Circular Concrete-filled Steel Tube Research Articles

Cyclic behavior of circular CFST stub columns with different inner constructions

Circular concrete-filled steel tube (CFST) columns, which are widely used in high-rise and super high-rise buildings, have good mechanical performance. Low-cyclic loading tests were conducted on eight large-sized specimens with different shear-to-span ratios to investigate the seismic performance of circular CFST stub columns with different inner constructions (an internal circular steel tube, built-in partitions, vertical stiffeners, circumferential stiffeners, and a built-in reinforcement cage). The results show that built-in partitions coordinated the common deformation of the internal and external steel tubes, providing the specimen with the best seismic performance. By setting an internal circular steel tube in circular CFST stub columns, a significant confining effect on the core concrete was generated and the deformation of the external steel tube was well developed, which significantly improved the seismic performance of the specimens. Setting circumferential stiffeners had a significant confining effect on the radial deformation of the steel tube, and placing vertical stiffeners between the circumferential stiffeners had an obvious effect on preventing local buckling and improving the flexural capacity of the steel tube. The energy dissipation capacity of the specimens with different inner constructions was better when the shear-to-span ratio was smaller. A method for calculating the peak bearing capacity of such circular CFST stub columns is proposed, which can accurately predict the peak bearing capacity of the columns and provide a basis for the calculation of their application in practical engineering.

Seismic behavior of circular CFST columns with different internal constructions

To improve the seismic performance of circular concrete-filled steel tube (CFST) columns with large dimensions, circumferential stiffeners, vertical stiffeners, and reinforcement cages were suggested to be set in the columns. Seven circular CFST column specimens with four internal constructions and two shear-span ratios were designed and tested under low-frequency cyclic loading. The failure modes, hysteretic behavior, bearing capacity, ductility, stiffness and strength degradation, energy dissipation capacity, and effect on the seismic performance based on different internal constructions and different shear-span ratios were herein discussed and clarified. The test results illustrated that the internal constructions could effectively postpone the local buckling of steel tube and improve the ultimate strength, ductility, and stiffness of circular CFST columns. A finite element (FE) model was developed to carry out the working mechanism and parametric analysis. Finally, a theoretical model to estimate the ultimate bearing capacity of CFST columns with different internal constructions was established, the results obtained from the model were in good agreement with the test results.

Strength prediction of circular CFST columns through advanced machine learning methods

Concrete-filled steel tube (CFST), well recognized for its excellent mechanical behaviour and economic efficiency, is widely used as a main load-carrying component in various kinds of structures. Machine learning (ML) is one of the promising artificial intelligence methods which just starts to be utilized for the advanced prediction of structural performances. This paper attempts to evaluate the feasibility of combining mechanism analysis to optimize ML models in predicting the axial compression strength of circular CFST columns. A comprehensive database containing 2,045 circular CFSTs under axial loading was established through extensive literature survey. Based on correlation analysis and mechanism analysis, input parameters for ML models were rationally selected. Then back-propagation neural network (BPNN), genetic algorithm (GA)-BPNN, radial basis function neural network (RBFNN), Gaussian process regression (GPR) and multiple linear regression (MLR) models were established. It was revealed that the established ML models, especially GPR, could reliably predict the strengths of CFST with higher accuracies and wider applicable ranges than existing methods in current design standards. By subdividing the database according to column slenderness, ML models achieved improved accuracy for strength prediction, whilst little effect on the model accuracy was generated by random subdivisions. This indicates that when adopting ML methods in structural engineering sector, optimization of the models can be expected on the basis of rational understanding towards the corresponding structural mechanism.

Prediction of the load-shortening curve of CFST columns using ANN-based models

Artificial neural network (ANN) as a machine learning (ML) technique has been successfully applied in engineering applications such as structural dynamics and structural design. It has also received considerable attention for the design of concrete-filled steel tube (CFST) columns. However, the application of ANN method to CFST columns is mainly restricted to the prediction of the ultimate strength. In this paper, a novel approach to predict and plot the complete axial load-shortening curve of concentrically loaded rectangular and circular CFST columns using ANN method is presented. To train the networks, a database including 392 test results of rectangular and circular CFST columns with their corresponding data points of the load-deflection curves is compiled. In addition, 1152 finite element models (FEMs) are generated and analysed using ABAQUS to expand the training data and address data gaps in the experimental database. The validity of the developed FEMs is verified by experimental data. Based on the trained ANN models, a MATLAB-based graphical user interface (GUI) is also developed to provide a convenient tool for users to predict and plot the axial load-shortening response of rectangular and circular CFST columns. Using the developed GUI, a parametric study is also conducted to verify the accuracy of the ANN models in predicting the behaviour of CFST columns fabricated with different geometric and material properties. The results show that the developed ANN models can accurately predict the load-deflection response of CFST columns.

Numerical investigation on circular concrete-filled double skin steel tube columns under torsion

This paper presents a finite element analysis (FEA) on the torsional behavior of circular concrete-filled double skin steel tube (CFDST) columns. A refined finite element model was established and verified by the test results of circular CFDST columns subject to torsion. The comparison results show that the finite element model can reasonably predict the torsional behavior of circular CFDST columns. Then, parametric analysis was conducted to investigate the effect of key parameters including concrete strength, yield strength and steel ratio of the outer steel tube, yield strength and thickness of the inner steel tube, and the hollow ratio on the torsional capacity of circular CFDST columns. The effect of end constraint, i.e. aspect ratio on the torsional capacity of circular CFDST columns was also discussed. When the aspect ratio is greater than 1.0, the aspect ratio has little effect on the torsional capacity and ductile deformation of circular CFDST columns. Yield strength and steel ratio of the outer steel tube have a great influence on the torsional capacity of circular CFDST columns, but the influence of concrete strength on the torsional capacity of circular CFDST columns is not obvious. Increasing the yield strength and thickness of the inner steel tube can effectively improve the torsional capacity of the circular CFDST columns when the hollow ratio is greater than 0.6. Finally, design formulas were proposed to predict the torsional capacity of the circular CFDST columns, and the comparison results show that the proposed formula has good accuracy.

Axial compressive behavior of circular concrete-filled steel tube stub columns prepared with spontaneous-combustion coal gangue aggregate

The compressive behavior of circular concrete-filled steel tube (CFST) stub columns prepared with spontaneous-combustion coal gangue aggregate (SCGA) was investigated to study the effect of SCGA substitution level by experimental test and finite element simulation. The test variables included SCGA replacement ratios (0%, 50% and 100%) and steel ratios (8.3%, 11.6% and 14.2%). The study concerns the failure mode, ultimate compressive strength, elastic stiffness, ductility of the specimens, and the confinement effect of the steel tube. The test results showed that the failure mode of specimens prepared with SCGA was similar to that of reference CFSTs; compressive strength, elastic stiffness and ductility of the specimens were reduced by 2.93–4.81% (8.25–10.18%), −16.17–8.24% (3.01–22.16%) and 7.78–10.61% (11.86–16.34%) respectively with the increasing replacement ratio of SCGA from 0% to 50% (100%). A finite element model was proposed based on ABAQUS software and benchmarked against the test data from this study. The numerical results indicated that the SCGA replacement ratio and the confinement effect were the major factors of the compressive behavior of CFSTs. After comparing the test data with the existing design codes, modified design equations considering replacement ratio and confinement effects was proposed to predict the ultimate compressive strength of the specimens prepared with SCGA. Accuracy of the derived modified equation was evaluated, with the mean value of 0.984 and a standard deviation of 0.076. The study provides an experimental basis of the utilization of SCGA in CFST stub columns.

Impact of diameter to thickness (D/t) on axial capacity of circular CFST columns: Experimental, parametric and numerical analysis

Twenty-four circular Concrete Filled Steel Tube (CFST) columns divided in to three series based on their cross-sectional dimensions were subjected to uni-axial compression and their behaviour is studied. This paper aims to develop the volume of experimental database as there is shortage in data that can assess the guidance from the codes and enhances their accuracy in determining the ultimate capacities and the behaviour of CFST specimens subjected to uni axial compression. This study consists of CFST specimens having outer diameter of 76 mm, 89 mm and 100 mm with same wall thickness of 3 mm having four Length to Diameter (L/D) ratios of 3, 4, 5 and 6. Impact of D/t on the parameters like confinement (ξ), strength index (SI), relative slenderness ratio (λ), percentage contribution of steel and concrete and ductility index (DI) were studied. Further, the axial compressive load values were compared with the predicted design values of codes, namely, Eurocode – 4 (EC4), American code (AISC 360-10), Australian code (AS5100), Chinese code (DBJ13-51) and American Concrete Institute (ACI-318). A design equation is proposed to calculate the ultimate axial load and the predicted results are near to the test results. To check the accuracy of the proposed equation, experimental results of 63 CFST circular columns from the literatures were compared with proposed equation and found the results to be conservative. At last, finite element analysis using ABAQUS was done to study the behaviour of column buckling, axial load and displacement curves. Results showed good agreement with experimental test results.

Axial Behavior of Concrete Filled-steel Tube Columns Reinforced with Steel Fibers

Concrete filled steel tube (CFST) columns are being popular in civil engineering due to their superior structural characteristics. This paper investigates enhancement in axial behavior of CFST columns by adding steel fibers to plain concrete that infill steel tubes. Four specimens were prepared: two square columns (100*100 mm) and two circular columns (100 mm in diameter). All columns were 60 cm in length. Plain concrete mix and concrete reinforced with steel fibers were used to infill steel tube columns. Ultimate axial load capacity, ductility and failure mode are discussed in this study. The results showed that the ultimate axial load capacity of CFST columns reinforced with steel fibers increased by 28% and 20 % for circular and square columns, respectively. Also, the circular CFST columns exhibited better ductility than the square CFST columns due to better concrete confinement. Circular and square CFST columns with steel fibers showed improved ductility by 16.3% and 12%, respectively. The failure mode of the square CFST columns were local buckling which occurred near the end of columns, while, for the circular CFST columns, local buckling occurred near the mid-height. Also, the study involved sectional analysis that captured the behavior of CFST columns very well. The sectional analysis showed that increasing steel fiber content to 2% increased the axial load capacity by 51 and 38% for circular and square CFST columns, respectively. Furthermore, sectional analysis showed that doubling section size increased axial load capacity by approximately 4 and 5 times for circular and square columns, respectively.

Short columns composed of concrete-filled steel tubes in a fire situation – Numerical model and the “air-gap” effect

Abstract The increase in temperature reduces the strength of steel and concrete, in such a way that it is essential to verify concrete-filled steel tube columns in fire situations. Numerical simulations, with lower costs than laboratory tests, have great importance in checking resistance and defining simplified methods for design practice. However, peculiarities of the thermal and mechanical behavior of heated confined concrete and the air-gap effect (a phenomenon inherent to concrete-filled steel columns) must still be better understood. Therefore, this study presents the development of a numerical model performed in the ABAQUS software (Dassault Systemes SIMULIA Corp., 2014) for the thermomechanical analysis of short columns composed of circular and square concrete-filled steel tubes considering the air-gap effect. The air-gap phenomenon is presented and analyzed according to possibilities of implementation to the numerical model and, finally, the proposed numerical model is validated with experimental results presented in the literature. According to the study results, the numerical model can be used to define and adjust simplified methods for verification of composite columns in fire situation. The importance of considering the air-gap effect in numerical modeling was confirmed, taking into account that disregarding its effect may result in overestimated responses of the steel tube resistance in fire situations. Moreover, it was suggested thermomechanical joint analysis and the use of the explicit solver as a strategy to minimize processing time.

Influence of parameters on the strength and behavior of concrete filled steel tube specimens subjected to axial compression and cyclic loading

The strength and behavior of Concrete Filled Steel Tube (CFST) columns subjected to uni-axial compression and cyclic loading are presented from the available literatures. Based on the experimental test results of 230 circular, 163 rectangular and 113 square specimens subjected to axial loading and 33 circular, 18 rectangular and 23 square specimens subjected to cyclic loading, the parameters affecting the strength and behavior of CFST columns are outlined. The effect of relative slenderness ratio (λ), confinement (ξ), Length to Diameter (L/D), Length to Breadth (L/B), Diameter to thickness (D/t), Breadth to thickness (B/t) on the axial capacity of columns are examined. The effect on ductility ratio (μ) due to axial load level (n), steel ratio (α) and member slenderness (ƛ) are also presented. The ultimate axial loads (Ne) of the columns are compared with the design values (Nc) predicted using various codes namely, Eurocode-4 (EC4), Australian standards (AS5100), Chinese code (DBJ13-51) and American code (AISC 360–10). Results showed that relative slenderness ratio and Length to Diameter of circular CFST specimens showed direct effect on the strength and behavior of the column. Circular sections with high steel ratio reflected high ductility compared to rectangular and square sections. Steel ratio and member slenderness show significant changes in ductility ratio as the values are increasing for all three types of sections. Axial load predictions of EC4 agrees well with the experimental test results and are conservative. While, results predicted by AS5100, DBJ13-51 and AISC 360 – 10 are unconservative.

Predicting the Ultimate Axial Capacity of Uniaxially Loaded CFST Columns Using Multiphysics Artificial Intelligence.

The object of this research is concrete-filled steel tubes (CFST). The article aimed to develop a prediction Multiphysics model for the circular CFST column by using the Artificial Neural Network (ANN), the Adaptive Neuro-Fuzzy Inference System (ANFIS) and the Gene Expression Program (GEP). The database for this study contains 1667 datapoints in which 702 are short CFST columns and 965 are long CFST columns. The input parameters are the geometric dimensions of the structural elements of the column and the mechanical properties of materials. The target parameters are the bearing capacity of columns, which determines their life cycle. A Multiphysics model was developed, and various statistical checks were applied using the three artificial intelligence techniques mentioned above. Parametric and sensitivity analyses were also performed on both short and long GEP models. The overall performance of the GEP model was better than the ANN and ANFIS models, and the prediction values of the GEP model were near actual values. The PI of the predicted Nst by GEP, ANN and ANFIS for training are 0.0416, 0.1423, and 0.1016, respectively, and for Nlg these values are 0.1169, 0.2990 and 0.1542, respectively. Corresponding OF values are 0.2300, 0.1200, and 0.090 for Nst, and 0.1000, 0.2700, and 0.1500 for Nlg. The superiority of the GEP method to the other techniques can be seen from the fact that the GEP technique provides suitable connections based on practical experimental work and does not rely on prior solutions. It is concluded that the GEP model can be used to predict the bearing capacity of circular CFST columns to avoid any laborious and time-consuming experimental work. It is also recommended that further research should be performed on the data to develop a prediction equation using other techniques such as Random Forest Regression and Multi Expression Program.

Novel Fuzzy-Based Optimization Approaches for the Prediction of Ultimate Axial Load of Circular Concrete-Filled Steel Tubes

An accurate estimation of the axial compression capacity of the concrete-filled steel tubular (CFST) column is crucial for ensuring the safety of structures containing them and preventing related failures. In this article, two novel hybrid fuzzy systems (FS) were used to create a new framework for estimating the axial compression capacity of circular CCFST columns. In the hybrid models, differential evolution (DE) and firefly algorithm (FFA) techniques are employed in order to obtain the optimal membership functions of the base FS model. To train the models with the new hybrid techniques, i.e., FS-DE and FS-FFA, a substantial library of 410 experimental tests was compiled from openly available literature sources. The new model’s robustness and accuracy was assessed using a variety of statistical criteria both for model development and for model validation. The novel FS-FFA and FS-DE models were able to improve the prediction capacity of the base model by 9.68% and 6.58%, respectively. Furthermore, the proposed models exhibited considerably improved performance compared to existing design code methodologies. These models can be utilized for solving similar problems in structural engineering and concrete technology with an enhanced level of accuracy.

Seismic behavior and damage assessment of concrete‐filled steel tube columns in diagrid structures

SummaryDiagrid structures have become an innovative and attractive choice for high‐rise buildings due to their high rigidity and esthetic appearance. Inclined concrete‐filled steel tube (CFST) columns, as the main load‐carrying members of diagrid structures, are supposed to be subjected to axial tension and compression loads. However, few studies have investigated the seismic behavior and damage assessment of CFST columns under axial cyclic loading. This study presents experimental results for eight circular CFST columns under axial cyclic loading. Based on the observed damage evolution, a damage assessment model for CFST columns subjected to axial cyclic loading was developed using a nonlinear combination of stiffness degradation and hysteretic energy dissipation. The contributions of the maximum response and cyclic loading effects to the damage were specified by introducing combination coefficients, which can be determined from a proposed equation based on the aspect ratio and confinement index of the CFST columns. Finally, the corresponding relationships between the range of the damage index and the degree of damage of CFST columns under different performance levels were established, which can provide the necessary basis for economic loss assessment and repair of earthquake‐damaged diagrid structures.

Loss assessment of rigid-frame bridges under horizontal and vertical ground motions

Previous investigations have attributed some reinforced concrete (RC) bridge piers suffered significant damages during large earthquakes due to vertical ground motion effects. This has motivated this study to investigate the use of concrete-filled steel tube (CFST) columns as a more resilient and sustainable alternative to resist combined horizontal and vertical ground motions. The classic performance-based earthquake engineering (PBEE) framework is used to examine the seismic performance of a rigid-frame bridge with construction options of circular RC, circular CFST, and square CFST columns. The hysteretic response of each column is calibrated to the results of hybrid testing of columns under combined horizontal and vertical ground motions. The incremental dynamic analysis (IDA) is conducted using a suite of 50 near-fault records. The results are used to compare the observed damage states and quantify the repair sustainability metrics including repair cost (economic), repair downtime (social), and repair carbon emissions (environmental), as well as the loss of resilience. The losses are integrated with the hazard curves to compute the expected annual losses, all defined as a set of decision variables in the PBEE framework. The results confirm the outstanding seismic performance of CFST columns and highlight their benefits in improving the resilience and sustainability of critical and post-disaster bridges that are prone to strong multi-directional ground motions.

Investigation of the axial compressive behaviour of CFRP-confined circular CFST stub columns with inner latticed steel angles

To investigate the compressive behaviour of carbon fibre-reinforced polymer (CFRP)-confined circular concrete-filled steel tube (CFST) stub columns with inner latticed steel angles, the axial compression test was carried out in this paper. The main parameters investigated included the diameter-to-thickness ratio of the outer steel tube, the cross-sectional area of the inner latticed steel angles and the number of CFRP layers. First, the failure modes, load-displacement curves, ultimate load results and load-strain curves were obtained experimentally. Second, the stress-strain curves of steel tubes, the confining stress on the concrete and the stress-strain curves of the concrete were analysed according to the experimental results. Finally, design equations for the ultimate load of CFRP-confined circular CFST stub columns with inner latticed steel angles were proposed. Research results showed that the latticed steel angles have an excellent effect for improving the ultimate load and ductility of CFSTs, and the CFRP have an obvious confining effect on the local buckling of steel tubes, thus improving the ultimate load of CFSTs with latticed steel angles. It can be concluded that the failure of CFRP-confined specimens were mainly due to the rupture of the CFRP caused by the local buckling of steel tubes, and the latticed steel angles could participate in the overall loading process and provide a certain confining effect on the concrete enclosed by the latticed steel angles. On the basis of experimental results, design equations for predicting the ultimate load of CFRP-confined CFST clumns with latticed steel angles were proposed and compared with experimental results, and conservative prediction results were acquired.

Experimental study on the steel–concrete bond behaviour of circular concrete-filled stainless steel tubes

The paper reports an experimental investigation on the bond stress–slip relationship in concrete-filled stainless steel tube (CFSST) structures. Push-out tests were conducted on twelve specimens covering two stainless steel grades, two concrete grades, two cross-sectional dimensions and two internal ring conditions (with and without the internal ring). The effects of these factors on the bond stress–slip relationship in CFSST members were analysed and discussed. A theoretical model, based on the assumption that no restraining force from the steel tube was applied to the concrete core, was developed to predict the bond stress–slip response of CFSSTs and was evaluated against the test results. An empirical model was also proposed based on regression analysis to replicate the experimental bond stress–slip relationships, which can be used as input data for analytical and numerical simulations in future studies of CFSST structures.

ANALYTICAL MODEL FOR THE PREDICTION OF THE ELASTIC RESPONSE OF CURVED T-STUBS

Composite steel-concrete columns utilise the advantages of both materials, by combining high strength and ductility of steel with the compressive strength of the concrete. But the wide adaptation of composite structures is limited, mainly because of the lack of cheap and easy to construct connections, as many of which require costly and timeconsuming on-site welding, when circular concrete filled steel tubes (CFST) are adopted. New connections, like those incorporating the use of blind bolts and curved end-plates, may represent a valuable alternative. Such joints can be adapted to circular CFST to eliminate on-site welding, but they require the creation of new curved T-stub components. This paper proposes an analytical model for the evaluation of bolt forces in the curved T-stubs within the elastic range. The model is then validated against experimental results of joints between circular CFST columns and steel beams, with both preloaded and snug tightened bolts. Analytical model shows good agreement with experimental data, but needs further development to take into account the prying forces.

Genetic algorithm hybridized with eXtreme gradient boosting to predict axial compressive capacity of CCFST columns

The study aimed to propose a robust method for predicting the axial compressive capacity (Nu) of circular concrete-filled steel tube (CCFST) columns. For this purpose, a hybrid intelligent system was designed by fusing the eXtreme Gradient Boosting (XGB) model with the Genetic Algorithm (GA). The GA was utilized to determine an optimal set of XGB’s hyperparameters to improve accuracy. A dataset of 509 tests from available literature was compiled for training and testing phases, which covered five input variables considering geometric and material properties of CCFST columns. The objective predictive model (GA-XGB) was validated against a benchmark model, namely GA-ANN, an artificial neural network (ANN) model optimized by the same metaheuristic algorithm. The simulation results in terms of error range and statistical indices indicated that the GA-XGB was superior to the GA-ANN model and other established mathematical approaches in predicting Nu of CCFST columns. The relative importance of individual features was further investigated to find the most significant input variable. Finally, a web app has been developed to make the proposed GA-XGB model applicable to practical design. The results confirmed that GA-XGB can be used in the design and behavior prediction of CCFST columns as a reliable and precise tool.

Finite Model Analysis and Practical Design Equations of Circular Concrete-Filled Steel Tube Columns Subjected to Compression-Torsion Load.

The objective of this study is to investigate the mechanical properties and the composite action of circular concrete-filled steel tube (CFST) columns subjected to compression-torsion load using finite element model analysis. Load–strain (T–γ) curves, normal stress, shear stress, and the composite action between the steel tubes and the interior concrete were analyzed based on the verified 3D finite element models. The results indicate that with the increase of axial force, the maximum shear stress at the core concrete increased significantly, and the maximum shear stress of the steel tubes gradually decreased. Meanwhile, the torsional bearing capacity of the column increased at first and then decreased. The torque share in the columns changed from the tube-sharing domain to the concrete-sharing domain, while the axial force of the steel tube remained unchanged. Practical design equations for the torsional capacity of axially loaded circular CFST columns were proposed based on the parametric analysis. The accuracy and validity of the proposed equations were verified against the collected experimental results.

Numerical Study of Circular Concrete Filled Steel Tubes Subjected to Pure Torsion

This study numerically explored the torsional behavior of circular concrete-filled steel tubes (CFST) under pure torsion. Numerical models of CFSTs were developed in ABAQUS. The models were validated by comparing with the experimental results available in the literature; then, these models were used for parametric study. Based on the obtained results, the mechanism of torsional moment transferring from steel plates to CFST was presented. The results obtained from the parametric study indicated that the compressive strength of concrete marginally improved the torsional moment capacity of the CFST while concrete prevented buckling and helped the steel tubes to work more effectively. The steel strength significantly affected the torsional moment capacity of the CFST. When the yield strength of steel increased from 235 to 420 MPa, the yield torsional moment of the CFST increased by approximately 50%. The yield torsional moment capacity of the steel tube had the strongest correlation with the yield moment of the CFST, followed by the ratio of diameter to thickness of the steel tube while the parameters related to the compressive strength of concrete exhibited a poor correlation with the yield torsional moment.