Analytical behavior and bearing capacity research on out-of-code tapered CFDST members under pure torsion and compression-torsion combination

Abstract

Facing the apparent enhancement of rated power of offshore wind turbines, the increasing rotor diameter and blade length gradually pose severe challenges to its support structure for resisting extreme loads. Due to the remarkable composite effect, the recently developed tapered concrete-filled double skin steel tubular (CFDST) members can provide a high-performance structural type and competitive application prospect compared to traditional steel tower barrels. Therefore, this paper conducted the numerical and theoretical research on out-of-code tapered CFDST members under pure torsion and compression-torsion combination. The finite element (FE) model was established to examine the full-range mechanism including torque versus deformation response (T-θ curve), interaction behavior and component contribution, indicating that four characteristic points can distinguish the T-θ curve under pure torsion, and slight contact pressure between the inner tube and sandwich concrete exists along the height direction. The parametric study was performed to investigate the influences of geometric-physical parameters, e.g., by increasing tapered angle (ψ) 0.2°, the torsional capacity obtains a reduction of 5%; hollow ratios (χ), axial compression ratios, and diameter-to-thickness (D/t) ratios of double-skin tubes obviously affect the torsion resistance; altering steel yield grade influences torsional capacity than concrete grade. Subsequently, a theoretical multilayered cylinder model as the simplified design tool was proposed to simulate full-range T-θ curves of tapered CFDST members. Moreover, a modified design method for calculating N-T correlation curve was developed and validated by introducing the refined equations of pure torsion resistance (Tu), where the design methods in current codes displayed a quite conservative prediction for tapered CFDST members with out-of-code parameters (e.g., hollow ratios or D/t ratios). The above research on out-of-code tapered CFDST members can offer valuable design guidelines for its engineering applications.