Concrete-filled Steel Tubular Beam-columns Research Articles

Behaviour and design calculations of rectangular CFST beam-columns with slender sections

In this paper, design calculations of Concrete-Filled Steel Tubular (CFST) columns with slender sections subjected to combined axial compression and bending are considered based on the Australian code (AS 5100), American code (AISC 360–16) and European code (EC 4) approaches. A Finite Element (FE) model is developed to predict the strength of rectangular CFST slender section beam-columns. The experimental data is collected and used to examine the accuracy of AS 5100, AISC 360–16 and EC 4 in predicting the strengths of CFST slender section beam-columns, and to validate the FE model. In addition, based on the test results, FE analysis results and limits specified in the codes, design procedures are developed to propose modifications to the current AS 5100 and EC 4 to improve their accuracy in designing CFST slender section columns. The effects of material strengths on the accuracy of the proposed methods are also investigated. The results indicate that, while AISC 360–16 provides conservative strength predictions, the current AS 5100 and EC 4 overestimate the strengths of CFST slender section beam-columns. The comparative study also shows that better strength predictions are achieved using the proposed design procedures.

Behaviour of square CFST beam-columns under combined sustained load and corrosion: FEA modelling and analysis

This paper presents a numerical investigation on concrete-filled steel tubular (CFST) beam-columns with square sections subjected to combined sustained load and corrosion. A general numerical method is used for the simulation of the time dependent behaviour of the core concrete, while the wall-thickness loss of the outer steel tube induced by corrosion is considered in this model simultaneously. The established finite element analysis (FEA) modelling is verified with test data and used for further investigations of the load-deformation relationship, load-transfer mechanism, stress and strain development, failure modes and ultimate states on CFST beam-columns when suffering combined sustained load and corrosion. Parametric studies are conducted to reveal the effects of key parameters, based on which simplified design method is suggested for square CFST beam-columns under combined sustained load and corrosion.

Behaviour of square CFST beam-columns under combined sustained load and corrosion: Experiments

This paper presents an experimental study on concrete-filled steel tubular (CFST) beam-columns with square sections under combined sustained load and chloride corrosion. A total of 11 specimens (including 7 CFST members and 4 reference hollow steel tubes designed for comparison) were tested under 120-day sustained load in accelerated chloride corrosion environment and then loaded to failure. The main test parameters include load ratio (ratio between the long-term sustained load and the ultimate capacity of the specimen under short-term loading) and corrosion depth. The failure modes, full-range load-displacement relationship, load-strain relationship and ductility were measured and compared. Based on the experimental observation, mechanism of CFST beam-column members under the combined effects of sustained load and chloride corrosion was discussed and compared to the reference hollow steel tubes. Traditional design methods were modified for the consideration of combined sustained load and corrosion, which were calibrated and evaluated against the test results.

Experimental assessment of the cyclic behaviour of concrete-filled steel tubular beam-columns with octagonal sections

This paper presents an experimental investigation on concrete-filled steel tubular beam-columns with octagonal cross-sections (OCFST) under cyclic lateral loading with or without axial load. A total of nine specimens with different parameters, including the axial load level (n) ranging from 0 to 0.5 and concrete compressive cylinder strength (fc′) varying from 30 MPa to 90 MPa, were tested. The failure modes, ultimate strengths, displacement ductility, effective flexural stiffness and cumulative dissipated energy are presented. The results indicate that OCFST beam-columns exhibit a ductile plastic mode and excellent energy dissipation. With the increase of axial load level, the ductility and the energy dissipation capacity decreases, while the ultimate bending capacity firstly increases then drops. Concrete grades seem to have limited influence on the ultimate strength and energy dissipation capacity. The comparison results of the ultimate bending moments and effective flexural stiffness between predictions using EN 1994-1-1 and AISC 360-16 and test results reveal their applicability to the design of OCFST beam-columns.

Rectangular concrete-filled steel tubular beam-columns using high-strength steel: Experiments and design

Concrete-filled steel tubular (CFT) columns and high-strength steel have been increasingly used in construction. However, the application of high-strength steel in CFT columns has not been permitted in many design codes. This paper reports an experimental investigation on the behavior of rectangular CFT beam-columns using high-strength steel. The influences of the in-fill concrete, eccentricity ratio and the width-to-thickness ratio on the resistance of the test columns are discussed. The contact behavior is studied using the finite element analysis. Chinese code DB 29-57 adopts the assumption that the concrete bears the axial load only, and the steel tube withstands the bending moment and part of the axial load. A new N-M interaction approach is developed based on the foresaid assumption to account for the plastic behavior of high-strength steel. The approach is verified by the experimental results and the available current design codes to be reasonably conservative, and it can be employed to design rectangular CFT beam-columns using high-strength steel.

Numerical modeling on post-local buckling behavior of circular and square concrete-filled steel tubular beam columns

In concrete-filled steel tubular (CFST) beam-columns under cyclic loadings, local buckling of restrained steel tube, crushing and slipping of the confined concrete could have significant influences on strength and stiffness degradation. This paper presents a numerical model based on fiber element discretization for simulating the deterioration behaviour of both rectangular and circular CFST beam columns under axial load and cyclic loading. Uniaxial stress-strain relationships for steel and concrete in CFST beam columns are proposed by introducing unified parameters to control the extent of deterioration. Axial contraction caused by local buckling was accounted by defining an inelastic region that promises plasticity and strength degradation. The numerical model was validated by comparing the simulation results and the associated experimental results. The influences of diameter-to-thickness ratio, concrete and steel strength ranging from normal strength to high strength on the deterioration behavior of circular and rectangular CFST beam-columns were analyzed. Finally, through recognizing the limit state to trigger global strength degradation of a CFST frame, the developed numerical model is applicable for simulating global behavior of CFST frames under collapse-level deformations.

Behavior of thin-walled dodecagonal section double skin concrete-filled steel tubular beam-columns

The behavior of thin-walled dodecagonal section double skin concrete-filled steel tubular beam-columns was investigated in this paper. A series of tests were carried out on specimens having the length of 2000mm. Both outer and inner tubes of test specimens were cold-formed steel dodecagonal hollow sections (DHS). The diameters of outer and inner steel tubes of test specimens were 400mm and 240mm, respectively. Each test specimen was tested to failure, and the behavior under loading was recorded. Finite element model was developed using ABAQUS and verified against the test results. Parametric study was conducted using the verified finite element model. The suitability of current design methods was evaluated.

High strength thin-walled rectangular concrete-filled steel tubular slender beam-columns, Part I: Modeling

High strength thin-walled rectangular concrete-filled steel tubular (CFST) slender beam-columns under eccentric loading may undergo local and overall buckling. The modeling of the interaction between local and overall buckling is highly complicated. There is relatively little numerical study on the interaction buckling of high strength thin-walled rectangular CFST slender beam-columns. This paper presents a new numerical model for simulating the nonlinear inelastic behavior of uniaxially loaded high strength thin-walled rectangular CFST slender beam-columns with local buckling effects. The cross-section strengths of CFST beam-columns are modeled using the fiber element method. The progressive local and post-local buckling of thin steel tube walls under stress gradients is simulated by gradually redistributing normal stresses within the steel tube walls. New efficient Müller's method algorithms are developed to iterate the neutral axis depth in the cross-sectional analysis and to adjust the curvature at the columns ends in the axial load–moment interaction strength analysis of a slender beam-column to satisfy equilibrium conditions. Analysis procedures for determining the load–deflection and axial load–moment interaction curves for high strength thin-walled rectangular CFST slender beam-columns incorporating progressive local bucking and initial geometric imperfections are presented. The new numerical model developed is shown to be efficient for predicting axial load–deflection and axial load–moment interaction curves for high strength thin-walled rectangular CFST slender beam-columns. The verification of the numerical model and parametric studies is given in a companion paper.

Strength and ductility of high strength concrete-filled steel tubular beam–columns

The ultimate strength and ductility of high strength thin-walled concrete-filled steel tubular (CFST) beam–columns with local buckling effects, are investigated in this paper, using a performance-based analysis (PBA) technique. The PBA technique accounts for the effects of geometric imperfections, residual stresses, strain hardening, local buckling and concrete confinement on the behavior of high strength thin-walled CFST beam–columns. The accuracy of the PBA technique is further examined by comparisons with experimental results. The PBA program is employed to study the effects of depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility of high strength thin-walled CFST beam–columns under combined axial load and biaxial bending. The results obtained indicate that increasing the depth-to-thickness ratio and axial load levels significantly reduces the stiffness, strength and ductility of CFST beam–columns. Increasing concrete compressive strengths increases the stiffness and strength, but reduces the axial ductility and section performance of CFST beam–columns. Moreover, the steel yield strength has a significant effect on the section and strength performance of CFST beam–columns but does not have a significant effect on their axial and curvature ductility.

Performance-based analysis of concrete-filled steel tubular beam–columns, Part II: Verification and applications

The theory and algorithms of a performance-based analysis (PBA) technique for the nonlinear analysis and performance-based design of thin-walled concrete-filled steel tubular (CFST) beam–columns with local buckling effects were presented in a companion paper. Initial local buckling and effective strength/width formulas for steel plates are incorporated in the PBA program to account for local buckling effects. Performance indices are used in the PBA program to quantify the section, axial ductility and curvature ductility performance of thin-walled CFST beam–columns. This paper presents the verification and applications of the PBA program developed. The axial load–strain curves, ultimate axial loads and moment–curvature curves for thin-walled CFST columns predicted by the PBA program are verified by experimental data. The PBA program is then utilized to investigate the effects of local buckling, depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility performance of thin-walled CFST beam–columns under axial load and biaxial bending. The PBA technique developed is shown to be efficient and accurate and can be used directly in the performance-based design of thin-walled CFST beam–columns and implemented in advanced analysis programs for composite columns and frames.

Performance-based analysis of concrete-filled steel tubular beam–columns, Part I: Theory and algorithms

This paper presents a performance-based analysis (PBA) technique based on fiber element formulations for the nonlinear analysis and performance-based design of thin-walled concrete-filled steel tubular (CFST) beam–columns with local buckling effects. Geometric imperfections, residual stresses and strain hardening of steel tubes and confined concrete models are considered in the PBA technique. Initial local buckling and effective strength/width formulas are incorporated in the PBA program to account for local buckling effects. The progressive local buckling of a thin-walled steel tube filled with concrete is simulated by gradually redistributing normal stresses within the steel tube walls. Performance indices are proposed to quantify the section, axial ductility and curvature ductility performance of thin-walled CFST beam–columns under axial load and biaxial bending. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis in a thin-walled CFST beam–column section to satisfy equilibrium conditions. The analysis algorithms for thin-walled CFST beam–columns under axial load and uni- and biaxial bending are presented. The PBA program can efficiently generate axial load–strain curves, moment–curvature curves and axial load–moment strength interaction diagrams for thin-walled CFST beam–columns under biaxial loads. The proposed PBA technique allows the designer to analyze and design thin-walled CFST beam–columns made of compact or non-compact steel tubes with any strength grades and normal and high-strength concrete. The verification and applications of the PBA program are given in a companion paper.

Seismic behaviour of square CFT beam–columns under biaxial bending moment

This paper investigates the behaviour of square concrete-filled steel tubular (CFT) beam–columns subjected to biaxial moment. Nine tests on beam–columns are reported here under a combined loading of constant axial load and cyclic lateral load applied at varying angles to the axis of the cross-section, referred to as ‘diagonal’ loading. The specimens were prepared in order to evaluate the influence of different parameters on the overall structural response, their ductility and their energy dissipation ability; the parameters included the effects of axial load ratio, width-to-thickness ratio, concrete compressive strength, slenderness ratio and load angle on the moment strength. The experimental results indicate that the ductility and energy dissipation ability of biaxially loaded square CFT columns decrease with increasing the axial load ratio. Their ductility and energy dissipation ability was also observed to decrease as the concrete compressive strength increased while the ductility was barely affected by the load angle. An increase in the load angle of biaxially bent square CFT beam–columns led to a slight decrease of the moment strength. Both EC4 and AIJ code provisions were shown to predict with reasonable accuracy the moment strength capacity observed in the tests, while the ACI-predicted moment strength gave to slightly conservative values. On the other hand, the LRFD code provisions greatly underestimated their moment strength.

Nonlinear analysis of short concrete-filled steel tubular beam–columns under axial load and biaxial bending

This paper presents a nonlinear fiber element analysis method for determining the axial load–moment strength interaction diagrams for short concrete-filled steel tubular (CFST) beam–columns under axial load and biaxial bending. Nonlinear constitutive models for confined concrete and structural steel are considered in the fiber element analysis. Efficient secant algorithms are developed to iterate the depth and orientation of the neutral axis in a composite section to satisfy equilibrium conditions. The accuracy of the fiber element analysis program is verified by comparisons of fiber analysis results with experimental data and existing solutions. The fiber element analysis program developed is employed to study the effects of steel ratios, concrete compressive strengths and steel yield strengths on axial load–moment interaction diagrams and the C -ratio of CFST beam–columns. The proposed fiber element analysis technique is shown to be efficient and accurate and can be used directly in the design of CFST beam–columns and implemented in advanced analysis programs for the nonlinear analysis of composite columns and frames.

Cyclic performance of fire-damaged concrete-filled steel tubular beam–columns repaired with CFRP wraps

This paper presents the results of cyclic tests of fire-damaged concrete-filled steel tubular (CFST) beam–columns repaired by unidirectional carbon fibre reinforced polymer (CFRP) composites. The test parameters included the cross-section type (circular and square), the number of CFRP layers (1 and 2) and the axial load level (0–0.78). Based on the experimental measurements, the repair effects of CFRP wraps on the ultimate lateral strength, flexural stiffness, dissipated energy and ductility of the beam–columns are studied. The test results showed that the ultimate lateral strength, flexural stiffness and ductility can be enhanced to some extent due to the CFRP confinement effect. However, the strength and stiffness of all damaged specimens have not been fully restored in the current tests. Since there exist several possible solutions for the repair/strengthening of CFST members to restore/enhance their strength or stiffness, it seems that an appropriate evaluation method should be developed to make a suitable selection among them based on future comparative study.

Behaviour of fire-exposed concrete-filled steel tubular beam columns repaired with CFRP wraps

This paper presents the test results of fire-exposed concrete-filled steel tubular (CFST) beam columns repaired by unidirectional carbon fibre reinforced polymer (CFRP) composites. Both circular and square specimens were tested to investigate the repair effects of CFRP composites on them. The test results showed that the load-bearing capacity of the fire-exposed CFST beam columns was enhanced by the CFRP jackets to some extent, while the influence of CFRP repair on stiffness was not apparent. The strength enhancement from CFRP confinement decreased with the increasing of eccentricity or slenderness ratio. Ductility enhancement was also observed except those axially loaded shorter specimens with rupture of CFRP jackets at the mid-height, occurred near the peak loads. However, the strengths of all repaired specimens have not been fully restored due to the long exposure time of them in fire. From the test results, it is recommended that, in repairing severely fire-damaged CFST members, slender members or those members subjected to comparatively large bending moments, other appropriate repair measures should be taken.

Local buckling of steel plates in concrete-filled thin-walled steel tubular beam–columns

The availability of high strength steels and concrete leads to the use of thin steel plates in concrete-filled steel tubular beam–columns. However, the use of thin steel plates in composite beam–columns gives a rise to local buckling that would appreciably reduce the strength and ductility performance of the members. This paper studies the critical local and post-local buckling behavior of steel plates in concrete-filled thin-walled steel tubular beam–columns by using the finite element analysis method. Geometric and material nonlinear analyses are performed to investigate the critical local and post-local buckling strengths of steel plates under compression and in-plane bending. Initial geometric imperfections and residual stresses presented in steel plates, material yielding and strain hardening are taken into account in the nonlinear analysis. Based on the results obtained from the nonlinear finite element analyses, a set of design formulas are proposed for determining the critical local buckling and ultimate strengths of steel plates in concrete-filled steel tubular beam–columns. In addition, effective width formulas are developed for the ultimate strength design of clamped steel plates under non-uniform compression. The accuracy of the proposed design formulas is established by comparisons with available solutions. The proposed design formulas can be used directly in the design of composite beam–columns and adopted in the advanced analysis of concrete-filled thin-walled steel tubular beam–columns to account for local buckling effects.

コンクリート充填鋼管柱の構造性能に関する調査研究

Concrete filled steel tubular (CFT) columns have become popular as structural member used for buildings, moreover, a large number of beam-column specimens have been tested. CFT structures are designed by either of the two standards, now. The superposed strength method is basically adopted for the design formulas in both of these standards, only considering the equilibrium of force. However, the lateral force-thrust-rotation angle relationships are not clear. Therefore, in this study, the structural performance of CFT columns have been examined based on the review of existing studies. As a result, the yield strength, the ultimate strength and the lateral force-thrust-rotation angle relationships of the CFT beam-columns have been clarified based on the existing studies.

Simplified design formula of slender concrete filled steel tubular beam-columns

The objective of this paper is to develop a simplified method that could predict the strength of concrete filled steel tube (CFT) columns applicable to high strength material under combined axial compression and flexure. The simplified method for determining the strength of CFT columns is based on the interaction curve of the section approached by a polygonal connection of the points. These points are determined by using symmetrical properties of the CFT section. For each point, a simple equation is proposed to determine the strength of the slender columns under compression and flexure. The simple equation was adjusted with results of elasto-plastic analysis results. Validation of the simplified method is undertaken by comparison with data from the test conducted at Kyushu University. These results confirm the fact that the simplified method could accurately and reliably predict the strength of CFT columns under combined axial compression and flexure.