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• Typical failure patterns of TLHR-CFDST columns for wind turbine towers are investigated. • Coupled mechanisms of columns under compression-bending-torsion loads and parameter effects are analysed. • Ultimate strength correlations of TLHR-CFDST columns for wind turbine towers are revealed. • A strength prediction method of TLHR-CFDST columns under combined loads is proposed. Concrete-filled double-skin steel tubular (CFDST) tower has emerged as a promising type of wind turbine support structure with excellent strength and stiffness. As the basic member in CFDST towers, tapered concrete-filled double-skin steel tubular columns with large hollow ratios (TLHR-CFDSTs) are frequently subjected to complex combined compression-bending-torsion loads. This paper presents a comprehensive analytical study on the coupled behaviour of the TLHR-CFDST column under combined loads and the relevant ultimate strength prediction method. A FE model was established to conduct numerical analyses and extensive parameter studies while the data from the previous experimental research was used to provide a verification basis. It is found that the column subjected to combined loads has three typical failure patterns, which are mainly governed by the bending-to-torsion ratio (m t). Columns with moderate m t undergo the bending-torsion failure and show obvious coupled performances. The load-deformation relation, the ultimate characteristics and the effects of important parameters, including axial compressive ratio (n), tapered angle (θ), hollow ratio (χ) and height-to-diameter ratio (λ) were discussed. The ultimate strength correlations of the column were revealed by analysing strength data within the parameter scopes commonly used in wind turbine towers. Lastly, a practical and accurate prediction method for the ultimate strength of the TLHR-CFDST column under combined compression-bending-torsion loads was proposed. [ABSTRACT FROM AUTHOR]

Concrete-filled double skin steel tubular columns with large hollow ratio (LHR-CFDST) have been commonly used in recent years. With the increasing size of the engineering structures, one efficient way to decrease the consumption of steel is to increase the diameter-to-thickness ratio. Few studies have been conducted to report on thin-walled LHR-CFDST columns. At the same time, local buckling may occur in these structures in the absence of stiffening measures. Based on this, an experimental and numerical analysis was carried out for the stiffened thin-walled LHR-CFDST columns under axial compression in this paper. Five thin-walled LHR-CFDST columns with different stiffening measures (ribs, studs) were tested. The failure modes, load versus shortening curves, local buckling behaviours and ductility were investigated. The test results indicates that the stiffened measures effectively delayed the appearance of local buckling and increased the peak load. Besides, the finite element models validated by the test results were applied to the subsequent mechanical mechanism analysis of the stiffened thin-walled LHR-CFDST. It can be seen that the stiffened measures enhance the confinement of the inner and outer steel tubes on the concrete and greatly improve the ultimate strength of the concrete. Finally, the predicted peak load formulae in the existing design specifications were modified by combining the characteristics of the specimens in this paper. The peak load values obtained by the improved formulae agreed well with the test results. • Five thin-walled CFDST columns with different stiffening forms (ribs, studs) were tested under axial compression. • The effects of stiffeners on the local buckling and peak load of specimens were revealed. • The validated finite element model was developed to further analyze the structural mechanisms. • The interaction between steel tubes and concrete were investigated. • The predicted peak load formulae in the existing design specifications were modified. [ABSTRACT FROM AUTHOR]

Tapered concrete-filled double-skin steel tubular columns with large hollow ratios (LHR-CFDSTs) possess light self-weight and good lateral rigidity. They exhibit tremendous potential in lateral load-critical structures, including large-span buildings, wind turbine towers and offshore infrastructures. In these structures, in addition to axial force and bending moment, columns are also commonly subjected to considerable torque. Although the compressive/flexural behaviour of tapered LHR-CFDST columns has attracted attention from researchers, studies on the torsional performance and mechanism are limited. This paper thus presents a comprehensive analytical study on the torsional behaviour of tapered LHR-CFDST columns on the basis of previous experimental research. Numerical analyses were conducted to investigate the influence of the key parameters. Full-range torsional characteristics of the column were revealed with regard to the typical torque–torsional angle relation, the stress development of the concrete and the contact between components. Load distributions as well as the effective twist length were discussed to understand the mechanism of the column under torsion. It is found that significant torsional deformation, oblique concrete cracks and struts, the compressive stress of concrete and the contact pressure all develop concentratedly within the effective twist length, which reflects the segments of the tapered column with a relatively weak torsion-resistant ability. Lastly, a torsional capacity prediction method was proposed by refining the previous theoretical model and it exhibits favourable accuracy for tapered LHR-CFDST columns with tapered angles higher than 1.50°. • Comprehensive numerical models of tapered LHR-CFDST columns are developed and verified. • Parametric effects and full-range torsional behaviour are analysed. • Load distribution and effective twist length are discussed to reveal torsional mechanism. • A refined torsional capacity prediction method of tapered LHR-CFDST columns is proposed. [ABSTRACT FROM AUTHOR]