Concrete-filled Steel Tubular Short Columns Research Articles

Compressive behavior of manufactured sand recycled coarse aggregate concrete-filled steel tubular short columns

To study the mechanical properties of manufactured sand recycled aggregate concrete-filled steel tubular (RACFST) columns, a total of 81 square short RACFST column specimens with different types of manufactured sand and natural sand were created and subjected to axial compression tests. The variables included sand type, core concrete strength, slenderness ratio, and steel tube thickness. The failure mode, axial load-displacement curve, axial compressive capacity, and deformation capacity of the specimens were analyzed in detail. The results indicate that the failure mode of the specimen is generally waist drum-type failure and shear-type failure, and both of them are local buckling. The load-displacement curve of the manufactured sand specimen is similar to that of the natural sand specimen, but the steel tube of the manufactured sand specimen yields later than that of the natural sand specimen. Generally, the limestone manufactured sand specimen has the highest axial compressive capacity and the best ductility ratio, while the pebble manufactured sand specimen has an axial compressive capacity similar to the natural sand specimen but a lower ductility ratio. The effects of concrete compressive strength, slenderness ratio, and steel tube thickness on specimens with manufactured sand are similar to those with natural sand. Based on the results of this experimental study, a formula for calculating the axial compressive capacity of square RACFST columns is provided.

Circular concrete-filled double-skin steel tubular short columns under axial compression: An improved theoretical model and design

Existing theoretical analysis methods for concrete-filled double-skin steel tubular (CFDST) short columns generally ignore the reduction of concrete strength due to brittleness. As a result, these methods cannot accurately capture full range behavior for CFDST columns containing high-strength concrete. By focusing on the reduction effect of concrete strength in CFDST columns due to concrete brittleness, this paper proposes a reduction factor for use in design, for the first time. Based on this reduction factor, a theoretical model for axially compressed circular CFDST short columns considering non-uniform confinement effect of the sandwiched concrete is established. A substantial experimental database of 168 CFDST columns with carbon circular steel tubes is then compiled. By comparing with this database, the proposed theoretical model is shown to achieve greater accuracy than existing approaches. Utilizing this model, the influence of column parameters on the performance of the studied CFDST members are investigated and the results are discussed. It is shown that concrete strength, hollow ratio, yield strength of the outer tube, and the diameter-to-thickness ratio of the outer and inner tubes are influential to the ultimate strength and ductility of CFDST columns. However, the yield strength of the inner tube only affects the ultimate strength of CFDST columns but has a negligible influence on the ductility. Finally, utilizing the experimental and theoretical data presented, the accuracy of existing design codes for concrete-filled steel tubes for the design of CFDST members with or without considering the reduction effect of concrete strength is assessed. The results indicate that Eurocode 4 provides a more reasonable prediction result.

3D numerical study of axial compression behavior of concrete-filled steel tubular columns from mesoscopic perspective

This study investigates the axial compression behavior of Concrete-Filled Steel Tubular (CFST) short columns from a mesoscopic perspective. Concrete is regarded as a three-phase composite material consisting of aggregates, mortar, and interfacial transition zones (ITZs). The Concrete Damaged Plasticity (CDP) model was employed to simulate the nonlinear mechanical response and fracture characteristics of concrete. Verified against experimental results, the numerical model excels in realistically simulating the uniaxial compression behavior of CFST short columns. Subsequently, factors influencing the macroscopic behavior of CFST short columns were investigated. Results indicate that the random aggregate model adopted for concrete enables a more realistic prediction of the behavior of CFST short columns. Additionally, circular section steel tubes demonstrate a stronger confinement effect compared to square ones. With increasing lateral confinement, the properties of CFST short columns, such as bearing capacity and ductility, are significantly enhanced.

An analytical model for axially compressed circular concrete-filled steel tubular (CFST) short columns with varied confinement factors

Accurate evaluation of the load-strain behavior of axially compressed concrete-filled steel tube (CFST) short columns is pivotal for engineering structures, and depends on precise prediction of the hoop-axial strain response. This study introduces a two-stage hoop-axial strain relationship that incorporates the initial Poisson's ratio and elastic limit to account for the initial linearity of concrete, and a reduction coefficient to take into account the effect of concrete strength on lateral deformation properties, based on a widely used hoop-axial equation. Experimental validation across a definite concrete strength range of 28 MPa to 125.3 MPa reveals improved predictions for the dilation rate of CFST columns subjected to axial compression, particularly for those columns fabricated with high-strength concrete (> 100 MPa). Subsequently, an analytical model consisting of the proposed two-stage strain relationship, a confined concrete model with inflection-point strain and residual strength, and the biaxial stress-strain relationship of steel tube is then established to estimate the compressive behavior of circular CFST short columns. Experimental verification over a range of confinements (confinement factor from 0.289 to 2.138) demonstrates the ability of the proposed analytical model to accurately capture the load-strain curves of circular CFST short columns, especially the post-peak behavior of those with confinement factors less than 1. The performance of the established analytical model in assessing the axial load-carrying capacity of circular CFST stub columns is investigated using a database assembled from available studies.

Axial compressive behavior of reinforced hollow circular high strength concrete filled steel tubular short columns

This study aims to investigate the mechanical behavior of reinforced hollow circular high-strength concrete-filled steel tubular (RHCFST) short columns under axial compression. The axial compression test of 6 RHCFST short columns was carried out, and the bearing capacity, ductility and failure modes of the columns were analyzed. The results revealed that the failure modes of RHCFST short columns primarily included mixed failure, shear failure, and crushing failure. Comparatively, the failure modes of specimens with ordinary reinforcement exhibited a shift towards mixed failure, significant improvement in ductility, and a decrease in the increase trend of ultimate bearing capacity with the growth of steel tube thickness. Based on the test results, finite element models were developed using ABAQUS software for parameter analysis. The results demonstrated a positive correlation between the short column’s bearing capacity and parameters such as steel strength, sandwich concrete strength, steel tube thickness, and ordinary reinforcement diameter. Additionally, the initial stiffness of the short column increased with the increase of steel tube thickness. Considering the mechanical properties and engineering applicability of the member, the recommended design of steel tube yield strength, sandwich concrete strength, steel tube thickness and ordinary reinforcement diameter were given. Furthermore, the axial bearing capacity calculation formulae for RHCFST short columns were proposed. The calculated values were found to closely match both experimental and simulated values, with errors falling within 10 %. Consequently, the formulae can offer a reliable prediction of the member’s axial bearing capacity.

Numerical simulation of axially loaded circular concrete-filled steel tubular short columns with localized corrosion

Concrete-filled steel tubular (CFST) columns are widely utilized in structures owing to their cost-effectiveness, high strength, ductility, and proven fire resistance. Despite these advantages, concerns persist regarding the performance of CFST columns with locally corroded steel tubes, impeding their widespread adoption in offshore structures. This research employs three-dimensional finite element (FE) simulations for circular CFST stub columns subjected to axial compression, validated against experimental data. Verification includes full load-displacement histories, ultimate axial strengths, and failure modes. The validated FE models are then employed to assess and compare the structural behavior of CFST columns with various shapes of localized corrosion, analyzing overall load-deformation curves, interaction stress-deformation responses, and composite actions. To acquire critical data for understanding stub column behavior, 81 specimens underwent non-linear finite element analysis, with local corrosion represented by artificial notches in steel tubes. Test parameters, such as notch orientations, lengths, depths, widths, and concrete compressive strength, are varied. Results elucidate the impacts of these parameters on different aspects of stub column responses with corroded steel tubes. Furthermore, the accuracy of existing design models in predicting the ultimate axial strengths of CFST columns with corroded steel tubes is assessed. Finally, through parametric analysis of test results, a practical model is proposed and verified for computing the peak resistance of a CFST stub column with a locally corroded steel tube.

Artificial Neural Network Models for Determining the Load-Bearing Capacity of Eccentrically Compressed Short Concrete-Filled Steel Tubular Columns

Artificial neural networks (ANN) have a great promise in predicting the load-bearing capacity of building structures. The purpose of this work was to develop ANN models to determine the ultimate load of eccentrically compressed concrete-filled steel tubular (CFST) columns of circular cross-sections, which operated on the widest possible range of input parameters. Short columns were considered for which the amount of deflection does not affect the bending moment. A feedforward network was selected as the neural network type. The input parameters of the neural networks were the outer diameter of the columns, the thickness of the pipe wall, the yield strength of steel, the compressive strength of concrete and the relative eccentricity. Artificial neural networks were trained on synthetic data generated based on a theoretical model of the limit equilibrium of CFST columns. Two ANN models were created. When training the first model, the ultimate loads were determined at a given eccentricity of the axial force without taking into account additional random eccentricity. When training the second model, additional random eccentricity was taken into account. The total volume of the training dataset was 179,025 samples. Such a large training dataset size has never been used before. The training dataset covers a wide range of changes in the characteristics of the pipe metal and concrete of the core, pipe diameters and wall thicknesses, as well as eccentricities of the axial force. The trained models are characterized by high mean square error (MSE) scores. The correlation coefficients between the predicted and target values are very close to 1. The ANN models were tested on experimental data for 81 eccentrically compressed samples presented in five different works and 265 centrally compressed samples presented in twenty-six papers.

Numerical investigation and design of UHPC-encased CFST stub columns under axial compression

Reinforced ultra-high-performance concrete (RUHPC)-encased concrete-filled steel tubular (CFST) columns in composite construction have exceptional fire and corrosion resistance, superior load-bearing capacity, and enhanced ductility compared to standalone RUHPC and CFST columns. However, research on their behavior is limited. This paper presents a 3D finite element (FE) simulation model for accurately predicting the behavior of axially loaded RUHPC-encased CFST short columns. The model accounts for the effects of the concrete confinement provided by the steel reinforcement and circular steel tube. The existing experimental data is used to validate the FE model. The validated model is then utilized to investigate the performance of RUHPC-encased CFST columns with various geometric and material properties. The range analysis technique is applied to an orthogonal design to assess the relative significance of the factors affecting the structural behavior. The findings reveal that RUHPC-encased CFST columns have high ultimate strength and ductility. The range analysis identifies the importance order of parameters as: the compressive strength of UHPC, the steel fiber concentration, tube diameter, stirrup spacing, tube thickness, and confined concrete’s compressive strength. Lastly, a simplified design model is developed to estimate the compressive capacity of RUHPC-encased CFST stub columns, providing accurate results that are verified by tests and numerical analysis.

Numerical analysis of circular steel–reinforced concrete-filled steel tubular stub columns

This paper presents a computational model for determining the axial responses of short circular steel–reinforced concrete-filled steel tubular (SRCFST) columns. A novel confinement model is formulated for the concrete core, which is effectively confined by the external circular steel tube and the embedded steel section. The modelling scheme of the confinement is programmed using a mathematical model that utilises fibre-element discretisation of column cross-sections. The numerical predictions are verified by experimental measurements and results obtained from finite-element analysis, demonstrating the accuracy of the modelling technology. In addition, existing concrete confinement models for concrete in circular concrete-filled steel tubular columns are assessed. The new confinement model is shown to be superior in replicating the responses of SRCFST columns. The influences of the design parameters on the column's performance are numerically investigated and ranked in order of importance through a sensitivity analysis. In this study, not only is the validity of current design standards in determining the axial load capacity of SRCFST columns examined but a new design formula is also proposed. The proposed confinement model can be employed in numerical procedures for the inelastic simulation of SRCFST columns; the design formula is suitable for use in practical design.

Mechanical behavior and design applicability of ultra-high-performance concrete-filled circular steel tubular short columns under axial compression

This paper presents the results of axial compression tests conducted on 12 short columns made of ultra-high-performance concrete-filled steel tubes (UHPCFST) with different diameter-to-thickness (D/t) ratios and steel fiber contents. The study aimed to investigate the interaction between infilled concrete and steel tubes as well as the confinement effect on the compressive behavior of UHPCFST short columns. Two types of load–displacement curves were observed in the test, which were related to the D/t ratio of the steel tube. Besides, the results showed that columns with smaller D/t ratios exhibited higher strength enhancement and better post-yield performance due to increased concrete confinement. Furthermore, the presence of steel fibers improved the post-yield ductility of the columns but did not significantly contribute to the strength enhancement. Owing to the longer elastic branch and less lateral dilation of UHPC compared to normal-strength concrete (NSC), the establishment of confining effect to UHPC core is relatively later than the yielding of steel tube, leading to a redistribution between axial stress and hoop stress. The test results were compared with current design codes, including Eurocode 4, Chinese code GB 50936, and American code AISC 360. The results showed that Eurocode 4 and GB 50936 overestimated the axial load-bearing capacity when considering the confinement effect of the steel tube, while the calculations by Eurocode 4 without considering the confinement effect and AISC 360 were closer to the test results. Therefore, the consideration of the confinement effect in design is not recommended and a reduction factor of UHPC is suggested to obtain a reliable prediction of the axial load-bearing capacity of UHPCFST short columns as safe as that of the CFST short columns infilled with NSC.

Performance of round-ended recycled aggregate CFST stub columns after fire exposure

The post-fire performance of round-ended recycled aggregate concrete-filled steel tubular (RE-RACFST) short columns has been investigated experimentally and numerically. The test parameters included cross-section aspect ratios, recycled aggregate replacements and steel ratios. Twelve short columns were first heated for 90 min following ISO 834 standard fire, followed by natural cooling. Subsequently, axial compressive tests were performed to obtain the damage mode, load versus axial deformation, and strain development of the columns. The test results revealed that a higher level of recycled aggregate content resulted in decreased bearing capacity and elastic stiffness of RE-RACFSTs, and an earlier onset of the confining effect. Furthermore, finite element (FE) models were established and benchmarked against the experimental results. Parametric investigations were then conducted to study the effects of key factors on the residual load-carrying capacity. Finally, according to the experimental and FE results, a simplified formula was proposed to estimate the post-fire residual capacity of RE-RACFST stub columns.

Unified Model for Axial Bearing Capacity of Concrete-Filled Steel Tubular Circular Columns Based on Hoek–Brown Failure Criterion

Concrete-filled steel tubular (CFST) composite columns can overcome the brittleness of concrete and improve the plastic deformation ability of concrete, thus improving its strength and deformation ability. At present, most of the published models for predicting the axial bearing capacity of CFST columns are empirical models based on the nonlinear fitting of experimental data, which has some limitations on the application of the models. Therefore, to establish a new unified theoretical model, a new ultimate compressive strength of core concrete was established by the Hoek–Brown failure criterion, and the conversion formula between the cube and cylinder compressive strength was also established in this paper. At the same time, the strength-reduction coefficient influenced by the slenderness ratio was also established. The newly established unified model can predict the axial bearing capacity of CFST columns with different steel types, concrete types, slenderness ratios, diameter-to-thickness ratios, and cross-sectional dimensions. At the same time, the newly established unified model can be applied to a wider range of test parameters. To determine the parameters in the proposed model and assess the models, a total of 798 test data were collected. Based on the test database, the existing models and the proposed model were evaluated. The results show that the proposed model has very high accuracy in predicting the test results of CFST short and long columns, and the average value (AV) and integral absolute error (IAE) are 1.012 and 0.094, respectively. In addition, the model proposed in this paper also has high accuracy in predicting the axial bearing capacity of CFST columns under high temperatures.

Test and design of rectangular steel-reinforced concrete-filled stainless steel tubular short columns

In this study, the compressive behaviour of rectangular steel-reinforced concrete-filled stainless steel tubular (SRCFSST) short columns was investigated. Compressive tests on 10 rectangular SRCFSST stub columns and 2 rectangular concrete-filled stainless steel tubular (CFSST) counterparts were performed to study the effect of steel ratios and steel sectional shape on the compressive performance of SRCFSST short columns. Compression test results showed that the rectangular SRCFSST column has higher carrying capacity, deformability, and ductility than the rectangular CFSST column. A refined finite element (FE) model of the rectangular SRCFSST stub column was created. According to the FE study, adding carbon profiled steel to rectangular CFSST significantly improved confinement of concrete. Thus, ductility and bearing capacity of the composite column were greatly improved. The carrying capacity, peak strain, and compressive stiffness of the rectangular SRCFSST short column were evaluated using FE parametric analysis. Finally, design methods of the carrying capacity, peak strain, and compressive stiffness of the SRCFSST short column were recommended or refined.

Behavior of embedded steel profile ultra-high performance concrete-filled stainless steel tubular short columns under axial compression

This study investigated the behavior of ultra-high performance concrete-filled stainless steel tubular columns with embedded steel profiles (ESUHPCFSST) under axial compression. Six short columns were designed with the parameters of diameter-to-thickness ratio and embedded steel profile ratio. Axial compression tests were conducted to investigate the failure modes, failure mechanisms, and the impact of relevant parameters on the mechanical properties of the specimens. A finite element model was developed and validated against the experimental results to enable a more comprehensive parameter analysis. The results indicate that the failure of the specimens always initiates from the steel profile flange and gradually extends to the external stainless steel tube. The initial stiffness, ultimate bearing capacity, and ductility of the specimens increase with decreasing diameter-to-thickness ratio and increasing embedded steel profile ratio. The strength of the embedded steel profile positively correlates with ultimate bearing capacity and ductility. Higher concrete strength improves ultimate bearing capacity but reduces ductility. Finally, a bearing capacity prediction model for ESUHPCFSST columns is proposed based on the experimental and finite element results, which guides the engineering application of such structures.

A unified design equation for square and rectangular concrete-filled steel tubular short columns

This paper develops a new unified design equation to estimate the axial compressive strength of square and rectangular concrete-filled steel tubular (CFST) short columns. The following four steps are involved: (i) compiling an experimental database consisting of 348 square and 113 rectangular CFST short columns, (ii) conducting 245 parametric analyses using benchmarked finite element (FE) models to address the gaps in the database, (iii) evaluating existing design equations and identifying their insufficiencies, and (iv) proposing and validating the new design equation. As compared with the existing design equations, the proposed design equation has the following advantages: (i) allowing a fast and convenient estimation of the axial compressive strength of square and rectangular CFST short columns, (ii) getting rid of the calculation of the section slenderness, and (iii) extending the codified material strength limits of the concrete infill and steel tube, i.e., it is suitable to square and rectangular CFST short columns with the compressive strength of the concrete infill up to 150 MPa and yield stress of steel tube up to 1000 MPa.

Artificial neural network model for strength predictions of CFST columns strengthened with CFRP

This paper presents an optimised Artificial Neural Network (ANN) model for predicting the ultimate axial strengths of concentrically loaded Concrete-Filled Steel Tubular (CFST) short and slender columns strengthened with Carbon Fibre-Reinforced Polymer (CFRP). Since experimental data on CFRP strengthened CFST columns is limited, an accurate Finite Element (FE) model is developed and used to provide additional numerical data. A multi-layered feed-forward back-propagation network is proposed, optimised, and trained using the results of 76 experimental tests and 450 generated FE models. The accuracy of the ANN model is assessed through comparing its computed results with experimental data. A reliability analysis is performed using Monte Carlo Simulation (MCS) to evaluate the safety of the solutions computed by the ANN model. ANN-based equations and Graphical User Interface (GUI) are developed based on the trained ANN model for the determination of the ultimate axial strengths of CFST columns. The results show that the developed ANN model is capable of accurately predicting the ultimate axial strengths of CFRP strengthened CFST columns with a high degree of accuracy.

Axial compressive performance of ultra-high strength concrete-filled dual steel tubular short columns with outer stiffened tubes and inner circular tubes

Previous literature shows that studies of ultra-high strength concrete-filled dual steel tubular (UHS-CFDST) short columns under concentric compression are rare, and there is no relevant research on ultra-high strength concrete-filled dual stiffened steel tubular (UHS-CFDSST) short columns. In addition, existing design specifications do not include ultra-high strength concrete (UHSC). Therefore, the axial compressive response and resistance of UHS-CFDSST short columns are investigated herein. First, the behaviour of CFDSST columns with normal-strength concrete (NSC) was replicated by finite element (FE) modelling using ABAQUS software. Additionally, the experimental results of square and circular ultra-high strength concrete-filled steel tubular (UHS-CFST) short columns were collected from the available literature, and finite element (FE) modelling is performed accordingly. The results showed that the applied concrete constitutive model is suitable for UHSC and the modelling strategy for UHS-CFST columns and CFDSST columns with NSC is suitable for UHS-CFDSST short columns. Accordingly, parametric studies were carried out on UHS-CFDSST to investigate the effects of geometrical and material properties of different steel and concrete components on the performance of these columns. It was found that the strength of the sandwiched concrete has the greatest effect on the resistance of the columns compared to the other parameters considered. Finally, the axial resistance of the UHS-CFDSST short columns was compared to the resistances calculated by using the European, British and Chinese codes, and it was found that the Chinese code gives the best results.

Experimental assessment on the size effects of square concrete-filled steel tubular columns under axial compression

In this study, eight specimens with the sectional widths (B) of 200–1000 mm and the width-to-steel tube thickness ratios (B/t) of 40 and 50 were tested to failure under axial compression to investigate the size effects of square concrete-filled steel tubular (CFST) short columns, specifically those on the failure mode, composite elastic modulus, peak axial strain, peak axial stress, ductility index, strength index, and stress development. The test results indicate that the peak axial stress, ductility index, and strength index decrease with the increasing specimen size, while the failure mode and composite elastic modulus maintain relatively steady. Under the peak axial load, the longitudinal to yield stress ratio of steel tubes increases with the increasing specimen size, while the trend of transverse to yield stress ratio is opposite. The size effect on the transverse stress in the steel tube reduces the tube confinement effect, thus degrades the compressive strength of the core concrete. Compared to the test results, both the EC4 and the GB50936 codes are unconservative in predicting the strength of large square CFST columns. Therefore, a model considering the ratio of width to thickness (B/t) and specimen size is proposed to evaluate the axial bearing capacity of square CFST short columns.

Structural Behavior of Short Thin-Walled Steel Columns Filled with High Strength Concrete Made of Recycled Aggregate

This paper introduces an experimental work to study the behavior of 12 recycled aggregate concrete-filled steel tubular (RACFST) short columns with high-strength concrete grade subjected to axial compression loading. These columns formed from six different cross-sections involving triangle, elliptical, and hexagon, whereas the other three sections included traditional forms for control purposes, involving square, rectangular, and circular. All RACFST columns used are made of mild steel plates. These columns were divided into two groups. The steel tube thickness was the only parameter modified to study its effect properly. In addition, the study included the search for the best effective section concerning the properties of stability and confinement, so these columns were designed so that the cross-sectional areas of steel tubes were approximately equal. Also, composite action and level of ductility for these columns were studied. The ultimate failure axial load, the reduction in the axial column length, the failure pattern of cross-section shape, and lateral displacement were recorded during the test. Regarding the two groups of columns with (1 and 2) mm thick steel plates, testing data obtained from RACFST columns with elliptical and circular cross-sections respectively showed better stability and confinement of concrete and the ability to withstand a higher ultimate failure stress. Moreover, columns with polygonal sections showed when the number of ribs for the steel plates of the model or increased the angle between the two sides is (90°) or more, column cross-sections can achieve more stability and confinement, respectively. In general , increasing the thickness of the steel tubes increases. Finally, for all columns, the higher values of the concrete contribution ratio (C.C.R) led to an increase in the strength index (S.I.) values. An increase in values of final failure stresses accompanied this increase.

Experimental assessment on the size effects of circular concrete-filled steel tubular columns under axial compression

In this study, eight circular concrete-filled steel tubular (CFST) short columns with diameters (D) of 275–1100 mm and the column diameter-to-steel tube thickness ratios (D/t) of 55 and 69 were tested under axial compression. The bearing capacities of the steel tube and confined concrete were analyzed separately to illustrate the size effect of each component of the composite column. The experimental results show that all test specimens suffered a shear failure. The peak stress, peak strain, ductility index, and strength index decrease with the increasing D, while the composite elastic modules maintain to be relatively constant with the various specimen dimensions. Under the peak load, the vertical stress in the steel tube increases, while the hoop stress decreases with the increasing D. Compared with the specimens with a small D/t ratio, the vertical stress in the steel tube with a larger D/t ratio is lower, while the hoop stress is higher. The confined concrete strength can be divided into the unconfined concrete strength and the increased concrete strength due to lateral confinement, and the reduction of the latter is the main cause of the size effect of the CFST columns. The current design codes such as EC4, AISC, AIJ, and GB50936 overestimate the peak bearing capacity of circular CFST short columns with large diameters, compared to the experimental results. Therefore, a model considering the size effect was established to evaluate the axial bearing capacity of circular CFST short columns.