Design and Testing of Same-Tower Six-Circuit Steel Tube Transmission Tower

Literature Overview

The paper by Zhang Zifu, Mo Zenglu, and Li Qinghua, published in Power System Technology in 2010, presents the design and full-scale testing of a same-tower six-circuit steel tube transmission tower. This structure is designed to carry six separate transmission circuits on a single tower, which is a highly efficient use of land resources in areas where transmission corridor space is limited. The authors, affiliated with the China Electric Power Research Institute and the China National Nuclear Corporation Power Planning and Design Institute, address the unique structural challenges of this design, including the determination of bearing capacity calculation methods for steel tube members with small slenderness ratios, structural optimization, and economic comparison with conventional angle steel towers. The successful full-scale test validates the structural integrity and rationality of the design.

Core Technical Content

Steel Tube Member Design and Small Slenderness Ratio Challenge

The use of steel tube members in transmission towers offers advantages in terms of aerodynamic efficiency, reduced wind load, and improved corrosion resistance compared to conventional angle steel members. However, steel tube members typically have smaller slenderness ratios compared to angle steel members, which presents a challenge in the bearing capacity calculation. The conventional design codes for steel structures are based on the assumption of moderate to high slenderness ratios, and their applicability to small slenderness ratio members requires validation.

The authors conducted model tests on steel tube members with different end conditions (joint types) to determine the actual bearing capacity and the appropriate calculation method. The test results showed that the conventional buckling formulas overestimate the bearing capacity of small slenderness ratio steel tube members, necessitating a modified calculation approach.

Structural Optimization

The structural optimization of the six-circuit steel tube tower involved several key aspects:

1. The arrangement of the six circuits on the tower was optimized to minimize the overall tower weight while maintaining adequate clearance between circuits and between circuits and the ground.
2. The tower height and cross-arm geometry were optimized to achieve the required electrical clearance under all operating conditions, including conductor swing under maximum wind and ice loading.
3. The member sizing was optimized using parametric analysis to minimize the total steel weight while satisfying the strength, stiffness, and stability requirements.
4. The foundation design was optimized to minimize the excavation volume and concrete quantity, taking advantage of the reduced wind load provided by the aerodynamic efficiency of steel tube members.

Economic Comparison

The economic comparison between the steel tube tower and the conventional angle steel tower of the same type revealed that the steel tube tower, despite having a higher unit price per ton of steel, offers a lower total cost due to several factors:

• Reduced steel weight due to aerodynamic advantages and optimized structural design.
• Lower foundation cost due to reduced wind load.
• Reduced maintenance cost due to inherent corrosion resistance of the steel tube members.
• Longer service life due to the absence of corrosion-prone bolted connections.

Engineering Practice Integration

The design and construction of a same-tower six-circuit steel tube transmission tower presents several unique challenges that require careful attention in engineering practice:

1. The welding quality of the steel tube members is critical, as the welded sleeve or flange connections must transmit both axial and bending forces. The weld inspection must include both visual inspection and non-destructive testing (UT or RT) to ensure full penetration and absence of defects.
2. The geometric accuracy of the steel tube members must be controlled within tight tolerances to ensure proper fit-up during assembly. Ovality of the tube cross-section should be limited to less than 1 percent of the nominal diameter.
3. The erection sequence must be carefully planned to minimize the temporary stresses during construction. The use of temporary bracing and sequential loading is essential to prevent local buckling of the slender tube members.
4. The connection design between the steel tube members and the cross-arms must accommodate the differential thermal expansion between the tower body and the conductor system.

The full-scale test of the tower was a critical validation step. The test loading was applied incrementally to simulate the design load cases, including vertical load, wind load, and ice load. The measurements included member strains, tower displacement, foundation settlement, and connection behavior. The test results confirmed that the structural behavior was consistent with the design predictions, validating the modified bearing capacity calculation method for small slenderness ratio steel tube members.

Study Insights and Implications

The most significant insight from this research is the demonstration that steel tube transmission towers are technically feasible and economically competitive for multi-circuit applications. The small slenderness ratio challenge, which was previously considered a barrier to the widespread adoption of steel tube towers, has been addressed through experimental validation and modified calculation methods.

For the transmission line engineering community, this research opens the door to more efficient use of transmission corridors, which is increasingly important in urban and environmentally sensitive areas. The economic comparison demonstrates that the long-term benefits of steel tube towers, including reduced maintenance and longer service life, outweigh the initial cost premium.

The practical implication is that engineers designing multi-circuit transmission towers should consider steel tube members as a viable alternative to conventional angle steel members. The key to successful implementation lies in the proper design of connections, the control of manufacturing tolerances, and the rigorous quality assurance of welding. The full-scale test results provide the confidence needed to adopt this technology in critical transmission infrastructure projects. This paper represents a significant advancement in the field of transmission tower design and should be referenced by engineers involved in the design of multi-circuit transmission structures.