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Under coupled bending-shear-torsion (B-S-T) loading combinations, the ultimate bearing capacity of concrete filled steel tube (CFST) short columns has not been studied. In this paper, the test on eight CFST short columns was carried out. The load-deformation relations and failure modes of CFST short columns were obtained and analyzed. Based on experimental results, the failure mechanism of CFST short columns were obviously affected by the ratio of bending moment to torque, and CFST short columns under coupled B-S-T loading combinations had good ductility. The ultimate bending-shear bearing capacity of CFST short columns was significantly decreased by increasing torsion moment. Moreover, nonlinear finite element analysis was performed, which generated an accurate prediction of the ultimate capacity of concrete filled steel tube short columns under coupled B-S-T. As a conclusion, a simplified design equation for calculating the coupled B-S-T bearing capacity of CFST short columns was proposed according the regression results of the finite element parametric analysis. [ABSTRACT FROM AUTHOR]

It has been shown that fire disaster normally occurs following an earthquake disaster, which might cause even more serious losses than the earthquake itself. This study investigates the post-earthquake fire performance of concrete-filled steel tube (CFST) columns. The experimental programme contains two parts - quasi-static tests and fire tests. During the quasi-static tests, the square CFST columns were loaded under constant compressive force and cyclic horizontal force, until reaching different degrees of seismic damage. Afterwards, the pre-damaged columns were loaded under a constant axial compression ratio (i.e. ratio of the applied compressive load to its bearing capacity) of 0.3 at elevated temperatures. The fire resistance time and failure modes of these pre-damaged CFST columns were reported. Finite element (FE) models were developed and validated against the test results; a total of 48 numerical results were generated in the parametric study using the validated model. In the parametric study, the influence of key parameters including the thickness of steel tube, the yield strength of steel tube, the axial compression ratio and pre-damage degrees on the fire resistance of square CFST columns were analysed. The experimental and numerical results are used to propose design rules for the fire performance of square CFST columns with different degrees of seismic damage. • Quasi-static tests and fire tests were carried out in the experimental programme. • Post-earthquake fire performance of square concrete-filled steel tube columns was investigated. • The design rules for the post-earthquake fire performance of square CFST columns were proposed. [ABSTRACT FROM AUTHOR]

Based on the completed experimental study, the structural response of steel tube confined RC (STRC) columns under pure torsion, bending-torsion and compression-bending-torsion load were investigated through the finite element method (FEM). Experimental results were used to verify the proposed finite element model and the verification results revealed that the assessment capability of the developed models was efficacious. The model considered the 3D equilibrium, compatibility, and constitutive laws of materials at the section and structural level. In addition, a parametric study was performed to visualize the influence of steel tube, section size, concrete, longitudinal reinforcement, stirrup, and axial force on the ultimate bearing capacity and the ultimate bearing capacity correlation of STRC columns. To this aim, a total of 72 STRC columns were modeled. It was noticed that the thickness of steel tube, the yield strength of longitudinal reinforcement, the diameter of longitudinal reinforcement (longitudinal reinforcement ratio) and axial force were the most crucial parameter, influencing the ultimate bearing capacity of STRC columns significantly. As well, it was overtly beheld from the study that the correlation curves of flexural and torsional capacity were similar under the influence of different parameters, and relevant equations of flexural and torsional capacity were proposed through curve fitting. • The finite element model was developed to investigate the behavior of STRC columns under combined load. • The parametric study was carried out and the most crucial parameters were figured out. • The correlation curves of ultimate bearing capacity were obtained through curve fitting. • The relevant equations of ultimate bearing capacity were proposed. [ABSTRACT FROM AUTHOR]

Recent developments in the high-output wind turbine have heightened the need for CFDST support towers. Tapered Concrete-Filled Double Skin Steel Tubular columns with Large Hollow Ratios (TLHR-CFDSTs) are the typical members in the wind turbine towers, which are usually subjected to torsion during the life cycle. It is necessary to pay more attention to these members under torsion due to their tapered shapes and thin-walled cross sections. This paper thus presents an experimental study on the torsional behaviour of the TLHR-CFDST column. Specimens with different tapered angles, hollow ratios, tubes' thicknesses and axial compression ratios were tested under pure torsion and compression–torsion. The typical failure modes, torque versus deformation relations, strain distributions and the influence of important parameters are revealed. Based on the experimental results, the mechanism of TLHR-CFDST columns under torsion is discussed. The applicability of existing torsional capacity calculation methods is evaluated and finally a simplified method for predicting the torsional capacity of TLHR-CFDST columns is proposed. • Tests on TLHR-CFDST columns under pure torsion and compression–torsion​ are reported. • Behaviour and parametric effects of TLHR-CFDST columns under torsion are revealed. • Torsional mechanism of TLHR-CFDST columns is discussed. • Theoretical method for predicting the torsional capacity of TLHR-CFDST columns is proposed. [ABSTRACT FROM AUTHOR]

This paper presents a pseudo-static test investigation and finite element analysis on the behavior of circular concrete-filled double skin steel tube (CFDST) members under axial compression and torsion. The hollow ratio (χ) and axial compression ratio (n) are considered in the test study. The ductility, hysteretic behavior, failure modes, and compression–torsion correlation of circular CFDST members are discussed. The comparison results show that the validated finite element model is capable to capture the behavior of circular CFDST members under compression and torsion. The effect of loading paths and various parameters on the compression–torsion correlation of circular CFDST members are analyzed. Parametric analyses are carried out to investigate the effect of key parameters, such as material properties, nominal steel ratio, the wall thickness of inner steel tube, and hollow ratio on the compression–torsion correlation of circular CFDST members. It can be seen that the circular CFDST members under compression and torsion possess good load-bearing capacity, energy dissipation, and ductility properties. The loading paths, yield strength of inner and outer steel tubes, nominal steel ratio, and the wall thickness of inner steel tubes have little effect on the compression–torsion correlation of the composite members. When the axial compression ratio is less than 0.2, the axial compression load can slightly improve the torque capacity of circular CFDST members. Finally, the compression–torsion correlation equation of these members was proposed by regression analysis, and design formulas with good accuracy for predicting the torque capacity of circular CFDST members under compression and torsion were proposed. • Test investigation and numerical analysis were conducted to study behavior of CFDST members under compression and torsion. • Parametric analyses and regression analysis were conducted to propose the Axial compression and torsion correlation equation of CFDST members. • Parametric analyses have been conducted considering key parameters. • The axial compression and torsion correlation equation of CFDST members were proposed via regression analysis. • Formulas with good accuracy for predicting the torque capacity of CFDST members under compression and torsion were proposed. [ABSTRACT FROM AUTHOR]

Steel tubes adopted in wind turbine towers generally sustain the combination of compression, bending, and torsion loads. The study of compressive behavior is crucial for investigating the combined bearing capacity of the tower and proposing the corresponding design method. Local buckling tends to affect compressive behavior due to the large diameter-to-thickness ratios of the tubes. Thus, the authors proposed a new form of tower based on longitudinally stiffened steel tubes, which were experimentally and analytically studied in this paper. A total of six tube specimens (four stiffened ones and two unstiffened ones) were tested under axial compression. The experimental results indicate that the local buckling of the tube can be substantially prevented by stiffeners and the T-type stiffener exhibits a superior effect. Finite element models were built and validated by the test results, and the parametric analysis was conducted, indicating that the diameter-to-thickness ratio of the steel tube, the yield strength of steel, and the proportion of stiffener area are the prime sensitive parameters affecting the ultimate strength. Ultimate compressive strength prediction methods in design specifications were evaluated. A practical method for calculating the ultimate strength of stiffened steel tubes was subsequently developed in this paper. • Six steel tube specimens with different stiffener types and diameter-thickness ratios were tested under axial compression. • Finite element models were established and validated by the test results. • Parametric analysis was conducted and the prime sensitive parameters affecting the ultimate strength were presented. • The applicability of calculation methods suggested by several specifications was evaluated. • The calculation method for the ultimate axial compressive strength was proposed by parametric and regression analysis. [ABSTRACT FROM AUTHOR]

Thin-walled steel tubes have been widely used in wind turbine towers. These towers are commonly subjected to combined compression-bending-torsion loads; thus, the steel tubes are prone to local buckling. Installing stiffeners has proven an effective method to improve the local stability of the thin-walled members. This paper presents an experimental and analytical study of thin-walled steel tubes strengthened by longitudinal stiffeners. Six specimens with different types of stiffeners and various diameter-thickness ratios were tested under a combined compression-bending-torsion load. The failure modes, load–displacement relationships, ultimate strength, and deformability were revealed. The validated finite element model was used to analyze the structural mechanisms of the stiffened steel tubes. The results show that stiffeners can substantially alleviate the local buckling of steel tubes, significantly improving their ultimate strength and plastic deformability. In addition, T-type stiffeners perform better than I-type stiffeners. The ultimate strength of the specimens is preliminarily evaluated using the methods recommended in existing design guidelines. The calculated ultimate strength of the unstiffened specimens shows a good agreement with the test results, whereas the ultimate strength of the stiffened specimens is underestimated. • Six specimens with different stiffener types and diameter-thickness ratios were tested under combined load. • The validated finite element model was developed to further analyze the structural mechanisms of the stiffened steel tubes. • The effects of stiffeners on the static behavior of steel tubes were revealed. • The applicability of the design methods recommended by the existing design guidelines was preliminarily evaluated. • The reasonable stiffener type and stiffening position were suggested. [ABSTRACT FROM AUTHOR]