Axial Compression Behavior of Hollow Sandwich CFST Transition Structures in Wind Turbine Hybrid Towers

Literature Overview

This paper by Wang Yuhang, Cao Feng, Zhou Xuhong, and colleagues from Chongqing University and CSSC Haizhuang Wind Power Co., Ltd., published in the journal "Progress in Steel Building Structures" (2023, Vol. 25, No. 4, pp. 25–36), addresses a critical structural challenge in the emerging field of wind turbine hybrid towers. The research is funded by the National Natural Science Foundation of China (Grant No. 52278144) and the Central Universities Basic Scientific Research Fund (2022CDJQY-009). The study proposes a novel hollow sandwich concrete-filled steel tube (CFST) transition structure to connect the upper steel tower section with the lower concrete tower section, and validates this concept through axial compression tests on four hollow sandwich CFST specimens and one conventional reinforced concrete (RC) comparison specimen.

Core Technical Concept and Structural Design

The transition zone in a steel-concrete hybrid wind turbine tower is subjected to complex combined loads including axial compression, bending, and torsion transmitted from the rotor and nacelle. Traditional reinforced concrete transition structures are prone to cracking and even catastrophic failure under these demanding conditions. The proposed hollow sandwich CFST transition structure employs an innovative configuration consisting of an outer steel shell, an inner steel shell, and a sandwiched concrete layer between them, with internal diaphragms and shear studs to ensure composite action between the steel components and the concrete core.

This design philosophy leverages the well-established confinement effect of steel tubes on concrete while introducing a double-skin concept that provides redundancy and improved ductility. The hollow sandwich configuration creates a layered composite action where both the inner and outer steel shells contribute to load-bearing capacity and lateral restraint of the concrete, effectively creating a sandwich panel effect that resists local buckling.

Experimental Program and Test Results

The experimental program consisted of five specimens: four hollow sandwich CFST transition specimens and one RC comparison specimen. The tests were conducted under monotonic axial compression to characterize the load-bearing capacity, deformation behavior, and failure modes of each configuration.

The key finding is that at the same steel tonnage, the hollow sandwich CFST configuration significantly improves both the load-bearing capacity and deformation capacity compared to the traditional RC structure. This represents a substantial structural efficiency gain, particularly important in wind turbine applications where material optimization directly impacts project economics and transportability.

Influence of Diaphragms and Shear Studs

The parametric study reveals that both diaphragms and shear studs play critical roles in enhancing the structural performance of the hollow sandwich CFST transition structure. The diaphragms serve to maintain the geometric stability of the inner and outer steel shells, preventing their premature local buckling under compressive loading. The shear studs, on the other hand, provide direct mechanical interlock between the steel shells and the sandwiched concrete, ensuring that the composite action is fully mobilized throughout the loading process.

From a welding and fabrication perspective, the installation of shear studs introduces specific quality considerations. Stud welding must be performed with precise control of heat input to avoid distortion of the thin steel shells. The weld quality of each stud must be verified through bend tests or pull-out tests, as inadequate stud welds would compromise the composite action and potentially lead to delamination between steel and concrete layers during service.

The study specifically notes that shear studs are more beneficial for enhancing structural integrity, producing more pronounced increases in ductility compared to diaphragms alone. This finding has direct implications for design optimization: when the design objective is to maximize energy dissipation capacity and post-peak deformation behavior, shear stud configuration should be prioritized.

Engineering Practice Implications

For wind turbine tower manufacturers and structural engineers, this research provides a validated structural solution for the challenging transition zone between steel and concrete tower segments. The hollow sandwich CFST approach offers several practical advantages: reduced concrete cracking risk, improved fatigue resistance at the steel-concrete interface, enhanced constructability through prefabrication of steel components, and potentially reduced foundation loads due to higher structural efficiency.

From a manufacturing standpoint, the fabrication of hollow sandwich CFST transition structures requires careful attention to dimensional tolerances of the steel shells, precise placement of diaphragms and shear studs during assembly, and controlled concrete placement to ensure complete filling of the sandwich cavity without voids. The welding of diaphragms to the steel shells must comply with applicable standards such as GB/T 150 or ASME Section VIII for pressure vessel-type fabrication, or more specifically, steel structure welding codes such as GB 50661 or AWS D1.1.

Key Questions and Reflections

Several questions arise from this research that warrant further investigation. First, the long-term durability of the hollow sandwich CFST transition under cyclic wind loading and potential corrosion of the inner steel shell in a concrete environment requires assessment. Second, the seismic performance of this configuration, particularly under the combined effects of earthquake-induced inertial forces and wind loads, has not been addressed. Third, the cost-effectiveness analysis comparing the hollow sandwich CFST approach with alternative transition solutions, including bolted flange connections and tapered transition sections, would strengthen the practical applicability of this concept.

The research demonstrates a sound engineering approach to solving a real-world structural problem through the combination of innovative design, experimental validation, and numerical analysis. The clear demonstration of performance superiority over conventional RC solutions positions this concept as a strong candidate for future wind turbine tower designs, particularly in applications where structural integrity and service life are paramount.

Study Insights and Outlook

The hollow sandwich CFST transition structure represents a meaningful advancement in wind turbine hybrid tower technology. The experimental evidence, supported by finite element analysis and theoretical modeling, provides confidence in the structural performance of this configuration. Future work should focus on cyclic loading tests to evaluate fatigue and seismic behavior, full-scale prototype testing, and detailed fabrication specifications that address welding quality, concrete placement procedures, and inspection protocols specific to this novel structural system. The integration of this technology into wind turbine design codes and manufacturing standards will be essential for its widespread adoption in the renewable energy sector.