Circular Concrete-filled Steel Tube Research Articles

Experimental study on circular CFST short columns with intermittently welded stiffeners

This paper deals with the experimental study on strength the strength and deformation characteristics of short circular Concrete Filled Steel Tube (CFST) columns. Effect of vertical stiffeners on the behavior of the column is studied under axial compressive loading. Intermittently welded vertical stiffeners are used to strengthen the tubes. Stiffeners are attached to the inner surface of tube by welding through pre drilled holes on the tube. The variable of the study is the spacing of the weld between stiffeners and circular tube. A total of 5 specimens with different weld spacing (60 mm, 75 mm, 100 mm, 150 mm and 350 mm) were prepared and tested. Short CFST columns of height 350 mm, outer tube diameter of 165 mm and thickness of 4.5 mm were used in the study. Concrete of cube compressive strength 41 N/mm2 and steel tubes with yield strength 310 N/mm2 are adopted. The test results indicate that the strength and deformation of the circular CFST column is found to be significantly influenced by the weld spacing. The ultimate axial load carrying capacity was found to increase by 11% when the spacing of weld is reduced from 350 mm to 60 mm. The vertical stiffeners are found to effective in enhancing the initial stiffness and ductility of CFST columns. The prediction models were developed for strength and deformation of CFST columns. The prediction is found to be in good agreement with the corresponding test data.

Axial Behaviour of Slender RC Circular Columns Strengthened with Circular CFST Jackets

This paper investigates the axial behavior of slender reinforced concrete (RC) columns strengthened with concrete filled steel tube (CFST) jacketing technique. It is realized by pouring self‐compacting concrete (SCC) into the gap between inner original slender RC columns and outer steel tubes. Nine specimens were prepared and tested to failure under axial compression: a control specimen without strengthening and eight specimens with heights ranging between 1240 and 2140 mm strengthened with CFST jacketing. Experimental variables included four different length‐to‐diameter (L/D) ratios, three different diameter‐to‐thickness (D/t) ratios, and three different SCC strengths. The experimental results showed that the outer steel tube provided confinement to the SCC and original slender RC columns and thus effectively improved the behavior of slender RC columns. The failure mode of slender RC columns was changed from brittle failure (concrete peel‐off) into ductile failure (global bending) after strengthening. And, the load‐bearing capacity, material utilization, and ductility of slender RC columns were significantly enhanced. The strengthening effect of CFST jacketing decreased with the increase of L/D ratio and D/t ratio but showed little variation with higher SCC strength. An existing expression of load‐bearing capacity for traditional CFST columns was extended to propose a formula for the load‐bearing capacity of CFST jacketed columns, and the predictions showed good agreement with the experimental results.

Experimental study of the cyclic behavior of concrete-filled double skin steel tube columns subjected to pure torsion

Based on a quasi-static test on six concrete-filled double skin steel tube columns subject to cyclic pure torsion, the torsion behavior of concrete-filled double skin steel tube columns with various section types, hollow ratios and steel ratios was studied. The failure modes, torsion-rotation angle hysteretic curves and strain distribution law of concrete-filled double skin steel tube columns under reversed cyclic loading are obtained. The test results show that the hysteretic curves of concrete-filled double skin steel tube columns under pure torsion are plump. The unloading stiffness was close to the initial elastic stiffness. The good energy dissipation capacity of concrete-filled double skin steel tube columns can be observed. Under a cyclic torsion moment, the torsion-resistance capacity of circular concrete-filled double skin steel tube columns was better than that of square and rectangular concrete-filled double skin steel tube columns. With the same thickness of the inner steel tube and the same section size and shape, the hollow ratio of concrete-filled double skin steel tube columns has little effect on the cyclic torsion behavior.

Experimental study on axially compressed circular CFST columns with improved confinement effect

The concrete-filled-steel-tube (CFST) column has been widely used in construction due to the benefit of composite action between inner concrete and exterior steel tube. Under axial compression, the steel tube is subjected to biaxial stress state in the hoop and axial direction, and the hoop stress component provides confinement for the inner concrete. However, the existence of axial stress component accelerates the buckling of steel tube, which hinders its confinement effect to inner concrete and accordingly column's axial compressive strength and ductility. This paper aims to enhance the axial compressive strength and ductility of CFST stub columns by improving its confinement effect provided by the steel tube. CFST stub columns axially loaded/unloaded were experimentally studied for different cases: a) varying concrete strength, b) lubrication on the contact between steel tube and concrete and c) corrugations in the steel tube that are introduced to intentionally weaken the tube vertically. It was found that axial compressive strength of CFST stub columns was effectively enhanced by reducing the axial stress in the steel tube. The introduction of corrugations in the steel tube led to largely concentrated axial deformation which reduced axial stress in other portions of the steel tube and consequently resulted in “tighter” hoop confinement to the inner concrete; both axial compressive strength and ductility were enhanced.

Investigation on the fiber based approach to estimate the axial load carrying capacity of the circular concrete filled steel tube (CFST)

External confining devices are often used to enhance the strength and ductility of reinforced concrete columns. Among the available external confining devices, steel tube is one of the most widely used in construction. However, steel tube has some drawbacks such as local buckling which needs to be considered when estimating the axial load carrying capacity of the concrete-filled-steel-tube (CFST) column. To tackle this problem in design, Eurocode 4 provided guidelines to estimate the effective yield strength of the steel tube material. To study the behavior of CFST column, in this paper, a non-linear analysis using a fiber-based approach was conducted. The use of the fiber-based approach allows the engineers to predict not only the axial load carrying capacity but also the complete load-deformation curve of the CFST columns for a known confining pressure. In the proposed fiber-based approach, an inverse analysis is used to estimate the constant confining pressure similar to design-oriented models. This paper also presents comparisons between the fiber-based approach model with the experimental results and the 3D non-linear finite element analysis.

Compressive behavior of post-heated circular CFST short columns externally strengthened with CFRP sheets

Due to the increasing usage of concrete-filled steel tube (CFST) members in structural engineering, there will be more chances of fire hazards on these structures in the near future. Externally bonding carbon fiber reinforced polymer (CFRP) composites has emerged as a popular method for repairing damaged steel-reinforced concrete members. However, limited research is available to evaluate the behavior of CFRP strengthened CFST members. This paper presents the results of an experimental investigation on the compressive behavior of the post-heated circular CFST short columns externally strengthened with CFRP sheets. A total of twenty-one specimens are tested to investigate the influence of temperature and the number of CFRP layers on the mechanical behavior of repaired specimens. The ultimate strength is obtained from monotone static tests. The extensometer technique based on gauge strain is used to measure strains of electric gauges glued to the external surface of specimens. The results indicate that the increase in the number of CFRP layers leads to a significant change in the mechanical properties of post-heated CFST columns. Furthermore, it is shown that increases in the number of CFRP layers remarkably enhance the ultimate strength and initial stiffness of specimens subjected to the same heat treatments, while deteriorate the ductility. Based on extensive experimental analysis, simplified formulae are proposed to estimate the compressive ultimate strength of all specimens tested, providing reasonably good correlation with the experimental results. Besides, the proposed formulae are compared with some existing empirical models, and validity of the proposed formulae is evaluated.

Size effect in circular concrete-filled steel tubes with different diameter-to-thickness ratios under axial compression

Axial compression tests of circular concrete-filled steel tubes with different diameters (219mm, 426mm, and 630mm) and ratios of tube diameter to steel thickness (55 and 88) were conducted to investigate the effect of size on the bearing capacity. The experimental results indicated that the peak nominal stress decreased as the size increased, and the decrease in the nominal stress due to the size effect increased at higher ratios of diameter to thickness. At the peak load moment, an increased specimen diameter corresponded to a decreased hoop stress in the steel tube as well as a decreased concrete strength due to the confinement effect of the steel tube. When the ratio of diameter to thickness increased, the extent of reduction of the hoop stress and the confining effect of the steel tube influenced by the increasing specimen size increased. However, the vertical stress in the steel tube was increased at increased size, and increases in the ratio of diameter to thickness improved the increase degree of the vertical stress of steel tube due to the enlargement of specimen size. Hence, the vertical bearing capacity of the steel tube was affected by both the specimen size and the ratio of diameter to thickness. Based on the size effect law (SEL) proposed by Bazant, and taken the effect of the ratio of diameter to thickness into consideration, a size-dependent formula to evaluate hoop stress in the steel tube was developed. A size-related model considering situations with different ratios of diameter to thickness was established in order to estimate the bearing capacity of large-size circular concrete-filled steel tubes. The model and experimental results showed good agreement.

An analysis-based model for axially loaded circular CFST columns

Concrete-filled-steel-tube (CFST) columns have been increasingly adopted in many modern structures due to the beneficial composite action between steel tube and core concrete. Previous experimental and theoretical studies have proved this composite action can be further improved by providing external confinement in the form of steel rings, tie bars, spirals or jackets. A theoretical model developed by the authors previously based on (1) an accurate hoop strain equation; (2) an actively confined concrete model; (3) a three-dimensional steel model; (4) Interaction of core concrete, steel tube and external confinement was adopted in this paper to conduct a parametric study, which aims at investigating the effects of steel tube yield strength, concrete compressive strength, steel ratio and external confinement on the uni-axial behaviour of CFST columns. From the results in parametric study, two sets of critical steel ratios based on two different levels of ductility at different steel yield and concrete compressive strengths have been obtained. In addition, the maximum column strength obtained from the model and previous experimental studies was compared with the design strength calculated using different design codes. New design equations that can predict the axial strength more satisfactorily have also been proposed.

Circular Concrete-Filled Steel Tubes Subjected to Coupled Tension and Chloride Corrosion

AbstractThis paper investigates the time-dependent behavior of concrete-filled steel tubular (CFST) tensile members subjected to coupled long-term loading and chloride corrosion. A series of experi...

Inelastic behavior of circular concrete-filled steel tubes: monotonic versus cyclic response

In this paper, a computational procedure for determining the response of circular concrete-filled steel tube (CFT) columns to monotonic loading is developed. Firstly, the basis of the proposed method is established by creating accurate three-dimensional nonlinear finite element models of these columns, which are validated by comparing their response results with those of experimental tests that are available in the literature. Then, these models are used for an extensive parametric study that determines the response to monotonic lateral loads of 192 circular CFT specimens with fairly broad values of diameter-to-thickness ratios, yield stress of steel tube, compressive strength of concrete core and axial load levels. On the basis of this response databank, empirical expressions are developed to estimate the force–displacement behavior of circular CFT columns under monotonic loading with simple yet reliable manner. The validity of these empirical expressions is verified by comparing their results with those of experimental tests. It is found that the proposed expressions can effectively describe the force–displacement behavior and capacity of circular CFT columns under monotonic loading conditions. These expressions are also used in pushover analysis of a simple frame consisting of steel beams and CFT columns in order to demonstrate their applicability and usefulness in simple practical problems. Finally, the proposed method, of determining the response of composite structures to monotonic lateral loading is compared with nonlinear time-history response analysis of these structures and useful conclusions are provided.

Finite element analysis of concrete-filled steel tube (CFST) columns with circular sections under eccentric load

The casting of concrete in concrete-filled steel tube (CFST) can, via the confinement effect of the steel tube, significantly increase the ductility of the concrete and, in the case of high-strength concrete (HSC), alleviate the shortfall in ductility of the HSC. This kind of structure is gaining popularity but its behaviour is quite complicated. In an axially loaded circular CFST column, the confinement is uniform and equi-biaxial (isotropic within the cross-section). But, when the circular CFST column is under eccentric load, the confinement becomes non-uniform and anisotropic. Such complicated confinement effect is not easy to analyse and for such analysis, a rigorous finite element (FE) method is generally needed. In this paper, a new FE model considering the lateral strain-axial strain relation of the confined concrete covering the full range from the initial elastic stage to the inelastic stage is developed for the analysis of circular CFST columns under eccentric load. The FE model is used to analyse a total of 95 CFST specimens tested by other researchers and the numerical results are compared to the published test results for verification.

Design Research and Application of D-shape Expansive Concrete-filled Steel Tube Supports in Roadway Support

In order to perfect the application of concrete-filled steel tube support (CFSTS) in roadway support, aiming at the shortcomings of the existing common CFSTS, this paper put forward the D-shape expansive concrete-filled steel tube support (D-ECFSTS). Firstly, this article uses the numerical simulation to carry on the optimization of circular steel tube, designs the D-shape steel tube, and carries on the comparative analysis of D-shape concrete-filled steel tube support (D-CFSTS) and circular concrete-filled steel tube support (CFSTS). Then, the scheme of expansive concrete is put forward, and the ratio of expansive concrete is tested by the test method. Finally, the D-ECFSTS was made by D-CFSTS and expansive concrete, then used in roadway support. The results show: The deformation of D-CFSTS is much smaller than that of circular CFSTS under the same load; The best ratio of expansive concrete includes that the ratio of cement, sand, gravel is 1:1.31:2.65, water cement ratio is 0.41, expansion agent ratio is 11% and water-reducing agent ratio is 0.5%; In the support of Pingdingshan coal mine roadway, the supporting effect of D-ECFSTS is much better than that of U36 support. So, D-ECFSTS is a good roadway support method.

Multi-physical field guided wave simulation for circular concrete-filled steel tubes coupled with piezoelectric patches considering debonding defects

The active interface debonding detection for concrete-filled steel tubes (CFSTs) using Piezoelectric Lead Zirconate Titanate (PZT) patches has been proposed and experimentally studied in recent years. In order to further investigate the guided wave propagation and the response of embedded PZT sensor, transient dynamic analysis on multi-physical field coupling models composed of surface-mounted PZT actuator, embedded PZT sensor and a circular CFST specimen considering the coupling effect between the PZT patches and CFST member and the influence of interface debonding defects is carried out numerically. The guided wave propagation within the CFST-PZT coupling system without and with interfacial debonding defects under single period impulse signal is simulated and compared. The voltage response time history of the embedded PZT sensor in coupling models under both sinusoidal and sweep frequency sinusoidal excitations are simulated as an alternative solver. Numerical results demonstrate that interface debonding leads to changes in guided wave propagation path, traveling time and the PZT sensor response. The sensitivity of a quantitative evaluation index called as the normalized wavelet packet energy of the output voltage of embedded PZT sensor to the initiation of interface debonding defect and its length and depth is illustrated.

Concrete-Filled Circular Steel Tubes with a Timber Infill under Axial Compression

Over the past two decades, there has been significant interest in research relating to concrete-filled tubes, and a corresponding penetration of this technology into practice. This paper aims to expound upon the effect of timber cores on the structural response of concrete-filled circular tubes under compression. A timber infill with different shapes and geometries surrounded by concrete and encased in a steel tube was employed. The effects of the combination of infill elements on the failure, axial capacity, ductility, and structural efficiency (weight versus capacity) are exhaustively set forth. For the specimens with the highest timber to concrete ratio, the capacity was enhanced by about two times the capacity of the hollow steel specimens. For these specimens a significant reduction in the total weight of the composite element was obtained relative to the fully concrete-filled specimens. These specimens showed the highest ductility among the other specimens. In addition, greater ratios of energy absorption to the mass were obtained for the specimens with different timber cores in comparison to the equivalent values for fully concrete-filled tubes, which is quite desirable in many practical scenarios. It is found that the use of timber as an inner core element in this new composite yields promising results in decreasing the weight and yet enhancing the capacity, ductility, and energy absorption, and can be a good alternative to double-skin concrete-filled steel tubes.

Wave propagation simulation and its wavelet package analysis for debonding detection of circular CFST members

In order to investigate the interface debonding defects detection mechanism between steel tube and concrete core of concrete-filled steel tubes (CFSTs), multi-physical fields coupling finite element models constituted of a surface mounted Piezoceramic Lead Zirconate Titanate (PZT) actuator, an embedded PZT sensor and a circular cross section of CFST column are established. The stress wave initiation and propagation induced by the PZT actuator under sinusoidal and sweep frequency excitations are simulated with a two dimensional (2D) plain strain analysis and the difference of stress wave fields close to the interface debonding defect and within the cross section of the CFST members without and with debonding defects are compared in time domain. The linearity and stability of the embedded PZT response under sinusoidal signals with different frequencies and amplitudes are validated. The relationship between the amplitudes of stress wave and the measurement distances in a healthy CFST cross section is also studied. Meanwhile, the responses of PZT sensor under both sinusoidal and sweep frequency excitations are compared and the influence of debonding defect depth and length on the output voltage is also illustrated. The results show the output voltage signal amplitude and head wave arriving time are affected significantly by debonding defects. Moreover, the measurement of PZT sensor is sensitive to the initiation of interface debonding defects. Furthermore, wavelet packet analysis on the voltage signal under sweep frequency excitations is carried out and a normalized wavelet packet energy index (NWPEI) is defined to identify the interfacial debonding. The value of NWPEI attenuates with the increase in the dimension of debonding defects. The results help understand the debonding defects detection mechanism for circular CFST members with PZT technique.

Analytical model and design formulae of circular CFSTs under axial tension

A circular concrete-filled steel tube (CCFST) component is subjected to tension in certain practical engineering projects. Several past experimental programs have stressed that the apparent strength and stiffness enhancement phenomena observed in tensile circular steel tubes can be attributable to the filled concrete. In this study, an analytical model for describing the strength and stiffness enhancement of the circular steel tubes is developed. The interface performance between the steel tube and filled concrete is considered in the model. Based on the proposed analytical model, simplified design formulae for strength and stiffness enhancement coefficients are proposed. These design formulae indicate that the confining and fibre-reinforcing effects lead to the strength improvement, while the confining effect, fibre-reinforcing effect, and tension-stiffening effects all contribute to the stiffness enhancement. Then, a design procedure for determining the axial tensile strength and stiffness of CCFSTs is provided, and a design example is also demonstrated. The accuracy of both the proposed analytical model and design formulae are validated by the results of two groups of axial tension tests, which demonstrates that the full-process load-strain behaviour can be accurately captured by a bilinear model based on the proposed analytical formulae. Finally, analytical solutions for the longitudinal stress distributions of the steel tube and filled concrete are provided for a better understanding of the tension-stiffening effect. In addition, the contributions of different effects to strength and stiffness enhancements are quantitatively evaluated for all the test specimens.

Experimental study on confining-strengthening, confining-stiffening, and fractal cracking of circular concrete filled steel tubes under axial tension

This paper studies the mechanical behavior of the circular concrete filled steel tubes (CCFTs) under axial tension. A group of three CCFTs with different steel ratios and in-filled concrete diameters are tested and compared with hollow steel tubes (HTs). Based on the experimental results in the present and previous studies, three key issues of CCFTs under tension, i.e. the confining-strengthening effect, the confining-stiffening effect, and the tension-stiffening effect, are studied at both the stress-resultant level (i.e., force-displacement relationship) and stress levels (i.e., stress-strain relationship). Due to the tendency of the shrinking and slipping of the steel tube in relation to the infilled concrete, the confining contact stress and bonding shear stress, in the vertical and tangential direction respectively, are distributed in the interface between the steel and concrete, which results in the confining and bonding effect of the CCFTs. At the stress-resultant level, the test results demonstrate that the strength and stiffness of CCFTs are average 10% and 29% larger than those of HTs, respectively. The strength increase is contributed by the confining-strengthening effect, and the stiffness enhancement is a combined result of the confining-stiffening and tension-stiffening effects. At the stress-strain level, the measured strains show that the steel tube of the CCFT is in the state of the bi-axial tensile stress state because of the difference of the Poisson's ratio between steel tube and infilled concrete, which contributes to the confining-strengthening and confining-stiffening effects of the CCFTs. In addition, a fractal cracking phenomenon of the core concrete of tensile CCFTs is observed and discussed.

ANN Model for Predicting the Compressive Strength of Circular Steel-Confined Concrete

Concrete filled steel tube is constructed using various tube shapes to obtain most efficient properties of concrete core and steel tube. The compressive strength of concrete is considerably increased by the lateral confined steel tube in circular concrete filled steel tube (CCFT). The aim of this study is to present an integrated approach for predicting the steel-confined compressive strength of concrete in CCFT columns under axial loading based on large number of experimental data using artificial neural networks. Neural networks process information in a similar way that the human brain does. Neural networks learn by example. The main parameters investigated in this study include the compressive strength of unconfined concrete (\(f_{\text{c}}^{'}\)), outer diameter (D) and length (L) of column, wall thickness (t) and tensile yield stress (\(F_{\text{y}}\)) of steel tube. Subsequently, using the idealized network, empirical equations are developed for the confinement effect. The results of proposed model are compared with those of existing models on the basis of the experimental results. The findings indicate the precision and efficiency of ANN model for predicting the capacity of CCFT columns.

Stress-transfer in concrete encased and filled tube square columns employed in top-down construction

Top-down construction is a construction technique in which pit excavation and structure construction are conducted simultaneously. Reducing construction time and minimizing noise and vibration which affect neighboring structures, the technique is widely employed in constructing downtown structures. While H-steel columns have been commonly used as core columns, concrete filled steel tube (CFT) columns are at the center of attention because the latter have less axial directionality and greater cross-sectional efficiency than the former. When compared with circular CFT columns, square CFT columns are more easily connected to the floor structure and the area of percussion rotary drilling (PRD) is smaller. For this reason, square CFT columns are used as core columns of concrete encased and filled square (CET) columns in underground floors. However, studies on the structural behavior and concrete stress transfer of CET columns have not been conducted. Since concrete is cast according to construction sequence, checking the stress of concrete inside the core columns and the stress of covering concrete is essential. This paper presents the results of structural tests and analyses conducted to evaluate the usability and safety of CET columns in top-down construction where CFT columns are used as core columns. Parameters in the tests are loading condition, concrete strength and covering depth. The compressive load capacity and failure behavior of specimens are evaluated. In addition, 2 cases of field application of CET columns in underground floors are analyzed.

Behaviour and design of hollow and concrete-filled spiral welded steel tube columns subjected to axial compression

Spiral welded tube (SWT) structures have found worldwide application in pipeline construction, wind turbine towers, foundation piles, and columns in tall buildings. However, the understanding of their fundamental behaviour is still insufficient and efficient analysis and design methods have not been precisely developed owing to the lack of experimental and numerical research on these types of structures. A distinct advantage of SWT is their streamlined manufacturing process, so that today large diameter SWT can be economically produced. Due to the application of SWT as structural members being relatively new, this paper presents an investigation into the behaviour of hollow and concrete-filled steel SWT columns when subjected to axial compressive loading. Parameters of particular interest affecting the strength and failure modes include the weld's spiral geometry and initial imperfections from the production process. To evaluate the behaviour of SWT columns, an accurately developed finite element model (FEM) which incorporates the effects of initial local imperfections and residual stresses using the commercial finite element program ABAQUS has been prepared. The FEM buckling behaviour of SWT is compared with that of longitudinally welded tubes (LWTs). Experimental laboratory testing is carried out on twenty columns under displacement-controlled loading conditions in order to calibrate and verify the accuracy of the model results. Furthermore, a design model is proposed for circular concrete-filled steel tube columns. In addition, comparisons with the prediction of axial load capacity using the proposed design model, Australian Standards, Eurocode, and American Institute of Steel Construction code provisions for hollow and concrete-filled SWT and LWT columns is also carried out.