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An innovative composite structural element, the dune sand concrete-filled circular steel tube (DS-CFCST) column combines the mechanical performance of concrete-filled steel tube (CFST) columns with the environmental and economic benefits of dune sand (DS) concrete. However, current experimental investigations into DS-CFCST columns' axial compressive behavior are limited. This study conducts a numerical analysis to examine the effects of varying DS replacement ratios and the influence of confinement on DS-CFCST stub columns. Finite element (FE) analysis reveals that DS-CFCST stub columns exhibit reduced ultimate bearing capacity compared to CFST columns, primarily due to weakened confinement effects at higher DS replacement ratios. A parametric study investigated the impacts of various design parameters on the ultimate axial bearing capacity of DS-CFCST stub columns. A practical design formula, based on equilibrium principles and the FE model, was developed. This formula simplifies the prediction of the ultimate load-bearing capacity of DS-CFCST stub columns using the superposition method. Its accuracy was validated by comparing it with experimental data and FE results. Lastly, a reliability analysis was performed, showing the DS-CFCST columns' reliability index sensitivity to variations in concrete strength, steel yield strength, steel content ratio, load effect ratio, load combination factor, and DS replacement ratio. [ABSTRACT FROM AUTHOR]

No study has been reported to use machine learning methods to predict the full-range test curves of circular CFST columns. In this paper, the long short-term memory (LSTM) network was introduced to calculate the axially compressive load–strain curves of the circular CFST columns according to an experiment database of limited scale. To improve the feasibility of input data for the recurrent neural network algorithm, data preprocessing methods and data configurations were discussed. The prediction results indicate that the LSTM network provides more accurate estimations compared with the artificial neural networks, random forest and support vector regression. Meanwhile, this method can be used to calculate the mechanical properties including the elastic modulus, ultimate bearing capacity, and the ductility of the columns with acceptable accuracy for engineering practice (the prediction error within 20%). For future research, it is expected that the machine learning method will be applied to predict the structural response of different members under various loading conditions. [ABSTRACT FROM AUTHOR]

Recycled aggregate concrete-filled steel tubular (RACFST) columns are widely recognized as efficient structural members that can reduce the environmental impact of the building industry and improve the mechanical behavior of recycled aggregate concrete (RAC). The objective of this study is to investigate the behavior of recycled aggregate concrete-filled circular steel tubular (RACFCST) stub columns subjected to the axial loading. Three-dimensional finite element (FE) models were established using a triaxial plastic-damage constitutive model of RAC considering the replacement ratio of recycled aggregates. The FE analytical results revealed that the decreased ultimate bearing capacity of RACFCST stub columns compared with conventional concrete infilled steel tubular (CFST) columns was mainly due to the weakened confinement effect and efficiency. This trend will become more apparent with the larger replacement ratio of recycled aggregates. A practical design formula of the ultimate bearing capacity of RACFCST stub columns subjected to axial load was proposed on the basis of the reasonably simplified cross-sectional stress nephogram at the ultimate state. The derivation process incorporated the equilibrium condition and the superposition theory. The proposed equation was evaluated by comparing its accuracy and accessibility to some well-known design formulae proposed by other researchers and some widely used design codes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of lightweight aggregate concrete-filled circular steel tubular (LACFCST) stub columns under axial compression. A finite 3D solid element model of the LACFCST stub column was established by adopting a plastic-damage constitutive model of lightweight aggregate concrete (LAC). The finite element model (FEM) analysis results revealed that the confinement effect of the steel tube on the infilled LAC was weaker than that on the infilled conventional concrete. A parametric study making use of 95 full-scale FEMs was conducted to investigate the influences of various design parameters of LACFCST stub columns on their ultimate axial bearing capacity and the composite actions. Moreover, a numerical model of the axial and transverse stress of steel tubes at the ultimate state of LACFCST columns was proposed using the regression method. Based on the equilibrium conditions and the proposed model, a practical design formula making use of an enhancement factor was derived to estimate the ultimate bearing capacity of LACFCST stub columns by using the superposition method. The validity of the proposed formula was verified against the experimental data of 49 LACFCST stub column specimens under the axial loading available in the literature. Meanwhile, the accuracy and conciseness of the proposed formula were evaluated by comparison with the formulas suggested by the existing design codes. [ABSTRACT FROM AUTHOR]

This paper studied the composite action of concrete-filled circular aluminum alloy tubular (CFCAT) stub columns under axial compression. A fine-meshed finite three-dimensional (3D) solid element model making use of a tri-axial plastic-damage constitutive model of concrete and elastoplastic constitutive model of aluminum alloy was established. A parametric study utilizing the verified finite element (FE) model was carried out and the analytical results were exploited to investigate the composite actions of concrete-filled circular aluminum alloy tubular stub columns subjected axial compression. Compared with the concrete-filled steel tube (CFCST) stub columns, the aluminum alloy tube exerted a weaker constraint effect on the infilled concrete due to its lower elastic modulus. Based on the FE analytical results and regression method, the composite action model of concrete-filled circular aluminum alloy tubular stub columns was proposed. By generalizing the stress nephogram of the concrete-filled circular aluminum alloy tubular stub column at the limit state, a design formula was proposed to estimate the ultimate bearing capacity the columns using the superposition method. The predicted results of the proposed formula show a good agreement with both the experimental and FE analytical results. The comparison between the proposed formula and current design methods indicates that the proposed formula is more accurate and convenient to use. [ABSTRACT FROM AUTHOR]

• High axial pressure cyclic loading test of stirrup cage confined CFT columns. • Different contact modes between stirrup cage and steel tube were studied. • The effect of the contact mode on the seismic behaviors were investigated. 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. [ABSTRACT FROM AUTHOR]

A refined numerical model was proposed to simulate the mechanical behavior of circular concrete-filled steel tubular (CFST) columns under fire conditions. The model was calibrated against the thermal and mechanical results from physical testing and showed good reliability. Based on the numerical analyses, the mechanical properties of steel-concrete interface and composite column for circular CFST with different confinement coefficients during the entire fire process were investigated in depth. Moreover, a series of parametric analyses were undertaken to investigate the influence of main factors, such as the load ratio, the steel tube ratio, the steel strength, the cross-sectional diameter and the concrete strength, on the mechanical behavior of the circular CFST column. Finally, a theoretical formulation and a simplified formulation for evaluating the ultimate bearing capacity of circular CFST columns with different length-diameter ratios under fire conditions were proposed, which showed an acceptable accuracy. • A refined 3D finite element model of circular CFST columns is developed and calibrated. • A series of parametric analyses are conducted. • The failure modes of circular CFST columns in fire are captured. • The formulations for evaluating the ultimate bearing capacity of circular CFST columns with different length-diameter ratios in fire are proposed. [ABSTRACT FROM AUTHOR]