• Journal of Korean Association for Spatial Structures
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• v.7 no.6
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• pp.69-74
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• 2007

Recently, to improve performance and strength of circular steel columns, application of concrete-filled steel tube(CFT) type are gradually increased. To accurately predict the plastic design of concrete-filled steel tube columns, a plasticity model is required which can be describe large deformation behavior of concretes and steels. In this study, elastic-plastic large deformation analysis is developed by using the plasticity model of structural steels, and accurate and validity of the developed program is verified by comparing between the experiment and the analysis for concrete-filled steel tube column. In concrete-filled steel tube columns, influence of initial deflection on ultimate strength behavior is investigated by using developed program.

• Journal of Korean Association for Spatial Structures
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• v.8 no.5
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• pp.59-65
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• 2008

Recently, to improve the load carrying capacity of column structures such as bridge piers, application to concrete-filled steel tube(CFT) type columns are increased more and more. To design the concrete-filled steel tube(CFT) columns in accuracy, influence of material and geometry properties and aspect ratio on ultimate strength of the concrete-filled steel tube column is investigated by experimental researches. In this investigation, the ultimate strength distribution of the concrete-filled steel tube column in accordance with diameter-thickness ratio(D/t) and steel-concrete area ratio(As/Ac) are clarified by the compressive tests. Futhermore, parametric experimental investigation on concrete target strength is performed. It was known from experimental observation that ultimate strength of concrete-filled steel tube column under axial compressive loading more depends on section properties of steel tube rather than concrete strength.

• Journal of the Korea institute for structural maintenance and inspection
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• v.13 no.2 s.54
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• pp.223-230
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• 2009

In this study we deal with bending behaviors of a concrete filled tubular(CFT) with various diameter-thickness ratios and concrete strengths. In finite element analysis using a commercial package(LUSAS), the bonding effect between concrete and steel in CFT structures is modeled by applying a joint element for the bonding surface. In order to consider the nonlinearity of concrete and steel tubes, stress-strain curves of the concrete and steel are used for the increased stresses in a plastic domain. The numerical results obtained from the proposed method show good agreement with the experimental data from load-displacement curves of a steel tube under distributed loads. Several parametric studies are focused on structural characteristics of CFT under bending effects for different diameter-thickness ratios and concrete strengths.

• Journal of Korean Society of Steel Construction
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• v.22 no.2
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• pp.185-195
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• 2010

This paper presents the experiment results for a CFT column confined by carbon fiber sheets(CFSs) under an axial load. Nine specimens were constructed and axial compression tests were conducted. The main experiment parameters were diameter-thickness ratio(D/t), reinforcing CFSa, and the attachment of a cushion gap between surface of steel tube and CFSs. The load-displacement curves of the specimens were obtained from the compression tests. Finally, it was concluded that the CFT column with a gap had grater ductility capacity improvement that the CFT column confined by CFSs.

• Journal of Korean Society of Steel Construction
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• v.24 no.1
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• pp.81-89
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• 2012

The concrete-filled tubular square column is superior to steel frame column in terms of fire resistance because of the thermal storage provided by the concrete. Studies have been conducted on CFT column reinforcement with steel bars or with the use of an internal tube to improve its structural load capacity and fire resistance. In fact, reinforced CFT columns have been increasingly used to deal with high axial force. The functional deterioration of columns due to fire damage needs to be measured precisely. In this study, the temperature distribution inside the columns in case of a fire was evaluated and the degree of deterioration in the load capacity of the concrete and reinforcing members associated with temperature distribution was identified in order to evaluate the overall residual strength of the columns.

• Journal of Korean Society of Steel Construction
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• v.27 no.2
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• pp.219-229
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• 2015

An experimental study was performed for thin-walled rectangular concrete-filled tubular (CFT) columns. The present study mainly focused on evaluation of the axial load-carrying capacity of concrete-filled tubular columns using high-strength steel and slender section. The test parameters were width-to-thickness ratio, concrete strength, steel yield strength, and the use of stiffeners. Five specimens were tested under monotonic axial loading. Although elastic local buckling occurred in the slender-section specimens with high-strength steel, the specimens exhibited considerable post-buckling reserve. The test results also satisfied the predictions of a current design code. The specimens strengthened with vertical stiffeners exhibited improved strength and ductility when compared with the un-stiffened specimens.

• Journal of Korean Society of Steel Construction
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• v.25 no.1
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• pp.71-80
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• 2013

Fire-resistant (FR) test data for a square concrete-filled steel tube (CFT) columns consisting of high-strength steel (fy>650MPa) and high strength concrete (fck>100MPa) under axial loads are insufficient. The FR behavior of square high-strength CFT members was investigated experimentally for two specimens having ${\Box}-400{\times}400{\times}15{\times}3,000mm$ with two axial load cases (5,000kN and 2,500kN). The results show that the FR performance of the high-strength CFT was rapidly decreased at earlier time (much earlier at high axial load) than expected due to high strength concrete spalling and cracks. In addition, a fiber element analysis (FEA) model was proposed and used to simulate the fiber behaviour of the columns. For steel and concrete, the mechanical and thermal properties recommended in EN 1994-1-2 are adopted. Test results were compared to those of numerical analyses considering a combination of temperature and axial compression. The numerical model can reasonably predict the time-axial deformation relationship.

• Journal of the Korea Concrete Institute
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• v.14 no.6
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• pp.864-873
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• 2002

Recently, CFT Column has reported a lot of study result, because a CFT column has certain superior structural properties as well as good productivity, execution efficiency, and improved rigidity over existing column. However, CFT column still has problems clearing the capacity evaluation between its steel tube member and high-strength concrete materials. Also, high-strength concrete filled steel square tube column(HCFT) examined numerical value explanatorily about transformation performance(M-ø) of when short-column receives equal flexure-moment from axial stress on research for concrete. hnd, with basis assumption, executed development of analysis program of moment-curvature relation for analytic analysis of transformation performance of HCFT section that get by an experiment. This study investigated to properties of structural(capacity, curvature), through a series of experiments for HCFT with key parameters, such as strength of concrete(600kgf/$\textrm{cm}^2$), D/t ratio, slenderness ratio(λ) and concrete kinds under eccentric load. And, I executed comparative analysis with AISC-LRFD, AIJ and Takanori Sato etc. and experiment result that is capacity design formula.

• Journal of the Korea institute for structural maintenance and inspection
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• v.13 no.3 s.55
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• pp.225-231
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• 2009

This study introduces the CFTA girder(Concrete-Filled and Tied Steel Tubular Arch Girder) which is a combined structural system of traditional CFT, arch, and prestress structures. Static load tests and structural behavior analyses were carried out for a 25m long CFTA girder. In the analysis, each load of 58kN, 88kN, 148kN, 207kN,and 298kN was applied incrementally at the positions of 1.0 m distances in both directions from the center of the girder. On each test, strain and displacement were measured. Linear static FEM analyses using Strand7 code were also performed to check the structural stability and to investigate the effects of prestressing(${\pm}$20%) and material property(Young's modulus) on the displacement and strain. The results of this study are summarized as follows: the initial strain & displacement under selfweight and prestressing were influenced with the variation of prestressing, but they were mainly effected only by Young's modulus when additional loads were applied.

• Computational Structural Engineering
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• v.9 no.1
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• pp.151-159
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• 1996

A finite element formulation based on the CFT(Compression Field Theory) concept such as the effect of compression softening in cracked concrete, and macroscopic and rotating crack models etc. was presented for the nonlinear behaviour of structural concrete. In this category, tangential or secant material stiffnesses for cracked concrete were also defined and discussed in view of the iterative solution schemes for nonlinear equations. Considering the computational efficiency and the ability of modelling the post-ultimate behaviour as major concerns, the incremental displacement solution algorithm involving initial material stiffnesses and the relaxation procedure for fast convergence was adopted and formulated in a type of 8-noded quadrilateral isoparametric elements. The analysis program NASCOM(Nonlinear Analysis of structrual Concrete by FEM : Monotonic Loading) developed baed on the CFT constitutive relationships and the incremetal solution strategy described enables the predictions of strength and deformation capacities in a full range. crack patterns and their corresponding widths, and yield extents of reinforcement. As the verfication purpose of NASCOM, the prediction of Cervenka's panel test results including the load resistance and the deformation history was made. A limited number of predictions indicate a good correlation in a general sense.