• Advances in concrete construction
• /
• 제6권5호
• /
• pp.485-507
• /
• 2018

Concrete-Filled Steel Tube (CFST) columns are an increasingly popular means to support great compressive loads in buildings. The residual strength capacity of CFST stub columns may be utilized to assess the potential damage caused by fire and calculate the structural fire protection for least post-fire repair. Ten specimens under room conditions and 10 specimens under fire exposure to the Eurocode smouldering slow-growth fire were tested to examine the effects of diameter to thickness D/t ratio and reinforcing bars on residual strength capacity, ductility and stiffness of CFST stub columns. On the other hand, in sixteen among the twenty specimens, three or six reinforcing bars were welded inside the steel tube. The longitudinal strains in the steel tube and load-displacement relationships were recorded throughout the subsequent compressive tests. Corresponding values of residual strength capacity calculated using AISC 360-10 and EC4 standards are presented for comparison purposes with the experimental results of this study. The test results showed that after exposure to $750^{\circ}C$, the residual strength capacity increased for all specimens, while the ductility and stiffness were slightly decreased. The comparison results showed that the predicted residual strength using EC4 were close to those obtained experimentally in this research.

• Steel and Composite Structures
• /
• 제31권6호
• /
• pp.559-573
• /
• 2019

Spiral spacing effect on axial compressive behavior of reinforced concrete filled steel tube (RCFST) stub column is experimentally investigated in this paper. A total of twenty specimens including sixteen square RCFST columns and four benchmarked conventional square concrete filled steel tube (CFST) columns are fabricated and tested. Test variables include spiral spacing (spiral ratio) and concrete strength. The failure modes, load versus displacement curves, compressive rigidity, axial compressive strength, and ductility of the specimens are obtained and analyzed. Especially, the effect of spiral spacing on axial compressive strength and ductility is investigated and discussed in detail. Test results show that heavily arranged spirals considerably increase the ultimate compressive strength but lightly arranged spirals have no obvious effect on the ultimate strength. In practical design, the effect of spirals on RCFST column strength should be considered only when spirals are heavily arranged. Spiral spacing has a considerable effect on increasing the post-peak ductility of RCFST columns. Decreasing of the spiral spacing considerably increases the post-peak ductility of the RCFSTs. When the concrete strength increases, ultimate strength increases but the ductility decreases, due to the brittleness of the higher strength concrete. Arranging spirals, even with a rather small amount of spirals, is an economical and easy solution for improving the ductility of RCFST columns with high-strength concrete. Ultimate compressive strengths of the columns are calculated according to the codes EC4 (2004), GB 50936 (2014), AIJ (2008), and ACI 318 (2014). The ultimate strength of RCFST stub columns can be most precisely evaluated using standard GB 50936 (2014) considering the effect of spiral confinement on core concrete.

• Steel and Composite Structures
• /
• 제36권5호
• /
• pp.587-602
• /
• 2020

In recent years, the composite columns have been widely used in the structures. These columns are mainly used to construct the structures with a large span and high floor height. Concrete filled tubes (CFTs) are a type of composite column, which are popular nowadays due to their numerous benefits. The purpose of this study is to investigate such frames at elevated temperatures. The method used in this research is based on section 2.2 of Eurocode 4. First, for the verification purpose, a comparison was made between the experimental results and the numerical model of the concrete filled tube. Then a composite frame was analyzed under fire temperature with different parameters. The results showed that the failure time decreased with increasing the friction of different models. Moreover, investigation of the concrete moisture content revealed that an increase in the concrete moisture content from 3% to 10% led to extended failure time for different models. For instance, in the second frame model, the failure time has increased up to 8%.

• 한국강구조학회 논문집
• /
• 제9권3호통권32호
• /
• pp.391-400
• /
• 1997

본 연구의 목적은 콘크리트충전 각형강관 기둥의 최대내력을 산정할 수 있는 내력식을 제안하는데 있다. 내력식을 제안하기 위해서 수치해석과 실험을 통해 최대내력를 정량적으로 평가하고 기존의 한계상태설계법의 규준식에 근거하여 적절한 보정방안을 제안하여 콘크리트 충전 각형강관 기둥의 최대내력을 산정할 수 있는 내력식을 제안하고자 한다.

• 한국강구조학회 논문집
• /
• 제9권1호통권30호
• /
• pp.121-128
• /
• 1997

It has been reported by the existing papers that the ultimate load capacity and energy absorption capacity of the CFST column are considerably higher than those resulting from a simple addition of the capacities of the concrete and the steel tube. It is normally believed that the confined effect for the infilled concrete due to the hoop tension of steel at the parameter of cross sections can remarkably improve the ductility and energy absorption capacities of the CFST columns. This paper provides the results of a study on the load-carrying capacities and energy absorption capacities of the CFST columns, a numerical analysis method, i. e. N-M interaction curves and Moment curvature relationships. The numerical approaches are verified by comparing with the existing test results and the circular and square steel tube sections are selected to clarify the amount of confinement effects to improve the ultimate deformable capacity(a ultimate strain value) of the infilled concrete. Then, an adequate value of the ultimate strain of the infilled concrete and an equation of the ultimate capacity of the CFST column are suggested.

• Steel and Composite Structures
• /
• 제23권5호
• /
• pp.517-527
• /
• 2017

In this paper, the effect of active confinement on the compressive behaviour of circular steel tube-confined concrete (STCC) and concrete-filled steel tube (CFST) columns is investigated. In STCC columns the axial load is only applied to the concrete core, while in CFST columns the load is carried by the whole composite section. A new method is introduced to apply confining pressure on fresh concrete by laterally prestressing steel tubes. In order to achieve different prestressing levels, short-term and long-term pressures are applied to the fresh concrete. Three groups of STCC and CFST specimens (passive, S-active and L-active groups) are tested under axial loads. The results including stress-strain relationships of composite column components, secant modulus of elasticity, and volumetric strain are presented and discussed. Based on the elastic-plastic theory, the behaviour of the steel tube is also analyzed during elastic, yielding, and strain hardening stages. The results show that using the proposed prestressing method can considerably improve the compressive behaviour of both STCC and CFST specimens, while increasing the prestressing level has insignificant effects. By applying prestressing, the linear range in the stress-strain curve of STCC specimens increases by almost twice as much, while the improvement is negligible in CFST specimens.

• Steel and Composite Structures
• /
• 제39권2호
• /
• pp.189-200
• /
• 2021

The compressive strength of circular concrete filled steel tubular (C-CFST) stubs strengthened with carbon fiber reinforced polymer (CFRP) is studied theoretically. According to previous experimental results, the failure process and mechanism of circular CFRP-concrete filled steel tubular (C-CFRP-CFST) stubs is analyzed, and the loading process is divided into 3 stages, i.e., elastic stage, elasto-plastic stage and failure stage. Based on continuum mechanics, the theoretical model of C-CFRP-CFST stubs under axial compression is established based on the assumptions that steel tube and concrete are both in three-dimensional stress state and CFRP is in uniaxial tensile stress state. Equations for calculating the yield strength and the ultimate strength of C-CFRP-CFST stubs are deduced. Theoretical predictions from the presented equations are compared with existing experimental results. There are a total of 49 tested specimens, including 15 ones for comparison of yield strength and 44 ones for comparison of ultimate strength. It is found that the predicted results of most specimens are within an error limit of 10%. Finally, simplified equations for calculating both yield strength and ultimate strength of C-CFRP-CFST stubs are proposed.

• 한국강구조학회 논문집
• /
• 제24권6호
• /
• pp.711-723
• /
• 2012

플랜지와 웨브에 서로 강도가 다른 이종강재를 사용한 CFT 합성구조의 거동특성을 파악하기 위하여, 플랜지는 건축용 800MPa급 강재인 HSA800, 웨브에는 일반강도 강재인 SM490 강재를 사용하여 실험연구를 수행하였다. 주요실험 변수는 강관의 강도 조합, 충전된 콘크리트의 강도, 콘크리트 충전효과이다. 이종강재간의 용접접합부는 낮은강도 강재에 적합한 용접부를 사용하여 접합부 성능을 검증하였다. 실험체의 거동특성을 평가하기 위해 편심압축 실험을 수행하였으며, 현행 설계기준들에 따른 예측결과와 비교하였다. 플랜지에 고강도 강재를 적용함에 따라 단면의 축강도 및 휨모멘트강도가 증가하였으며, 부재 강도를 충분히 발현한 이후 용접부에서 파괴가 일어났다. 실험결과 현행 설계기준을 적용하여 합성단면의 축력-모멘트 상관관계 및 유효휨강성을 안전측으로 예측 가능하였다.

• 한국강구조학회 논문집
• /
• 제23권4호
• /
• pp.439-454
• /
• 2011

내부구속 중공 콘크리트 충전 강관(ICH CFT: Internally Confined Hollow Concrete Filled Tube) 기둥의 비선형 해석모델을 제안하고 기존 연구자의 실험 결과를 이용하여 검증하였다. 제안된 모델은 콘크리트의 구속효과와 재료비선형성을 고려하였다. 검증결과, 제안된 해석 모델은 ICH CFT 기둥의 거동을 예측하는데 합리적이고 신뢰할 수 있는 결과를 보여주었다. 제안된 모델을 이용하여 매개변수 연구를 수행하였으며, 기둥의 거동에 영향을 미치는 주요인자로서 콘크리트의 강도, 중공비, 내부강관의 두께를 매개변수로 선택하였다. 해석결과, 콘크리트 강도와 내부강관의 두께는 기둥의 축강도와 모멘트 저항능력에 큰 영향을 주었으나, 중공비의 변화는 축강도에만 영향을 미치는 결과를 보여주었다.

• 한국콘크리트학회:학술대회논문집
• /
• 한국콘크리트학회 2008년도 추계 학술발표회 제20권2호
• /
• pp.145-148
• /
• 2008

현행 AISC-LRFD, ACI 318과 국내의 설계지침에서는 CFT 기둥의 탄성계수와 항복강도 산정 시구속효과에 의한 축 강성의 증가를 반영하고 있지 않고 있다. 또한 AISC-LRFD와 ACI 318의 제시된 탄성계수와 항복강도의 산정방법으로 계산된 값이 차이가 큰 문제점이 있다. 본 연구에서는 CFT(Rectangular) 기둥을 강관의 두께 및 충진 콘크리트의 강도에 따른 시험체를 9개 제작하여, 실험을 통하여 AISC-LRFD, ACI 318과 국내의 설계지침과 비교하였다. 본 연구에서의 실험결과 현행 AISC-LRFD, ACI 318과 국내의 설계지침의 CFT 기둥의 항복강도는 실제 CFT 기둥의 항복강도와는 큰 차이가 있는 것으로 나타났다.