Experimental Study and Numerical Simulation of Steel Tube-Confining Concrete K-Type Intersection Nodes

Literature Overview

This paper by Gao Chunyan and Wang Jiali, published in the Chinese Journal of Applied Mechanics in 2021, investigates the mechanical behavior of K-type welded intersection nodes (相贯节点) in steel tube-confined concrete (STC) lattice wind turbine towers. The research was conducted at the School of Civil Engineering, Inner Mongolia University of Science and Technology, and is supported by the National Natural Science Foundation (Project 51368042) and the Inner Mongolia Natural Science Foundation (Project 2018MS05041). Five full-scale K-type welded intersection node models were extracted from prototype lattice wind turbine towers and subjected to experimental testing and theoretical analysis.

Experimental Setup and Test Configuration

The study extracted five K-type welded intersection node models from actual lattice wind turbine tower prototypes, ensuring that the test specimens represent real structural configurations rather than idealized laboratory models. The nodes were subjected to loading tests to investigate the failure process, load-deformation relationships, and bearing capacity under different geometric parameter conditions.

Geometric Parameters Studied

Key Experimental Findings

Failure Mode Characteristics

The experimental results revealed that the circular STC K-type intersection node exhibits a distinctive failure behavior compared to hollow circular steel tube intersection nodes. The primary finding is that the chord tube does not show obvious macroscopic deformation before the loss of bearing capacity, and brace failure (腹杆失效) is the dominant failure mode. This is a significant finding because it indicates that the failure is localized at the brace rather than the chord, which has important implications for damage detection and structural health monitoring.

Comparison with Hollow Steel Tube Nodes

The study provides a valuable comparison between STC nodes and conventional hollow steel tube nodes:

The concrete core within the STC node provides a confinement effect that enhances the local stability of the steel tube at the intersection region. This confinement reduces the stress concentration around the intersection line (相贯线), which is a critical advantage for fatigue-resistant design in wind turbine applications.

Numerical Simulation and Parametric Analysis

A finite element model was established to systematically analyze the influence of various geometric parameters on the node failure mode and ultimate bearing capacity. The finite element analysis identified two controlling failure modes for the circular STC K-type intersection node:

1. Brace failure (腹杆失效): The brace tube fails through local buckling or plastic collapse at the intersection region.
2. Chord shear failure (弦杆冲剪破坏): The chord tube fails through punching shear failure at the intersection line.

Failure Mode Transition Criteria

The finite element analysis identified the wall thickness ratio τ and the brace-chord angle θ as the key indicators controlling the transition between failure modes. The study recommends limiting τ ≤ 1 and θ ≤ 45° to avoid node failure in STC lattice wind turbine towers.

Welding Process Considerations

The K-type intersection node involves the welding of brace tubes to the chord tube at the intersection line. The welding process for these nodes presents several challenges:

• The intersection line geometry varies along the weld, requiring adapted welding procedures for different sections
• The presence of the concrete core within the chord tube affects heat dissipation during welding
• Residual stresses from welding can interact with the concrete-steel interface, potentially affecting the confinement effectiveness
• The weld quality at the intersection is critical for fatigue performance in wind turbine applications

From a welding engineering perspective, the stress concentration reduction provided by the concrete core may partially compensate for weld imperfections, but this should not be relied upon as a substitute for proper welding quality control. The weld procedure should be qualified according to applicable standards (such as GB/T 985, ISO 15614, or AWS D1.1) with specific consideration for the STC configuration.

Engineering Practice Integration

The research findings have direct application to the design and fabrication of lattice wind turbine towers using STC members. The recommended parameter limits (τ ≤ 1, θ ≤ 45°) provide clear design guidelines for engineers. The identification of brace failure as the dominant failure mode suggests that damage detection efforts should focus on the brace tubes rather than the chord tubes, which has implications for inspection planning and maintenance strategies.

The stiffness enhancement provided by the concrete core is particularly valuable for wind turbine towers, where excessive deflection can affect the aerodynamic performance of the blades. The reduced stress concentration at the intersection line improves the fatigue life of the node, which is critical for the long service life (typically 20-25 years) required for wind turbine structures.

Study Insights and Recommendations

This research provides valuable experimental and numerical insights into the behavior of STC K-type intersection nodes under loading. The comparison with hollow steel tube nodes demonstrates the clear advantages of the STC configuration in terms of stiffness, stress distribution, and failure mode. The identified failure mode transition criteria based on τ and θ provide practical design guidelines that can be directly incorporated into structural design codes for lattice wind turbine towers. The emphasis on brace failure as the dominant failure mode has important implications for structural health monitoring and maintenance planning, directing inspection resources to the most critical locations.