Design of Lattice CFST Wind Turbine Towers

Design Challenges and Motivation

The paper by Wang Haijun and Kong Xiangyu, published in Journal of Shenyang University of Technology (Vol. 37, No. 3, 2015), addresses the design of lattice steel tube concrete-filled steel tube (CFST) wind turbine towers. As wind turbine hub heights continue to increase to capture stronger and more consistent wind resources, conventional conical tubular towers face increasing challenges in terms of structural efficiency, material economy, and constructability. The authors propose and analyze a lattice CFST tower configuration as an alternative structural solution for 3 MW wind turbines.

The key challenges driving this research include:

• Increasing tower height leads to disproportionately increasing bending moments at the base.
• Conical tubular towers require ever-increasing wall thickness at the base, leading to excessive steel usage.
• Transport and erection of large-diameter tubular sections become increasingly difficult at greater heights.
• Economic efficiency decreases as the structural weight grows faster than the power generation benefit.

Design Methodology

The authors employed two design approaches for the lattice CFST tower: the unified theory of CFST and the simplified space truss method. The design was based on a specific 3 MW wind turbine project, with the tower designed to meet the performance requirements defined by applicable standards.

Lattice Tower Configuration

The lattice tower consists of three column legs made of CFST members, interconnected by diagonal and horizontal web members forming a spatial truss structure. The three-column configuration was selected for its favorable combination of structural efficiency, constructability, and geometric simplicity.

Structural Design Considerations

The design of lattice CFST towers involves several distinct structural considerations that differ from conventional tubular towers:

1. Column leg design: Each column leg is designed as a CFST member, with the concrete core providing additional axial capacity and stiffness while the steel tube provides tensile strength and confinement.
2. Web member design: The diagonal and horizontal web members transfer shear forces and provide lateral stability. These members are typically designed as steel tubes or steel sections, depending on the specific loading requirements.
3. Joint design: The connection nodes between column legs and web members are critical for structural integrity. The joints must transfer axial forces, shear forces, and bending moments between the intersecting members.
4. Foundation design: The lattice configuration distributes the base reactions across three discrete points rather than a continuous ring, requiring careful foundation design to handle the concentrated loads.

Finite Element Analysis Results

The ANSYS finite element analysis compared the lattice CFST tower with a conventional conical tubular tower in terms of mechanical performance and steel usage. The key findings include:

Mechanical Performance Comparison

Economic Comparison

The lattice CFST tower demonstrated better economic performance in terms of steel usage. The space truss configuration allows for more efficient material utilization because:

• The concrete core in the CFST column legs provides additional compressive capacity without proportionally increasing steel usage.
• The lattice configuration avoids the need for thick-walled base sections that are required in tubular towers to resist high bending moments.
• The modular nature of the lattice structure facilitates prefabrication and transportation, potentially reducing construction costs.

Engineering Practice Implications

From a steel pipe fabrication and welding perspective, the lattice CFST tower introduces several technical considerations:

The CFST column legs require careful coordination between steel pipe manufacturing, concrete pouring, and structural assembly. The steel pipe must be fabricated to tight dimensional tolerances to ensure proper fit-up with the web members at the joints. The concrete filling process must be carefully controlled to avoid voids and ensure full confinement of the steel tube.

The joint design is particularly critical. The intersection of CFST column legs with web members creates complex stress states that require careful weld detailing. Potential welding challenges include:

• Welding of web members to the curved surface of the CFST column legs, which requires precise fit-up and possibly custom-cut connection plates.
• The need for full-penetration welds at critical joints to ensure structural continuity and fatigue resistance.
• The challenge of welding in confined spaces within the lattice structure, which may limit the choice of welding processes.

The study demonstrates that lattice CFST towers are a viable and economically attractive alternative to conventional tubular towers for medium and large wind turbines. The combination of CFST material efficiency with the structural advantages of lattice configurations offers a promising path for reducing the material cost and improving the constructability of tall wind turbine towers. Future work should focus on fatigue performance under cyclic wind loading, dynamic response under extreme wind events, and long-term durability of the concrete core within the exposed lattice environment.