A shake table test is conducted for the three-story reinforced concrete building structure using 0.28 g, 0.5 g, 0.75 g, and 1.0 g of seismic input motions based on the Gyeongju earthquake. Computational efforts are made in parallel to explore the mechanical details in the structure. For engineering practice, the elastic modulus of concrete and rebar in the dynamic analysis is reduced to 38% and 50%, respectively, to calibrate the structure's natural frequencies. The engineering approach to the reduced modulus of elasticity is believed to be due to the inability to specify the flexibility of the actual boundary conditions. This aspect may lead to disadvantages of nonlinear dynamic analysis that can distort local stress and strain relationships. The initial elastic modulus can be applied directly without the so-called engineering adjustment with infinite element models with spring and spring-dashpot boundary conditions. This has the advantage of imposing the system flexibility of the structure on the sub-boundary conditions of springs and damping devices to control its sensitivity in a serial arrangement. This can reflect the flexibility of realistic boundary conditions and the effects of system damping (such as the gap between a concrete footing and shake table, loosening of steel anchors, etc.) in scalar quantities. However, these spring and dashpot coefficients can only be coordinated based on experimental results, making it challenging to select the coefficients in-prior to perform an experimental test.

The boundary reaction method(BRM) is a substructure time domain method, it removes global iterations between frequency and time domain analyses commonly required in the hybrid approaches, so that it operates as a two-step uncoupled method. 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. In the time domain analysis, the near-field soil is modeled to simulate the wave radiation problem. This paper evaluates the performance of the BRM according to modeling extent of near-field soil for the nonlinear SSI analysis of base-isolated NPP structure. For this purpose, parametric studies are performed using equivalent linear SSI problems. The accuracy of the BRM solution is evaluated by comparing the BRM solution with that of conventional SSI seismic technique. The numerical results show that the soil condition affects the modeling range of near-field soil for the BRM analysis as well as the size of the basemat. Finally, the BRM is applied for the nonlinear SSI analysis of a base-isolated NPP structure to demonstrate the accuracy and effectiveness of the method.

This study intends to evaluate the conservativeness of the fixed-base analysis as compared to the soil-structure interaction (SSI) analysis for the seismically isolated model of a nuclear power plant in Korea. To that goal, the boundary reaction method (BRM), combining frequency-domain and time-domain analyses in a twofold process, is adopted for the SSI analysis considering the nonlinearity of the seismic base isolation. The program KIESSI-3D is used for computing the reaction forces in the frequency domain and the program MIDAS/Civil is applied for the nonlinear time-domain analysis. The BRM numerical model is verified by comparing the results of the frequency-domain analysis and time-domain analysis for the soil-structure system with an equivalent linear base isolation model. Moreover, the displacement response of the base isolation and the horizontal response at the top of the structure obtained by the nonlinear SSI analysis using BRM are compared with those obtained by the fixed-base analysis. The comparison reveals that the fixed-base analysis provides conservative peak deformation for the base isolation but is not particularly conservative in term of the floor response spectrum of the superstructure.

이 연구는 기초의 묻힘이 면진 원전구조물의 응답에 미치는 효과를 지표기초와 비교하여 평가하였다. 면진장치의 비선형 성을 고려한 비선형 SSI 해석은 진동수영역해석과 시간영역해석의 복합법인 경계반력법(BRM)을 이용하여 수행하였다. BRM 해석모델은 BRM을 이용한 등가선형 SSI 해석결과를 재래의 주파수영역 SSI 해석결과와 비교함으로 검증하였다. 마 지막으로 비선형 SSI해석에 의한 묻힌기초 모델의 면진장치의 변위 및 구조물 응답을 지표기초의 해석결과와 비교하였다. 비교결과, 면진장치의 변위응답은 묻힌기초효과를 고려할 경우 감소할 수 있음을 알 수 있었다.

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.

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.

이 논문에서는 경계반력법을 이용한 비선형 지반-구조물 상호작용 해석을 위해 LS-DYNA나 MIDAS/Civil 등의 유한요소해석 프로그램과 연계하는 방법을 제시하였다. 경계반력법 적용시 유한요소프로그램에서 구조물과 지반은 선형 또는 비선형 유한요소를 이용하여 모델링하였다. 유한요소의 해석모델 외부의 무한영역으로 전달되는 탄성파를 최대한 흡수하기위해 유한요소 모델의 외측에 LS-DYNA의 경우에는 PML(Perfectly Matched Layer) 요소를, MIDAS/Civil의 경우에는 점성감쇠-스프링 요소를 적용하였다. 비선형 유한요소는 구조물영역에만 적용되는 것으로 가정하였다. 이 연구에서는 입사지진파에 의한 경계반력은 KIESSI-3D 프로그램을 이용하여 계산하였다. 선형 지반-구조물 상호작용 문제에 대해 일반적인 KIESSI-3D의 해석결과와 BRM해석결과를 비교하여 제시된 방법의 효율성을 제시하였다. 또한 수치적 비교를 통해 비선형 구조에 대해 보수적인 응답을 보이는 선형 SSI문제에 대하여 얻은 경계반력이 비선형 지반-구조물 상호작용해석에 효과적으로 적용 가능함을 알 수 있었다.

단단한 암반이 아닌 지반에 건설된 구조물의 지진응답을 정확하게 평가하기 위해서는 지반-구조물 상호작용(SSI)의 영향을 고려하여야 한다. 최근에는 USNRC SRP 3.7.2와 같은 원전설계분야의 규제지침 강화로 인해 SSI 해석의 필요성이 증가되고 있다. 이 연구에서는 대형 3차원 SSI 해석에 KIESSI-3D 프로그램을 사용한 사례를 제시하고 해석시간 및 정확성에 대한 토의를 하였다. 이를 위하여 대형 3차원 구조물 예제에 대한 SSI 해석을 수행하고 해석결과와 해석시간을 ACS/SASSI 프로그램과 비교하였다. 비교결과 KIESSI-3D 프로그램과 ACS/SASSI 프로그램에 의해 구한 구조응답의 전달함수가 거의 동일함을 보여 KIESSI-3D 프로그램의 정확성을 확인할 수 있었다. 아울러 해석시간 측면에서는 다중 쓰레드를 사용한 KIESSI-3D 프로그램(8개 쓰레드)이 단일 쓰레드를 사용한 ACS/SASSI 프로그램 보다 약 30배～2000배 빠른 성능을 보였다.

이 연구에서는 감절점쉘요소의 개념에 근거한 새로운 4절점 곡면 쉘요소를 제시하였다. 회전장이 독립변수로 도입된 범함수에 의하여 면내회전자유도를 도입함으로써 개발된 쉘요소에서는 절점당 6자유도를 갖도록 하였다. 아울러 쉘요소의 면내거동 개선을위하여 4개의 비적합변위형에 의한 비적합변위를 면내방향의 변위성분에 추가하였으며, 면외거동 개선을 위하여 대체전단변형률장이 적용되었다. 이 연구에서의 비적합변위형의 수치적 구현에 있어서 일정한 변형률상태를 표현할 수 있도록 하기 위하여 비적합변위형의 직접수정법이 적용되었다. 이렇게 정식화된 쉘요소 강성행렬의 수치적분에 있어서는 부피적분을 위하여 9점 적분법이 사용되었다. 개발된쉘요소는 바람직하지 못한 영에너지모드를 갖지 않으며, 일정한 변형률 상태를 표현할 수 있음을 확인하였다. 개발된 4절점 곡면 쉘 요소에 대한 다양한 수치예제를 통한 검증 결과, 전반적으로 양호한 거동을 보여주고 있음을 확인하였다.

This paper addresses simplified method for nonlinear soil–structure interactions (SSI) analysis of base-isolated nuclear power plant (NPP) structure under incident seismic waves. The accuracy and applicability of the method are evaluated by comparing with nonlinear time history responses obtained by the boundary reaction method (BRM).

This paper addresses simplified method for nonlinear soil–structure interactions (SSI) analysis of base-isolated nuclear power plant (NPP) structure under incident seismic waves. The accuracy and applicability of the method are evaluated by comparing with nonlinear time history responses obtained by the boundary reaction method (BRM).