Numerical analysis of the axially loaded concrete filled steel tube columns with debonding separation at the steel-concrete interface

Chen Shiming,Zhang Huifeng 국제구조공학회 2012 Steel and Composite Structures, An International J Vol.13 No.3

The interaction between steel tube and concrete core is the key design considerations for concrete-filled steel tube columns. In a concrete-filled steel tube (CFST) column, the steel tube provides confinement to the concrete core which permits the composite action among the steel tube and the concrete. Due to construction faults and plastic shrinkage of concrete, the debonding separation at the steel-concrete interface weakens the confinement effect, and hence affects the behaviour and bearing capacity of the composite member. This study investigates the axial loading behavior of the concrete filled circular steel tube columns with debonding separation. A three-dimensional nonlinear finite element model of CFST composite columns with introduced debonding gap was developed. The results from the finite element analysis captured successfully the experimental behaviours. The calibrated finite element models were then utilized to assess the influence of concrete strength, steel yield stress and the steel-concrete ratio on the debonding behaviour. The findings indicate a likely significant drop in the load carrying capacity with the increase of the size of the debonding gap. A design formula is proposed to reduce the load carrying capacity with the presence of debonding separation.

Experimental and analytical behaviour of cogged bars within concrete filled circular tubes

Tilak Pokharel,Huang Yao,Helen M. Goldsworthy,Emad F. Gad 국제구조공학회 2016 Steel and Composite Structures, An International J Vol.20 No.5

Recent research on steel moment-resisting connection between steel beams and concrete filled steel tubes has shown that there are considerable advantages to be obtained by anchoring the connection to the concrete infill within the tube using anchors in blind bolts. In the research reported here, extensive experimental tests and numerical analyses have been performed to study the anchorage behaviour of cogged deformed reinforcing bars within concrete filled circular steel tubes. This data in essential knowledge for the design of the steel connections that use anchored blind bolts, both for strength and stiffness. A series of pull-out tests were conducted using steel tubes with different diameter to thickness ratios under monotonic and cyclic loading. Both hoop strains and longitudinal strains in the tubes were measured together with applied load and slip. Various lead-in lengths before the bend and length of tailed extension after the bend were examined. These dimensions were limited by the dimensions of the steel tube and did not meet the requirements for “standard” cogs as specified in concrete standards such as AS 3600 and ACI 318. Nevertheless, all of the tested specimens failed by bar fracture outside the steel tubes. A comprehensive 3D Finite Element model was developed to simulate the pull-out tests. The FE model took into account material nonlinearities, deformations in reinforcing bars and interactions between different surfaces. The FE results were found to be in good agreement with experimental results. This model was then used to conduct parametric studies to investigate the influence of the confinement provided by the steel tube on the infilled concrete.

Investigations of different steel layouts on the seismic behavior of transition steel-concrete composite connections

Qi, Liangjie,Xue, Jianyang,Zhai, Lei Techno-Press 2019 Advances in concrete construction Vol.8 No.3

This article presents a comparative study of the effect of steel layouts on the seismic behavior of transition steel-concrete composite connections, both experimental and analytical investigations of concrete filled steel tube-reinforced concrete (CFST-RC) and steel reinforecd concrete-reinforced concrete (SRC-RC) structures were conducted. The steel-concrete composite connections were subjected to combined constant axial load and lateral cyclic displacements. Tests were carried out on four full-scale connections extracted from a real project engineering with different levels of axial force. The effect of steel layouts on the mechanical behavior of the transition connections was evaluated by failure modes, hysteretic behavior, backbone curves, displacement ductility, energy dissipation capacity and stiffness degradation. Test results showed that different steel layouts led to significantly different failure modes. For CFST-RC transition specimens, the circular cracks of the concrete at the RC column base was followed by steel yielding at the bottom of the CFST column. While uncoordinated deformation could be observed between SRC and RC columns in SRC-RC transition specimens, the crushing and peeling damage of unconfined concrete at the SRC column base was more serious. The existences of I-shape steel and steel tube avoided the pinching phenomenon on the hysteresis curve, which was different from the hysteresis curve of the general reinforced concrete column. The hysteresis loops were spindle-shaped, indicating excellent seismic performance for these transition composite connections. The average values of equivalent viscous damping coefficients of the four specimens are 0.123, 0.186 and 0.304 corresponding to the yielding point, peak point and ultimate point, respectively. Those values demonstrate that the transition steel-concrete composite connections have great energy dissipating capacity. Based on the experimental research, a high-fidelity ABAQUS model was established to further study the influence of concrete strength, steel grade and longitudinal reinforcement ratio on the mechanical behavior of transition composite connections.

Repair of Buckled Concrete Filled Steel Tube Columns Subjected to Axial Compression

Ahmed Almasslawi,Talha Ekmekyapar,Baraa J.M. AL-Eliwi 대한토목학회 2020 KSCE Journal of Civil Engineering Vol.24 No.5

Concrete filled steel tube (CFST) columns are the main loaded elements in the structures as they provoke the superior mechanical properties of constituent materials. However, concrete filled double steel tube (CFDST) columns refer to a new type of composite elements which have very high level of fire resistance and the potential to be implemented in high-rise structures. Columns are exposed to high stresses due to the increasing loads of daily life. Therefore, unpredictable deformations may occur and affect the performance and safety of structures. This article first studies the effectiveness of a concrete filled circular steel tube (CFCST)-based technique for repairing buckled and stressed CFST columns, then it studies the advantages of CFDST columns in improving the capability of composite members. The CFCST-based repairing system is to place the deformed CFST column in a larger diameter steel tube, and then the concrete is poured in the gap between the deformed column and the larger tube. Buckled CFST specimens with different tube tkicknesses were repaired and tested to failure under axial compression. The performance of repaired CFST columns was comparedwith that of undamaged counterpart columns. Based on findings, it can be concluded that the repair technique restored the capacity of the deformed columns from 97% to 100% of the capacity of the undamaged counterpart columns which confirm the effectiveness of repairing using CFCST-based technique. Results of the study provide significant information to the available test data concerning repairing of CFST members.

An approach for partial strengthening of circular RC columns using outer steel tube

Ju-young Hwang,Hyo-Gyoung Kwak 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.38 No.6

This paper introduces an improved design equation to evaluate the resisting capacity of circular reinforced concrete (RC) columns partially strengthened with outer steel tube. When RC column members are required to be strengthened according to the change in the loadings considered and/or the deterioration progress in columns, wrapping up RC column with steel circular tube, which takes the form of concrete filled steel tube (CFST), has been popularly considered because of its structural advantage induced from the confinement effect. However, the relatively high construction cost of steel tube is restricting its use to the required region, while deriving the shape of a partial CFST column. To evaluate the resisting capacity of a partial CFST column, numerical analyses need to be performed, and a numerical model proposed in the previous study for the numerical analysis of full CFST columns is used to conduct parametric studies for the introduction of a design equation. The bond-slip effect developed along the interface between the in-filled concrete and the exterior steel tube is taken into consideration and the validity of the numerical model has been established through correlation studies between experimental data and numerical results for partial CFST circular columns. Moreover, parametric studies make it possible to introduce a design equation for determining the optimum length of outer steel tube which produces partial CFST circular columns.

중심축력을 받는 콘크리트 충전강관 기둥의 역학적 거동 특성에 관한 연구

박정민,김화중 한국콘크리트학회 1995 콘크리트학회지 Vol.7 No.5

본 연구는 콘크리트 충전강관을 고층 건물의 구조부재로 이용하기 위한 연구의 일환으로서 강관의 폭두께비, 세장비와 충전콘크리트의 강도를 주요 변수로 하여 강관이 콘크리트를 단순 구속하는 경우의 재하조건으로서 일련의 실험을 콘크리트 충전강관 기둥의 역학적인 거동 특성을 고찰하였다. 얻어진 결론을 요약하면 다음과 같다. (1)구속 콘크리트의 파괴양상은 단주의 경우 시험체 단부에서의 압괴에 의한 $45^{\circ}$정도의 사인장 파괴가 이루어졌으며 장주의 경우 횡방향 휨 파괴 양상을 나타내었다. (2)원형강관으로서 콘크리트를 구속함으로서 변형능력의 향상과 동시에 콘크리트의 연성 효과를 증대시킬 수 있었다. (3)강관의 세장비, 폭두께비, 콘크리트의 강도를 고려하여 콘크리트의 구속계수를 이용하여 강관에 의해 구속된 내부 콘크리트와 충전 강관 기둥의 최대내력 산정식을 제안하였다. This study investigated to the properties of structural behaviors through a series of experiment with the key parameter, such as diameter-to-thickness(D/t) ratio, selenderness ratio of steel t~ube and strength of concrete under loading condition simple confined concrete by steel tube as a fundmental study on adaptability with structural members in high-rise building. The obtained results are sumnarised as follow. (1) The fracture mode of confined concrete was presented digonal tension fracture in the direction of $45^{\circ}$ with compression failure at the end of specimen in stub column, but the fracture mode of long column was assumed an aspect of bending fracture transversely. (2) The deformation capacity and ductility effect was increased by confine steel tube for concrete. (3) 'The emprical formula to predict the ultimate capacity of confined concrete by steel tube and concrete filled steel tube column using restraint of concrete considered D / t ratio, selenderness ratio of steel tube anti strength of' concrete were proposed.

Experimental and analytical investigation of high-strength concrete-filled steel tube square columns subjected to flexural loading

유정한,Kyung-Soo Chung,Jin-Ho Kim 국제구조공학회 2013 Steel and Composite Structures, An International J Vol.14 No.2

The concrete-filled steel tube (CFT) columns have several benefits of high load-bearing capacity, inherent ductility and toughness because of the confinement effect of the steel tube on concrete and the restraining effect of the concrete on local buckling of steel tube. However, the experimental research into the behavior of square CFT columns consisting of high-strength steel and high-strength concrete is limited. Six full scale CFT specimens were tested under flexural moment. The CFT columns consisted of high-strength steel tubes (fy = 325 MPa, 555 MPa, 900 MPa) and high-strength concrete (fck = 80 MPa and 120 MPa). The ultimate capacity of high strength square CFT columns was compared with AISC-LRFD design code. Also,this study was focused on investigating the effect of high-strength materials on the structural behavior and the mathematical models of the steel tube and concrete. Nonlinear fiber element analyses were conducted based on the material model considering the cyclic bending behavior of high-strength CFT members. The results obtained from the numerical analyses were compared with the experimental results. It was found that the numerical analysis results agree well with the experimental results.

Experimental study on creep behavior of fly ash concrete filled steel tube circular arches

Wu T. Yan,Bing Han,Jin Q. Zhang,Hui B. Xie,Li Zhu,Zhong J. Xue 국제구조공학회 2018 Steel and Composite Structures, An International J Vol.27 No.2

Fly ash can significantly improve concrete workability and performance, and recycling fly ash in concrete can contribute to a cleaner environment. Since fly ash influences pozzolanic reactions in concrete, mechanical behaviors of concrete containing fly ash differ from traditional concrete. Creep behaviors of fly ash concrete filled steel tube arch were experimentally investigated for 10% and 30% fly ash replacement. The axes of two arches are designed as circular arc with 2.1 m computed span, 0.24 m arch rise, and their cross-sections are all in circular section. Time dependent deflection and strain of loading and mid-span steel tube were measured, and long term deflection of the model arch with 10% fly ash replacement was significantly larger than with 30% replacement. Considering the steel tube strain, compressive zone height, cross section curvature, and internal force borne by the steel tube, the compressive zone height and structural internal forces increased gradually over time due to concrete creep. Increased fly ash content resulted in more significant neutral axis shift. Mechanisms for internal force effects on neutral axis height were analyzed and verified experimentally.

Strength Calculation of Short Concrete-filled Steel Tube Columns

Anatoly Leonidovich Krishan,Mariia Anatolyevna Astafeva,Elvira Petrovna Chernyshova 한국콘크리트학회 2019 International Journal of Concrete Structures and M Vol.13 No.2

The aim of this work is to propose a technique to calculate the strength of short concrete-filled steel tube columns under the short-term action of a compressive load, based on the phenomenological approach and the theoretical positions of reinforced concrete mechanics. The main dependencies that allow the realization of the deformation calculation model in practice are considered. A distinctive feature of the proposed approach is the method of the multipoint construction of deformation diagrams for a concrete core and steel shell. In this case, two main factors are taken into account. First, the steel shell and the concrete core work under conditions of a complex stress state. Since the proposed dependencies to determine the strength and the ultimate relative strain of volumetrically compressed concrete are obtained phenomenologically, they are more versatile than the commonly used empirical formulas. In particular, they can be used for self-stressing, fine-grained and other types of concrete. Second, with a step-by-step increase in the relative deformation, the lateral pressure on a concrete core and a steel shell constantly change. Thus, the parametric points of the concrete and steel deformation diagrams also change at each step. This circumstance was not taken into account in earlier calculations. A comparison of the theoretical and experimental results indicates that the practical application of the developed calculation procedure gives a reliable and fairly stable estimate of the stress–strain state and the strength of concrete-filled steel tube columns.

콘크리트 충전강관 기둥부재의 내화성능에 관한 해석 및 실험적 연구

권인규(Kwon In-Kyu) 대한건축학회 2010 大韓建築學會論文集 : 構造系 Vol.26 No.11

Concrete filled Steel Tube(CFST) is one of solution in high-rise buildings in terms of higher structural stability and an enhanced fire resistance according to higher heat capacity of concrete. Many studies on the CFST have been conducted since 1990 in the Europe, Japan, and Canada. But in Korea, the study on the fire resistance of the CFST is focused on establishment of equation of fire resistance. But the relationships between temperature of perimeter of tube and inner concrete filled at the tube are still uncertain.. Therefore, to compare the temperature differences in the tube and concrete filled steel tube, the analysis and fire test with the same model of steel tube and concrete filled steel tube were conducted and got the followings; the temperatures from steel perimeter and concrete filled into the tube and the comparison of the CFST between analysis and fire test showed the very same values during 18 minutes and the temperatures of concrete are not excceed 100℃.