• The Journal of the Acoustical Society of Korea
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• v.34 no.6
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• pp.444-454
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• 2015

Measurements of bottom backscattering strengths in a frequency range of 6-14 kHz were made on the shallow water off the southern Gyeonggi Bay in Yellow Sea in May 2013, as part of the KIOST-HYU joint acoustics experiment. Geological surveys for the experimental area were performed using multi-beam echo sounder, sparker system, and grab sampling to investigate the bottom topography, sub-bottom profile and composition of surficial sediment, respectively. In this paper, the backscattering strengths as a function of grazing angle (in range of $28^{\circ}{\sim}69^{\circ}$) were estimated and compared to the predictions obtained by Lambert's law and APL-UW scattering model. Finally, the effects of geoacoustic parameters corresponding to the experimental area on the backscattering strengths are discussed.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.15 no.2
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• pp.209-218
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• 2002

The Vector Random Decrement technique has been developed as an efficient method for transforming ambient responses into free decays of linear structures. It is shown that the VRD functions nay contain as much information about the modes as the really measured free decay responses. In this paper, the theory of the VRD technique is extended by applying the concept of the mode shape ratio into the Ibrahim Time Domain modal parameter identification algorithm. The VRD function is not shifted in the correction procedures for constant time shifts of the proposed VRD technique. Thus, a number of points equal to the largest of the time shifts used in the vector triggering condition are not deleted. In the VRD functions, any influence of the input to the system is averaged out. The proposed technique is compared with the traditional VRD technique by assessment of the modal parameters. The applicability of the VRD technique has been justified through a simulation study and a study of the response of a laboratory beam model subject to ambient loads.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.3
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• pp.237-244
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• 2016

This paper presents a nonlinear Moving Least Squares(MLS) difference method for material nonlinearity problem. The MLS difference method, which employs strong formulation involving the fast derivative approximation, discretizes governing partial differential equation based on a node model. However, the conventional MLS difference method cannot explicitly handle constitutive equation since it solves solid mechanics problems by using the Navier's equation that unifies unknowns into one variable, displacement. In this study, a double derivative approximation is devised to treat the constitutive equation of inelastic material in the framework of strong formulation; in fact, it manipulates the first order derivative approximation two times. The equilibrium equation described by the divergence of stress tensor is directly discretized and is linearized by the Newton method; as a result, an iterative procedure is developed to find convergent solution. Stresses and internal variables are calculated and updated by the return mapping algorithm. Effectiveness and stability of the iterative procedure is improved by using algorithmic tangent modulus. The consistency of the double derivative approximation was shown by the reproducing property test. Also, accuracy and stability of the procedure were verified by analyzing inelastic beam under incremental tensile loading.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.30 no.3
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• pp.191-198
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• 2017

To study the behavior of structures subject to blast loads it is important to calculate the loads due to the explosives accurately, especially in the case of interior explosions. It is known that numerical method based on computational fluid dynamics can estimate relatively accurate blast load due to the interior explosion including reflection effect. However, the numerical method has disadvantages that it is difficult to model the analysis and it takes much time to analyze it. Therefore, in this study, the analytical method which can include the reflection effect of the interior explosion was studied. The target structures were set as the slabs of residential buildings subject to interior explosion that could lead to massive casualties and progressive collapses. First, the numerical method is used to investigate the interior explosion effect and the maximum deflection of the slab which was assumed to be elastic, and compared with the analytical method proposed in this study. In the proposed analytical method, we determine the weighting factor of the reflection effect using the beam theory so that the explosion load calculation method becomes more accurate.

• Journal of Korean Society of Steel Construction
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• v.23 no.3
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• pp.357-365
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• 2011

The current trends in steel construction intend to use tapered sections to minimize as much as possible the use of excess material. This can be done by choosing the cross-sections to be as economical as possible, leaving the classical approach of using prismatic members. In addition, it is important to predict the buckling behavior of tapered member with large depth-to-thickness ratio in order to prevent the collapse of PEB system subjected to overloads. An experimental investigation of buckling behavior of tapered beam was presented. The primary test parameter was depth-to-thickness ratio and taper ratio. Using initial stiffness and load-carrying capacity proposed by current provision, the simple plastic hinge method using modified Yoda's model and finite element analysis, the prediction of a moment-rotation curve of linearly tapered member was presented. Moreover, comparisons between analytical and experimental data for moment-rotation curves were accomplished.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.245-248
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• 2008

Concrete structures show time-dependent behavior due to creep and shrinkage of concrete and the uncertainties of creep and shrinkage are very huge. To reduce uncertainties of creep and shrinkage, it is substantially necessary to perform the long-term creep and shrinkage tests, but actual construction process doesn't allow it due to the limited time. Even though the tests are performed in laboratory, the values obtained from the tests could be different from the actual values in construction site because of the different environment between the laboratory and construction site and the model uncertainty itself. It is difficult to predict the long-term behaviors of concrete structures properly if the assumed creep coefficient obtained from Codes or the results of experiments is different from the real characteristics of concrete creep. In this study, for predicting the long-term behavior, the creep coefficients in reinforced concrete beams are estimated using creep sensitivity analysis from the measured deflections with time. And estimated creep coefficients using creep models of ACI Committee 209 and CEB-FIP MC90 are compared.

• Journal of the Korean Geotechnical Society
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• v.28 no.2
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• pp.57-70
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• 2012

For soft ground, a pile foundation is typically used as a substructure of transmission tower. However, differential settlement between the foundations can cause structural damage of transmission tower. The connected-pile foundation is a type of group foundation consisting of four foundations connected with beams, and it was suggested in USA and Japan. In this study, a series of 1/8 scale model pile tests were performed to investigate the effect of load direction and stiffness of connecting beam on the responses of connected-pile foundation. As a result, the load capacities of the connected-pile foundation were larger than those of the conventional group pile foundation. For example, under the given test conditions in this paper, the resistibility against differential settlement was improved significantly for connected-pile foundation and its efficiency was maximized when the stiffness of connecting beams is about 25% of the mat foundation.

• Progress in Superconductivity and Cryogenics
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• v.17 no.2
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• pp.36-40
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• 2015

Electron cyclotron resonance (ECR) ion source is an essential component of heavy-ion accelerator. For a given design, the intensities of the highly charged ion beams extracted from the source can be increased by enlarging the physical volume of ECR zone [1]. Several models for ECR ion source were and will be constructed depending on their operating conditions [2-4]. In this paper three simulation models with 3, 4 and 6 solenoid system were built, but it's not considered anything else except the number of coils. Two groups of optimization analysis are presented, and the evolution strategy (ES) is adopted as an optimization tool which is a technique based on the ideas of mutation, adaptation and annealing [5]. In this research, the volume of ECR zone was calculated approximately, and optimized designs for ECR solenoid magnet system were presented. Firstly it is better to make the volume of ECR zone large to increase the intensity of ion beam under the specific confinement field conditions. At the same time the total volume of superconducting solenoids must be decreased to save material. By considering the volume of ECR zone and the total length of solenoids in each model with different number of coils, the 6 solenoid system represented the highest coil performance. By the way, a certain case, ECR zone volume itself can be essential than the cost. So the maximum ECR zone volume for each solenoid magnet system was calculated respectively with the same size of the plasma chamber and the total magnet space. By comparing the volume of ECR zone, the 6 solenoid system can be also made with the maximum ECR zone volume.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.21 no.10
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• pp.1184-1193
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• 2010

The upper frequency of international EMC regulations is being expanded above 1 GHz. Radiated emissions above 1 GHz are different from those below 1 GHz that is existing upper regulation frequency, and which have lower field strength and sharper and tilted beam-width, relatively. In this paper, an effective evaluation method to be used above 1 GHz is studied using an offset parabola antenna system having a double-ridged horn antenna as a feed. First, simple model is proposed for calculating antenna factor and field uniformity of the parabola antenna system, and then real radiated emission and radiated susceptibility measurements are performed using a constant noise emitter and the suggested antenna system. The results show that the proposed antenna system has higher gain and power efficiency, and wider field uniformity relative to a conventional double-ridged horn antenna. Therefore, it is confirmed that the proposed system can be effectively used for EMC measurements above 1 GHz.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.5
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• pp.1013-1021
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• 1994

The stiffness and mass matrices are developed for free torsional vibration analysis in linearly tapered thin-walled I-beams that takes into account the effect of warping torsion. The approximate shape functions are used for formulating stiffness and mass matrices. Significant improvements of accuracy and efficiency of free vibration analysis are achieved by using the stiffness and mass matrices developed in this study. Frequencies of free vibration of tapered members are compared with solutions based upon stepped representation of beam element and also are verified with model tests. The stiffness and mass matrices presented in this study can be used for the free vibration analysis of tapered and prismatic thin walled I-beams and space structures involving warping torsion.