• Wind and Structures
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• 제14권2호
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• pp.85-112
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• 2011

Wind turbine structures are long slender columns with a rotor and blade assembly placed on the top. These slender structures vibrate due to dynamic environmental forces and its own dynamics. Analysis of the dynamic behavior of wind turbines is fundamental to the stability, performance, operation and safety of these systems. In this paper a simplied approach is outlined for free vibration analysis of these long, slender structures taking the soil-structure interaction into account. The analytical method is based on an Euler-Bernoulli beam-column with elastic end supports. The elastic end-supports are considered to model the flexible nature of the interaction of these systems with soil. A closed-form approximate expression has been derived for the first natural frequency of the system. This new expression is a function of geometric and elastic properties of wind turbine tower and properties of the foundation including soil. The proposed simple expression has been independently validated using an exact numerical method, laboratory based experimental measurement and field measurement of a real wind turbine structure. The results obtained in the paper shows that the proposed expression can be used for a quick assessment of the fundamental frequency of a wind turbine taking the soil-structure interaction into account.

• 대한조선학회논문집
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• 제30권1호
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• pp.125-133
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• 1993

중간구속조건을 갖는 보 및 보-기둥의 진동에 관한 기존연구들의 대부분은 Euler 보이론틀에서 다루었다. 세장비가 작은 경우 또는 세장비가 큰 경우일지라도 고차진동에 대해서는 Timoshenko 이론들에 의한 해석이 요구되나, Timoshenko 보 및 보-기둥에 관한 연구사례는 적은 편이다. 본 연구에서는 Timoshenko 보-기둥에 대해서 양단경계조건을 병진스프링 회전스프링 구속으로, 중간구속조건을 임의 갯수의 집중질량 병진스프링 회전스프링으로 일반화하여 정식화한 다음 엄밀해법을 제시하고, 아울러 엄밀해법의 연산부담이 매우 큰 점을 고려하여 적정한 Rayleigh-Ritz 해석에 대해서도 검토하였다. Rayleigh-Ritz 해석에 있어서는 우선 기준계의 고유함수를 이용하는 방법이 고려될 수 있으나 이 경우에도 Euler 보이론과는 달라서 연산부담이 역시 큰 편이다. 따라서 기준계의 고유함수와 같은 성질을 갖는 다항식을 도출하고 이를 이용하는 Rayleigh-Ritz 해석의 유용성에 관해 검토했다. 한편, 본 연구의 대상계와 같은 복합계에 대해서는 최적설계관점에서 설계변수변경에 따른 재해석문제 또한 중요한 과제임을 고려하여, 특성다항식 이용 Rayleigh-Ritz 방법에 기초하여 계산되는 동특성 1차 감도의 유용성도 검토되었다. 수치 계산예를 통해 전기 특성 다항식을 이용한 Rayleigh-Ritz 해석이 정확도면에서 엄밀해와 부합성이 양호하고, 계산 효율은 매우 높음이 확인되었다.

• 한국건설관리학회논문집
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• 제5권5호
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• pp.143-150
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• 2004

본 연구는 다기준의사결정문제인 건설 프로젝트의 공법선정문제에 있어서 퍼지계층분석법 및 그 결과 생성되는 각 공법별 퍼지 점수의 상대평가를 위해 퍼지 선호도 함수를 적용하여 상대평가를 하고자 하였다. 지금까지의 건설 공법 선정에 대한 퍼지계층분석법에 대한 연구는 대부분 최종적인 공법선정을 위해서 각 공법별로 도출된 퍼지 점수의 순위결정기법으로 절대평가를 수행하였다. 그러나 이러한 기법은 비교 대상공법에 대한 일관적인 순위결과를 산출해 내지 못하므로 결과적으로 공법선정과정의 전반적인 타당성을 저해하는 문제점을 지니고 있었다. 따라서 본 연구에서는 건설 분야의 MCDM(Multiple Criteria Decision Making)에 관한 기존의 연구들을 바탕으로 순위 결정기법에 R.P.R(Relative Preference Ratio)기법을 도입하여 각 공법에 대한 퍼지 점수를 상대 평가함으로써 체계적이고 효율적인 건설 공법선정기법을 제시하였고, 이를 창호공법의 선정문제에 적용하여 그 타당성을 살펴보았다.

• Advances in nano research
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• 제5권4호
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• pp.281-301
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• 2017

In this disquisition, an exact solution method is developed for analyzing the vibration characteristics of magneto-electro-elastic functionally graded (MEE-FG) beams by considering porosity distribution and various boundary conditions via a four-variable shear deformation refined beam theory for the first time. Magneto-electroelastic properties of porous FG beam are supposed to vary through the thickness direction and are modeled via modified power-law rule which is formulated using the concept of even and uneven porosity distributions. Porosities possibly occurring inside functionally graded materials (FGMs) during fabrication because of technical problem that lead to creation micro-voids in FG materials. So, it is necessary to consider the effect of porosities on the vibration behavior of MEE-FG beam in the present study. The governing differential equations and related boundary conditions of porous MEE-FG beam subjected to physical field are derived by Hamilton's principle based on a four-variable tangential-exponential refined theory which avoids the use of shear correction factor. An analytical solution procedure is used to achieve the natural frequencies of porous-FG beam supposed to magneto-electrical field which satisfies various boundary conditions. A parametric study is led to carry out the effects of material graduation exponent, porosity parameter, external magnetic potential, external electric voltage, slenderness ratio and various boundary conditions on dimensionless frequencies of porous MEE-FG beam. It is concluded that these parameters play noticeable roles on the vibration behavior of MEE-FG beam with porosities. Presented numerical results can be applied as benchmarks for future design of MEE-FG structures with porosity phases.

• Geomechanics and Engineering
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• 제28권2호
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• pp.197-207
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• 2022

The seismic holding behaviors of plate anchor embedded into submerged coarse-grained soils were investigated considering different anchor inclinations. The limit equilibrium method and the Pseudo-Dynamic Approach (PDA) were employed to calculate the inertia force of the soils within the failure rupture. In addition, assuming the permeability of coarse-grained soils was sufficiently large, the coefficient of hydrodynamic force applied on the inclined plate anchor is obtained through adopting the exact potential flow theory. Therefore, the seismic holding resistance was calculated as the combination of the inertia force and the hydrodynamic force within the failure rupture. The failure rupture can be developed due to the uplift loads, which was assumed to be an arc of a circle perpendicular to the anchor and inclines at (π/4 - φ/2). Then, the derived analytical solutions were evaluated by comparing the static breakout factor N_γ to the published experimental and analytical results. The influences of soil and wave properties on the plate anchor holding behavior are reported. Finally, the dynamic anchor holding coefficients N_γd, were reported to illustrate the anchor holding behaviors. Results show that the soil accelerations in x and z directions were both nonlinear. The amplifications of soil accelerations were more severe at lower normalized frequencies (ωH/V) compared to higher normalized frequencies. The coefficient of hydrodynamic force, C, of the plate anchor was found to be almost constant with anchor inclinations. Finally, the seismic anchor holding coefficient oscillated with the oscillation of the inertia force on the plate anchor.

• 한국강구조학회 논문집
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• 제14권1호
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• pp.23-29
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• 2002

거더와 가로보를 포함하는 많은 교량시스템과 콘크리트 상판은 특별직교이방성 판처럼 거동한다. 그런데 Navier나 Levy의 해로 해결할 수 없는 경계조건을 가졌거나, 불규칙 단면을 가진 경에에 해석해를 얻기 것이 매우 어렵다. 더구나 이러한 경우, 고유치 문제에 대한 수치해법은 너무 복잡하다. 그러나 불규칙 단면을 갖는 보와 탑에 대해, 첫 번째 진동모드에 해당하는 고유진동수를 계산하는 방법이 1974년 '김'에 의해 개발되어 발표되었다. 최근에 이 방법은 2차원 문제로 확장되었고 전단변형이 고려된 복합판에 적용되어 왔다. 본 연구에서는 이 방법을 다양한 경계조건을 갖는 특별직교이방성 복합적층판에 적용하여 타당성을 검토하고 그 해석결과를 제시한다.

• 재활간호학회지
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• 제18권1호
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• pp.38-45
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• 2015

Purpose: This study was attempted in order to verify the effects of moxibustion on aged back pain and activities of daily living. Methods: A research design is quasi-experiment with pre-post test design in nonequivalent control group. As for data collection and experimental treatment, it accidentally sampled totally 50 people with 24 people for the experimental group and 26 people for the control group who appeal for chronic back pain aged over 60 who visited 4 senior centers where are located in N city from November 7, 2012 to December 3, 2012. Experimental treatment was carried out moxibustion totally 12 times by 3 times for 4 weeks in the experimental group. Data analysis was made by using SPSS program. As an analytical technique, the experimental group and the control group were analyzed the homogeneity verification with t-test, Fisher's exact test and $x^2$ test and the hypothesis verification with t-test. Results: The back pain level was reduced in the experimental group than the control group. The disability of daily living activities was reduced in the experimental group than in the control group. Conclusion: Moxibustion was considered to be likely useful therapeutic method for effects of chronic back pain and disability of daily living activities in aged.

• Nuclear Engineering and Technology
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• 제28권5호
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• pp.431-439
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• 1996

In the process of simplifying the complex fate of the plant into a binary state, the information loss is inevitable. To minimize the information loss, the quantification of plant safety status has been formulated through the combination of the probability density function arising from the sensor measurement and the membership function representing the expectation of the state of the system. Therefore, in this context, the safety index is introduced in an attempt to quantify the plant status from the perspective of safety. The combination of probability density function and membership function is achieved through the integration of the fuzzy intersection of the two functions, and it often is not a simple task to integrate the fuzzy intersection due to the complexity that is the result of the fuzzy intersection. Therefore, a methodology based on the Algebra of Logic is used to express the fuzzy intersection and the fuzzy union of the arbitrary functions analytically. These exact analytical expressions are then numerically integrated by the application of Monte Carlo method. The benchmark tests for rectangular area and both fuzzy intersection and union of two normal distribution functions have been performed. Lastly, the safety index was determined for the Core Reactivity Control of Yonggwang 3&4 using the presented methodology.

• Earthquakes and Structures
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• 제19권3호
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• pp.215-226
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• 2020

A self-centering wall (SCW) is a lateral resistant rocking system that incorporates posttensioned (PT) tendons to provide a self-centering capacity along with dampers to dissipate energy. This paper investigates the rocking responses of a SCW with base viscous dampers under a sinusoidal-type pulse considering yielding and fracture behaviour of the PT tendon. The differences in the overturning acceleration caused by different initial forces in the PT tendon are computed by the theoretical method. The exact analytical solution to the linear approximate equation of motion is also provided for slender SCWs. Finally, the effects of the ductile behaviour of PT tendons on the rocking response of a SCW are analysed. The results demonstrate that SCWs exhibit two overturning modes under pulse excitation. The overturning region with Mode 1 in the PT force cases separates the safe region of the wall into two parts: region S1 with an elastic tendon and region S2 with a fractured tendon. The minimum overturning acceleration of a SCW with an elastic-brittle tendon becomes insensitive to excitation frequency as the PT force increases. After the plastic behaviour of the PT tendon is considered, the minimum overturning acceleration of a SCW is increased significantly in the whole range of the studied w_g/p.

• Structural Engineering and Mechanics
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• 제61권5호
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• pp.563-576
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• 2017

This paper is a continuation of the investigations started in the paper by Akbarov, S.D., Guliyev, H.H and Yahnioglu, N. (2016) "Natural vibration of the three-layered solid sphere with middle layer made of FGM: three-dimensional approach", Structural Engineering and Mechanics, 57(2), 239-263, to the case where the three-layered sphere is a hollow one. Three-dimensional exact field equations of elastodynamics are employed for investigation and the discrete-analytical method is employed for solution of the corresponding eigenvalue problem. The FGM is modelled as inhomogeneous for which the modulus of elasticity, Poison's ratio and density vary continuously through the inward radial direction according to power law distribution. Numerical results on the natural frequencies are presented and discussed. These results are also compared with the corresponding ones obtained in the previous paper by the authors. In particular, it is established that for certain harmonics and for roots of certain order, the values of the natural frequency obtained for the hollow sphere can be greater (or less) than those obtained for the solid sphere.