Zhuojin Pipe Fitting Co., LtdPure Bending Performance of Circular Hollow Steel Tube Concrete Composite Members
Literature Overview
This research by Ren Qingxin, Wei Qiuyu, and Ding Jinan from Shenyang Jianzhu University investigates the pure bending behavior of circular hollow steel tube concrete composite members through finite element analysis. The study is funded by multiple national and provincial programs, reflecting its significance in structural engineering research. The authors validate their numerical model against existing experimental data, then systematically analyze the moment-deflection behavior, identify four characteristic points, examine the working mechanism at each point, and derive simplified calculation formulas for pure bending capacity.
Core Technical Findings
Hollow Rate Effect on Bending Capacity
The central finding of this paper is the quantification of how the hollow rate (the ratio of inner diameter to outer diameter, or more precisely the void area ratio) affects the bending capacity of the composite member. The study examines steel tube diameters of 80 mm, 100 mm, and 120 mm, corresponding to hollow rates of 0.137, 0.221, and 0.325 respectively.
| Outer Diameter (mm) | Hollow Rate | Capacity Reduction vs. Solid (%) |
|---|---|---|
| 80 (reference) | 0.137 | 0% (baseline) |
| 100 | 0.221 | 5.2% |
| 120 | 0.325 | 19.5% |
The non-linear relationship between hollow rate and capacity reduction is significant. The transition from 0.137 to 0.221 results in only a 5.2% capacity loss, while the increase from 0.221 to 0.325 causes a 19.5% reduction. This suggests that there is an optimal hollow rate range (approximately 0.15-0.25) where self-weight reduction is maximized with minimal structural penalty.
Four Characteristic Points in Moment-Deflection Curve
The authors define four characteristic points on the moment-deflection curve that represent distinct stages of the member's behavior:
- Elastic limit point: The member behaves linearly with both steel tube and concrete in elastic range.
- Yield point of steel tube: The outer fibers of the steel tube reach yield, initiating plastic deformation.
- Crushing point of concrete: The concrete at the compression zone reaches its crushing strength.
- Ultimate point: Maximum moment capacity is reached, typically associated with significant plastic deformation of the steel tube.
Working Mechanism Analysis
At each characteristic point, the stress distribution and deformation mechanism differ significantly. In the elastic stage, both materials contribute proportionally to load resistance. As the steel tube yields, the load redistributes to the concrete core, which continues to carry increasing compression. The hollow center creates a stress concentration at the inner boundary of the steel tube, but the concrete core effectively bridges this discontinuity. The interaction between steel and concrete is enhanced by the confining pressure developed at the interface, which increases the concrete's effective compressive strength.
Simplified Calculation Formula
Using the superposition principle and limit equilibrium principle, the authors derive a simplified formula for pure bending capacity. The formula accounts for:
- The contribution of the steel tube as a thin-walled section
- The contribution of the concrete core under confined conditions
- The interaction effect between steel and concrete
- The geometric effect of the hollow section
The calculated results show good agreement with experimental data, validating the proposed formula for practical design applications.
Engineering Practice Integration
From a structural engineering perspective, this research addresses a practical design challenge: how to optimize the cross-section of steel tube concrete members for bending applications while reducing self-weight. The hollow steel tube concrete concept offers several advantages:
- Weight reduction: By hollowing the center of the steel tube, the dead load of the member is reduced without proportionally reducing its load-bearing capacity.
- Material efficiency: The hollow rate can be optimized for specific loading conditions, particularly in bending-dominant applications.
- Constructability: The hollow center can accommodate reinforcement bars or utility conduits, adding functional versatility.
- Cost optimization: Reduced steel consumption in the tube wall (compensated by the hollow geometry) can lower material costs.
However, several practical considerations must be addressed in engineering implementation:
- The hollow section requires additional manufacturing steps (e.g., internal boring or extrusion) compared to solid tubes.
- Concrete placement into the hollow section requires careful pumping and vibration procedures to avoid voids.
- The interface bond between the steel tube and concrete in a hollow section may be less effective than in a solid section due to the reduced contact area.
- Fire protection requirements may be affected by the hollow geometry, as the reduced concrete cover could compromise fire resistance.
Key Questions and Reflections
The research raises several important questions for practical application. First, the study focuses on pure bending, but in real structures, members are typically subjected to combined loading (axial force plus bending). The interaction between axial compression and bending in hollow steel tube concrete members requires further investigation. Second, the long-term behavior under sustained loading, including creep and shrinkage effects on the hollow section, is not addressed. Third, the seismic performance of these members under cyclic loading is critical for earthquake-prone regions and should be evaluated.
The finite element methodology used (ABAQUS with validated material constitutive models) is appropriate for this type of analysis. The validation against existing experimental data provides confidence in the numerical predictions. However, the study would benefit from sensitivity analysis examining the influence of material property variations (concrete strength scatter, steel grade variations) on the predicted capacity.
Summary and Implications
This study provides valuable quantitative insights into the pure bending behavior of circular hollow steel tube concrete composite members. The finding that hollow rates up to approximately 0.22 can be achieved with minimal capacity reduction (5.2%) offers a practical design guideline for weight-optimized structural members. The proposed simplified calculation formula, validated against experimental data, provides engineers with a convenient tool for preliminary design calculations. The research contributes to the advancement of efficient steel-concrete composite structural systems, particularly for applications where self-weight reduction is a primary design objective while maintaining adequate bending capacity and ductility.