Zhuojin Pipe Fitting Co., LtdPerformance Study of Skew-Grid Steel Tube Concrete Planar Intersecting Nodes
Literature Overview
The paper by Wan Haitao, Yu Dianlong, Han Xiaolei, Zhang Hanwen, and Wang Yifan, published in Journal of Henan University (Natural Science) (2017, Vol. 47, No. 6, pp. 739-744), investigates the structural performance of planar intersecting nodes in skew-grid steel tube concrete structures. This research is particularly significant for engineers designing space structures such as stadium roofs, airport terminals, and large-span industrial buildings where steel tube concrete members are increasingly employed for their superior load-bearing capacity and fire resistance.
Research Methodology and Specimen Design
The authors designed four planar intersecting node specimens to evaluate the structural performance of skew-grid configurations. The specimens were tested under controlled loading conditions, and the experimental results were compared with finite element analysis (FEA) using ABAQUS software.
Specimen Configuration
| Parameter | Description |
|---|---|
| Number of specimens | 4 |
| Node type | Planar intersecting (K-joint variant with skew grid) |
| Chord member | Circular steel tube filled with concrete |
| Brace members | Circular steel tubes (hollow or concrete-filled) |
| Skew angles | Multiple configurations studied |
| Loading modes | Symmetric and asymmetric loading |
The skew-grid configuration differs from conventional orthogonal grid structures in that the brace members intersect the chord member at non-orthogonal angles. This introduces complex stress distributions at the node, including significant membrane stresses, bending moments, and potential local buckling of the chord wall.
Finite Element Analysis and Parameter Study
The FEA model was developed using ABAQUS with appropriate material models for both the steel tube and the concrete infill. The key parameters investigated include:
- Node configuration: Different geometries of the skew-grid intersection
- Skew angle: The angle between the brace member and the chord member axis
- Loading mode: Symmetric versus asymmetric loading conditions
Key Findings from FEA and Experimental Comparison
| Comparison Metric | Agreement Level | Notes |
|---|---|---|
| Load-displacement skeleton curve | Strong agreement | FEA captures the elastic, yielding, and post-peak behavior |
| Load-strain curve | Strong agreement | Strain distribution in chord and brace members matches well |
| Load capacity | Strong agreement | Maximum load predictions within acceptable engineering tolerance |
The study confirms that the skew-grid steel tube concrete planar intersecting nodes possess good structural rationality and strong load-bearing capacity. Importantly, the research demonstrates that the loading mode (symmetric or asymmetric) has minimal influence on the overall structural response and failure mode.
Technical Analysis of Node Behavior
The structural behavior of skew-grid nodes can be understood through the following mechanical principles:
Stress Distribution at the Node
At the intersection of the skew brace and the chord member, complex stress states develop:
- Membrane stresses: Concentrated at the intersection cutout, particularly in the direction perpendicular to the brace axis
- Bending stresses: Generated by the eccentricity of the brace load relative to the chord centroid
- Shear stresses: Transmitted through the chord wall thickness, particularly critical near the intersection
Failure Modes
Based on the research findings and general knowledge of steel tube concrete node behavior, the following failure modes are possible:
| Failure Mode | Description | Governing Parameter |
|---|---|---|
| Chord wall local buckling | Inward buckling of the chord wall at the intersection | Diameter-to-thickness ratio (d/t) |
| Chord wall yielding | Plastic deformation of the chord wall | Chord wall thickness and material strength |
| Brace member yielding | Plastic deformation of the brace member near the connection | Brace member cross-section and material strength |
| Concrete crushing | Local crushing of concrete infill | Concrete strength and confinement ratio |
| Weld failure | Failure of the fillet weld connecting brace to chord | Weld size and quality |
Engineering Practice Implications
From a practical design perspective, several recommendations emerge from this research:
- Design standards: Engineers should follow the relevant Chinese standards (GB 50017, GB 51246) and international standards (CIDECT, Eurocode 3 Part 1-8) for the design of steel tube concrete nodes. The skew-grid configuration requires special attention to the interaction effects that may not be fully captured by simplified design formulas.
- Finite element verification: Given the complexity of stress distributions in skew-grid nodes, FEA verification is strongly recommended for critical connections. The research demonstrates that ABAQUS-based models can accurately predict node behavior when properly calibrated.
- Construction considerations: The concrete infill of the steel tubes must be carefully placed and compacted to ensure full density and bond with the steel tube. Incomplete concrete placement can significantly reduce the load-bearing capacity of the node.
- Welding quality: The fillet welds connecting the brace members to the chord member are critical for load transfer. Weld quality should be verified through non-destructive testing (MT, PT, or UT) in accordance with relevant standards.
Study Insights and Reflections
This research contributes valuable experimental and numerical data to the understanding of skew-grid steel tube concrete nodes. The finding that loading mode has minimal influence on structural response is particularly encouraging from a design perspective, as it simplifies the analysis and allows for more flexible design solutions.
However, I would note that the study is limited to four specimens, which may not fully capture the range of possible behaviors under different geometric and material conditions. Future research should consider:
- A broader parametric study covering a wider range of skew angles, diameter-to-thickness ratios, and concrete strengths
- Cyclic loading tests to evaluate the seismic performance of skew-grid nodes
- Full-scale testing to validate the results of component-level tests
- Investigation of the effect of concrete infill density and placement quality on node performance
In conclusion, the skew-grid steel tube concrete planar intersecting node configuration is a viable and structurally sound solution for space structures. The research provides confidence that these nodes can be reliably designed and constructed when appropriate analysis methods and quality control measures are employed.