Zhuojin Pipe Fitting Co., LtdCreep Analysis of Axially Compressed Concrete-Filled Steel Tube Members
Literature Overview and Research Context
The paper by Wang Yuanfeng and Han Bing from Northern Jiaotong University presents a theoretical framework for analyzing creep behavior in axially compressed CFST members. Published in 2000 in the China Journal of Highway and Transport, this work was funded by the Ministry of Railways Science and Technology Development Fund. The research is grounded in the secondary flow theory of concrete creep and the creep theory of concrete under multiaxial stress states, adapted specifically to the loading characteristics of CFST members under axial compression.
Core Technical Content
The authors derive creep calculation formulas for axially compressed CFST members that account for the unique features of CFST creep behavior while capturing the influence of multiple factors including steel ratio, material strength, and stress magnitude. The derived formulas enable iterative computation of creep deformation over time, and the calculated results show good agreement with experimental data from existing literature.
The theoretical framework rests on several key assumptions and derivations:
- The secondary flow theory provides the basis for time-dependent deformation modeling, where creep strain is expressed as a function of stress history and elapsed time.
- Multiaxial stress creep theory is adapted to account for the confining effect of the steel tube on the core concrete, which fundamentally alters the creep behavior compared to plain concrete.
- The steel ratio (ratio of steel cross-sectional area to total cross-sectional area) is identified as a critical parameter influencing creep magnitude.
- Material strength levels of both steel and concrete significantly affect the creep response.
- The applied stress level relative to material strength governs the rate and magnitude of creep deformation.
| Influencing Factor | Effect on Creep | Direction |
|---|---|---|
| Steel ratio | Higher steel ratio reduces creep | Inverse |
| Steel strength | Higher strength slightly reduces creep | Inverse |
| Concrete strength | Higher strength reduces creep | Inverse |
| Applied stress | Higher stress increases creep | Direct |
| Confinement effect | Lateral confinement reduces creep | Inverse |
Significance for CFST Structural Design
Creep is a critical time-dependent deformation mechanism that affects the long-term performance of CFST structures, particularly in bridge engineering and long-span structures where sustained loads are present. For CFST arch bridges, creep deformation can lead to significant changes in geometry over time, affecting load distribution, stress levels, and overall structural safety. The formulas developed by Wang and Han provide engineers with a practical tool for predicting long-term deformation and incorporating creep effects into design calculations.
From a steel pipe manufacturing standpoint, this research underscores the importance of controlling the quality of both the steel tube and the core concrete. The steel tube acts as a confining element that restrains the concrete, and the effectiveness of this confinement depends on the bond between the steel tube inner surface and the concrete. Any gaps, voids, or poor contact between the tube wall and concrete would reduce the confining effect and increase creep deformation. This has direct implications for concrete placement procedures in CFST fabrication: proper vibration, adequate slump, and careful pouring sequences are essential to ensure full contact and minimize voids.
The iterative calculation approach described in the paper is practical for engineering applications but requires careful implementation. Engineers should be aware that creep is nonlinear and time-dependent, and simplified linear models may underestimate long-term deformation. The influence of environmental factors such as humidity and temperature, which are not explicitly discussed in the paper, should also be considered in real-world applications.
Study Insights and Practical Considerations
This 2000-era research remains relevant because creep is a fundamental material behavior that continues to affect CFST structures. Modern design codes such as GB 50935 and AISC 360 incorporate creep considerations, but the theoretical depth presented by Wang and Han provides a foundation for understanding the underlying mechanisms. Engineers working on CFST structures should integrate creep predictions into their design workflow, particularly for long-span bridges, tall building cores, and heavy-load industrial structures where sustained loading is a significant design consideration.