Regulatory Guide (RG) 1.60 presents the response spectra for the seismic design, especially for the safe shutdown earthquake (SSE), of nuclear power plants. This guide is applicable to a two-step process involving the issuance of construction permits and operating licenses (10 CFR Part 50) as well as the issuance of combined construction and operating licenses (COLs), early site permits (ESPs), and standard plant design certifications (10 CFR Part 52) [1]. New reactor designs, however, require modified design response spectra (MDRS) by broadening the high-frequency range from design response spectra (DRS) in RG 1.60. In order to generate artificial time histories to meet the acceptable criteria described in NUREG-0800 [2], it9s necessary to develop the power spectral density of the MDRS. In this paper, we generate the artificial earthquake time histories of the MDRS for further research.

For important structures such as nuclear power plants, In-Structure Response Spectrum (ISRS) analysis is essential because it evaluates the safety of equipment and components installed in the structure. Because most structures are asymmetric, the response can be affected by eccentricity. In the case of seismically isolated structures, this effect can be greater due to the difference between the center of mass of the structure and the center of rigidity of the isolator layer. Therefore, eccentricity effects must be considered when designing or evaluating the ISRS of seismically isolated structures. This study investigated the change of the ISRS of an isolated structure by assuming accidental eccentricity. The variables that affect the ISRS of the isolated structure were analyzed to see what additional impact they had due to eccentricity. The ISRS of the seismically isolated structure with eccentricity was amplified more than when there was non-eccentricity, and it was boosted more significantly in specific period ranges depending on the isolator’s initial stiffness and seismic intensity. Finally, whether the displacement requirement of isolators can be applied to the variation of the ISRS due to eccentricity in the design code was also examined.

In order to evaluate the earthquake safety of equipment in structures, it is essential to analyze the In-Structure Response Spectrum (ISRS). The ISRS has a peak value at the frequency corresponding to the structural vibration mode, but the frequency and amplitude at the peak can vary because of many uncertain parameters. There are several seismic design criteria for ISRS peak-broadening for fixed base structures. However, there are no suggested criteria for constructing the design ISRS of seismically isolated structures. The ISRS of isolated structures may change due to the major uncertainty parameter of the isolator, which is the shear stiffness of the isolator and the several uncertainty parameters caused by the nonlinear behavior of isolators. This study evaluated the effects on the ISRS due to the initial stiffness of the bi-linear curve of isolators and the variation of effective stiffness by the input ground motion intensity and intense motion duration. Analyzing a simplified structural model for isolated base structure confirmed that the ISRS at the frequency of structural mode was amplified and shifted. It was found that the uncertainty of the initial stiffness of isolators significantly affects the shape of ISRS. The variation caused by the intensity and duration of input ground motions was also evaluated. These results suggested several considerations for generating ISRS for seismically isolated structures.

The design response spectrum presented in the seismic design standard reflects the characteristics of the tectonic environment at a site. However, since the design response spectrum does not represent the ground motion with a specific earthquake magnitude or distance, input ground motions for response history analysis need to be selected reasonably. It is appropriate to use observed ground motions recorded in Korea for the seismic design. However, recently recorded ground motions in the Gyeongju (2016) or Pohang (2017) earthquakes are not compatible with the design response spectrum. Therefore, it is necessary to convert the recorded ground motion in Korea to a model similar to the design response spectrum. In this study, several approaches to adjust the spectral acceleration level at each period range were tested. These are the intrinsic and scattering attenuation considering the earthquake environment, magnitude, distance change by the green function method, and a rupture propagation direction's directivity effect. Using these variables, the amplification ratio for the representative natural period was regressed. Finally, the optimum condition compatible with the design response spectrum was suggested, and the validation was performed by converting the recorded ground motion.

Seismic qualification of equipment including piping is performed by using floor response spectra (FRS) or in-structure response spectra (ISRS) as the earthquake input at the base of the equipment. The amplitude of the FRS may be noticeably reduced when obtained from coupling analysis because of interaction between the primary structure and the equipment. This paper introduces a method using a modal synthesis approach to generate the FRS in a coupled primary-secondary system that can avoid numerical instabilities or inaccuracies. The FRS were generated by considering the dynamic interaction that can occur at the interface between the supporting structure and the equipment. This study performed a numerical example analysis using a typical nuclear structure to investigate the coupling effect when generating the FRS. The study results show that the coupling analysis dominantly reduces the FRS and yields rational results. The modal synthesis approach is very practical to implement because it requires information on only a small number of dynamic characteristics of the primary and the secondary systems such as frequencies, modal participation factors, and mode shape ordinates at the locations where the FRS needs to be generated.

On November 15, 2017, Pohang earthquake occurred. Its local magnitude was announced to be ML=5.4 by Korea Meteorological Administration (KMA). Ground motion data recorded at KMA stations were obtained from their data bases. From the data, horizontal and vertical response spectra, and V/H ratio were calculated. The horizontal spectra were defined as GMRotI50 spectra. From the statistical analysis of the GMRotI50 spectra, a mean plus one standard deviation spectrum in lognormal distribution is obtained. Regression analysis is performed on this curve to determine the shape of spectrum including transition periods. Applying the same procedure, the shape and transition periods of vertical spectrum were obtained. These results were compared with the Korean standard design spectra, which were developed from domestic and overseas intra-plate earthquake records, and Gyeongju earthquake response spectra. The response spectra of Pohang earthquake were found to be almost identical with the newly proposed design spectra. Even the V/H ratios showed good agreement. These results confirmed that the method adopted when developing the standard design spectra were valid and the developed design spectra were reliable.

On September 12, 2016, Gyeongju earthquake occurred. Its local magnitude was announced to be ML=5.8 by Korea Meteorological Administration (KMA). Ground motion data recorded at KMA, EMC and KERC stations was obtained from their data bases. From the data, horizontal and vertical response spectra, and V/H ratio were calculated. The horizontal spectrum was defined as geometric mean spectrum, GMRotI50. From the statistical analysis of the geometric mean spectra, a mean plus one standard deviation spectrum in lognormal distribution is obtained. Regression analysis is performed on this curve to determine the shape of spectrum including transition periods. Applying the same procedure, the shape and transition periods of vertical spectrum was obtained. These results were compared with the Korean standard design spectra, which were developed from domestic and overseas intraplate earthquake records. The response spectra of Gyeongju earthquake were found to be almost identical with the newly proposed design spectra. Even the V/H ratios showed good agreement. These results confirmed that the method adopted when developing the standard design spectra were valid and the developed design spectra were reliable.

In the companion papers (I, II), site-specific response analyses were performed at more than 300 domestic sites and a new site classification system and design response spectra (DRS) were proposed using the results of the site-specific response analyses. In this paper, the proposed site classification system and the design response spectra are compared with those in other seismic codes and verified by different methods. Firstly, the design response spectra are compared with the design response spectra in Eurocode 8, KBC 2016 and MOCT 1997 to estimate quantitative differences and general trends. Secondly, site-specific response analyses are carried out using VS-profiles obtained using field seismic tests and the results are compared with the proposed DRS in order to reduce the uncertainty in using the SPT-N value in site-specific response analyses in the companion paper (I). In addition, site coefficients from real earthquake records measured in Korean peninsula are used to compare with the proposed site coefficients. Finally, dynamic centrifuge tests are also performed to simulate the representative Korean site conditions, such as shallow depth to bedrock and short-period amplification characteristics. The overall results showed that the proposed site classification system and design response spectra reasonably represented the site amplification characteristic of shallow bedrock condition in Korea.

In the companion paper (I – Database and Site Response Analyses), site-specific response analyses were performed at more than 300 domestic sites. In this study, a new site classification system and design response spectra are proposed using results of the site-specific response analyses. Depth to bedrock (H) and average shear wave velocity of soil above the bedrock (VS,Soil) were adopted as parameters to classify the sites into sub-categories because these two factors mostly affect site amplification, especially for shallow bedrock region. The 20 m of depth to bedrock was selected as the initial parameter for site classification based on the trend of site coefficients obtained from the site-specific response analyses. The sites having less than 20 m of depth to bedrock (H1 sites) are sub-divided into two site classes using 260 m/s of VS,Soil while the sites having greater than 20 m of depth to bedrock (H2 sites) are sub-divided into two site classes at VS,Soil equal to 180 m/s. The integration interval of 0.4 ~ 1.5 sec period range was adopted to calculate the long-period site coefficients (Fv) for reflecting the amplification characteristics of Korean geological condition. In addition, the frequency distribution of depth to bedrock reported for Korean sites was also considered in calculating the site coefficients for H2 sites to incorporate sites having greater than 30 m of depth to bedrock. The relationships between the site coefficients and rock shaking intensity were proposed and then subsequently compared with the site coefficients of similar site classes suggested in other codes.

Korea is part of a region of low to moderate seismicity located inside the Eurasian plate with bedrock located at depths less than 30 m. However, the spectral acceleration obtained from site response analyses based on the geologic conditions of inland areas of the Korean peninsula are significantly different from the current Korean seismic code. Therefore, suitable site classification scheme and design response spectra based on local site conditions in the Korean peninsula are required to produce reliable estimates of earthquake ground motion. In this study, site-specific response analyses were performed at more than 300 sites with at least 100 sites at each site categories of SC, SD, and SE as defined in the current seismic code in Korea. The process of creating a huge database of input parameters - such as shear wave velocity profiles, normalized shear modulus reduction curves, damping curves, and input earthquake motions - for site response analyses were described. The response spectra and site coefficients obtained from site response analyses were compared with those proposed for the site categories in the current code. Problems with the current seismic design code were subsequently discussed, and the development and verifications of new site classification system and corresponding design response spectra are detailed in companion papers (II-development of new site categories and design response spectra and III-Verifications)

Response spectra of a building are made with a SDOF system taking into account a first mode shape, even though higher modes may affect on the dynamic responses of a high-rise building. A soft soil layer under a building also affects on the responses of a building. In this study, seismic responses of a MDOF system were investigated to examine the effects of higher modes on the response of a tall building by comparing them with those of a SDOF system including the kinematic interaction effect. Study was performed using a pseudo 3D finite element program with seven bedrock earthquake records downloaded from the PEER database. Effects of higher modes on the seismic responses of a tall building were investigated for base shear force and base moment of a MDOF system including story shear forces and story moments. Study results show that higher modes of a MDOF system contribute to a reduction of base shear force up to 1/4-1/5 of KBC and base moment. The effect of higher modes is more significant on the base shear force than on the base moment. Maximum story shear force and moment occurred at the top part of a building rather than at a base in the cases of tall buildings differently from short buildings, and higher modes of a tall building affected on the base forces making them almost constant at the base. A soft soil layer also affects some on the base shear force of a high-rise building independently on the soft soil type, but a soft soil effect is prominent on the base moment.

실제 구조요소나 구조시스템의 비선형응답은 단순화된 형태의 분할선형 이력모델 보다는 완만한 곡선이력모델로 나타내는 것이 보다 정확하다. 본 논문에서는 완만한 곡선이력거동을 적용한 일정연성도 비탄성 응답스펙트럼을 작성하는 방법을 제시하였다. 가속도, 변위 및 입력에너지에 대한 비탄성 응답스펙트럼에 대한 곡선형이력거동의 완만한 정도의 영향을 평가하였다. 해석결과로부터 곡선형이력거동의 완만도가 증가할수록 비탄성 응답스펙트럼은 감소하는 경향을 나타냄을 알 수 있었다.

Structures with Coulomb-friction damping system have strong nonlinearity that the dynamic behavior is highly affected by the relative magnitude between frictional force and excitation load. In this study, normalized response spectra of the structures with non-dimensional friction force are obtained through nonlinear time history analyses of the mass-normalized single degree of freedom systems using ground motion data designed by ATC-40 and Korean Building Code 2005(KBC 2005) on each slte. The variation of the control performance of Coulomb-frictional damping system is investigated in terms of the dynamic load and the structural natural period, of which effects were not considered in the previous studies. Finally, Design spectra of the structure with Coulomb-frictional damper considering the safety of structural design is proposed though linear curve fitting

Site response analyses were performed based on equivalent linear technique using shear wave velocity profiles of 162 sites collected around the Korean peninsula. The site characteristics, particularly the shear wave velocities and the depth to the bedrock, are compared to those in the western United States. The results show that the site-response coefficients based on the mean shear velocity of the top 30m (V_{S30}) suggested in the current code underestimates the motion in short-period ranges and overestimates the motion in mid-period ranges. The current Korean code based on UBC is required to be modified considering site characteristics in Korea for the reliable estimation of site amplification. From the results of numerical estimations, new regression curves were derived between site coefficients (F_{a}\;and\;F_{v}) and the fundamental site periods, and site coefficients were grouped based on site periods with reasonable standard deviations compared to site classification based on V_{S30}. Finally, new site classification system and modification of design response spectra are recommended considering geotechnical characteristics in Korea.

건축구조물의 비선형 지진응답해석에서 입력지진동은 구조물의 탄소성 지진응답을 좌우하는 중요한 요소이다. 지진동파형은 지진발생과 전파경로에 따른 여러 가지 인자에 의해 그 특성이 결정되기 때문에 구조물의 지진응답해석에서 일반성을 갖는 입력지진동을 선정하는 것은 매우 어려운 문제이다. 본 논문은 내진설계용 스펙트럼에 대응하는 인공지진동파형을 작성한 후, 작성된 인공지진동에 대한 탄소성 응답스펙트럼 특성을 분석한 것이다. 여기서 작성된 인공지진동파형은 과거의 지진에서 얻어진 기록지진동파형을 이용하여 기록지진동과 동일한 위상각을 가지며, 감쇠정수 h=5%일 때의 내진설계용 스펙트럼과 거의 일치하도록 작성되었다. 작성된 인공지진동은 원 기록지진동과 동일한 위상각을 가지며, 주기 T=0.02{\sim}10.0sec 범위에서 설계용 스펙트럼과 매우 근접하게 작성되었다. 인공지진동을 입력한 1자유도계의 탄성 및 탄소성 지진응답해석을 수행하여 탄소성 응답스펙트럼 및 탄소성 응답특성을 분석하였다. 본 논문에서 작성된 인공지진동은 건축구조물의 탄소성 지진응답해석용 입력지진동으로 충분히 타당성이 있다고 사료된다.

동반논문(II-지반분류 개선방법)에서는 지반의 고유주기를 기준으로 지반을 분류하는 방법이 국내 지반조건에 적합한 지반분류 방법으로 제시하였다. 그러나, 지반분류 방법을 개선하여도 해석결과의 평균 스펙트럼 가속도 값과 재산정된 응답스펙트럼과 차이가 나타나는 부분이 존재한다. 이는 설계응답스펙트럼을 작성에 필요한 증폭계수를 계산하는 방법을 변경해야 해결할 수 있다. 본 논문에서는 국내 지반조건에 적합하도록 증폭계수를 재산정하기 위한 적분구간 변경에 대하여 검토하였다. 검토 결과 장주기 영역의 증폭계수 F_v의 적분구간은 현재 주기 0.4초{\sim}2.0초에서 주기 0.4초{\sim}1.5초로 변경할 경우 해석결과의 평균 스펙트럼 가속도 값과 설계응답스펙트럼이 비교적 잘 일치하는 결과를 얻을 수 있었고, 기존의 방법보다 국내 지반특성에 더 적합한 설계응답스펙트럼을 작성할 수 있었다.

동반논문 (I)에서는 국내 지반특성에 적합하도록 국내 내진설계기준이 개선되어야 한다는 결론을 얻었다. 본 논문에서는 우수한 지반분류 방법을 찾기 위하여 상부 토층 30m의 평균 전단파속도(V_{S30}), 지반의 고유주기(T_G) 및 기반암 깊이를 이용한 지반분류 방법에 대하여 심도있게 검토하였다. 증폭계수(F_a,\;F_v)의 표준편차, 해석결과의 평균 스펙트럼 가속도와 재산정된 응답스펙트럼을 비교한 결과 각각의 방법에서 큰 차이가 발생하지 않아 특정한 방법이 우수하다고 판단하기 힘들었다. 그러나, T_G를 이용한 방법에서 RRS 값의 증폭구간이 좁은 구간에 집중되는 경향을 보여 지진시 유사한 거동특성을 나타내는 지반을 같은 지반그룹으로 분류할 수 있는 장점이 있었다. 또한, 증폭계수와 T_G의 상관관계를 나타내는 추세선의 경우, V_{S30} 방법 보다 입력 가속도의 증가에 따른 지반의 비선형성 효과를 더욱 명확하게 나타낼 수 있었다. 마지막으로, V_{S30}을 이용하여 지반을 분류할 경우 기반암이 30m 보다 얕은 곳에 존재하는 경우에도 무조건 심도 30m까지 기반암의 전단파속도를 가정하여 계산해야 하나, T_G를 이용할 경우 이러한 불확실성을 제거할 수 있어 우수한 방법으로 판단된다. 본 논문에서는 지반의 고유주기를 이용한 방법을 기반암 깊이가 얕은 국내지반특성에 적합한 지반분류 방법으로 제안하였다.

본 논문에서는 국내 162개 지반에 대한 전단파속도 주상도, 기반암 깊이 및 지반의 동적변형특성을 획득하여 등가선형해석을 수행한 후 미국 서부해안지역의 지반 특성과 비교 검토하였다. 검토 결과 국내의 일반적인 특성을 가지는 지반과 미국 서부해안지역의 지반은 기반암 깊이와 고유주기가 매우 다름을 확인하였다. 지진응답 해석 결과 단주기 증폭계수 F_a의 경우 1997 UBC 기준의 값보다 크게 산정되었고, 장주기 증폭계수 F_v는 작게 나타나 국내 지반특성에 적합한 증폭계수는 현재 국내 내진설계기준 값과는 매우 다른 경향을 보였다. 따라서, 증폭계수를 재산정하고 설계응답스펙트럼을 개선해야 할 필요성을 확인하였다. 본 논문에서는 현재 이용되고 있는 내진설계기준과 국내 지반특성과의 차이점 파악에 중점을 두었고, 개선방법에 대한 내용은 동반논문(II 지반분류 개선방법, III 설계응답스펙트럼 개선방법)에서 심도있게 논의하였다.

원자력발전소의 기기, 배관 시스템과 같은 부속구조물의 동적 응답을 먼기 위해 사용되는 층응답스펙트럼은 일반적으로 주구조물과 부속구조물의 동적 상호작용이 반영되지 않고 만들어진다. 본 연구에서는 기기와 구조물의 동적 상호작용이 고려된 해석을 통해 층응답스펙트럼을 생성시키는 해석법을 기술하였다. 이 방법은 기기를 모사하는 단자유도계와 기기가 놓여있는 구조물의 임피던스로 분할되는 부분구조 해석법을 적용하여 기기의 응답을 구한다. 단자유도계의 진동수, 감쇠비 및 질량 특성을 변화시키면서 최대 동적 응답을 계산함으로써 일련의 층응답스펙트럼을 작성한다. 전형적인 원자력발전소의 원자로 구조물에서 본 방법을 고려한 층응답스펙트럼과 기기를 포함한 전체 해석으로부터 작성된 층응답스펙트럼과 비교함으로써 본 연구의 타당성을 확인하였다. 기기-구조물 상호작용 효과를 확인하기 위하여 구조물 질량의 1% 이내인 기기에 대하여 기술된 방법과 기존 방법을 각각 적용하여 최대 응답값을 비교하였다. 그 결과 지배 진동수 부근에서 기기-구조물 상호작용을 고려한 응답이 그렇지 않은 경우인 기존방법의 응답에 비하여 저감되는 현상을 보였다.

비탄성 지진해석은 구조물-지반 체계의 비선형 거동 때문에 내진설계를 위해 필요하고, 합리적인 내진설계를 위해서 지반-구조물 상호작용을 고려한 성능에 기준한 설계의 중요성도 인식되고 있다. 이 연구에서는 11개 중약진과 5개 강진 기록을 최대 가속도 0.075g, 0.15g, 0.2g와 0.3g로 조정하여 연약지반에 세워진 단자유도계에 대한 탄성과 비탄성 지진응답해석을 지반의 비선형성을 고려하여 수행하였다. 의사3차원 동적해석 프로그램을 사용하여 주파수 영역에서 지진하중을 암반에 작용시켜 구조물-지반 체계에 대한 지진응답해석을 한번에 수행하였다. 연구결과에 의하면 비선형 지반-구조물 상호작용 영향을 고려하는 것과 설계기준에 따라 내진설계를 하는 것보다는 여러 가지 지반조건을 고려하여 성능에 기준한 내진설계를 수행하는 것이 필요하다. 또한 약진에 의한 연약지반의 비선형성이 비선형 지반에 의한 지진파의 증폭 때문에 탄성과 비탄성 지진응답에 심하게 영향을 미쳤는데 특히 탄성지진응답에서 두드러졌다.?