Axial Compressive Bearing Capacity of High-Strength Steel Tubes in UHV Transmission Towers

Literature Overview

This 2010 paper by Yang Longyu, Li Zhengliang, Wei Lei, Kang Dongchang, Ma Zhenyi, and Duan Bei from Chongqing University addresses the design challenge of using high-strength steel tubes in ultra-high voltage (UHV) transmission towers, where existing Chinese design codes lack sufficient parameters. Funded by the National Natural Science Foundation (50678181) and State Grid Corporation of China (SGKJ[2007]413), the study combines finite element analysis, experimental testing, and comparison with existing design methods to establish the axial compressive bearing capacity of high-strength steel tube members. Published in the Journal of Xi'an University of Architecture and Technology (Vol. 42, No. 2, pp. 201–204), the paper bridges structural engineering research and practical power infrastructure design.

Core Technical Content

Background and Motivation

China's UHV transmission grid (500 kV and above) requires transmission towers with unprecedented span lengths and load capacities. Conventional carbon steel tubes (e.g., Q345, Q390) become uneconomical at the required dimensions because the increased member sizes lead to excessive self-weight and material consumption. High-strength steel tubes (e.g., Q420, Q460, Q550) offer higher yield strength, allowing smaller cross-sections for the same load capacity. However, the Chinese design codes at the time did not adequately address the design of high-strength steel tube members, particularly regarding:

• The validity of existing stability formulas for higher strength grades
• The effect of residual stresses from the manufacturing process on buckling behavior
• The appropriate slenderness ratio range for efficient design

Design Method Comparison

The authors compare several existing design approaches for the axial compressive bearing capacity of steel tube members:

Key Findings

1. Existing code methods are applicable: The Chinese code design method can be used for high-strength steel tube members, provided appropriate adjustments are made to account for the higher strength grade. The strength reduction factor (φ) decreases with increasing slenderness ratio (λ) and the characteristic curve shape remains valid.
2. Optimal slenderness ratio range: For UHV transmission tower applications, the optimal slenderness ratio range for high-strength steel tube members is 40–80. Below λ = 40, the member is in the inelastic buckling regime and the full strength advantage of high-strength steel is not utilized. Above λ = 80, the member is in the elastic buckling regime where the strength grade has minimal influence on bearing capacity, making the use of high-strength steel economically unjustified.
3. Finite element validation: The FE models, incorporating geometric imperfections, residual stresses, and material nonlinearity, confirmed that the code-based calculations provide conservative estimates of bearing capacity for high-strength steel tubes.

Slenderness Ratio and Bearing Capacity Relationship

Engineering Practice Integration

Application in UHV Transmission Tower Design

The practical implications of this research for power transmission tower design are significant. In a typical 500 kV or 750 kV transmission tower, the main tower members (legs, braces, and cross-arms) are subjected to combined axial compression, bending, and wind loading. The use of high-strength steel tubes allows:

• Reduced member cross-sections, lowering self-weight by 15–25%
• Reduced foundation loads, decreasing foundation cost
• Reduced transportation and erection requirements, since smaller members can be fabricated in standard lengths
• Extended service life, as high-strength steels typically exhibit better fatigue resistance

However, the design must account for:

• Welding effects: High-strength steels are more susceptible to hydrogen-induced cracking and reduced toughness in the heat-affected zone (HAZ). Preheating, controlled welding sequences, and post-weld heat treatment (PWHT) may be required.
• Cold-forming effects: If the tube is cold-bent or cold-formed, the strain-hardened regions may have reduced ductility and fracture resistance.
• Corrosion protection: High-strength steels may require more robust corrosion protection systems (e.g., hot-dip galvanizing with increased coating thickness) because the consequences of corrosion-induced section loss are more severe in high-strength members.

Quality Control Considerations

For high-strength steel tube members in transmission towers, the following quality control measures are recommended:

Study Insights

This paper addresses a practical gap in Chinese design codes that had been identified by the power industry but had not been systematically resolved through research. The finding that existing code methods are applicable to high-strength steel tubes, with appropriate modifications, is reassuring for practicing engineers who need to justify the use of higher-strength materials in safety-critical infrastructure. The identification of the optimal slenderness ratio range (40–80) provides a clear design guideline: members outside this range should be evaluated for alternative materials or cross-sections.

From a broader perspective, this research reflects a global trend in structural engineering: the pursuit of material efficiency through the use of higher-strength steels, coupled with rigorous validation through testing and numerical analysis. The challenge remains to balance the economic and performance benefits of high-strength materials against the increased demands on fabrication quality, welding procedures, and corrosion protection. For transmission tower engineers, the message is clear: high-strength steel tubes are a viable and efficient choice, provided that the design is informed by proper research and that fabrication quality is rigorously controlled.