Zhuojin Pipe Fitting Co., LtdExperimental and FEM Study of Square Hollow Sandwich Steel Tube Concrete Compression-Torsion Members
Literature Overview
This paper by Guo Lixiang, Li Ting, Yang Jian, and Huang Hong, published in Industrial Construction (Volume 45, Issue 7, 2015, pp. 148-152), presents experimental and finite element analysis of square hollow sandwich concrete-filled steel tube (CHS-SCFST) members subjected to combined axial compression and torsion. The study investigates how hollow ratio, slenderness ratio, and axial compression ratio influence torsional resistance and failure behavior. Seven hollow sandwich specimens and one solid steel tube concrete reference specimen were tested. The research is supported by the National Natural Science Foundation of China (Grants 51008122 and 51378206) and Jiangxi Provincial Science and Technology Programs.
Core Technical Findings
The experimental results reveal several critical behaviors that are directly relevant to structural steel tube design and welding practice:
- All specimens exhibited good ductility, which is essential for seismic-resistant structural applications.
- The sandwich concrete layer developed diagonal shear cracks at approximately 45 degrees, consistent with principal tensile stress orientation under torsion.
- The concrete did not undergo crushing failure, indicating that the steel tube confinement effectively restrained the sandwich concrete layer.
- The relationship between axial compression ratio (n) and torsional capacity is non-monotonic: for n ≤ 0.3, increasing axial compression ratio increases torsional resistance; for n > 0.3, increasing axial compression ratio decreases torsional resistance.
This non-monotonic behavior is particularly significant for engineers designing composite steel tube structures. The transition point around n = 0.3 represents a critical design threshold where the beneficial confining effect of axial load on the sandwich layer transitions to a detrimental effect due to progressive crushing and loss of confinement interaction.
Technical Parameters and Test Configuration
| Parameter | Range / Value | Notes |
|---|---|---|
| Specimen type | Square hollow sandwich SCFST | 7 hollow + 1 solid reference |
| Slenderness ratio | Variable | Primary parameter |
| Hollow ratio | Variable | Primary parameter |
| Axial compression ratio (n) | 0 to >0.3 | Key transition at n = 0.3 |
| Failure mode | Diagonal cracking of sandwich concrete | 45-degree cracks |
| Ductility | Good | All specimens |
| Analysis method | FEM simulation | Torque-rotation curves |
Welding and Fabrication Considerations
From a steel tube fabrication and welding perspective, this research has direct implications for the manufacturing of hollow sandwich composite steel tube members:
- Welding of sandwich layer: The hollow sandwich configuration requires internal welding or mechanical connection of the inner steel tube to the outer steel tube. The quality of these internal welds directly affects the composite action and the 45-degree shear crack initiation. Field weld inspection (PT or MT) of internal weld joints is essential, though technically challenging.
- Geometric tolerances: The hollow ratio is a critical parameter, and dimensional accuracy of the inner tube relative to the outer tube affects the sandwich concrete layer thickness uniformly. Variations in weld bead profile at the tube junction can locally reduce the sandwich thickness, creating stress concentration points.
- Material selection: For seismic applications requiring good ductility, the steel grade should be selected to ensure adequate elongation and yield ratio. Common grades such as Q345 or Q355 (per GB/T 1591) provide the necessary ductility while maintaining structural strength.
FEM Validation and Engineering Implications
The FEM simulation reproduced the torque-rotation curves with good agreement, validating the numerical model for parametric studies. This is particularly valuable because full-scale compression-torsion testing of composite steel tube members is expensive and destructive. Engineers can leverage validated FEM models to explore design parameter combinations beyond what experimental programs can cover.
The key engineering insight is that for hollow sandwich steel tube columns in combined compression-torsion loading, the design should target an axial compression ratio below 0.3 to maximize torsional capacity. This aligns with general seismic design philosophy of keeping members in the elastic or early plastic range under combined loading.
Reflections and Study Insights
The 45-degree crack pattern in the sandwich concrete is a classic torsional failure signature, confirming that the sandwich layer behaves as a structural element rather than merely a filler. This validates the design concept of hollow sandwich composite tubes where the sandwich concrete actively participates in load transfer. For welding engineers, the implication is that the internal weld joints connecting the inner and outer tubes must be designed and inspected to the same standard as external welds, even though they are inaccessible after assembly.
The transition behavior at n = 0.3 is a critical design parameter that should be incorporated into design codes for composite steel tube structures. Current Chinese standards (GB 50017 and CECS 28) do not specifically address hollow sandwich configurations, and this research provides valuable data to inform future code revisions. The combination of experimental validation and FEM modeling offers a robust methodology that can be extended to other cross-sectional geometries and loading conditions.