• Composites Research
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• 제23권2호
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• pp.17-23
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• 2010

스텔스 기술은 적진에서 항공기나 함정의 생존 가능성을 향상시키고 임무 수행 능력을 향상 시킬 수 있다. 본 논문의 목적은 섬유강화 복합재료를 이용하여 하중지지가 가능한 곡면부 형상을 갖는 저피탐지 구조를 제안하고 군사적 활용을 위한 전방위 스텔스 플랫폼의 개발 가능성을 보여주는 것이다. 본 연구에서는 곡면을 갖는 물체의 레이더 반사면적을 줄이기 위해서 기존의 circuit analog 흡수체에 기반을 둔 전자파 흡수구조를 개발하였다. 먼저 상용 3차원 전자기장 해석 프로그램을 이용하여 사각 주기격자 패턴의 전도성 고분자 층을 갖는 전자파 흡수구조를 설계하고 성능을 해석하였다. 다음으로 섬유강화 복합재료와 전도성 고분자 재료를 이용하여 설계된 반원통형 전자파 흡수구조를 제작하였다. 저항성 시트로 작용하는 주기격자 패턴층을 제작하기 위해서 PEDOT를 기반으로 하여 폴리우레탄을 바인더로 갖는 전도성 고분자 페이스트를 사용하였다. 마지막으로 제작된 RAS의 전자파 흡수 성능을 평가하기 위해 POSTECH의 compact range 장비를 이용하여 레이더 반사면적을 측정하였다.

• 한국강구조학회 논문집
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• 제23권1호
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• pp.83-97
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• 2011

본 논문에서는 콘크리트 채움 U형 강재보와 철근콘크리트 기둥으로 구성된 접합부의 내진 상세를 개발하였다. 접합부 내진성능을 평가하기 위하여 세 개의 보-기둥 접합부 실험체를 반복주기하중에 대하여 실험하였다. 보춤과 기둥 단면 형상을 실험 변수로 하였다. 합성보의 춤은 슬래브 두께를 포함하여 610mm, 710mm이며, 철근콘크리트 기둥은 사각단면과 원형단면이 사용되었다. 접합부를 보강시키기 위하여 사각단면 기둥과 원형단면 기둥에 각각 대각 철근과 외다이어프램 강판을 사용한 특수 상세가 사용되었다. 실험 결과 실험체는 강도와 변형능력, 에너지 소산에 있어서 우수한 성능을 보여주었다. 변형능력은 특수모멘트골조 기준인 4% 이상의 층간변위각을 발휘하였다.

• 한국화재소방학회:학술대회논문집
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• 한국화재소방학회 2010년도 춘계학술논문발표회 논문집
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• pp.389-394
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• 2010

A concrete filled square steel tube (CFT) is composed of the external steel material, which its strength is reduced in fire due to sudden temperature increase, and the internal concrete with high thermal capacity that can ensure the fire resistance performance of the structure. Therefore, research about the influence factors of the structural performance of CFT column is required in order to apply CFT column to a fire resisting structure, and additional research about influence for each condition is also necessary. Among the influence factors, the boundary condition between column and beam is important structurally, and it is one of the major factors that determine overall fire resisting performance. This study performed a fire experiment under loading in order to analyse the influences of CFT column to the boundary condition. As the results of the experiment, fire resistance time of 106 minutes was ensured for the clamped-end condition but 89 minutes for the hinge-end condition in case of the 360 cross section. And, fire resistance time of 113 minutes was ensured for the clamped-end condition but 78 minutes for the hinge-end condition in case of the 280 cross section. The difference in the fire resistance performance according to changes in the boundary conditions showed a tendency that larger change effect on the fire resistance performance was derived from smaller cross section area.

• Steel and Composite Structures
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• 제1권1호
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• pp.51-74
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• 2001

A series of tests on concrete-filled SHS (Square Hollow Section) stub columns (twenty), columns (eight) and beam-columns (twenty one) were carried out. The main parameters varied in the tests are (1) Confinement factor (${\xi}$) from 1.08 to 5.64, (2) concrete compression strength from 10.7MPa to 36.6MPa, (3) tube width to thickness ratio from 20.5 to 36.5. (4) load eccentricity (e) from 15 mm to 80 mm and (5) column slenderness (${\lambda}$) from 45 to 75. A mechanics model is developed in this paper for concrete-filled SHS stub columns, columns and beam-columns. A unified theory is described where a confinement factor (${\xi}$) is introduced to describe the composite action between the steel tube and filled concrete. The predicted load versus axial strain relationship is in good agreement with stub column test results. Simplified models are derived for section capacities and modulus in different stages of the composite sections. The predicted beam-column strength is compared with that of 331 beam-column tests with a wide range of parameters. A good agreement is obtained. The predicted load versus midspan deflection relationship for beam-columns is in good agreement with test results. A simplified model is developed for calculating the member capacity of concrete-filled SHS columns. Comparisons are made with predicted columns strengths using the existing codes such as LRFD (AISC 1994), AIJ (1997), and EC4 (1996). Simplified interaction curves are derived for concrete-filled beam-columns.

• Steel and Composite Structures
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• 제19권6호
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• pp.1403-1419
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• 2015

When precast concrete infill panels are connected to steel frames at discrete locations, interaction at the structural interface is neither complete nor absent. The contribution of precast concrete infill panels to the lateral stiffness and strength of steel frames can be significant depending on the quality, quantity and location of the discrete interface connections. This paper presents preliminary experimental and finite element results of an investigation into the composite behaviour of a square steel frame with a precast concrete infill panel subject to lateral loading. The panel is connected at the corners to the ends of the top and bottom beams. The Frame-to-Panel-Connection, FPC4 between steel beam and concrete panel consists of two parts. A T-section with five achor bars welded to the top of the flange is cast in at the panel corner at a forty five degree angle. The triangularly shaped web of the T-section is reinforced against local buckling with a stiffener plate. The second part consists of a triangular gusset plate which is welded to the beam flange. Two bolts acting in shear connect the gusset plate to the web of the T-section. This way the connection can act in tension or compression. Experimental pull-out tests on individual connections allowed their load deflection characteristics to be established. A full scale experiment was performed on a one-storey one-bay 3 by 3 m infilled frame structure which was horizontally loaded at the top. With the characteristics of the frame-to-panel connections obtained from the experiments on individual connections, finite element analyses were performed on the infilled frame structures taking geometric and material non-linear behaviour of the structural components into account. The finite element model yields reasonably accurate results. This allows the model to be used for further parametric studies.

• Steel and Composite Structures
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• 제25권1호
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• pp.79-92
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• 2017

Initial damage to a stud due to corrosion, fatigue, unexpected overloading, a weld defect or other factors could degrade the shear capacity of the stud. Based on typical push-out tests, a FEM model and theoretical formulations were proposed in this study. Six specimens with the same geometric dimensions were tested to investigate the effect of the damage degree and location on the static behavior and shear capacity of stud shear connectors. The test results indicated that a reduction of up to 36.6% and 62.9% of the section area of the shank could result in a dropping rate of 7.9% and 57.2%, respectively, compared to the standard specimen shear capacity. Numerical analysis was performed to simulate the push-out test and validated against test results. A parametrical study was performed to further investigate the damage degree and location on the shear capacity of studs based on the proposed numerical model. It was demonstrated that the shear capacity was not sensitive to the damage degree when the damage section was located at 0.5d, where d is the shank diameter, from the stud root, even if the stud had a significant reduction in area. Finally, a theoretical formula with a reduction factor K was proposed to consider the reduction of the shear capacity due to the presence of initial damage. Calculating K was accomplished in two ways: a linear relationship and a square relationship with the damage degree corresponding to the shear capacity dominated by the section area and the nominal diameter of the damaged stud. This coefficient was applied using Eurocode 4, AASHTO LRFD (2014) and GB50017-2003 (2003) and compared with the test results found in the literature. It was found that the proposed method produced good predictions of the shear capacity of stud shear connectors with initial damage.

• Journal of Preventive Medicine and Public Health
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• 제54권6호
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• pp.422-430
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• 2021

Objectives: Pregnant women are especially vulnerable to respiratory infections such as coronavirus disease 2019 (COVID-19), but insufficient research has investigated pregnancy and its outcomes in women with COVID-19. This cross-sectional study compared birth outcomes related to COVID-19 between Bangladeshi pregnant women with and without COVID-19. Methods: The study was conducted at 3 tertiary referral hospitals in Dhaka, Bangladesh, from March to August 2020. Pregnant women admitted for delivery at these hospitals with laboratory results (reverse-transcription polymerase chain reaction) were analyzed. Using convenience sampling, we included 70 COVID-19-positive and 140 COVID-19-negative pregnant women. Trained and experienced midwives conducted the interviews. Data were analyzed using the t-test, the chi-square test, and univariate and multivariable linear and logistic regression. Results: Pregnant women with COVID-19 were more likely to give birth to a preterm baby (adjusted odds ratio [aOR], 2.15; 95% confidence interval [CI], 1.06 to 4.37) and undergo a cesarean section (aOR, 3.27; 95% CI, 1.51 to 7.07). There were no significant differences in birth weight, premature rupture of membranes, and the Apgar score at 1 minute or 5 minutes post-delivery between women with and without COVID-19. All the newborn babies who were born to COVID-19-positive women were COVID-19-negative. Conclusions: Our study suggests that pregnant women with COVID-19 were more likely to give birth to a preterm baby and undergo a cesarean section. For this reason, physicians should be particularly cautious to minimize adverse birth outcomes among pregnant women with COVID-19 and their newborn babies.

• 한국산학기술학회논문지
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• 제22권2호
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• pp.617-624
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• 2021

각형강관 기둥에 콘크리트를 충전하여 사용하는 콘크리트충전 강관구조가 구조부재로 사용되면 기둥 부재의 내력과 변형 능력이 증가되어 높은 효율성을 가진 구조물 구현이 가능해 진다. 콘크리트충전 강관구조에 대한 국내의 설계 기준은 대한건축학회에서 2005년에 제정한 후, 2009년과 2016년에 각각 개정되었다. 연구 목적은 콘크리트충전 각형강관 단주를 대상으로 일축 압축실험을 실시하여 압축내력 및 변형능력에 주는 영향을 파악하고, 국내의 건축구조기준의 기준식을 검증하여 차후 수정 및 보완에 필요한 자료를 제공하는데 있다. 실험에서 강관은 냉간가공으로 제작된 각형강관을 사용하였고, 시험체는 강관의 폭두께비를 변수로 총 26개를 제작하여 중심 압축실험을 실시하였다. 실험결과 콘크리트충전 각형강관 단주의 압축내력과 변위관계 및 파괴모드를 얻었고, 실험결과를 분석하여 콘크리트의 충전효과와 폭두께비의 영향을 파악하였다. 충전된 콘크리트의 압축내력은 일축응력 상태보다 9%정도 증가하였는데, 이것은 차후 건축구조기준에 반영할 필요가 있다. 실험결과를 건축구조기준과 비교한 결과, 냉간가공된 각형강관의 경우 건축구조기준의 콤팩트단면 한계폭두께비 2.26은 다소 과대 평가하고 있기 때문에 수정이 필요하며, 보수적으로 보완한 계수 1.35로 제한하여 보다 더 안정적인 설계식을 제안하였다.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2004년도 하계학술대회 논문집 C
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• pp.1960-1962
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• 2004

In this paper, we have studied shielding characteristics of high conductivity or high permeability materials on ELF magnetic fields generated from single or three phase AC line. Perm alloy has been selected as high permeability material and copper as high conductivity material. Four-plate shield (square section) was considered as a shielding shape. We found copper showed stable shielding effects more than perm alloy.

• Structural Engineering and Mechanics
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• 제7권2호
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• pp.127-145
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• 1999

This paper deals with the development of an approach for evaluating the squash load and rigidity of unprotected concrete filled steel columns at elevated temperatures. The current approach of evaluating these properties is reviewed. It is shown that with a non-uniform temperature distribution, over the composite cross-section, the calculations for the squash load and rigidity are tedious in the current method. A simplified approach is proposed to evaluate the temperature distribution, squash load, and rigidity of composite columns. This approach is based on the model in Eurocode 4 and can conveniently be used to calculate the resistance to axial compression of a concrete filled steel column for any fire resistance time. The accuracy of the proposed approach is assessed by comparing the predicted strengths against the results of fire tests on concrete filled circular and square steel columns. The applicability of the proposed approach to a design situation is illustrated through a numerical example.