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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]