Flexural strength of prestressed concrete members with unbonded tendons

이득행,김강수 국제구조공학회 2011 Structural Engineering and Mechanics, An Int'l Jou Vol.38 No.5

It is difficult to accurately predict the flexural strength of prestressed members with unbonded tendons, unlike that of prestressed members with bonded tendons, due to the unbonded behavior between concrete and tendon. While there have been many studies on this subject, the flexural strength of prestressed members with unbonded tendons is still not well understood, and different standards in various countries often result in different estimation results for identical members. Therefore, this paper aimed to observe existing approaches and to propose an improved model for the ultimate strength of prestressed members with unbonded tendons. Additionally, a large number of tests results on flexural strength of prestressed members with unbonded tendons were collected from previous studies, which entered into a database to verify the accuracy of the proposed model. The proposed model, compared to existing approaches, well estimated the flexural strength of prestressed members with unbonded tendons, adequately reflecting the effects of influencing factors such as the reinforced steel ratio, the loading patterns, and the concrete strength. The proposed model also provided a reasonably good estimation of the ultimate strength of over-reinforced members and high-strength concrete members.

Web-shear capacity of prestressed hollow-core slab unit with consideration on the minimum shear reinforcement requirement

이득행,박민국,오재율,김강수,임주혁,서수연 사단법인 한국계산역학회 2014 Computers and Concrete, An International Journal Vol.14 No.3

Prestressed hollow-core slabs (HCS) are widely used for modern lightweight precast floor structures because they are cost-efficient by reducing materials, and have excellent flexural strength and stiffness by using prestressing tendons, compared to reinforced concrete (RC) floor system. According to the recently revised ACI318-08, the web-shear capacity of HCS members exceeding 315 mm in depth without the minimum shear reinforcement should be reduced by half. It is, however, difficult to provide shear reinforcement in HCS members produced by the extrusion method due to their unique concrete casting methods, and thus, their shear design is significantly affected by the minimum shear reinforcement provision in ACI318-08. In this study, a large number of shear test data on HCS members has been collected and analyzed to examine their web-shear capacity with consideration on the minimum shear reinforcement requirement in ACI318-08. The analysis results indicates that the minimum shear reinforcement requirement for deep HCS members are too severe, and that the web-shear strength equation in ACI318-08 does not provide good estimation of shear strengths for HCS members. Thus, in this paper, a rational web-shear strength equation for HCS members was derived in a simple manner, which provides a consistent margin of safety on shear strength for the HCS members up to 500 mm deep. More shear test data would be required to apply the proposed shear strength equation for the HCS members over 500 mm in depth though.

Simplified Strength Design Method for Allowable Compressive Stresses in Pretensioned Concrete Members at Transfer

이득행,황진하,김강수,김지상,정원석,오홍섭 대한토목학회 2014 KSCE JOURNAL OF CIVIL ENGINEERING Vol.18 No.7

The allowable compressive stress of the pretensioned precast concrete members at transfer is the most important factor for determining its bed staying time at the Precast Concrete (PC) factory, and it consequently has a large influence on the productivity of pretensioned PC members. The current design code provisions have specified the allowable compressive stresses of concrete at prestress transfer in an empirical manner, and it was simply expressed only as the function of concrete compressive strength at the time of transfer. In contrast, the Strength Design Method (SDM) can theoretically determine the allowable compressive stresses of pretensioned concrete members considering the effect of influential factors, such as eccentricity ratio of tendons, sectional types, level of prestressing forces, and size of concrete section. In this study, a SDM based approach was proposed to simplify the calculation process of the SDM for the each type of concrete cross-section. In addition, four rectangular pretensioned concrete specimens were fabricated and tested to investigate their allowable prestress. It appeared that ACI318-11 and EC2-02 code models provided unsafe estimations on the allowable compressive stress of pretensioned concrete members with the low eccentricity ratios but excessively conservative results for those with high eccentricity ratios. The SDM and the simplified SDM proposed in this study showed reasonable accuracy and safety margin on the allowable compressive stresses of the pretensoined concrete specimens at prestress transfer.

나자르바예프대학교 토목환경공학과 구조공학연구실

이득행 한국구조물진단유지관리공학회 2019 한국구조물진단유지관리공학회지 Vol.23 No.1

철근콘크리트 부재의 전단강도 산정모델

이득행,한선진,김강수 한국구조물진단유지관리공학회 2020 한국구조물진단유지관리공학회 논문집 Vol.24 No.5

This study presents a shear strength estimation model, in which the shear failure of a reinforced concrete (RC) member is assumed to be governed by the flexure-shear mechanism. Two shear demand curves and corresponding potential capacity curves for cracked tension and uncracked compression zones are derived, for which the bond mechanism developed between reinforcing bars and surrounding concrete is considered in flexural analysis. The shear crack concentration factor is also addressed to consider the so-called size effect induced in large RC members. In addition,unlike exising methods, a new formulation was addressed to consider the interaction between the shear contributions of concrete and stirrup. To verify the proposed method, an extensive shear database was established, and it appeared that the proposed method can capture the shear strengths of the collected test specimens regardless of their material properties, geometrical features, presence of stirrups, and bond characteristics. 이 연구에서는 철근콘크리트 부재의 전단파괴가 휨-전단 메커니즘에 지배된다는 가정을 바탕으로 인장측과 압축측에 대한 2개의 전단요구곡선들과 이에 대응되는 잠재전단강도곡선들을 각각 도출하였으며, 이를 기반으로 전단강도 산정모델을 제안하였다. 제안모델에서는 철근과 콘크리트의 부착거동을 고려하여 휨균열폭과 철근의 국부응력증가분을 산정하였다. 또한, 휨균열로부터 발전되는 지배전단균열의 생성과 균열진전거동을 이론적으로 모사하기 위하여 균열집중계수를 도입하였으며, 이를 통해 단면높이가 큰 철근콘크리트 부재에서 관측되는 크기효과를 반영하였다. 또한, 기존의 해석모델과는 다르게 전단철근과 콘크리트의 전단기여분 사이의 상호작용을 고려할 수 있는 새로운 형태의 수식을 개발하였다. 제안모델의 검증을 위하여 방대한 전단실험체들을 기존문헌으로부터 수집하였으며, 이를 통해 해석모델을 검증한 결과는 제안모델이 실험체들의 재료, 크기 및 철근의 부착특성에 관계없이 실험결과를 정확하게 평가할 수 있음을 보여주었다.

Simplified P-M interaction curve model for reinforced concrete columns exposed to standard fire

이득행,천나래,김민수,이정민,오재열,김강수 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.19 No.5

In the authors’ previous study, an axial force-flexural moment (P-M) interaction curve model was proposed to evaluate fire-resisting performances of reinforced concrete (RC) column members. The proposed method appeared to properly consider the axial and flexural strength degradations including the secondary moment effects in RC columns due to fire damage. However, the detailed P-M interaction curve model proposed in the authors’ previous study requires somewhat complex computational procedures and iterative calculations, which makes it difficult to be used for practical design in its current form. Thus, the aim of this study was to develop a simplified P-M interaction curve model of RC columns exposed to fire considering the effects of fire damage on the material performances and magnitudes of secondary moments. The simplified P-M interaction model proposed in this study was verified using 66 column fire test results collected from literature, and the verification results showed that the proposed simplified method can provide an adequate analysis accuracy of the failure loads and fire-resisting times of the RC column specimens.

Application of direct tension force transfer model with modified fixed-angle softened-truss model to finite element analysis of steel fiber-reinforced concrete members subjected to Shear

이득행,황진하,주현진,김강수 사단법인 한국계산역학회 2014 Computers and Concrete, An International Journal Vol.13 No.1

Steel fiber-reinforced concrete (SFRC) is known as one of the efficient modern composites that can greatly enhance the material performance of cracked concrete in tension. Such improved tensile resistance mechanism at crack interfaces in SFRC members can be heavily influenced by methodologies of treatments of crack direction. While most existing studies have focused on developing the numerical analysis model with the rotating-angle theory, there are only few studies on finite element analysis models with the fixed-angle model approach. According to many existing experimental studies, the direction of principal stress rotated after the formation of initial fixed-cracks, but it was also observed that new cracks with completely different angles relative to the initial crack direction very rarely occurred. Therefore, this study introduced the direct tension force transfer model (DTFTM), in which tensile resistance of the fibers at the crack interface can be easily estimated, to the nonlinear finite element analysis algorithm with the fixed-angle theory, and the proposed model was also verified by comparing the analysis results to the SFRC shear panel test results. The secant modulus method adopted in this study for iterative calculations in nonlinear finite element analysis showed highly stable and fast convergence capability when it was applied to the fixed-angle theory. The deviation angle between the principal stress direction and the fixed-crack direction significantly increased as the tensile stresses in the steel fibers at crack interfaces increased, which implies that the deviation angle is very important in the estimation of the shear behavior of SFRC members.

파형웨브가 적용된 프리스트레스트 합성거더에 대한 실험적 연구

이득행(Lee Deuck-Hang),김강수(Kim Kang-Su),오재열(Oh Jae-Yeul),임주혁(Lim Ju-Hyuk),최성모(Choi Sung-Mo),김진호(Kim Jin-Ho) 대한건축학회 2010 大韓建築學會論文集 : 構造系 Vol.26 No.11

The demands for long span structures with reduction of story height have greatly increased as building structures become much larger and higher in recent years. Although the development of flexural members for reducing story height or realizing long span has been studied by many researchers and engineers, there is still a lack of efficient systems that meet these two demands simultaneously. Thus, this study proposed a prestressed composite girder with corrugated web suitable for long span and reduction of story height. The corrugated webs utilized in this development make the accordion effect introducing the larger effective prestressing force to top and bottom flanges, which causes the larger upward camber reducing the member deflection. The concepts of effective moment of inertia and effective web area is proposed to reflect the accordion effect of the corrugated web before composite. Five full-scaled specimens, with key variables of prestressing, welding length of web, the number of drape points, and the use of shear connectors, were tested to investigate their flexural behavior and to verify the performance of the proposed composite girder. The test results verified that the proposed prestressed composite girder had an excellent flexural performance with greate flexural strength and stiffness.

이중잠재강도모델을 이용한 부착영향평가

이득행(Lee. Deuckhang) 한국콘크리트학회 2021 한국콘크리트학회 학술대회 논문집 Vol.33 No.1

T형 및 역T형 단면을 가지는 프리텐션부재의 프리스트레스 도입시 허용 압축응력

이득행(Deuck-Hang Lee),이정연(Jeong-Yeon Lee),임주혁(Joo-Hyuk Lim),김강수(Kang-Su Kim) 한국콘크리트학회 2011 콘크리트학회논문집 Vol.23 No.3