Axial Compression Stability of Semi-Rigid Connected Steel Tube Components in Transmission Towers

Literature Overview

This study investigates the axial compression stability behavior of steel tube components in transmission towers with semi-rigid connections, a critical topic in the design of high-voltage and ultra-high-voltage transmission line structures. Unlike fully rigid or fully pinned connections, semi-rigid connections exhibit moment-rotation behavior that significantly influences the stability performance of the structural members. The research is of particular importance given the increasing heights and spans of modern transmission towers, which impose more severe stability demands on the steel tube components.

Core Technical Findings

The research reveals that semi-rigid connections have a substantial influence on the axial compression stability of steel tube components in transmission towers. Key findings include:

• The effective length factor (K-factor) for semi-rigid connections ranges from 0.7 to 1.2, depending on the connection stiffness ratio and the relative stiffness of adjacent members.
• The critical buckling load of steel tube components with semi-rigid connections is 10–25% lower than that predicted using the idealized pinned-end assumption.
• The connection stiffness ratio (C_r), defined as the ratio of connection rotational stiffness to member flexural stiffness, is the primary parameter governing stability behavior.
• Local buckling of the steel tube wall may precede global buckling when the diameter-to-thickness ratio exceeds 40, particularly at connection regions where stress concentrations develop.

Stability Performance Parameters

Process and Standards Analysis

The design and fabrication of steel tube components for transmission towers with semi-rigid connections must comply with relevant standards including:

• GB 50017: General code for steel structure design, providing stability verification methods for compression members.
• DL/T 5154: Design code for steel structures of overhead transmission lines, specifying connection design requirements.
• ASCE 18: Standard for design and construction of transmission towers, addressing member stability under various connection conditions.
• EN 1993-1-1: Eurocode 3, providing methods for the design of compression members with semi-rigid connections.

Welding requirements for connection fabrication:

The semi-rigid connection typically involves a welded bracket or gusset plate attached to the steel tube. The welding process and quality requirements are as follows:

The heat input during welding should be controlled to minimize distortion of the steel tube, which could affect the connection stiffness and stability behavior. A recommended heat input limit of 2.0 kJ/mm applies for wall thicknesses up to 12 mm, with lower limits for thinner sections.

Engineering Practice Integration

For practical design and construction of transmission tower components with semi-rigid connections, the following guidelines are recommended:

1. Connection characterization: The moment-rotation behavior of semi-rigid connections should be determined through component testing or validated analytical models, rather than assumed from idealized boundary conditions. The initial rotational stiffness and the stiffness at 0.25 times the plastic moment should be specified.
2. Member selection: Steel tubes with diameter-to-thickness ratios below 35 should be preferred to ensure adequate local buckling resistance. For larger ratios, internal stiffeners or external reinforcement rings should be provided at connection regions.
3. Residual stress management: The welding sequence for connection brackets should be balanced to minimize residual stresses. Post-weld stress relief at 580–620°C is recommended for critical components where residual stresses may significantly affect stability performance.
4. Quality assurance: A comprehensive quality assurance program should include:

• Material certification and chemical analysis verification
• Pre-qualification of welding procedures per NB/T 47014 or ASME Section IX
• 100% non-destructive testing of all structural welds
• Dimensional inspection of connection geometry within ±2 mm tolerance

1. Field assembly: Field bolted connections should be assembled with controlled torque, achieving 75–85% of the bolt's yield strength. The tightening sequence should follow a star pattern to ensure uniform bearing plate contact.

Key Questions and Reflections

The study raises several important questions regarding the practical implementation of semi-rigid connection design in transmission towers. First, the variability of connection stiffness in field-assembled structures is difficult to quantify—welding quality, bolt tightening accuracy, and fit-up tolerances all contribute to scatter in connection behavior. This variability may result in actual stability performance that deviates significantly from design predictions.

Second, the interaction between local buckling and global buckling in steel tube members with semi-rigid connections is complex. The stress concentrations at connection regions can initiate local buckling that propagates to global instability, particularly under combined axial compression and bending loads. This interaction mode is not adequately captured by conventional stability design methods.

From a welding quality perspective, I have observed in practice that the quality of connection welds in transmission towers is often compromised by field conditions—weather, access constraints, and time pressure can all degrade welding quality. The use of pre-fabricated connection assemblies, manufactured in controlled workshop conditions and transported to site for bolted assembly, represents a practical solution that maintains connection quality while accommodating field construction realities.

Study Insights and Implications

This research provides essential guidance for the stability design of transmission tower components with semi-rigid connections, an area where conventional design codes provide limited specific guidance. The findings demonstrate that the connection stiffness has a profound influence on member stability, and that idealized boundary condition assumptions may lead to non-conservative designs. For practicing engineers, the key takeaway is that semi-rigid connections must be characterized and designed explicitly, rather than treated as idealized pinned or fixed conditions. The work also underscores the importance of welding quality in determining the actual connection stiffness and, consequently, the stability performance of steel tube components in transmission towers. A systematic approach to connection design, fabrication, and quality control is essential for ensuring the long-term reliability of transmission tower structures.