Concrete-filled Steel Tube Short Columns Research Articles

Evolution of Confinement Stress in Axially Loaded Concrete-Filled Steel Tube Stub Columns: Study on Enhancing Urban Building Efficiency

In the context of green building and sustainable urban development, understanding the mechanical behavior of structural components like concrete-filled steel tube (CFST) columns is crucial due to their improved load-bearing capacity, energy efficiency, and optimized material usage, which enhance structural resilience and sustainability. This research addresses the complex development of confining stress and its impact on the concrete core (CC) behavior within these columns, which are essential for urban infrastructure. Through extensive numerical studies, this study proposes a model to estimate the confining stress in axially loaded CFST short columns. Study findings reveal that the confinement effectiveness is influenced by variables such as compressive strength (CS) of the concrete, cross-sectional shape, and depth-to-wall thickness percentage. Additionally, the confinement is also significantly affected by the yield strain of steel εy/εc to the peak strain of unconfined concrete εc. A three-dimensional finite element model (FEM) was built for the simulation of the columns’ nonlinear behavior and was rigorously validated against experimental data. This model aids in the design and implementation of more efficient and resilient urban structures, supporting the principles of sustainable construction. The study underscores the importance of structural integrity in sustainable urban development and provides valuable insights for improving the design of green building materials.

Performance of lightweight coconut shell concrete-filled circular steel tube columns under axial compression

The current study aimed to use coconut shell concrete, a structural lightweight concrete, as an infill material in concrete-filled steel tube (CFST) columns and test it under axial compression. Testing was done on eighteen short, intermediate and long coconut shell CFST columns and six normal-weight CFST short columns for comparison. For both types of columns, the axial load-displacement curves and modes of failure were examined. By varying the length-to-diameter and diameter-to-thickness ratios, the axial capacity of steel tubes filled with coconut shell concrete was assessed. The composite action was verified from the results of the confinement index, strength index and the contribution of the coconut shell concrete as infill concrete. Structural efficiency and energy absorption of the lightweight CFST column was contrasted with its counterpart column. The contribution of coconut shell concrete to the strength of the CFST column was the highest at 61.36% and more significant than that of normal-weight CFST columns. The coconut shell CFST columns were 23.63% lighter than the normal-weight columns, contributing to its higher structural efficiency. These columns also had 8.12% more energy absorption than normal-weight columns. Hence, the results of this investigation revealed that coconut shell concrete has the potential to be utilized in CFST columns. Further, compared to the experimental ultimate loads, the predictions made by the existing codes, EC4 and ANSI/AISC 360 are conservative.

Experimental study of ultra-high-performance fibre-reinforced concrete (UHPFRC)-encased CFST short columns under axial and eccentric compression

This study presents a comprehensive experimental programme and analysis of ultra-high-performance fibre-reinforced concrete (UHPFRC)-encased concrete-filled steel tube (CFST) columns subjected to axial and eccentric compression. Twelve short columns were tested, with six of them utilising UHPFRC encasement while the other six employing normal strength fibre-reinforced concrete (FRC) encasement for comparison purpose. The study focused on the effects of load eccentricity ratio, concrete encasement type, and steel tube configuration. Key findings from the test results including failure modes, load-deflection behaviour, moment-curvature relationships and curvature ductility were examined. The findings showed that UHPFRC-encased CFST columns exhibited significantly enhanced strength and bending capacity, while FRC-encased columns showed remarkable ductility. A notable shift in failure mode was observed with increasing load eccentricity ratio, transitioning from crushing of concrete to compression, and then to balanced failure. Furthermore, applicability of prevalent design codes such as EC4, AIJ, and ACI 318 in calculating the peak loads of UHPFRC- and FRC-encased CFST columns was assessed. The plastic stress distribution methods in EC4 and AIJ overestimated peak loads, while ACI 318's strain compatibility approach accurately predicted FRC-encased columns but not UHPFRC-encased columns. To address this, modifications of the parameters α and β were incorporated into the ACI design method to reflect the relative brittle nature of UHPFRC, resulting in accurate predictions.

The influence of basalt fiber on the mechanical performance of concrete-filled steel tube short columns under axial compression

With rapid economic and social development, both concrete-filled steel tube (CFST) composite structures and basalt fiber (BF) have been widely applied in the field of civil engineering. To investigate the laws and characteristics of the influence of chopped BF on the mechanical properties of CFST columns and further promote the application of BF in CFST structures, the axial compressive bearing capacity test of 18 CFST short columns was carried out, and the influence of BF of different lengths on their structural mechanical properties was analyzed. The test results were compared with the theoretical calculation results and the finite element analysis results to verify the reasonableness of the test results. The results reveal that the axial compressive bearing capacity of the CFST short column after adding BF is significantly improved compared to the ordinary CFST short column, in which the bearing capacity and the ductility coefficient are increased by approximately 8.1% and 31.6%, respectively, on average. In addition, changing the length of BF has less effect on the bearing capacity of CFST short columns, the rate of increase in bearing capacity decreases with an increase in the steel ratio of CFST, and the coefficient of ductility increases with the increase in the steel ratio.

Axial compression experiment and numerical simulation of CFST short columns at an ultra early age

To study the axial compression performance of ultra-early-age concrete-filled steel tube (CFST) short columns, axial compression tests were carried out on short columns of CFSTs with different wall thicknesses of steel tubes and different concrete ages. The load-displacement curves and failure modes of CFST short column specimens under different concrete ages were obtained. The influence of wall thickness and concrete age on the mechanical properties of ultra-early-age CFST specimens was analyzed. The calculation formula for the bearing capacity of mature CFST short columns in GB50936–2014 was extended, and a new calculation formula for calculating the bearing capacity of ultra-early-age CFST short columns was proposed. Considering that there is no known concrete stress-strain expression for ultra-early-age CFST short columns in existing literature, this paper has adjusted the constitutive model of the core concrete in ordinary CFSTs and advanced a stress-strain relationship suitable for the core concrete in ultra-early-age CFST short columns.

Axially-compressed behavior of carbon-FRP strengthening CFT short columns having circumferential void defects

To clarify the viability of externally bonding carbon-fiber-reinforced polymer (FRP) for strengthening the structural integrity of concrete-filled steel tube (CFT) short columns having circumferential void defects, this study comprehensively explores the strengthening effectiveness of various wrapping parameters (i.e., carbon-FRP strength, wrapping layers, and wrapping ratio) on the axially-compressed CFT short columns with various void ratios, material strengths, and geometric parameters by numerical method. Detailed mechanical responses, encompassing failure patterns, load-shortening (N-Δ) curves and stress distribution, are delved to reveal the strengthening mechanisms. Finally, strength prediction formulas for these composite columns are proposed rooted in the ring extension and local force transmission theories. Results show that the presence of initial voids delays the occurrence of steel-concrete contact stress (p) and weakens the p-value at peak load, thus reducing the axial resistance of column. Particularly when the void ratio exceeds 0.2%, the longitudinal strain when the concrete contacts with outer tube is beyond the concrete's peak compressive strain, thus the shape of the N-Δ curve is obviously changed, exhibiting two peaks. Externally bonding carbon-FRP strips can obviously improve the peak load of the column with a void ratio ≤ 0.2%. If the void ratio exceeds 0.2%, externally bonding carbon-FRP strips can also improve the column's second peak load, even exceeding the strength of pure CFT column, while it has no effect on the first load. The formulas proposed herein have a satisfactory accuracy based on the tested validation, which are expected to guide the design of strengthening scheme in engineering.

Axial compression behavior and stress–strain relationship of slurry-wrapping treatment recycled aggregate concrete-filled steel tube short columns

Abstract The utilization of recycled concrete can effectively solve the problems of large accumulations of construction waste and the constant consumption of natural aggregates. Slurry-wrapping modification and concrete-filled steel tubes (CFSTs) are effective approaches to enhance the utilization ratio of recycled concrete. The study involved 4 ordinary CFST columns and 16 slurry-wrapping recycled aggregate concrete-filled steel tube (SRACFST) columns. The variable parameters included the slurry-wrapping recycled aggregate replacement ratio and the section form. The result shows that the ultimate strength of the specimens reached the minimum and maximum at 25 and 100% aggregate replacement ratio. The favorable strength performance of the SRACFST columns is attributed to the dominant positive effect of the slurry-wrapping treatment in reducing matrix porosity over the negative impact of the recycled aggregate’s complex interface transition zone. Due to the hoop factor, the residual bearing ratio peaks at a 25% substitution ratio. Four design standards were used to predict the ultimate strength, with GB50936 and EC4 yielding more accurate estimations, while AISC360-10 and AIJ provided conservative estimations. This study presents the equations of the stress–strain curve for the whole axial compression process, which can be directly applied in the theoretical and numerical analysis of RACFST columns.

Confinement-based direct design method for fibre reinforced polymer confined CFST short columns

The paper presents a detailed analysis of the behaviour of circular and square concrete filled steel tube (CFST) short columns strengthened externally with carbon or glass fibre reinforced polymer sheets (CFRP or GFRP, respectively). A thorough review of existing test information is presented and discussed, and the most salient parameters in terms of the overall strength are identified. There are a large number of influential and inter-related parameters which affect the load-carrying capacity, including the geometry, cross-sectional shape, type of steel, concrete strength, boundary and loading conditions, and type of FRP. It is shown that existing design approaches do not reliably predict the strength for the full range of possible parameters. Therefore, this paper proposes a new design model to calculate the axial compressive strength of FRP-confined concrete filled steel tubular (CFST) short columns with either a circular or square cross-section. The method accounts for the various complexities which affect the behaviour, yet presents a user-friendly, performance-based design expression. It is based on an evaluation of the lateral confining pressure provided by the both the FRP and the steel tube to the concrete core. This is employed in the confinement-based direct resistance calculations. The paper validates the approach by comparing its capacity predictions with a large database of experimental results and alternative design models available in the literature. The results show that the proposed model provides much accurate strength predictions with greater reliability for the full range of parameters examined, than existing methods.

Axial compressive behavior and design of high-strength square concrete-filled steel tube short columns with embedded GFRP tubes

This paper proposed a hybrid column by embedding glass fiber-reinforced polymer (GFRP) tube into high-strength square concrete-filled steel tube (CFT), named as high-strength square steel-concrete-GFRP concrete (SCFC) column. A total of 13 high-strength square SCFC short columns were tested. The test parameters included diameter and thickness of GFRP tube, width and thickness of steel tube, yield stress of steel, and compressive strength of concrete. Results showed that high-strength square SCFC short columns exhibited post-yield strengthening behavior and stable residual strength. The axial strength, confining effect, and ductility were improved because of the embedded GFRP tube. Detailed finite element models were developed and benchmarked to investigate the strength contribution of each component and conduct parametric studies. It was shown that: (i) the concrete infill and the steel tube contributed >90% to the overall compressive strength and (ii) the strength improvement by embedding GFRP tube was closely related to the section size ratio, confinement ratio of the GFRP tube, and confinement ratio of the steel tube. Evaluation of the current equations showed that they were not accurate enough to estimate the axial compressive strength of high-strength square SCFC short columns. New equations were then proposed.

Behaviour and design of high-strength octagonal CFST short columns subjected to combined compression and bending

A systematic experimental and numerical investigation on the behaviour of octagonal concrete-filled steel tube short columns fabricated with S690 high-strength steel and C90 high-strength concrete, and subjected to combined compression and bending is reported in this paper. Experiments were firstly performed on twelve short column specimens with different plate width-to-thickness ratios. The experimental results and observations including the ultimate loads, load versus displacement responses and failure modes of the specimens were discussed. In supplementary to the experimental investigation, numerical investigation using a validated finite element model was performed to examine the behaviour of the structures with a wide range of plate width-to-thickness ratios, yield strength of high-strength steel and compressive strength of high-strength concrete that subjected to various combinations of compression and bending. Based on the experimental and numerical results, the applicability of existing design approaches in standards was also evaluated. The assessment shows that the design approaches provide conservative strength predictions for the high-strength octagonal concrete-filled steel tube short columns under combined compression and bending and can be applied to provide safe designs for the structures.

A theoretical model for predicting mechanical properties of circular concrete-filled steel tube short columns

Circular concrete-filled steel tube (CCFST) short columns have superior mechanical properties and broad application prospects. To rapidly predict its mechanical properties, this paper introduced both the similarities and differences of existing models, evaluated the influence of debonding effect and radial stress on CCFST short columns, selected mechanical properties for materials, considered the stress-path dependence of concrete and the strain hardening of steel tubes, and proposed a theoretical model calculated by an incremental-iterative program. Results suggest that ignoring the radial stress and the de-bonding effect has little effect on the mechanical properties of CCFST short columns without local buckling. As the axial strain increases, the influence of the stress-path dependence on the concrete increases. The concrete in CCFST short columns has no stress-path dependence when the confinement factor is greater than 2.5. In the prediction of hoop strains, load components, load-axial strain curves, and axial load capacity, the proposed model is effective and accurate and is superior to the existing models.

A unified model for compressive strength of concrete-filled steel tube short columns with various symmetric sections

In concrete-filled steel tube (CFST) short columns with square or circular section, the contribution of steel tube to the confinement and the load-carrying capacity of CFST short columns are affected by the column section shape. However, conventional confinement factor (CF) defines only the strength ratio of steel tube section to concrete section. In the present study, the effect of section shape on the composite action between the steel tube and concrete of CFST short columns was investigated. To consider the effect of section shape, a new confinement factor (MCF) was proposed, and a critical value for the new confinement factor (CMCF) was estimated to predict failure mode of CFST short columns under axial compression. Based on the MCF, a new section of the CFST short column with round corner (RCO-CFST) was proposed to improve the lateral confinement of CFST short column with square section. To verify the validity of the proposed MCF and CMCF, 14 CFST short column specimens with various section shapes were tested. Based on the MCF and CMCF, a finite element model for RCO-CFST short columns was proposed. Further, a unified model for the load-carrying capacity of CFST short columns with circular, square, and round corner sections was also proposed. The proposed model was compared to 411 existing CFST short column test results. The proposed model predicted well the test results of the existing CFST short columns regardless of section shape and material strength, which showed better prediction results than the current design codes for the CFST short columns using high-strength materials.

Performance of Bolt-Welded CFST Short Columns with Different Initial Imperfections: Experimental and Numerical Studies

Gap between the steel tube and core concrete of Concrete-Filled Steel Tube (CFST) members is regarded as an initial imperfection, which may exist in the CFST arch bridge due to construction process problems. In this paper, based on the design scheme of the bolt-welded joints in arch rib columns of an extra-long span CFST arch bridge, the effects of two types of initial imperfections, interfacial gap and spherical-cap gap, on the mechanical properties of Bolt-Welded CFST (BWCFST) short column under axial compression were investigated. Axial compression tests were conducted on three column specimens with spherical-cap gap, three specimens with interfacial gap, and one normal specimen without imperfection. A Finite Element Analysis (FEA) model was developed to further investigate the behavior of the BWCFST short columns, and was validated by the experiment.

Axial compressive behavior of rubberized concrete-filled steel tube short columns

With different volume replacement ratios of rubber fine aggregate and thicknesses of steel tube, 12 rubberized concrete-filled steel tube (RuCFST) columns were fabricated. Then axial compressive tests were completed. The test results show that RuCFST columns exhibit lower axial compressive capacity and higher ductility than normal CFST columns. The steel tube of the RuCFST with a high rubber replacement ratio is more prone to local buckling and the axial load-displacement curves of RuCFST columns tends to exhibit hardening post-peak response. Based on the experimental results, the relationship between the critical value of the confinement coefficient and the rubber replacement ratio of RuCFST columns was obtained. Then, the axial compressive strength of RuCFST columns was investigated by Eurocode4, AISC360-16 and GB 50936-2014 codes. The results show that the predictions by those codes are conservative. Finally, by considering the higher ductility of RuCFST columns and full use of material properties, a modified formula to calculate the compressive strength of RuCFST columns was proposed and then validated by the experimental results.

Unified solutions on axial bearing capacity of T-shaped concrete-filled steel tube short columns with binding bars

Based on the unified strength theory, according to the characteristics of axial bearing capacity of T-shaped concrete-filled steel tubular stub short columns with binding bars, the cross shaped section was divided into 3 rectangular sections which have binding bars and one rectangular section which has no bars. Considering intermediate principal stress, the ration of width to thickness of pipe to the bearing capacity of steel tube was in consideration too, the paper analyzed force mechanism. In the end, the mathematical formulation of bearing capacity for the T-shaped concrete-filled steel tube stub column with binding bars was obtained. The formula calculation values are in good agreement with the test value of the relevant literature.

Unified solutions on axial bearing capacity of round-ended concrete-filled steel tube short columns with binding bars

Based on the unified strength theory, according to the characteristics of axial bearing capacity of round-ended concrete-filled steel tubular stub short columns with binding bars, the cross shaped section was divided into a rectangular section which has bars and two round-ended sections which have no bars. Considering intermediate principal stress, the ration of width to thickness of pipe to the bearing capacity of steel tube was in consideration too, the paper analyzed force mechanism. In the end, the mathematical formulation of bearing capacity for the round-ended concrete filled steel tube stub column with binding bars was obtained. The formula calculation values were in good agreement with the value of the relevant literature. Theoretical formulas are expected to be helpful for engineering applications.

Optimization design of rectangular concrete-filled steel tube short columns with Balancing Composite Motion Optimization and data-driven model

Concrete-filled steel tube (CFT) are widely used as critical members for various types of structures such as bridges, high-rise buildings etc. However, there is a lack of proper models in standards to calculate the capacity of CFT members especially for high strength steel and concrete. This leads to various experiments and simulations conducted and provided in literature and a data-driven is a potential candidate with such plenty of data. The developed model used Artificial Neural Network, ANN, and this model well performed on the test set with R2 is up to 0.9899. Consequently, the ANN model is incorporated with a novel optimization algorithm, the Balancing Composite Motion Optimization - BCMO, recently proposed by Le-Duc et al. This new algorithm is compared with other existing algorithms including: Differential Evolution, Dual Annealing and Second-harmonic generation, to observe the differences among these algorithms. The parameter study of the number of individuals and the maximum generations of the BCMO also conducted for further investigations. Finally, taking the advantage of computationally cost saving of the BCMO, the ANN is again conducted with the inputs is the length and the load applied on the short columns and the output is the objective functions. This ANN is a high accuracy model with R2 is 0.9984, which aimed to provide the designer a rough prediction of the Objective function, which especially useful when the monetary unit cost of materials used is available.

Analytical Assessment on the Structural Behaviour of CFST and CFDST Short Columns using Ansys

Composite sections are much of the time used in structures on account of the straightforwardness and speed of erection, and prevalent in fire situation. In concrete consumed chambers the unfilled space is either filled by plain concrete or fortified concrete. In order to decrease the dead weight of composite portions and to assemble the detainment and toughness of the Concrete In-Filled Double Skinned Steel Tubular (CFDST) fragments were grasped. Two reference models, for instance, Concrete Filled Steel Tube (CFST) and Concrete In-Filled Double Skin Steel Tubes (CFDST) were endorsed and were used as references for the parametric examination. The game plan of furrowed steel plates is to improve the bond quality between the strong and the steel tubes and to manufacture the store passing on cutoff of the fragment. The parameters considered in the examination consolidated the strong assessment, thickness of the steel plate and steel grade. The composite structure under scrutiny was impersonated with a tri dimensional numerical model using the ANSYS programming, which relies upon the Finite Element Method (FEM). Examination were done reliant on the center point compressive weight. Examination of the results provoked the assurance that changing a single parameter didn't assemble as far as possible considering the way that the failure was moved to other essential parts in the composite sectionThe decisive results got from the examination of composite portion are improvement or abatement breaking point of fragment that influences the nature of the area in view of changes in the material properties. Sort of composite fragment is included to consider for feasibly picked as building structure. Finally, the stack passing on cutoff of the wrinkled composite sections are higher than the sustained fragment

Explicit Simulation of Circular CFST Stub Columns with External Steel Confinement under Axial Compression

Concrete-filled steel tube (CFST) structural members have been widely used in engineering projects for their superior strength and ductility. However, the different lateral dilation characteristics between concrete infill and steel tube have caused imperfect composite interaction during the early loading stage. To overcome this issue, external steel confinements in the form of rings and spiral were previously suggested to minimise the lateral expansion of the steel tube and enhance the concrete confinement effects. This study presented the analytical behaviour of circular CFST short columns with an external ring or spiral confinements which are subjected to axial loading. An explicit finite element (FE) model was developed and verified based on previous experimental findings. Besides that, this study analysed the failure modes, axial load–strain relationship, stress distributions, and bond strength of the composite column components. Parametric analysis was also undertaken to evaluate the impact of material strengths, total steel ratio, and diameter-to-thickness ratio. The results suggest that the use of external steel confinement can enhance the compressive behaviour of CFSTs better than increasing the thickness of the steel tube when using the same steel ratio. Finally, simplified design formulations were developed to accurately calculate the ultimate capacity of CFST columns with and without external steel confinement.

Structural behavior of concrete-filled steel tube short columns stiffened by external and internal continuous spirals

Concrete-filled steel tube (CFST) column is one of the most effective forms of composite members in which the steel tube provides both axial strength and confining pressure to improve the compressive capacity and the ductility of the core concrete. This paper presents an evaluation of the structural behavior of CFST short columns strengthened by external and internal continuous spirals. A total of 16 CFST specimens were tested under axial compression. Specimens were stiffened as follows: (1) external continuous spiral (ECS) welded to the exterior surface of the steel tube, (2) internal continuous spiral (ICS) welded to the interior surface of the steel tube, and (3) unwelded internal continuous spiral (UICS) placed longitudinally inside the steel tube. Self-compacting concrete (SCC) was used in the experimental study. The main parameters in this study are the diameter of spiral bar, the number of spiral turns, and the location of the continuous spiral. From the test results, it was concluded that toughness, elastic strength, and ductility were improved significantly as a result of stiffening the CFST columns by external and internal spirals. ECS, ICS, and UICS improved the compression capacity up to 46.8%, 48.7%, and 47.9%, respectively, as compared with the control specimen. A theoretical model to estimate the ultimate load of stiffened CFST columns was developed and close correlations were found between the model and the experimental results.