Secondary Stress Influencing Factors in 1000kV Double-Circuit Steel Pipe Towers

Literature Overview

Published in Power System Technology (Vol. 34, No. 2, 2010, pp. 20–23), this paper by Li Maohua, Yang Jingbo, Li Zhengliang, Han Junke, and Li Feng from Chongqing University and China Electric Power Research Institute investigates the secondary stress distribution in China's first 1000 kV AC ultra-high voltage (UHV) double-circuit steel pipe tower (SZT2 tower). The study combines a full-scale prototype test with two computational approaches—a truss model using transmission tower design software and a beam-rod hybrid truss model using ANSYS finite element software—to identify the distribution patterns and influencing factors of secondary stresses.

Core Technical Content

Research Background

In ultra-high voltage transmission tower design, the primary stress (主应力) is calculated using standard truss analysis assuming pin-jointed connections. However, actual joints exhibit partial rigidity due to the finite bending stiffness of the connected members, generating secondary stresses (次应力) that can be significant in large-diameter steel pipe towers. For the SZT2 tower, the maximum secondary stress in main chord members reaches 30% of the primary stress, necessitating explicit consideration in the design process.

Methodology and Comparative Analysis

Three analytical approaches were employed:

1. Truss model (杆单元桁架模型): Standard transmission tower design software assuming ideal pin joints—provides primary stress only
2. Beam-rod hybrid model (梁-杆混合桁架模型): ANSYS finite element model incorporating beam elements for members with significant bending stiffness and rod elements for slender members—captures secondary stress effects
3. Full-scale prototype test (真型试验): Physical testing of the complete SZT2 tower under representative loading conditions

Key Findings

1. Maximum secondary stress location: The tower body slope change locations (塔身变坡处) exhibit the highest secondary stress concentrations. These geometric discontinuities create local stiffness mismatches that amplify secondary stress effects.
2. Slenderness ratio effect: Under otherwise identical conditions, the secondary stress influence decreases with increasing member slenderness ratio (长径比). Slender members behave more like ideal truss elements with negligible bending stiffness, reducing the secondary stress contribution.
3. Quantitative magnitude: The maximum secondary stress in SZT2 tower main chord members reaches 30% of the primary stress, which is substantial and cannot be neglected in design.
4. Model validation: The beam-rod hybrid model results correlated well with the full-scale test measurements, validating the computational approach for secondary stress prediction.

Technical Parameters and Design Considerations

Influence Factor Ranking

Engineering Practice Integration

Design Recommendations for UHV Steel Pipe Towers

Based on the research findings, the following design guidelines should be applied:

1. Secondary stress allowance: Design codes should explicitly permit or require consideration of secondary stresses in UHV steel pipe towers, with a recommended allowance of 20%–30% above primary stress for capacity verification.
2. Slope change optimization: Tower body slope transitions should be designed with gradual geometric changes rather than abrupt angle changes to minimize stiffness discontinuities.
3. Member slenderness optimization: Where practical, increasing member slenderness ratios (within buckling constraints) can reduce secondary stress concentrations without compromising primary load capacity.
4. Hybrid modeling approach: The beam-rod hybrid finite element model should be adopted as the standard verification method for UHV steel pipe tower design, with the truss model serving as a preliminary screening tool.
5. Prototype testing protocol: Full-scale testing should be conducted for the first tower of each new design, with strain measurements at slope change locations and mid-span of longest members.

Fabrication and Welding Quality Considerations

For steel pipe towers subject to significant secondary stresses:

• Butt welds in main chord members at slope change locations should be 100% radiographically inspected
• Welding residual stresses should be reduced by post-weld heat treatment or vibration stress relief at critical joints
• Dimensional tolerances for tube straightness should be maintained within 1:1000 to ensure proper joint alignment and minimize unintended bending moments
• Bolted connections at slope change locations should be preloaded to specification to ensure joint rotational stiffness consistency

Key Reflections

This research addresses a fundamental limitation in traditional transmission tower design methodology—the assumption of ideal pin-jointed connections. The finding that secondary stresses can reach 30% of primary stresses in UHV steel pipe towers represents a significant design consideration that may have been overlooked in earlier practice. The beam-rod hybrid modeling approach provides a practical compromise between computational efficiency and accuracy, making it suitable for routine design verification. The identification of slope change locations as critical zones for secondary stress concentration offers clear guidance for both design optimization and inspection prioritization. For engineers involved in UHV transmission line projects, this work establishes the quantitative basis for incorporating secondary stress effects into the design process, ensuring adequate safety margins in structures subject to extreme environmental and electrical loading conditions. The correlation between full-scale testing and hybrid modeling validates the computational approach for future design applications where physical testing may be impractical.