Zhuojin Pipe Fitting Co., LtdLoad-Bearing Capacity of Steel Pipe Nodes in 750kV Lattice Structures
Literature Overview
The paper by Guo Hongchao, Jian Zheng, Si Jianhui, Liu Maoshe, Lang Xuhai, and Hu Feng from Xi'an University of Technology and China Power Engineering Consulting Group Northwest Electric Power Design Institute (published in Journal of Xi'an University of Technology, Vol. 30, No. 1, 2014) presents experimental research on the load-bearing capacity of steel pipe nodes in 750kV lattice structures. This work was supported by the National Natural Science Foundation of China (Grant 51308454) and multiple other funding programs. The research was conducted in conjunction with the Hami South 750kV Substation Project.
Core Technical Findings
The authors conducted full-scale tests on complex K-type and KT-type steel pipe nodes used in 750kV substation lattice structures. The test results analyzed the load-bearing performance, bearing capacity, connection plate stress distribution, and deformation amplitude of the nodes.
The key findings are:
- Under design loads, all five test specimens showed that both the members and connection plates remained in elastic working conditions.
- At 1.5 times the design load (overload stage), the members did not show significant phenomena, while the connection plates entered plastic deformation in individual locations.
- The plastic zones were small and developed slowly, indicating a high safety margin for the nodes.
| Test Stage | Load Level | Member Behavior | Connection Plate Behavior | Safety Assessment |
|---|---|---|---|---|
| Design load | 1.0 × F_d | Elastic | Elastic | Fully satisfactory |
| Overload | 1.5 × F_d | Elastic | Local plastic (small zone) | High safety margin |
| Ultimate | > 1.5 × F_d | Not tested | Progressive plastic | Not applicable |
Interpretation of Technical Points
The K-type and KT-type nodes are complex welded or bolted connections that are critical for the structural integrity of 750kV substation lattice structures. These nodes connect multiple steel pipe members at various angles, and their behavior under load is governed by the interaction of member bending, axial force, and connection plate deformation.
The finding that members remain elastic even at 1.5 times the design load is particularly significant. This indicates that the steel pipe members have sufficient cross-sectional capacity and that the design is not overly conservative. The connection plates, which are the weakest link in the node system, show local plastic deformation at 1.5 times the design load, but the plastic zones are small and develop slowly. This behavior is characteristic of ductile materials and indicates that the nodes have adequate reserve strength.
The stress distribution in the connection plates is a critical parameter. In welded nodes, the stress concentration at the weld toes can initiate cracks under cyclic loading. In bolted nodes, the stress distribution depends on the bolt preload and the contact pressure between the connection plates. The test results provide valuable data for validating finite element analysis models.
Engineering Practice Integration
In the design and construction of 750kV substation lattice structures, several measures derived from this study should be implemented:
- Node design verification: The design of K-type and KT-type nodes should be verified using both analytical methods and finite element analysis. The finite element models should be calibrated against the test results from this study.
- Weld quality control: For welded nodes, the weld quality is critical. Non-destructive testing (NDT) including ultrasonic testing (UT) and magnetic particle testing (MT) should be performed on all critical welds. The acceptance criteria should be stringent, typically requiring no cracks, no undercuts deeper than 0.5 mm, and no porosity exceeding the specified limits.
- Bolt preload control: For bolted nodes, the bolt preload must be controlled to ensure proper contact pressure between the connection plates. Torque wrenches or direct tension indicators should be used to verify the bolt preload.
- Connection plate thickness: The connection plate thickness should be sufficient to prevent local buckling under compressive loads. The minimum thickness should be determined based on the stress distribution results from the test.
From a quality control perspective, the inspection of steel pipe nodes should include:
- Visual inspection of welds and bolt connections
- Dimensional verification of connection plates
- NDT of critical welds
- Bolt preload verification for bolted connections
Key Questions and Reflections
A significant question is how the behavior of these nodes would change under cyclic loading conditions, such as those caused by wind or seismic activity. The static test results provide information on the ultimate load capacity, but the fatigue behavior under cyclic loading is equally important for the long-term safety of the structure.
Another consideration is the effect of corrosion on the node behavior. In outdoor environments, the connection plates and welds are susceptible to corrosion, which can reduce the effective cross-sectional area and initiate cracks. The design should account for corrosion allowance, and the maintenance program should include regular inspection and coating repair.
Study Insights and Implications
This research provides valuable experimental data for the design and verification of steel pipe nodes in 750kV substation lattice structures. The full-scale testing approach ensures that the results are directly applicable to practical design. The finding that the nodes have a high safety margin under design loads gives confidence in the structural integrity of the substation structures.
For structural engineers and designers, the key takeaway is that the K-type and KT-type steel pipe nodes are reliable components for 750kV substation structures. The connection plates are the critical elements that control the node behavior, and their design should be given special attention.
In summary, this study establishes that the steel pipe nodes in 750kV substation lattice structures have adequate load-bearing capacity and safety margins. The full-scale testing provides reliable data for design verification, and the findings support the continued use of these node types in high-voltage substation structures. The research contributes to the safety and reliability of electrical infrastructure by providing experimental validation of design assumptions.