• 대한조선학회논문집
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• 제46권1호
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• pp.31-42
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• 2009

Common structural rule (CSR) doesn' t provide any other specific regulations for permanent means of access (PMA) platform structure in a cargo oil tank. The PMA platform is recommended to comply with scantling requirement of local support member. However, it leads to too conservative scantlings compared with actual loads imposed on the platform. This paper proposes a strength assessment procedure for the PMA structure based on a nonlinear ultimate strength. The ultimate strength is evaluated in a sufficiently conservative way. The first linear buckling mode is used as an initial imperfection shape and its magnitude is determined using the definitions of DNV PULS. Since the same imperfection mode as the failure mode of the ultimate limit state is assumed, it can accelerate the failure. Au ultimate strength capacity curve obtained from a series of nonlinear FE analysis is compared with actual stresses calculated by CSR cargo hold analysis.

• 대한토목학회논문집
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• 제12권2호
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• pp.43-54
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• 1992

낮은 아치는 동하중 재하시 재료는 탄성범위 내에 있더라도 큰 변형이 발생할 수 있으며, 좌굴 가능성이 높기 때문에 선형해석으로는 정확한 거동을 구명하기 어렵다. 본 연구에서는 이에 따라 낮은 아치의 동적 비선형 해석방법 및 좌굴판단기준을 제시하였으며, 제시된 방법을 토대로 낮은 아치의 동적 비선형 해석을 수행하고 임계좌굴하중을 구하였다. 형상의 비선형성은 Lagrangian 운동좌표를 고려하여 해석하였으며 동적운동방정식을 풀기 위하여 유한요소법을 사용하였다. 이 때, 동적 운동방정식의 시간적분으로 Newmark 해법을 채택하였다. 프로그램은 만재 방사형 등분포하중을 받는 낮은 원호 아치를 해석하여 그 결과치를 다른 연구결과와 비교하여 검증하였다. 모형해석을 통해서는 큰 동하중을 받는 원호 아치는 기하학적 비선형 거동을 고려하여 해석되어야 하며, 아치가 낮아질수록 좌굴발생 가능성이 높아짐을 알았다. 여러가지 형상의 아치에 대한 좌굴해석을 실시하여 임계 좌굴하중을 구하였으며 기존의 연구와 비교하여 정확성을 확인하였다. 원호 아치의 거동을 본 연구에서 사용한 무차원 매개변수를 이용하여 해석한 결과로 부터, 동일한 형상매개변수를 가진 아치들은 실제하중을 하중매개변수로 환산하여 같은 하중매개변수를 재하했을 때 시간매개변수에 따라 처짐비를 기준으로 같은 거동을 함을 알았으며, 좌굴현상도 같은 하중매개변수에서 나타남을 확인하였다. 또한, 포물선형상의 아치와 연직하중이나 집중하중이 재하된 경우의 해석에도 개발된 프로그램이 유용하게 사용될 수 있음을 해석예를 통하여 밝혔다.

• 한국전산구조공학회논문집
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• 제29권5호
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• pp.437-445
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• 2016

현재 많은 산업에서 구조물의 온도환경 유지를 위한 단열재로 폴리우레탄 폼이 사용되며, 수명 동안 정적 및 동적의 다양한 하중이 이에 부과된다. 폴리우레탄 폼은 고분자재료로써 다공성이며, 단열성능은 내부기공의 크기에 크게 의존한다. 또한, 폴리우레탄 폼의 기계적 거동은 변형률 속도 및 온도에 대한 의존성이 큰 동시에 압축에 대하여 큰 비선형 연성거동을 보인다. 이러한 비선형 연성 압축거동 중에 폴리우레탄 폼은 변형률의 증가에 따라 기공율과 탄성계수의 감소를 보인다. 따라서 본 연구에서는 상기 특성들을 포함한 폴리우레탄 폼의 변형률 속도 및 온도 의존 비선형 압축거동을 모사하기 위하여 온도 의존 손상 점소성 구성방정식이 개발되었다.

• 한국추진공학회지
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• 제26권3호
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• pp.10-21
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• 2022

우주 발사체 구조인 추진제 탱크는 지상운송, 발사대기, 이륙 및 비행 과정 동안 다양한 정적 및 동적 하중이 작용하여 이에 대해 구조건전성을 보유해야 하며 더불어 추진제를 많이 싣기 위해서 크고 가벼워야 한다. 이런 특성으로 본 연구의 구조 대상인 추진제 탱크 실린더는 얇은 두께를 가지게 되어 실린더 설계에서 압축하중에 의한 좌굴이 중요하게 고려된다. 하지만 기존의 수립된 NASA 및 유럽 등의 좌굴 설계 기준은 상당히 보수적인 값으로 최신 설계 및 제작 기술을 반영하지 못하고 있다. 본 연구에서는 초기 결함이 반영된 다양한 해석 모델을 이용하여 비선형 좌굴 해석을 수행하고 실린더 구조의 새로운 좌굴 설계 기준 수립 방안을 제시한다. 결론적으로 공통격벽 추진제 탱크 실린더 구조의 효과적인 경량 설계가 구현될 수 있음을 확인하였다.

• Steel and Composite Structures
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• 제42권4호
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• pp.539-552
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• 2022

Concentrically braced frames (CBFs) possess high stiffness and strength against lateral loads; however, they suffer from low energy absorption capacity against seismic loads due to the susceptibility of CBF diagonal elements to bucking under compression loading. To address this problem, in this study, an innovative damper was proposed and investigated experimentally and numerically. The proposed damper comprises main plates and includes a flange plate angled at θ and a trapezius-shaped web plate surrounded by the plate at the top and bottom sections. To investigate the damper behaviour, dampers with θ = 0°, 30°, 45°, 60°, and 90° were evaluated with different flange plate thicknesses of 10, 15, 20, 25 and 30 mm. Dampers with θ = 0° and 90° create rectangular-shaped and I-shaped shear links, respectively. The results indicate that the damper with θ = 30° exhibits better performance in terms of ultimate strength, stiffness, overstrength, and distribution stress over the damper as compared to dampers with other angles. The hysteresis curves of the dampers confirm that the proposed damper acts as a ductile fuse. Furthermore, the web and flange plates contribute to the shear resistance, with the flange carrying approximately 80% and 10% of the shear force for dampers with θ = 30° and 90°, respectively. Moreover, dampers that have a larger flange-plate shear strength than the shear strength of the web exhibit behaviours in linear and nonlinear zones. In addition, the over-strength obtained for the damper was greater than 1.5 (proposed by AISC for shear links). Relevant relationships are determined to predict and design the damper and the elements outside it.

• Smart Structures and Systems
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• 제31권2호
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• pp.131-140
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• 2023

As well-known, the extended Kalman filter (EKF) is a powerful tool for parameter identification with limited measurements. However, traditional EKF is not applicable when the external excitation is unknown. By using least-squares estimation (LSE) for force identification, an EKF with unknown input (EKF-UI) approach was recently proposed by the authors. In this approach, to ensure the influence matrix be of full column rank, the sensors have to be deployed at all the degrees-of-freedom (DOFs) corresponding to the unknown excitation, saying collocated measurements are required. However, it is not easy to guarantee that the sensors can be installed at all these locations. To circumvent this limitation, based on the idea of first-order-holder discretization (FOHD), an improved EKF with unknown input (IEKF-UI) approach is proposed in this study for the simultaneous identification of structural parameters and unknown excitation. By using projection matrix, an improved observation equation is obtained. Few displacement measurements are fused into the observation equation to avoid the so-called low-frequency drift. To avoid the ill-conditioning problem for force identification without collocated measurements, the idea of FOHD is employed. The recursive solution of the structural states and unknown loads is then analytically derived. The effectiveness of the proposed approach is validated via several numerical examples. Results show that the proposed approach is capable of satisfactorily identifying the parameters of linear and nonlinear structures and the unknown excitation applied to them.

• International Journal of Naval Architecture and Ocean Engineering
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• 제12권1호
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• pp.241-257
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• 2020

The rapid proliferation of oil/gas drilling and wind turbine installations with jack-up rig-formed structures increases structural safety requirements, due to the greater risks of operational collisions during use of these structures. Therefore, current industrial practices and regulations have tended to increase the required accidental collision design loads (impact energies) for jack-up rigs. However, the existing simplified design approach tends to be limited to the design and prediction of local members due to the difficulty in applying the increased uniform impact energy to a brace member without regard for the member's position. It is therefore necessary to define accidental load estimation in terms of a reasonable collision scenario and its application to the structural response analysis. We found by a collision probabilistic approach that the kinetic energy ranged from a minimum of 9 MJ to a maximum 1049 MJ. Only 6% of these values are less than the 35 MJ recommendation of DNV-GL (2013). This study assumed and applied a representative design load of 196.2 MN for an impact load of 20,000 tons. Based on this design load, the detailed design of a leg structure was numerically verified via an FE analysis comprising three categories: linear analysis, buckling analysis and progressive collapse analysis. Based on the numerical results from this analysis, it was possible to predict the collapse mode and position of each member in relation to the collision load. This study provided a collision strength assessment between attendant vessels and a jack-up rig based on probabilistic collision scenarios and nonlinear structural analysis. The numerical results of this study also afforded reasonable evaluation criteria and specific evaluation procedures.

• Structural Engineering and Mechanics
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• 제54권5호
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• pp.855-875
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• 2015

Many novel materials exhibit a property of different elastic moduli in tension and compression. One such material is graphene, a wonder material, which has the highest strength yet measured. Investigations on buckling problems for structures with different moduli are scarce. To address this new problem, firstly, the nondimensional expression of the relation between offset of neutral axis and deflection curve is derived based on the phased integration method, and then using the energy method, load-deflection relation of the rod is determined; Secondly, based on the improved constitutive model for different moduli, large deformation finite element formulations are developed and combined with the arc-length method, finite element iterative program for rods with different moduli is established to obtain buckling critical loads; Thirdly, material mechanical properties tests of graphite, which is the raw material of graphene, are performed to measure the tensile and compressive elastic moduli, moreover, buckling tests are also conducted to investigate the buckling behavior of this kind of graphite rod. By comparing the calculation results of the energy method and finite element method with those of laboratory tests, the analytical model and finite element numerical model are demonstrated to be accurate and reliable. The results show that it may lead to unsafe results if the classic theory was still adopted to determine the buckling loads of those rods composed of a material having different moduli. The proposed models could provide a novel approach for further investigation of non-linear mechanical behavior for other structures with different moduli.

• 대한토목학회논문집
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• 제10권2호
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• pp.103-112
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• 1990

본 연구는 내진설계시 많은 불확실성을 내포하는 지반의 거동특성을 규명하고, 적용대상지반의 확충 및 경제성 제고를 위하여 토사지반의 동적지반특성 평가 및 지반-구조물 거동 특성을 고찰하였다. 예제해석은 토사지반에 원전 containment 구조물이 설치된 경우를 가상하여 지진하중에 대한 지반-구조물 시스템의 거동을 반무한체해석과 유한요소해석으로 분석하였다. 이는 토사지반에 원전이 건설될 경우에 고려해야 할 안정성 및 경제성 분석의 일환으로 수행되었으며, 토사지반의 큰 비선형거동을 정확하게 해석에 반영하기 위한 해석 software와 지반입력 data의 합리적인 평가방안 등을 예제해석을 통하여 분석하였다. 예제해석결과를 종합해 볼 때 토사지반의 동적거동의 정확한 분석을 위하여 비선형 유한요소해석은 Seed & Idriss 모델이, 선형 유한요소해석은 지진하중에 대한 1차원 지반거동시 변형율에서의 동적지반특성을 이용한 방법이, 반무한체해석은 정적하중시 변형율에서의 동적지반특성을 이용한 방법이 가장 합리적으로 동적지반특성을 평가하는 것으로 추천할 수 있다.

• Journal of Power Electronics
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• 제18권6호
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• pp.1830-1843
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• 2018

In this paper, the generation of a reference current and voltage signal based on a Kalman filter is offered for a 3-phase 4wire UPQC (Unified Power Quality Conditioner). The performance of the UPQC is improved with source voltages that are distorted due to harmonic components. Despite harmonic and frequency variations, the Kalman filter is capable enough to determine the amplitude and the phase angle of load currents and source voltages. The calculation of the first state is sufficient to identify the fundamental components of the current, voltage and angle. Therefore, the Kalman state estimator is fast and simple. A Kalman based control strategy is proposed and implemented for a UPQC in a distribution system. The performance of the proposed control strategy is assessed for all possible source conditions with varying nonlinear and linear loads. The functioning of the proposed control algorithm with a UPQC is scrutinized and validated through simulations employing MATLAB/Simulink software. Using a FPGA SPATRAN 3A DSP board, the proposed algorithm is developed and implemented. A small-scale laboratory prototype is built to verify the simulation results. The stated control scheme for the UPQC reduces the following issues, voltage sags, voltage swells, harmonic distortions (voltage and current), unbalanced supply voltage and unbalanced power factor under dynamic and steady-state operating conditions.