Behaviour of concrete-filled circular steel tubular K-joints in wind turbine towers

The advantages of concrete-filled steel tubular lattice towers in stress efficiency, material consumption, and industrial construction are significant for the future development of ultra-long blades, ultra-high towers, and large-capacity units. The stress state of the joints is a key component of a wind tower structure. Therefore, experiments were conducted on four concrete-filled circular steel tubular (CFCST) K-joints in a lattice wind turbine tower, along with one hollow circular steel tubular (HCST) K-joint for comparison. The test results revealed that the CFCST K-joints failed due to local and overall buckling of the compression web, whereas no plastic deformation occurred on the chord. The stiffness of the CFCST K-joints significantly improved, and the stress concentration around the intersecting line was significantly reduced. Existing structural codes, both domestic and international, underestimate the bearing capacity of CFCST K-joints because the contribution of the core concrete cannot be accounted for. Finite element analyses indicate that the bearing capacity of the CFCST K-joints is dependent on the failure mode, primarily influenced by the thickness ratio τ and the angle θ. To prevent joint failure in the CFCST lattice wind turbine towers, specifically suggest τ ≤ 1 and θ ≤ 45°. Based on the bearing capacity calculation method proposed by CIDECT for HCST K-joints, design equations were obtained to predict the punching shear bearing capacity of CFCST K-joints using the least-squares method. The established formula exhibited high accuracy through regression verification.