• Proceedings of the Korea Concrete Institute Conference
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• 2000.04a
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• pp.527-532
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• 2000

Cracks in reinforced concrete members are important element in structural analysis and design. It is clear from the test results that shear strength of cracked member is remarkably degraded compared with uncracked one. However, considerable amount of shear resistance by such mechanisms as aggregate interlock and dowel action is still active. There are various approaches to shear transfer estimation including finite element analysis, fracture mechanics, upper bound theory of plasticity, etc., but working out comprehensive and consistent models and manageable equations is rather difficult and remains to be improved. Shear transfer problems under cyclic loading and effective compressive strength of cracked concrete have not been adequately investigated and need further systematic research.

• Structural Engineering and Mechanics
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• v.55 no.2
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• pp.245-261
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• 2015

A three-dimensional (3-D) method of analysis is presented for determining the free vibration frequencies of eccentric hemi-spherical shells of revolution with variable thickness. Unlike conventional shell theories, which are mathematically two-dimensional (2-D), the present method is based upon the 3-D dynamic equations of elasticity. Displacement components $u_r$, $u_{\Theta}$, and $u_z$ in the radial, circumferential, and axial directions, respectively, are taken to be periodic in ${\theta}$ and in time, and algebraic polynomials in the r and z directions. Potential and kinetic energies of eccentric hemi-spherical shells with variable thickness are formulated, and the Ritz method is used to solve the eigenvalue problem, thus yielding upper bound values of the frequencies by minimizing the frequencies. As the degree of the polynomials is increased, frequencies converge to the exact values. Convergence to three or four-digit exactitude is demonstrated for the first five frequencies of the shells. Numerical results are presented for a variety of eccentric hemi-spherical shells with variable thickness.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.23 no.55
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• pp.25-31
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• 2000

To make a better decision about when to shutdown a nuclear power plant, we build a decision model using influence diagrams. We proceed the analysis adopting a bayesian approach. Firstly, an accident arrival rate is assumed to be known and this assumption is relaxed later. We perform our analysis on the cases of exponential time to accidents, and gamma distribution for the arrival rate. An optimal shutdown time is obtained considering the trade-off between the costs incurred by an accident due to late shutdown and the possible loss of revenues due to the early shutdown. We also derive the upper bound of the failure rate where we may operate the plant.

• Journal of Institute of Control, Robotics and Systems
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• v.14 no.6
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• pp.554-557
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• 2008

We analyze the stability property of a class of nonlinear time-delay systems with time-varying delays. We present a time-delay independent sufficient condition for the global asymptotic stability. In order to prove the sufficient condition, we exploit the inherent property of the considered systems instead of applying the Krasovskii or Razumikhin stability theory that may cause the mathematical difficulty of analysis. We prove the sufficient condition by constructing two sequences that represent the lower and upper bound variations of system state in time, and showing the two sequences converge to an identical point, which is the equilibrium point of the system. The simulation results illustrate the validity of the sufficient condition for the global asymptotic stability.

• Structural Engineering and Mechanics
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• v.55 no.1
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• pp.29-46
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• 2015

A three-dimensional (3-D) method of analysis is presented for determining the natural frequencies of a truncated shallow and deep conical shell with linearly varying thickness along the meridional direction free at its top edge and clamped at its bottom edge. Unlike conventional shell theories, which are mathematically two-dimensional (2-D), the present method is based upon the 3-D dynamic equations of elasticity. Displacement components $u_r$, $u_{\theta}$, and $u_z$ in the radial, circumferential, and axial directions, respectively, are taken to be periodic in ${\theta}$ and in time, and algebraic polynomials in the r and z directions. Strain and kinetic energies of the truncated conical shell with variable thickness are formulated, and the Ritz method is used to solve the eigenvalue problem, thus yielding upper bound values of the frequencies by minimizing the frequencies. As the degree of the polynomials is increased, frequencies converge to the exact values. Convergence to four-digit exactitude is demonstrated. The frequencies from the present 3-D method are compared with those from other 3-D finite element method and 2-D shell theories.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.1
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• pp.154-161
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• 2001

A UBET analysis has been carried out to predict the forming load and the extruded length of forward and backward extrusion of hexagonal and trochoidal wrench colts. For the upper bound load and the average length of the extruded billets are determined by minimizing the total energy consumption rate which is a function of unknown velocities and parameters at each element. Experiments are carried out with antimony-lead billets at room temperature using hexagonal and trochoidal shaped punches. The theoretical predictions of the forming load and the extruded length are in good agreement with the experimetal results.

• Geomechanics and Engineering
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• v.16 no.6
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• pp.569-575
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• 2018

The stability of shallow cavities with an arbitrary profile is a difficult issue in geotechnical engineering. This paper investigates this problem on the basis of the upper bound theorem of limit analysis and the Hoek-Brown failure criterion. The influence of pore pressure is taken into consideration by regarding it as an external force acting on rock skeleton. An objective function is constructed by equating the internal energy dissipation to the external force work. Then the Lagrange variation approach is used to solve this function. The validity of the proposed method is demonstrated by comparing the analytical solutions with the published research. The relations between shallow and deep cavity are revealed as well. The detaching curve of cavity roof with elliptical profile is obtained. In order to facilitate the application of engineering practice, the numerical results are tabulated, which play an important role in tunnel design and stability analysis of roof. The influential factors on potential collapse are taken into consideration. From the results, the impact of various factors on the extent of detaching is seen intuitively.

• Journal of the Korean Geotechnical Society
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• v.22 no.9
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• pp.23-36
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• 2006

This study presents the development of a new closed-form analytical solution for the ultimate bearing capacity of an embedded wall in a granular mass. The closed-form analytical solution consists of upper and lower bound solutions (UB and LB). The calculated values from these bound solutions were compared with the author's two-dimensional laboratory wall model loading test and finite element analysis in the plastic region. The comparison showed that ultimate bearing loads from both the model test and finite element analysis are located between UB and LB. In particular, the ultimate bearing load from LB showed good agreement with the ultimate bearing load values from both the model test and finite element analysis. However, the calculated value from the conventional empirical form subjected to plane-strain conditions was shown to be much smaller than the LB.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.8
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• pp.1927-1939
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• 1995

An experimental investigation is conducted to find out the large scale coherent structures in the intermediate wake past a rotating cylinder with a single tripping wire attached. Relation between the vortex shedding frequency and the spin rate of rotating cylinder and the effects of the tripping wire on the flow characteristics were studied by using spectral analysis and conditional phase average technique, respectively. It is found that the vortex shedding frequency is bound to a certain range and varies regularly as spin rate increases. The coherent structures are compared with those of the plain rotating cylinder in the case of spin rate of 1.0. Distance between the upper and lower center of vortices increase and the vortex shedding time is delayed, the velocity fluctuation energy decreases near the center line of vortices and it spreads out to the outer region. The Reynolds shear stress increases highly in the upper region and the turbulent wake width expands with strong entrainment process.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.19 no.5
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• pp.399-407
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• 2007

Two reliability models, AFDA(Approximate Full Distribution Approach) and Monte-Carlo simulation method, are directly developed to study on both hydraulic failure mode of berm recession and structural failure mode of armour breakage of berm breakwaters. By comparing the present results with the results of other researcher, it may be confirmed that two reliability models can be straightforwardly applicable to berm breakwaters. Relative influence of each random variable on hydraulic and structural failure probabilities could be properly analyzed. The upper bound and the lower bound of failure probability can be evaluated by using bi-modal bounds of the multiple failure mode analysis, from which it may be possible to investigate some kinds of dependence into between two failure modes. Finally, it may also be found that the structural failure mode of armour breakage could become a main failure mode of berm breakwaters in the condition of more than any allowable berm recession.