). Four structural modeling schemes, i.e. lumped-mass stick model (LMSM), solid-based finite element model (Solid FEM), multi-layer shell model (MLSM), and beam-truss model (BTM), are developed to simulate the seismic behaviors of the containment structure. A full three-dimensional finite element model (full 3D FEM) is additionally constructed to verify the previous numerical models. A set of input ground motions with response spectra matching to the US NRC 1.60 design spectrum is generated to perform linear and nonlinear time-history analyses. Floor response spectra (FRS) and floor displacements are obtained at the different elevations of the structure since they are critical outputs for evaluating the seismic vulnerability of RCB and secondary components. The results show that the difference in seismic responses between linear and nonlinear analyses gets larger as an earthquake intensity increases. It is observed that the linear analysis underestimates floor displacements while it overestimates floor accelerations. Moreover, a systematic assessment of the capability and efficiency of each structural model is presented thoroughly. MLSM can be an alternative approach to a full 3D FEM, which is complicated in modeling and extremely time-consuming in dynamic analyses. Specifically, BTM is recommended as the optimal model for evaluating the nonlinear seismic performance of NPP structures. Thereafter, linear and nonlinear BTM are employed in a series of time-history analyses to develop fragility curves of RCB for different damage states. It is shown that the linear analysis underestimates the probability of damage of RCB at a given earthquake intensity when compared to the nonlinear analysis. The nonlinear analysis approach is highly suggested for assessing the vulnerability of NPP structures.

일체형교대교량의 지진취약도 평가에 적합한 지진강도지표

응웬두이두안(Nguyen, Duy-Duan),삼다니아자드(Azad, Md Samdani),최병호(Choi Byung H.),이태형(Lee, Tae-Hyung) 한국방재학회 2020 한국방재학회논문집 Vol.20 No.6

본 논문의 목적은 일체형교대교량의 지진취약도를 평가하는데 적합한 지진강도지표(IM)를 판별하는 것이다. 90개의 지진가속도기록과 20가지 IM을 고려한 비선형시간이력해석을 수행하여 교각의 횡방향변위를 관찰하였다. 교량의 내진성능과 상관도가높은 IM을 판별하기 위해 결정계수, 표준편차, 상관계수 등 다양한 통계인자를 고려하였다. 수치해석결과 상관도가 높은IM은 고유주기에서의 스펙트럼가속도, 스펙트럼속도, 스펙트럼변위와 ASI, EPA, PGA, A95 등으로 나타났다. 상관도가 높은IM를 기준으로 한 교량의 지진취약도곡선을 도출하였다. The purpose of this study is to identify efficient earthquake intensity measures (IMs) for evaluating the seismic vulnerability of integral abutment bridges. A total of 90 ground motion records and 20 typical IMs were employed for the numerical analyses. A series of nonlinear time-history analyses was performed on the bridges to observe the lateral displacement of the bridge piers. Statistical parameters such as the coefficient of determination, standard deviation, and correlation coefficient were calculated to identify the strongly correlated IMs with the seismic performance of the bridges. The numerical results show that the efficient IMs are spectral acceleration, spectral velocity, spectral displacement at the fundamental period, acceleration spectrum intensity, effective peak acceleration, peak ground acceleration, and A95. Moreover, a set of fragility curves of the bridges was developed with respect to the efficient IMs.

박스형 터널의 지진 취약도 평가

박두희,Duy-Duan Nguyen,이태형,Van-Quang Nguyen 한국지반공학회 2018 한국지반공학회논문집 Vol.34 No.11

In this study, the seismic response of cut-and-cover box tunnels is evaluated from pseudo-static analyses and the fragility curves are derived. A series of site profiles were used to evaluate the effect of soil conditions. A total of 20 ground motions were used. The fragility curves were developed as functions of peak ground acceleration for three damage states, which are minor, moderate, and extensive states. The damage indices, defined as the ratio of the elastic moment to the yield moment, correlated to three damage states, were used. The curves are shown to greatly depend on the site profile. The curves are further compared to those derived in previous studies. The widely used empirically derived curves are shown not to account for the site effects, and therefore underestimate the response for soft sites.

Prediction of Axial Compression Capacity of Cold-Formed Steel Oval Hollow Section Columns Using ANN and ANFIS Models

Trong-Ha Nguyen,Ngoc-Long Tran,Duy-Duan Nguyen 한국강구조학회 2022 International Journal of Steel Structures Vol.22 No.1

The steel oval hollow section (OHS) provides an aesthetic architecture and a greater local buckling strength. However, the existing design codes do not specify the eff ective width in calculating the load-bearing capacity of OHS members. This study aims to predict the axial compression capacity (ACC) of cold-formed steel OHS columns using artifi cial neural network (ANN) and adaptive neural fuzzy inference system (ANFIS) models. A total of 128 data sets collected from the literature were utilized to develop the ANN and ANFIS models. The performance of the two machine learning models was compared with three existing design codes. The results demonstrated that the developed ANN and ANFIS models predicted the ACC of steel OHS columns more accurately compared to the existing formulas. Specifi cally, the ANN model revealed a superior performance with the highest coeffi cient of determination and the smallest root means square errors. Moreover, the formulas based on ANN and ANFIS models, which accommodates all input parameters, were proposed to predict the ACC of coldformed steel OHS columns. The thickness of the cross-section was the most infl uential parameter on the ACC of the OHS column. By contrast, the column length negatively aff ected the ACC value of the steel column. Finally, a graphical user interface tool was developed to readily calculate the ACC of the steel OHS columns.

Practical ANN Model for Estimating Buckling Load Capacity of Corroded Web-Tapered Steel I-Section Columns

Trong-Ha Nguyen,Van-Tien Phan,Duy-Duan Nguyen 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.6

This study develops an artificial neural network (ANN) to estimate the critical buckling load (CBL) of corroded web-tapered steel I-section (WTSI) columns in pre-engineered steel buildings. A total of 387 datasets are employed to develop the ANN model. The datasets are generated from the proposed analytical model and Newton–Raphson method. The input parameters of the developed ANN model contain the cross-sectional dimensions of the steel column (i.e., the top and bottom flange width, top and bottom flange thickness, maximum section height, minimum section height, and web thickness), elastic modulus of material, and the column height. Meanwhile, the CBL is the output parameter of the ANN model. A predictive process for the CBL of the corroded WTSI columns has been proposed based on the ANN model and previous corrosion model. Results reveal that the ANN model showed an excellent performance in predicting the CBL of the corroded steel columns. The R2 values of the training, testing, and validation data are 0.99975, 0.99916, and 0.99951, respectively. The root-mean-squared errors of the training, testing, and validation data are 96.705 (kN), 103.402 (kN), and 103.200 (kN), respectively. Additionally, the a20-index is very close to 1.0. Moreover, a graphical user interface tool is constructed to facilitate the CBL calculation of the corroded WTSI columns.

Prediction of Critical Buckling Load of Web Tapered I-Section Steel Columns Using Artificial Neural Networks

Trong-Ha Nguyen,Ngoc-Long Tran,Duy-Duan Nguyen 한국강구조학회 2021 International Journal of Steel Structures Vol.21 No.4

The web tapered I-section steel (WTIS) columns have been widely used in civil and industrial steel structures. However, the existing theoretical and empirical equations demonstrate a signifi cant discrepancy in estimating the critical axial load of the WTIS columns. This study aims to develop eff ective artifi cial neural networks (ANNs) for predicting the critical buckling load of the WTIS columns. A database of 269 fi nite element models of WTIS columns was generated, after verifying with experimental results, to develop the ANN model. The results of the proposed ANN model were also compared with those of existing formulas, highlighting that the ANN model in this study predicts the critical buckling load of the WTIS columns more accurately than the existing formulas. Moreover, the infl uences of input parameters on the critical buckling load of the WTIS columns were thoroughly investigated. An ANN-based formula, which considers input variables, was thereafter proposed to estimate the critical buckling load of the WTIS columns. Additionally, a graphical user interface tool has been developed for simplifying the design practice of the WTIS columns.

Seismic performance evaluation of reactor containment building considering effects of concrete material models and prestressing forces

Thusa Bidhek,Nguyen Duy-Duan,Azad Md Samdani,이태형 한국원자력학회 2023 Nuclear Engineering and Technology Vol.55 No.5

The reactor containment building (RCB) in nuclear power plants (NPPs) plays an important role in protecting the reactor systems from external loads as well as preventing radioactive leaking. As we witnessed the nuclear disaster at Fukushima Daiichi (Japan) in 2011, the earthquake is one of the major threats to NPPs. The purpose of this study is to evaluate effects of concrete material models and presstressing forces on the seismic performance evaluation of RCB in NPPs. A typical RCB designed in Korea is employed for a case study. Detailed three-dimensional nonlinear finite element models of RCB are developed in ANSYS. A series of pushover analyses are then performed to obtain the pushover curves of RCB. Different capacity curves are compared to recognize the influence of different material models on the nonlinear behavior of RCB. Additionally, the effects of prestressing forces on the seismic performances of the structure are also investigated. Moreover, a set of damage states corresponding to damage evolutions of the structures is proposed in this study.