Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Ring Steel Skeleton Concrete Filled Steel Tube Columns
Literature Overview
This paper, published in the Journal of Xi'an University of Architecture and Technology (Natural Science Edition) in 2022, presents experimental investigation on the axial compression performance of ring steel skeleton–concrete filled steel tube (CFST) columns. The study was conducted by researchers from State Grid Jiangsu Electric Power Construction Branch, China Energy Engineering Group Jiangsu Electric Power Design Institute, and Southeast University, supported by the State Grid Corporation of China Science and Technology Program. A total of 16 specimens were tested in horizontal (lying) loading configuration: 4 conventional CFST columns and 12 ring steel skeleton CFST columns. The research addresses a practical engineering need in power transmission tower design, where high-strength composite columns are increasingly required to reduce member cross-sectional dimensions while maintaining or improving load-bearing capacity.
Core Findings and Technical Analysis
The experimental results reveal that the introduction of internal ring steel skeletons substantially enhances the structural performance of CFST columns. Compared with conventional CFST columns, the ring steel skeleton CFST columns achieved a maximum increase of 20.39% in axial load-bearing capacity and approximately 82.27% improvement in axial stiffness. This is a significant finding, particularly the stiffness improvement which far exceeds the bearing capacity gain, indicating that the ring steel skeleton primarily functions to constrain lateral deformation of the steel tube wall.
| Parameter | Conventional CFST | Ring Skeleton CFST | Improvement |
|---|---|---|---|
| Axial bearing capacity | Baseline | Up to +20.39% | Moderate increase |
| Axial stiffness | Baseline | Up to +82.27% | Substantial increase |
| Failure mode | Local buckling of tube wall | Controlled progressive failure | Fundamentally altered |
The paper identifies two key structural components within the ring steel skeleton system: the internal ring steel plates and the overall circumferential plate. The experimental evidence demonstrates that the overall circumferential plate has a relatively minor influence on bearing capacity within the tested spacing range, yet contributes significantly to axial stiffness enhancement. The internal ring steel plates, on the other hand, participate in load-bearing throughout the entire loading process, providing continuous contribution to both capacity and stiffness.
Failure Mode Analysis and Structural Behavior
One of the most important contributions of this research is the detailed characterization of failure mode changes. Conventional CFST columns typically fail through localized outward buckling of the steel tube wall, which initiates at the column ends and propagates inward. The introduction of internal ring steel skeletons fundamentally alters this failure mechanism. The ring plates act as internal stiffeners that distribute the confining pressure more uniformly along the column length, preventing premature local buckling and promoting a more ductile, progressive failure pattern.
The strain distribution analysis provides critical insight into the load-sharing mechanism. Strain gauges placed at various locations on the steel tube wall show that the ring steel plates effectively redistribute stress concentrations, particularly at mid-span where conventional CFST columns are most vulnerable. The horizontal (lying) test configuration was selected deliberately to simulate the actual boundary conditions of transmission tower legs, which are subjected to combined axial and lateral loads. This testing approach is more representative of real service conditions than vertical compression tests.
Engineering Practice Implications
From a practical design perspective, this research has direct relevance to the design of transmission tower main legs and cross-bracing members. The key design parameters identified include the spacing of ring steel plates, the thickness of ring plates relative to tube wall thickness, and the overall circumferential plate configuration. The finding that spacing has limited effect on bearing capacity but significant effect on stiffness suggests that stiffness-critical applications (such as dynamic load scenarios) benefit more from denser ring plate arrangements.
For welding and fabrication considerations, the internal ring steel skeleton introduces additional weld joints within the column assembly. These welds must be designed to withstand the full range of stresses developed during both normal loading and potential overload conditions. The weld quality control should follow the standards applicable to pressure vessels and structural steel, with particular attention to the circumferential welds connecting ring plates to the tube wall, which are susceptible to lack of fusion and porosity defects.
Key Reflections
The magnitude of stiffness improvement (82.27%) compared to bearing capacity improvement (20.39%) warrants careful interpretation. This suggests that the ring steel skeleton primarily functions as a geometric stiffener rather than a load-bearing reinforcement. In design practice, this means the ring skeleton is most beneficial for columns where deflection or vibration control is critical, not merely for increasing ultimate strength. Engineers should evaluate whether the added fabrication complexity and welding cost justify the performance gains for their specific application.