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This paper introduces a novel cross-sectional design for concrete-filled steel tubular (CFST) columns termed "concrete-filled double square steel tube" (CFDSST). The CFDSST columns incorporate an inner square steel tube positioned diagonally at 45° relative to the outer square steel tube, which serves as a stiffener for the outer tube. A three-dimensional finite element simulation model is presented to accurately predict the behavior of axially loaded CFDSST short columns. This model considers the influence of concrete confinement by the steel tubes. Experimental data are employed to validate the accuracy of the finite element model. The validated model is then utilized to assess the performance of the proposed columns, considering various geometric and material properties. The results highlight the substantial enhancement in structural performance achieved by the innovative CFDSST column configuration compared to conventional square CFST columns. The integration of the inner CFST components enhances both buckling resistance and the confinement mechanism of the column. This contributes to the increased load-bearing capacity, ductility, and residual strength of the columns. Notably, the steel tube thickness, the yield strength, and the concrete compressive strength greatly affect the performance of the column. Lastly, a simplified design model is formulated to predict the compressive capacity of the CFDSST stub columns, providing accurate outcomes validated by numerical analysis. • A new configuration of CFST columns is introduced. • Finite-element simulation model for CFDSST stub columns is developed. • The behavior of CFDSST stub columns is parametrically studied. • The proposed finite-element simulation and design models provide accurate predictions. [ABSTRACT FROM AUTHOR]

Concrete-filled steel tubular (CFST) columns are widely utilized in structures owing to their cost-effectiveness, high strength, ductility, and proven fire resistance. Despite these advantages, concerns persist regarding the performance of CFST columns with locally corroded steel tubes, impeding their widespread adoption in offshore structures. This research employs three-dimensional finite element (FE) simulations for circular CFST stub columns subjected to axial compression, validated against experimental data. Verification includes full load-displacement histories, ultimate axial strengths, and failure modes. The validated FE models are then employed to assess and compare the structural behavior of CFST columns with various shapes of localized corrosion, analyzing overall load-deformation curves, interaction stress-deformation responses, and composite actions. To acquire critical data for understanding stub column behavior, 81 specimens underwent non-linear finite element analysis, with local corrosion represented by artificial notches in steel tubes. Test parameters, such as notch orientations, lengths, depths, widths, and concrete compressive strength, are varied. Results elucidate the impacts of these parameters on different aspects of stub column responses with corroded steel tubes. Furthermore, the accuracy of existing design models in predicting the ultimate axial strengths of CFST columns with corroded steel tubes is assessed. Finally, through parametric analysis of test results, a practical model is proposed and verified for computing the peak resistance of a CFST stub column with a locally corroded steel tube. • Finite-element simulation model for circular CFST stub columns with locally corroded steel tubes is developed. • The influence of different shapes of localized corrosion on the performance of circular CFST stub columns is investigated. • The behavior of circular CFST stub columns with locally corroded steel tubes is parametrically studied. • The proposed finite-element simulation and design models provide accurate predictions. [ABSTRACT FROM AUTHOR]

Tapered concrete-filled double-skin steel tubular columns with large hollow ratio (LHR-CFDST) have a good application potential in wind turbine towers, where they are commonly subjected to the combined action of compression, bending, shearing and torsion. Up to now, research work on the behaviour of tapered LHR-CFDST columns under combined loads is seldom reported and relevant experimental data are limited. This paper presents an experimental and numerical study on the tapered LHR-CFDST column under combined compression-bending-shear-torsion loads. Seven specimens with different parameters including the tapered angle, the hollow ratio, the torsion-to-bending ratio, the axial compression ratio and the height-to-diameter ratio were tested. The experimental results in terms of failure modes, load versus deformation relations and strain responses were discussed. Reliable finite element models were established to conduct numerical analysis in an extended parameter range. The mechanism of columns and the effects of torsion-to-bending ratios, axial compression ratios, height-to-diameter ratios, tapered angles, hollow ratios, diameter-to-thickness ratios of steel tubes and the material strength were revealed. The suitability of the existing ultimate capacity prediction method was evaluated for tapered LHR-CFDST columns. It is found that the failure mode of columns under compression-bending-shear-torsion loads is related to the torsion-to-bending ratio. The columns behave in a ductile manner under moderate axial compression. Tapered angle has a slight effect on the bending capacity of the column, whereas the capacity declines with the increase of hollow ratio and torsion-to-bending ratio. The existing method gives a conservative prediction for the ultimate capacity of tapered LHR-CFDST columns. • Tests on tapered LHR-CFDST columns under combined compression-bending-shear-torsion loads are reported. • Behaviour and mechanism of tapered LHR-CFDST columns under combined loads are discussed. • Numerical analyses are conducted and effects of important parameters are revealed. • Suitability of the existing ultimate capacity prediction method is evaluated. [ABSTRACT FROM AUTHOR]

• 1) Completing the monotonic axial loading test on the STCLAC stub columns. • 2) Proposing a formula that calculated the equivalent lateral confining stress. • 3) Proposing formulae that calculated the compressive stress and the corresponding strain for confined LAC. • 4) Proposing the constitutive model for confined LAC. To study the constitutive model of lightweight aggregate concrete (LAC) constrained by the circular steel tube, the monotonic axial loading test was carried out on steel tube confined LAC (STCLAC) stub columns, considering the effects of the diameter-to-thickness (D / t) ratio (30.4, 19 and 12.7) and steel grade (Q235 and Q355). The test results showed that the steel tubes of STCLAC columns were bulging outward as a whole without local buckling during the loading process. The D / t ratio for the STCLAC column is recommended that be greater than 19.0. Due to friction stress accumulating at the interface between the steel tube and the LAC core, the steel tube is subjected to longitudinal stress. When the confinement factor (ξ) is less than 2.313, the transverse stress of the steel tube reached its yield strength before the peak load, otherwise it reached the proportional stress without yielding. Using the average transverse stresses along the height of the steel tube, this study calculated the equivalent lateral confining stress of the STCLAC column. The calculation formulas of the compressive stress and corresponding strain of confined LAC were proposed based on the test results. And the constitutive model was also proposed, which is more capable of predicting the stress–strain relationship of the constrained LAC by a circular steel tube. [ABSTRACT FROM AUTHOR]