Zhuojin Pipe Fitting Co., LtdHysteretic Model for Circular Hollow Sandwich Steel Tube Concrete Compression-Bending Members Under Cyclic Loading
Literature Overview
Chen Xuejia, Chen Mengcheng, and Huang Cheng (Nanchang Institute of Technology and East China Jiaotong University) present a study on the hysteretic behavior of circular hollow sandwich steel tube concrete (CHS-SC) compression-bending members under reversed loading. Published in Railway Construction (Volume 51, Issue 4, 2011, pages 141-144), this work employs finite element analysis to simulate the complete load-displacement response, incorporating concrete stiffness and strength degradation under cyclic loading as well as the Bauschinger effect in steel. The research was supported by the National Natural Science Foundation of China (Project No. 50668006).
Core Technical Findings
The finite element model successfully reproduces the experimental load-displacement curves, validating the modeling approach. The parametric analysis covers the nominal steel ratio, axial compression ratio, slenderness ratio, and hollow ratio. A simplified hysteretic relationship model is proposed for practical engineering application.
| Parameter | Effect on Skeleton Curve Peak Load | Effect on Post-Peak Degradation |
|---|---|---|
| Nominal steel ratio | Positive correlation | Reduced degradation |
| Axial compression ratio | Positive up to optimum, then negative | Increased degradation |
| Slenderness ratio | Negative correlation | Increased degradation |
| Hollow ratio | Negative correlation | Increased degradation |
Technical Interpretation of Key Points
The hollow sandwich configuration introduces a unique mechanical behavior compared to conventional filled steel tube concrete members. The hollow core creates a layered constraint mechanism where the outer steel tube provides confinement to the inner concrete annulus, while the inner steel tube provides additional confinement to the core concrete. Under cyclic loading, the interaction between these components creates complex stress redistribution patterns.
The Bauschinger effect in the steel components is particularly significant in the hollow sandwich configuration because the inner and outer steel tubes experience different levels of strain due to their distinct geometric positions. This differential strain behavior means that upon load reversal, the inner tube may exhibit more pronounced strength degradation than the outer tube. The finite element model must accurately capture this phenomenon to predict realistic hysteretic loops.
The proposed simplified hysteretic model offers a practical tool for seismic analysis of structures employing CHS-SC members. The model accounts for:
- Stiffness degradation under repeated loading cycles
- Strength deterioration with increasing displacement amplitude
- Pinching effects caused by concrete cracking and steel yielding
- Residual displacement accumulation
Process and Standards Analysis
From a materials and fabrication standpoint, the CHS-SC member requires careful attention to the following technical aspects:
| Fabrication Aspect | Technical Requirement | Relevant Standard |
|---|---|---|
| Outer tube manufacturing | ERW or SSAW, minimum 10 mm wall thickness recommended | GB/T 8163, API 5L |
| Inner tube manufacturing | Seamless preferred for dimensional accuracy | GB/T 8162, ASTM A53 |
| Concrete placement | Pumped or vibrated through access openings | GB 50204 |
| Tube-to-tube connection | Ring stiffeners or welded end plates | GB/T 50017 |
| Weld quality | Full-penetration welds at tube junctions | SY/T 4103 |
The hollow ratio directly affects constructability. Excessive hollow ratios complicate concrete placement and vibration, potentially leading to honeycombing and voids that compromise the confinement mechanism. Engineering practice suggests maintaining a hollow ratio below 0.5 for reliable concrete filling.
Integration with Engineering Practice
The CHS-SC member concept finds application in bridge piers, offshore platforms, and seismic-resistant building columns where enhanced energy dissipation is required. The parametric insights from this study directly inform the design of such members:
- For seismic zones with high ductility demands, a nominal steel ratio above 0.20 is recommended to ensure adequate post-yielding deformation capacity.
- The axial compression ratio should be limited to 0.40-0.60 to balance compressive capacity with ductility requirements.
- Slenderness ratios exceeding 15 require additional lateral bracing to prevent elastic buckling before the hysteretic capacity is fully mobilized.
The simplified hysteretic model proposed in this study can be directly implemented in nonlinear time-history analysis software, enabling engineers to assess the seismic performance of CHS-SC structures without resorting to computationally intensive fiber-section models.
Key Questions and Reflections
The study appropriately acknowledges the limitations of the simplified model, particularly its reduced accuracy for very large displacement demands beyond 4% drift. The question of how the hollow ratio affects the long-term durability of the CHS-SC member, particularly regarding corrosion of the inner tube surface, remains open. Furthermore, the transition from the simplified model to a full structural analysis requires calibration of equivalent member properties, which may not be straightforward for the asymmetric constraint provided by the hollow sandwich geometry.
Study Insights and Implications
This research bridges the gap between complex finite element simulation and practical engineering design by providing a simplified yet physically motivated hysteretic model. For welding engineers, the study reinforces the importance of achieving full fusion at all steel-to-steel interfaces, as the hysteretic performance depends critically on the integrity of these connections under repeated loading. The parametric study methodology, combining numerical simulation with experimental validation, represents a rigorous approach that should be emulated in future research on novel composite structural members.