Static behavior of large-diameter stiffened steel tubes for wind turbine towers under combined compression-bending-torsion load

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.