Circular Concrete-filled Steel Tube Columns Research Articles

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.

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.

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.

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.

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.

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.

Performance of new CFST square column-to-foundation connections for cyclic loads

Although the structural behavior of circular Concrete Filled Steel Tube (CFST) columns-to-foundation connections has been studied substantially, there are limited studies on connections for square CFST columns, which are more in demand in high-rise buildings. The acceptable behavior of these columns for their use in high-rise buildings require sufficient stiffness for controlling the drift, safe and efficient transfer of imposed loads, and adequate ductility for withstanding earthquake loads. In the present study, authors aim to develop two such column-to-foundation connections for the square CFST columns. The performance of the two square CFST column-to-foundation connections was assessed experimentally as well as numerically under cyclic lateral loading. The comparison of the connections was made with one of the prevalent square CFST column-to-foundation connections and a conventional RC connection to study the inelastic deformation capacity of connections under cyclic loads. The test results illustrate the efficient and improved behavior of the suggested connections. Numerical models were also developed, and the experimentally obtained results were predicted and compared. The developed numerical models were capable of predicting the experimental response of connections to large drift levels with acceptable accuracy. The numerical results were reasonably close to the experimental response.

Hysteresis Performance of Through Bolted Connections in Steel Beam to Circular CFST Column

Concrete Filled Steel Tubes (CFSTs) with through bolt connections are demonstrated to be satisfactory for seismic performance. Due to better confinement effect, circular CFST columns possess higher strength to weight ratio compared to square CFST. The only limiting factor in application of circular CFST in construction is the difficulty in joining with the beams. This paper presents an experimental study on hysteretic behavior of interior joint between circular CFST column and steel beam using extended end plates with through bolt. The study focuses on the comparison of connection using flat extended end plate with straight bolts and curved extended end plate with split bolt assembly for circular CFST columns. The experimental results were analysed in terms of strength, stiffness, ductility, failure mode and energy dissipation for the evaluation of cyclic performance. The results indicate that the seismic resistance agrees with seismic recommendations of American Institute of Steel Construction (AISC) and the split bolt assembly increases the performance remarkably. For comprehension of mechanism and prediction of shear capacity of the panel zone for curved endplates, a mathematical model was generated. This includes the influence of axial load, prestressing induced by bolt tightening, anchorage and the concrete strut action. The prediction accuracy was compared with the test result and it showed sound correlation.

Revisiting the composite action in axially loaded circular CFST columns through direct measurement of load components

The composite action in circular concrete-filled steel tube (CFST) columns under axial compression is one of the most fundamental problems in the research area of steel-concrete composite structures. However, it cannot be easily investigated through experiments as the load components of the steel tube and concrete infill cannot be measured in a conventional axial loading test. In this study, an innovative testing method was devised to directly measure the load components of circular CFST columns under axial compression. Six innovative specimens with varying concrete strength and diameter-to-thickness (D/t) ratio and their conventional counterparts were tested. Although some difference existed between the load-deformation responses of the innovative and corresponding conventional specimens, it was essentially insignificant. The development of hoop stresses in the steel tube tends to reduce the corresponding axial stresses, but this effect can be offset by the strain hardening of the steel tube. Strain hardening of the steel tube should be considered when evaluating the confinement effect at the ultimate load, especially for columns with low concrete strength and small D/t ratio. The average values of the coefficients related to the axial and hoop stresses of the steel tube at the ultimate load, computed using available tests results, are 0.92 and −0.14, respectively. The proposed axial load capacity formula with these two values can reasonably predict the ultimate loads of circular CFST columns under axial compression.

Ultimate axial capacity prediction of CCFST columns using hybrid intelligence models - a new approach

This study aims to propose a new intelligence technique of predicting the ultimate capacity of axially loaded circular concrete-filled steel tube (CCFST) columns. A hybrid system based on one of the evolution algorithm – Genetic Algorithm (GA), fused with a well-known data-driven model of multivariate adaptive regression splines (MARS), namely G-MARS, was proposed and applied. To construct the MARS model, a database of 504 experimental cases was collected from the available literature. The GA was utilized to determine an optimal set of MARS's hyperparameters, to improve the prediction accuracy. The compiled database covered five input variables, including the diameter of the circular cross section-section (D), the wall thickness of the steel tube (t), the length of the column (L), the compressive strength of the concrete (fc), and the yield strength of the steel tube (fy). A new explicit formulation was derived from MARS in further analysis, and its estimation accuracy was validated against a benchmark model, G-ANN, an artificial neural network (ANN) optimized using the same metaheuristic algorithm. The simulation results in terms of error range and statistical indices indicated that the derived formula had a superior capability in predicting the ultimate capacity of CCFST columns, relative to the G-ANN model and the other existing empirical methods.

Structural performance of nano-silica based blended CFST stub circular column

The objective of this study is to find the optimum percentage replacement of cement with the nano-silica in a concrete mix to improve the structural performance Concrete Filled Steel Tube (CFST) stub circular column under the axial compressive load. The CFST stub columns with circular cross-sectional area of conventional concrete mix (without nS) and nano-silica blended concrete mix (with nS-CFST) had been experimentally and numerically studied. The amorphous silica is a major component of the cementitious materials, and it reacts with the calcium hydroxide, which is comprised of the calcium silicate hydration. The addition of this nano-silica affects the different properties of the concrete and increases its performance. On the other hand, CFST is known for the composite behavior generated between the hollow tube of steel and a core constructed of concrete, which improves the collected performance of the member. In present study, the concrete mix of M25 had been modified with 4%, 6%, 8% and 10% of nano silica. The structural behavior of stub columns (with / without nS) are assessed under axial compressive load. Also, the CFST stub columns were then modelled into ABAQUS for finite element analysis. The nano-silica blended concrete in CFST stub columns showed an improved the structural performance than the conventional cement concrete stub circular CFST column.

Post-Fire Exposure Behavior of Circular Concrete-filled Steel Tube Column under Axial Loading

Concrete-filled steel tube (CFST) columns are a composite member which mainly consists of steel tube infilled with concrete that are increasingly being used as load-carrying members these days. In construction industry, CFST columns are being preferred for the development of tall buildings and long-span bridges. This paper presents an experimental investigation on concrete-filled steel tube (CFST) columns which were post heated and were subjected to axial loading. The thickness of the casing steel was 4 mm and 5 mm and diameter was 100 mm, 125 mm and 150 mm and were infilled with concrete of grade M30 which were used in the present study. This study also represents the behavior of CFST columns for two cooling regimes (annealing and quenching) after exposure to elevated temperatures of 600 °C and 800 °C. The results obtained from experimental analysis were compared to each other in terms of load-deformation pattern, ultimate load capacity, residual strength index, secant stiffness and ductility index. The test results showed that as compared with water quenching, annealing is slightly better for post fire cooling of CFST columns. Also, the results obtained by the experimental investigation were compared with each other on the basis of two cooling regimes.

Hybrid BART-based models optimized by nature-inspired metaheuristics to predict ultimate axial capacity of CCFST columns

The goal of this study was to investigate a novel approach of predicting the ultimate capacity of axially loaded circular concrete-filled steel tube (CCFST) columns. A hybrid intelligent system, namely GAP-BART, was developed based on the Bayesian additive regression tree (BART) combining with three nature-inspired optimization algorithms such as Genetic Algorithm (GA), Artificial Bee Colony (ABC), and Particle Swarm Optimization (PSO), and then applied. Three sub-hybrid models of the system were built, abbreviated as G-BART, A-BART, and P-BART, respectively, using 504 experimental data collected from published research. The compiled database covered five input variables, including the diameter of the circular cross-section—section (D), the wall thickness of the steel tube (t), the length of the column (L), the compressive strength of the concrete ( $$f_{\text{c}}^{'}$$ ), and the yield strength of the steel tube (fy). The coefficient of determination (R2) values of (0.9971, 0.9982, and 0.9986) and (0.9891, 0.9923 and 0.9931) were achieved for training and testing of G-BART, A-BART, and P-BART models, respectively. The P-BART model performed the lowest RMSE and MAE values for the training and testing set of (66.85 kN and 49.60 kN) and (141.24 kN and 102.04 kN), respectively. These results indicated that although the proposed models were able to estimate ultimate axial capacity with high accuracy, the P-BART model had the best performance among them. For benchmarking, the obtained results were validated against several mathematical approaches as well as other AI techniques (MARS, ANN). The findings of the comparative analysis clearly showed superior ability to predict the CFST ultimate axial capacity relative to the benchmark models. The relative importance of each predictor was investigated to find the most significant input variables. The results confirmed that the hybrid GAP-BART system can serve as a reliable and accurate tool for the design of CCFST columns and to predict their performance.

Cyclic loading tests of stirrup cage confined concrete-filled steel tube columns under high axial pressure

This paper presents a cyclic loading test study on 2 full-scale stirrup cage confined circular concrete-filled steel tube (CFT) columns and 3 full-scale stirrup cage confined square CFT columns under high axial pressure and low-cycle fatigue horizontal loading. The effect of the contact mode between the stirrup cage and the steel tube wall on the hysteretic behaviours of the specimens was investigated. The failure mode, hysteretic energy dissipation capacity, skeleton curve, elastic stiffness, maximum bearing capacity, ductility index, stiffness degradation, and residual deformation of the confined composite columns were analysed. The results indicate that using stirrup cage in the CFT columns can considerably enhance the seismic performance of CFT columns under high axial force ratio. Welding the stirrup cage to the inner wall of the steel tube can further strengthen the confinement effect of the stirrup cage and steel tube on the infilled concrete while also reducing the amplitude of the drum bending degree and the length of the local buckling region of the steel tube. Therefore, the overall stiffness, maximum bearing capacity and hysteretic energy dissipation capacity of the specimens are improved. Stirrup cage confined square CFT columns with the same design parameters have a greater bending stiffness, maximum bearing capacity and hysteretic energy dissipation capacity compared with those of stirrup cage confined circular CFT columns. The final failure mode of the stirrup cage confined square CFT columns is more favoured than that of the stirrup cage confined circular CFT columns for seismic design; namely, no ductile metal fracture of the steel tube wall on the tensile side was observed.

Hysteresis performance of through bolted connections in CFST frame

This paper presents an experimental study on the seismic performance of composite structural frame systems consisting of concrete-filled steel tubes (CFSTs) and steel beams. The modified, extended end-plate connections were fabricated by straight through bolt for square columns and split bolt for the circular column with a curved endplate. Half-scale CFST composite moment resisting frames were tested under displacement-controlled quasi-static lateral cyclic load. Circular CFST columns possess better confinement than square CFST columns, but the application in construction is restricted due to the difficulty in joining with the steel beams. The present study investigates the strength, stiffness degradation, ductility, failure mode, and energy dissipation characteristics corresponding to the inter-story drift. The results from the tests indicate that the seismic resistance is at par with the American Institute of Steel Construction (AISC) seismic recommendations for composite special moment-resisting frame, and hence the split-bolt assembly can be used as an effective alternate for connection in circular columns. This work also tries to impart the experimental database on the effect of increasing the number of the bay in the resisting direction. Single-bay and double-bay frames were compared to quantify the improvement in seismic performance.