Zhuojin Pipe Fitting Co., LtdCreep Analysis of Steel Pipe Concrete Axially Compressed Members
Literature Overview
The paper by Yang Yang, Wang Qicai, Zhang Rongling, Cui Jianlong, and Ge Yong, published in Sichuan Building Science (2015, Vol. 41, No. 2, pp. 70-72), presents a theoretical and analytical study on the creep behavior of steel pipe concrete members under axial compression. Supported by the Changjiang Scholars and Innovative Research Teams Program (IRT1139) and the Lanzhou Jiaotong University Youth Science Fund (2012028), this research was conducted at Lanzhou Jiaotong University. The study derives a stress redistribution calculation method based on deformation compatibility conditions, incorporating a converted elastic modulus for concrete.
Core Technical Points
Creep in steel pipe concrete members is a time-dependent deformation phenomenon that significantly affects the long-term structural performance. Unlike steel, which exhibits relatively minor creep under typical structural loads, concrete undergoes substantial creep deformation that can lead to:
- Progressive stress redistribution between steel and concrete
- Increased deflection in columns and beams
- Reduced effective prestress in prestressed members
- Potential long-term instability under sustained loads
The key innovation of this paper is the introduction of a converted elastic modulus (E_concrete,eq) for the concrete core, which accounts for the time-dependent material behavior in a simplified manner. By applying deformation compatibility conditions between the steel pipe and concrete core, the authors derive a method for calculating stress redistribution over time.
Creep Mechanism and Stress Redistribution
| Time Period | Creep Coefficient (φ) | Stress Redistribution | Structural Effect |
|---|---|---|---|
| 0-7 days | 0-0.5 | Minimal redistribution | Initial elastic response |
| 7-28 days | 0.5-1.5 | Moderate redistribution | Concrete creep dominates |
| 28 days-1 year | 1.5-3.0 | Significant redistribution | Steel carries more load |
| 1 year-10 years | 3.0-5.0 | Major redistribution | Steady-state reached |
| Beyond 10 years | 5.0-7.0 | Minor additional changes | Long-term equilibrium |
The creep coefficient (φ) represents the ratio of creep strain to elastic strain and is a function of time, concrete age at loading, concrete grade, and environmental conditions. For lightweight or low-strength concrete, the creep coefficient is typically higher, which means more significant stress redistribution occurs.
Influence Factors Analysis
The study systematically examines the effects of several key parameters on creep behavior:
Steel ratio (ρ): A higher steel ratio means a larger proportion of the cross-section is steel, which has much lower creep. This results in less overall creep deformation but also less stress redistribution, as the steel already carries a larger share of the load.
Stress level (σ): Higher initial stress levels lead to greater creep strains due to the non-linear nature of concrete creep. The creep strain is approximately proportional to the stress level for stresses below 0.5 times the compressive strength, but becomes significantly non-linear at higher stress levels.
Material grade: Higher-strength concrete generally exhibits lower creep due to its denser microstructure and lower porosity. However, the confinement effect of the steel pipe is more pronounced in higher-strength concrete, partially offsetting the reduced creep.
Stress Redistribution Methodology
The proposed method involves the following steps:
- Calculate the initial elastic stress distribution in steel and concrete based on the applied axial load and deformation compatibility
- Determine the creep strain of concrete at the given time using the creep coefficient and initial elastic strain
- Apply deformation compatibility to calculate the additional strain in the steel pipe due to concrete creep
- Calculate the stress redistribution: stress transferred from concrete to steel equals the creep strain multiplied by the steel elastic modulus
- Update the stress state and iterate if necessary for non-linear behavior
This iterative approach captures the progressive nature of stress redistribution and is particularly important for high stress levels where concrete creep is non-linear.
Engineering Practice Implications
For steel pipe concrete structures in practice, creep effects are most significant in:
- Tall buildings where columns are subjected to long-term gravity loads
- Long-span bridges where creep affects deflection and camber
- Nuclear power plants where concrete components are under sustained high loads
- Underground structures where creep affects long-term stability
The steel pipe fabrication process directly influences creep behavior through several mechanisms:
- Pipe surface quality: A smooth inner surface may reduce friction but also reduce bond effectiveness, affecting stress transfer
- Welding residual stresses: Residual stresses from pipe fabrication welding can affect the initial stress state and subsequent creep behavior
- Pipe straightness and dimensional accuracy: Deviations from ideal geometry can create secondary bending moments that interact with creep deformation
Key Questions and Reflections
The primary limitation of the proposed method is the assumption of uniform stress distribution across the cross-section, which is valid for axially loaded columns but not for eccentrically loaded or bent members. Additionally, the method does not explicitly account for shrinkage, which occurs simultaneously with creep and can have significant effects on the long-term behavior of concrete structures.
Another important consideration is the effect of temperature on creep. In high-temperature environments (such as industrial plants or fire conditions), concrete creep is significantly accelerated, which could lead to unexpected stress redistribution and potential structural failure. The method proposed in this paper is applicable primarily to ambient temperature conditions.
Study Insights and Implications
This research provides a practical and theoretically sound method for calculating stress redistribution due to creep in steel pipe concrete members. The introduction of a converted elastic modulus is a clever approach that simplifies the time-dependent analysis while maintaining reasonable accuracy. For steel pipe manufacturers and structural engineers, the key implication is that creep effects must be considered in the design and assessment of steel pipe concrete structures, particularly for long-term serviceability and durability. The method can be implemented in structural analysis software to enable more accurate long-term performance prediction, and the stress redistribution data can inform maintenance and inspection strategies for existing structures.