• 한국정밀공학회지
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• 제28권3호
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• pp.349-356
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• 2011

This paper replaces a conventional austenitic stainless steel sheet to a ferritic stainless steel for the cost reduction of a pulsator cover of a washing machine. However, ferritic stainless steel has poor formability in comparison with austenitic one. The low formability of ferritic steel results in problems during stamping such as fracture, wrinkling, shape inaccuracy and so on. Design modification of the stamping tool is carried out with the aid of the finite element analysis for multistage stamping process of the pulsator cover. The simulation results show that fracture occurs on top of the product while wrinkles are generated by the excess metal near the wing part. Modification of the initial stamping die is performed to improve metal flow and to eliminate problems during the stamping process. Simulation with the modified design fully demonstrates that safe forming is possible without inferiorities.

• 한국지반공학회논문집
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• 제20권5호
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• pp.67-78
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• 2004

주변의 뒷채움 흙과 파형강판 벽체의 상호작용을 통해 외력을 지지하는 지중강판구조물에서는 상부의 차량하중에 의해서 토피 지반에서 전단파괴, 또는 인장파괴가 발생하여 구조물의 파손이나 붕괴를 유발할 수 있다. 현재 적용 중인 설계기준들에서는 이를 방지하기 위하여 상부아치 위로 확보해야 하는 최소한의 토피 두께(토피고)를 규정하고 있으나, 이들은 표준형 파형강판으로 제작한 지간 7.7m 이하의 구조물에 대한 수치해석에 바탕을 두고 있으므로, 지간이 그 이상인 장지간 구조물에는 적용기가 어렵다. 이 연구에서는 원형 및 낮은 아치형 단면의 지중강판구조물에 대하여 강판 부재와 뒷채움 지반을 함께 모사한 수치해석을 실시하여 장지간 구조물에 대한 최소토피고의 크기를 분석하였다. 해석 결과, 현재의 시방기준은 장지간 구조물의 최소토피고를 과다산정하는 경향을 보였으며, 지간이 10m 이상일 경우의 최소토피고는 지간의 크기 및 단면형상에 관계없이 1.5m 정도로 나타났다. 최소토피고를 만족하지 못하는 경우에는 토피 지반에 활하중 분산효과가 뛰어난 콘크리트 보강 슬래브를 적용할 수 있는데, 원형 단면의 장지간 구조물에 대한 수치해석을 통해서 보강 슬래브가 토피 지반 위에 재하되는 활하중에 의한 강판 부재의 축력과 휨모멘트를 크게 감소시키는 것을 확인할 수 있었다. 따라서, 토피 두께가 기준에 미달하더라도 적절한 규격의 보강 슬래브를 설치하는 지중강판구조물은 기존의 절차에 따라 축력지배구조로서 설계 및 해석이 가능하다.

• 한국산학기술학회논문지
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• 제19권6호
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• pp.581-586
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• 2018

주철 맨홀뚜껑은 제조과정에서 환경오염을 유발하며, 작업환경은 매우 열악하다. 또한, 맨홀 뚜껑의 높이와 도로 표면의 높이가 일치하지 않은 경우에는 많은 불편을 초래하며 안전에도 문제를 야기한다. 본 연구에서는 주철 맨홀뚜껑을 대체할 수 있고, 도로 노면 공사 시 도로 노면 높이와 맨홀 뚜껑의 높이를 동일하게 맞추기 용이한 높이 조절이 가능한 강재 맨홀 뚜껑 구조를 제안하였다. 요구되는 품질 성능을 만족시키기 위한 맨홀 뚜껑의 구조에 대한 설계변수를 파악하기 위하여 구조 해석을 수행하였다. 요구되는 내하중 용량을 만족하는 맨홀뚜껑을 제작하기 위하여 ㄷ자 모양의 보강대에 대하여 최적 설계를 수행하였다. 높이조절부의 원통과 하부 프레임 원통은 강판을 굽힘 가공하여 원형의 형상으로 만든 후 용접에 의하여 제작하였다. 또한, 보강대도 강판을 굽힘 가공하여 제작하였다. 높이 조절용 홈은 CNC 밀링머신에 의하여 가공하였다. 4차례의 시제품을 제작하여 내하중 실험을 수행하였으며, 시험의 결과를 반영하여 새로이 제작하였다. 내하중 시험에서 용접부의 파손은 발생하지 않았으며, 주로 변형은 높이조절 홈과 가셋트 판이 접촉하는 부위에서 발생하였다. 450kN의 부하 하중에 의해 1~2.7mm의 변형이 발생하지만, 하중을 제거한 후에 거의 잔류변형이 없으며, 맨홀뚜껑의 분해 및 재조립이 가능하기 때문에 내하중 평가도 만족하는 것으로 판정받았다.

• Structural Engineering and Mechanics
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• 제56권6호
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• pp.899-916
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• 2015

Upgrading or strengthening of existing reinforced concrete (RC) infrastructure is an emerging demand nowadays. Near Surface Mounted (NSM) technique is very promising approach for flexural strengthening of RC members. However, premature failure such as concrete cover separation failure have been a main concern in utilizing this technique. In this study, U-wrap end anchorage with carbon fiber reinforced polymer (CFRP) fabrics is proposed to eliminate the concrete cover separation failure. Experimental programs were conducted to the consequence of U-wrap end anchorage on the flexurally strengthened RC beams with NSM-steel. A total of eight RC rectangular beam specimens were tested. One specimen was kept unstrengthened as a reference; three specimens were strengthened with NSM-steel bars and the remaining four specimens were strengthened with NSM-steel bars and U-wrap end anchorage using CFRP fabrics. A 3D non-linear finite element model (FEM) was developed to simulate the flexural response of the tested specimens. It is revealed that NSM-steel (with and without end-anchors) significantly improved the flexural strength; moreover decreased deflection and strains compared with reference specimen. Furthermore, NSM-steel with end anchorage strengthened specimens revealed the greater flexural strength and improve failure modes (premature to flexure) compared with the NSM-steel without end anchorage specimens. The results also ensured that the U-wrap end anchorage completely eliminate the concrete cover separation failure.

• Structural Engineering and Mechanics
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• 제30권6호
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• pp.713-732
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• 2008

Serviceability and durability of the concrete members can be seriously affected by the corrosion of steel rebar. Carbonation front and or chloride ingress can destroy the passive film on rebar and may set the corrosion (oxidation process). Depending on the level of oxidation (expansive corrosion products/rust) damage to the cover concrete takes place in the form of expansion, cracking and spalling or delamination. This makes the concrete unable to develop forces through bond and also become unprotected against further degradation from corrosion; and thus marks the end of service life for corrosion-affected structures. This paper presents an analytical model that predicts the weight loss of steel rebar and the corresponding time from onset of corrosion for the known corrosion rate and thus can be used for the determination of time to cover cracking in corrosion affected RC member. This model uses fully the thick-walled cylinder approach. The gradual crack propagation in radial directions (from inside) is considered when the circumferential tensile stresses at the inner surface of intact concrete have reached the tensile strength of concrete. The analysis is done separately with and without considering the stiffness of reinforcing steel and rust combine along with the assumption of zero residual strength of cracked concrete. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel rebar. The capability of the model to produce the experimental trends is demonstrated by comparing the model's predictions with the results of experimental data published in the literature. The effect of considering the corroded reinforcing steel bar stiffness is demonstrated. A sensitivity analysis has also been carried out to show the influence of the various parameters. It has been found that material properties and their inter-relations significantly influence weight loss of rebar. Time to cover cracking from onset of corrosion for the same weight loss is influenced by corrosion rate and state of oxidation of corrosion product formed. Time to cover cracking from onset of corrosion is useful in making certain decisions pertaining to inspection, repair, rehabilitation, replacement and demolition of RC member/structure in corrosive environment.

• 콘크리트학회논문집
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• 제25권5호
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• pp.573-581
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• 2013

이 논문은 강섬유보강콘크리트와 GFRP (glass fiber reinforced polymer)사이의 부착 특성을 조사하기 위한 실험적 연구를 수행하였다. 실험 주요 변수로는 보강근 지름, 섬유혼입량, 피복두께 및 콘크리트의 압축강도를 설정하였다. 부착파괴는 주로 콘크리트 피복에서의 쪼갬으로 인하여 유발되며, 이러한 콘크리트의 쪼갬파괴는 보강근과 콘크리트 사이의 변형 차이로 유발되는 인장력때문에 발생한다. 따라서, 보강근과 콘크리트 사이의 부착파괴를 방지하기 위하여, 콘크리트 피복부위의 인장강도를 향상시켜야 한다. 실험결과를 살펴보면, 섬유혼입량 증가는 부착강도를 크게 향상시키고 있으며, 피복두께는 최종 파괴모드를 변화시킴을 확인할 수 있었다. 보강근의 지름 또한 최종 파괴모드를 변화시킴을 확인할 수 있었다. 일반적으로 보강근의 지름은 부착특성에 영향을 미치는 것으로 알려져 있으나, 섬유혼입량은 부착특성에 큰 영향이 없는 것으로 알려져 있다. 콘크리트 압축강도의 증가는 보강근과 콘크리트 사이의 부착강도를 증가시켰으며, 이는 압축강도의 증가가 직접적으로 인장강도의 증가를 유발하기 때문이라고 판단된다.

• Structural Engineering and Mechanics
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• 제61권6호
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• pp.693-700
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• 2017

Concrete cover cracking due to the corrosion of steel reinforcing bars is one of the main causes of deterioration in Reinforced Concrete (RC) structures. The oxidation level of the bars causes varying levels of expansion. The rebar expansions could lead to through-thickness cracking of the concrete cover, where depending on the cracking characteristics, the service life of the structures would be affected. In this paper, the effect of geometrical and material parameters, i.e., concrete cover thickness, reinforcing bar diameter, and concrete tensile strength, on the required pressure for concrete cover cracking due to corrosion has been investigated through detailed numerical simulations. ABAQUS finite element software is employed as a modeling platform where the concrete cracking is simulated by means of eXtended Finite Element Method (XFEM). The accuracy of the numerical simulations is verified by comparing the numerical results with experimental data obtained from the literature. Using a previously proposed empirical equation and the numerical model, the time from corrosion initiation to the cover cracking is predicted and then compared to the respective experimental data. Finally, a parametric study is undertaken to determine the optimum ratio of the rebar diameter to the reinforcing bars spacing in order to avoid concrete cover delamination.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2004년도 춘계학술발표회
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• pp.425-432
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• 2004

In this paper, the results of the numerical analysis for the minimum depth of soil cover have been compared with those of currently suggested codes. Based on this comparison, the minimum depth of soil cover for the structures with long spans was suggested. Results showed that the actual depth of the soil cover required against soil failure over a circular and low-profile arch structure does not vary significantly with the size of the span and for the circular structure, the minimum depth of the soil cover was about 1.5m, and for the low-profile arch structures, below about 1.6m. And the previously established code in which the minimum depth of soil cover is defined to linearly increase with the increase in the span (CHBDC, 2001) was very conservative. For the structure with the relieving slab, the maximum live load thrust was reduced by about 36 percent and the maximum moment about 81 percent. The numerical analysis gave more conservative estimation of the live-load thrusts than the other design methods.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2005년도 봄학술 발표회 논문집(I)
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• pp.323-326
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• 2005

Uniaxial tension test of Glass Fiber Reinforced Polymer (GFRP) bar reinforced concrete prisms was performed. The objective was to investigate the adequate cover thickness of the GFRP rebars. The tension stiffening effect of GFRP bar reinforced concrete was also studied. The test variables included rebar types (conventional steel rebar and two different GFRP rebars) and cover thicknesses (five different cover thicknesses ranging between 1-3db). Normal strength concrete was used. Cracking patterns on concrete surface and cracking loads were careful1y observed during the direct tensile test. The test results indicated that the adequate cover thickness of the GFRP rebars may even be larger than that of the steel rebars and that the cover thickness of 2db commonly specified for the GFRP rebars may not be large enough. The tension stiffening effect of the GFRP rebars was also quantified and documented from the test results.

• Steel and Composite Structures
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• 제26권3호
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• pp.329-345
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• 2018

This paper systematically investigated the mechanical performance of the weak-axis cover-plate connection, including a beam end monotonic loading test and a column top cyclic loading test, and a series of parametric studies for exterior and interior joints under cyclic loading using a nonlinear finite element analysis program ABAQUS, focusing on the influences of the shape of top cover-plate, the length and thickness of the cover-plate, the thickness of the skin plate, and the steel material grade. Results showed that the strains at both edges of the beam flange were greater than the middle's, thus it is necessary to take some technical methods to ensure the construction quality of the beam flange groove weld. The plastic rotation of the exterior joint can satisfy the requirement of FEMA-267 (1995) of 0.03 rad, while only one side connection of interior joint satisfied ANSI/AISC 341-10 under the column top cyclic loading. Changing the shape or the thickness or the length of the cover-plate did not significantly affect the mechanical behaviors of frame joints no matter in exterior joints or interior joints. The length and thickness of the cover-plate recommended by FEMA 267 (1995) is also suitable to the weak-axis cover-plate joint. The minimum skin plate thickness and a design procedure for the weak-axis cover-plate connections were proposed finally.