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탄소나노튜브가 혼입된 UHPC 센서의 자기감지 성능평가 Evaluation of Self-Sensing Performance of UHPC with Carbon Nanotube

https://doi.org/10.4334/JKCI.2020.32.6.505

이상훈(Sang-Hoon Lee) ; 한선진(Sun-Jin Han) ; 장범수(Beom Soo Jang) ; 김강수(Kang Su Kim) ; 서수연(Soo-Yeon Seo)

In this study, a material test was conducted to investigate the self-sensing performance of ultra-high performance concrete (UHPC) with carbon nanotubes (CNTs), based on which the correlation between the damage of specimens and the fractional change in resistance (FCR) was analyzed in detail. In addition, small UHPC sensors were fabricated for practical application, and their performance was evaluated based on which gauge factors of the UHPC sensors were derived. The experimental results showed that the sectional shape of the UHPC sensor had an insignificant effect on the self-sensing performance of the sensor and that the electrodes with copper plates were more efficient to estimate the damage than were those with copper wires.

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개구부를 사각 및 팔각형의 철근으로 보강한 철근콘크리트 보의 전단성능에 관한 연구 Study on Shear Performance of RC Beams with Web Openings Strengthened with Square and Octagonal-shaped-Reinforcements

https://doi.org/10.4334/JKCI.2020.32.6.513

김형국(Hyeong-Gook Kim) ; 이범식(Bum-Sik Lee) ; 정찬유(Chan-Yu Jung) ; 이정윤(Jung-Yoon Lee) ; 김길희(Kil-Hee Kim)

The application of web opening to RC beams causes problems in terms of the usability of structures due to the cracks and deflection of structural members in which stresses are concentrated around web openings. Conventional web opening reinforcing methods require additional reinforcement such as diagonal reinforcing bars for strengthening web openings. Difficulty in fixing the location of the diagonal reinforcing bars and low work-ability are pointed out as their disadvantages. Furthermore, because of inaccuracies in the design and prediction of the strength of RC members with web openings, it is common not to place web openings in the joints or plastic hinge regions of structures subjected to bending moment. In this study, a new web opening reinforcement was proposed, that has excellent work-ability and is efficient in strengthening the shear performance and controlling cracks of RC beams. To evaluate the effect of the proposed web opening reinforcement in improving the shear performance and controlling cracks in the plastic hinge regions of RC beams, loading tests were conducted with the shape and position of the reinforcement as variables. Test results showed that a specimen with the proposed web opening reinforcement has a shear performance equal to or higher than that of a specimen with existing bent-type web opening reinforcement. Moreover, all reinforcing bars constituting the proposed web opening reinforcement resist shear cracks in positive and negative drifts. When installing web opening in the plastic hinge region, it is possible to secure the required shear capacity of RC beams by reducing the diameter of web openings.

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모르타르 마감두께를 고려한 조적채움벽의 전단강도 평가 Shear Strength of Masonry Infill Walls Considering Mortar Plaster Thickness

https://doi.org/10.4334/JKCI.2020.32.6.525

황유진(Yu-Jin Hwang) ; 강혜령(Hye-Ryeong Kang) ; 이상현(Sang-Hyun Lee)

In this study, the effects of the mortar plaster thickness on shear strength of a masonry infill wall were investigated. Specimens of the masonry wall were collected from existing school buildings and shear strength tests were conducted. The test results were compared with the masonry wall specifications of the ‘seismic performance evaluation and retrofit manual for school facilities (2019)’. In addition, finite element (FE) analysis was performed to analyze the shear strength variation with regard to the material conditions of the masonry walls and the mortar plaster. The experimental and analytical results indicated that the manual underestimates the shear strengths compared to the experimentally obtained values, which means that masonry wall strength can be significantly increased due to mortar plaster. Finally, FE analysis considering the mortar plaster provided results relatively similar to the experimentally obtained ones.

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버트레스형 구조체로 보강된 모멘트골조의 콘크리트 격막 면내강성에 따른 내진거동특성 Seismic Behavior of Moment Frame Retrofitted by External Buttress-Type Structure according to In-Plane Stiffness of Concrete Diaphragm

https://doi.org/10.4334/JKCI.2020.32.6.533

강혜령(Hye-Ryeong Kang) ; 노정태(Jung-Tae Noh) ; 이상현(Sang-Hyun Lee)

The seismic reinforcement method that involves attaching a frame with high stiffness to the outside of an existing building yields a structure that is hard to consider as a perfectly integrated one. The assumption of slabs as rigid diaphragms could be used to differently evaluate the actual behavior of seismic reinforced structures. Therefore, this paper proposes a diaphragm modeling method that properly evaluates seismic reinforcement effects by comparing characteristics of building behavior according to the diaphragm assumption. The behavior of frames is analyzed according to the diaphragm modeling method, which uses rigid and semi-rigid elements. As a result, a moment frame with relatively even stiffness shows similar performance curves and results. The semi-rigid diaphragm of the moment frame with shear walls at both ends shows brittle behavior due to the shear failure of the center column, unlike the case of the rigid diaphragm. To evaluate the diaphragm effects for an actual building, a non-seismically designed school building extended by a concrete wall structure is considered by performing pushover analysis. Results show similar behavior characteristics regardless of type of diaphragm in the longitudinal direction. However, the semi-rigid diaphragm model in the transverse direction shows brittle behavior because of shear failure of columns at the end sides of the existing building. This failure is caused by the mode shape, in which the deformation at both ends is relatively large compared to the center due to the in-plane deformation of the slab. Therefore, if a high stiffness member is installed or reinforced in a part of building, the semi-rigid diaphragm model can be used to accurately evaluate the response of building.

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수평접합부 수직보강철근 이음상세에 따른 프리캐스트 콘크리트 벽체의 내진성능 Seismic Capacity of Precast Concrete Wall Corresponding to Connection Detail of Vertical Bars at Horizontal Joint

https://doi.org/10.4334/JKCI.2020.32.6.541

서수연(Soo-Yeon Seo) ; 김승훈(Seung-Hun Kim) ; 차정우(Jeong-Woo Cha) ; 임병호(Byeong-Ho Lim)

The purpose of this study was to determine the effects of labor intensity and fatigue on the sleep quality of clinical nurses and provide basic data for preparation of nursing interventions to improve sleep quality. Methods: Data were collected from 198 nurses in 2 general hospitals with more than 500 beds in D-city. Data analysis was performed using stepwise regression with the SPSS/WIN 22.0 program. Results: The significant factors influencing sleep quality of the clinical nurses were fatigue, absolute labor intensity, and shift type. Explanatory power was 66% in the regression model. Conclusions: Findings indicate that nurses with higher level of fatigue, and greater absolute labor intensity which implies extended work hours, had lower sleep quality for all the three shifts. To improve the sleep quality of clinical nurses, an intervention program for fatigue management is needed. In addition, hospitals should establish an internal legal standard to compensate overtime work with breaks. At the same time, as shift work is inevitable for nurses, it is necessary to develop a work system that can minimize the disturbance of daily rhythms.

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철근콘크리트 벽식 아파트의 비구조재 내진설계를 위한 지반가속도 증폭 현상 분석 Analysis of Ground Acceleration Amplification for Seismic Design of Non-Structural Member in RC Wall-type Apartment Buildings

https://doi.org/10.4334/JKCI.2020.32.6.553

강수민(Su-Min Kang) ; 김희도(Hui-Do Kim)

In Korea, the importance of seismic design for non-structural elements has been highlighted due to recent earthquake damage cases in Gyeongju and Pohang. In this study, for the non-structural seismic design of high-rise RC wall-type structures mainly used in residential buildings in Korea, design codes were investigated and analysis on the maximum floor acceleration of high-rise RC wall-type structures was conducted through nonlinear seismic analysis. According to the nonlinear analysis, in high-rise RC wall-type apartments, the amplification of the acceleration for each floor of the structure does not show a linear amplification as in the design codes. In particular, it can be seen that the acceleration amplification appears large in the middle floors, and the acceleration amplification is different even within the same plan. In the case of high-rise RC wall-type apartments, the higher-order natural mode corresponding to the 4th to 6th mode has a great influence on the acceleration amplification, and the acceleration amplification in the same floor has a significant difference due to the rectangular plan shape.

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프리캐스트콘크리트 보를 위한 홈이 있는 이음접합부의 거동 Behavior of Midspan Splice Joints with Groove for Precast Concrete Beams

https://doi.org/10.4334/JKCI.2020.32.6.565

이승제(Seung-Jae Lee) ; 엄태성(Tae-Sung Eom) ; 강수민(Su-Min Kang) ; 김대진(Dae-Jin Kim) ; 김승일(Seung-Il Kim)

This study investigated the structural performance of a PC beam and its splice that was developed for the floor system of underground garage in apartment buildings. The PC beams were spliced on top of the PC wall column, or in midspan where the moment due to gravity load is almost zero. To improve the structural integrity at the splice joint, a groove was cut at the end of the PC beams, additional reinforcement was placed, and then in-situ concrete was filled. In addition, using in-situ concrete topping, the whole floor structure including the topping was designed as a continuous beam. Four point loading tests of four PC beam specimens were performed to investigate the structural performance at the splice joints with groove in midspan. The tests showed that the behavior of the PC beam joints was affected by the length of the grooves (or the bond resistance on the concrete interface inside the groove). The specimens with sufficient groove lengths showed the flexural strengths greater than the nominal strengths. On the other hand, the specimens with insufficient groove lengths showed concrete shear failure in the groove and topping as bond splitting cracks developed horizontally along the topping bars.

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PC 모멘트 저항 골조의 반응수정계수 Response Modification Factor of Precast Reinforced Concrete Moment Resisting Frame

https://doi.org/10.4334/JKCI.2020.32.6.575

무하마드 하룬(Muhmmad Haroon) ; 변현우(Hyun-Woo Byun) ; 이범식(Bum-Sik Lee) ; 김길희(Kil-Hee Kim) ; 이정윤(Jung-Yoon Lee)

Under severe seismic demands if structures are designed to remain within the elastic range, the results sections may highly be uneconomical because the maximum response acceleration is usually several times the maximum ground acceleration, depending on the structural stiffness and the magnitude of damping. Therefore, current design codes recommend much lower design seismic loads compared to the inertia loads for elastic response of structures by using the force reduction or response modification factor based on the equal energy dissipation concept. The response modification factor is used to reduce the actual base shear force to the design level force during an earthquake excitation. Various design codes i.e. ACI318-19, ASCE 7-10 etc. have come up with different R-factor values for different structural systems including the pre-cast moment frames. However, the Korean Concrete Institute design codes provide the R-factor values for cast-in place concrete moment frames along with other lateral load resisting systems but ignore the precast concrete structures. In this study, efforts are made to estimate the response modification factor for the precast RC moment resisting frames using nonlinear static push-over analysis. The types of the frames considered in this study are precast RC frames with strength and deformation capacity equal to 100 %, 75 % and 50 % of that of a monolithic RC frame. The result of this study shows that the R-factors of the three precast moment resisting frames were 6.5, 6.3 and 5.4, respectively.

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파쇄벽돌을 굵은 골재로 사용한 콘크리트의 가격 최소화를 위한 배합설계 Mix Proportioning of Concrete Containing Brick Chips as Coarse Aggregate for Cost Minimization

https://doi.org/10.4334/JKCI.2020.32.6.583

밀리언 타페세(Million Tafesse) ; 김락현(Rak-hyun Kim) ; 양범주(Beomjoo Yang) ; 김형기(Hyeong-Ki Kim)

Mix proportioning in terms of cost minimization was conducted for concrete containing brick chips as coarse aggregate. First, the physical and chemical characteristics of the brick chips were evaluated in detail, including computed tomography scanning, direct compressive strength measurement, and X-ray diffraction analysis, to further understand their effect on the properties of concrete. Second, the strength of brick chips concrete with various mix proportions was measured, and the proportioning of mixtures for material cost minimization based on the equivalent strength design was carried out using linear regression of the results following the local market price in the project area. By using the proposed proportioning method for brick chips concrete, it was possible to obtain a cost reduction of 5~24 % for mixtures with strengths of 25~33 MPa.

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철근 UHPC 비정형 솔리드 및 트러스 보의 적용을 위한 개념적 접근 Conceptual Approach to Applications of Reinforced UHPC Non-uniform Solid and Truss Beams

https://doi.org/10.4334/JKCI.2020.32.6.593

조창근(Chang-Geun Cho) ; 김호연(Hoyeon Kim) ; 김형기(Hyeong-Ki Kim)

This research is a conceptual approach to examine the possibility of using nonuniform concrete structural members in architectural and building structures by applying the ultra high-performance concrete (UHPC). From conventional reinforced concrete beams with rectangular cross-sections, made using 3-dimensional finite element simulation and the strut-tie approach, reinforced UHPC nonuniform solid and truss beams were redesigned with emphasis on structural efficiency as well as the artistic value of the architectural forms. Flexible and nonuniform formwork technologies were newly attempted to manufacture nonuniform concrete or UHPC members. Specimens of reinforced UHPC nonuniform solid and truss beams were manufactured and tested by beam loading tests; both types of solid and truss beams stably responded due to the control of multiple micro-cracks and the high compressive characteristic of UHPC during fully loading stages.

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