Zhuojin Pipe Fitting Co., LtdThermal Stress Analysis of CFST Expansion Concrete Under Seasonal Temperature Variations
Literature Overview
This paper by Ren Zhigang, Hu Shuguang, Ding Qingjun, Lv Linnu, and He Yongjia (2008), published in the Journal of Wuhan University of Technology, addresses a critical but often overlooked aspect of CFST design: the thermal stress induced by seasonal temperature differences in circular CFST members. Supported by the National Natural Science Foundation of China (Grant No. 50508034), the research derives two-dimensional thermal stress equations in cylindrical coordinates using linear thermoelastic theory and applies finite element analysis to evaluate the deformation and stress states under summer and winter temperature conditions.
Core Technical Approach
The fundamental innovation of this work is the recognition that the expansion performance design of CFST members, which traditionally focuses on compensating early-age concrete shrinkage, must also account for seasonal thermal cycling. The authors derive the governing equations for thermal stress in a circular CFST member under axisymmetric conditions:
| Governing Equation Component | Expression | Physical Meaning |
|---|---|---|
| Thermal strain | ε_T = α·ΔT | Linear thermal expansion |
| Radial equilibrium | dσ_r/dr + (σ_r - σ_θ)/r = 0 | Force balance in radial direction |
| Compatibility | dε_θ/dr + (ε_r - ε_θ)/r = 0 | Geometric continuity |
| Constitutive (steel) | σ = E_s(ε - α_s·ΔT) | Hooke's law with thermal correction |
| Constitutive (concrete) | σ = E_c(ε - α_c·ΔT) | Hooke's law with thermal correction |
The key insight is that the differential thermal expansion coefficients of steel (α_s ≈ 12×10⁻⁶/°C) and concrete (α_c ≈ 10×10⁻⁶/°C) create interfacial stresses that can be significant under large temperature gradients.
Thermal Stress Analysis Results
The FEA analysis reveals that under seasonal temperature variations, the following stress patterns develop:
| Condition | Steel Tube Radial Stress | Concrete Radial Stress | Interface Stress |
|---|---|---|---|
| Summer (uniform heating) | Compressive | Tensile | Compressive |
| Winter (uniform cooling) | Tensile | Compressive | Tensile |
| Summer (gradient heating) | Complex distribution | Complex distribution | Variable |
| Winter (gradient cooling) | Complex distribution | Complex distribution | Variable |
The study demonstrates that seasonal temperature differences can induce thermal stresses that partially counteract or reinforce the stresses from concrete shrinkage and creep. In summer conditions, the differential expansion of the steel tube relative to the concrete core creates an additional confining effect, while in winter, the differential contraction can relieve some of the confining stress. This cyclic behavior has implications for the long-term durability of the steel-concrete interface.
Implications for Steel Pipe Manufacturing and Quality Control
From the perspective of steel pipe manufacturing, this research highlights several quality control considerations:
- Pipe straightness and roundness: Thermal stresses in CFST members are sensitive to geometric imperfections. A non-circular or non-straight steel tube will develop non-uniform thermal stresses that can accelerate fatigue cracking. Manufacturing tolerances for roundness should be within ±1% of the nominal diameter, and straightness within 1:1000 of the pipe length.
- Weld quality in longitudinal seams: The longitudinal weld in a welded steel tube is a potential crack initiation site under cyclic thermal loading. The weld metal and HAZ may have different thermal expansion coefficients than the base metal, creating localized stress concentrations. Full-penetration welding with proper preheat and interpass temperature control is essential.
- Surface quality and coating integrity: The steel tube surface in contact with concrete must be free of scale, rust, and other contaminants that could affect the thermal and mechanical bond. Any protective coating applied to the steel tube must be compatible with the thermal cycling conditions and must not delaminate under the induced stresses.
Coupled Effects and Design Recommendations
The paper emphasizes that seasonal thermal effects should not be considered in isolation but should be coupled with:
- Concrete shrinkage and creep: Long-term volume changes that interact with thermal strains
- Load level: Axial and bending loads that modify the stress state
- Expansion agent characteristics: The type and dosage of expansive admixture used in the concrete mix
- Temperature gradient distribution: Non-uniform temperature profiles within the member cross-section
The coupled analysis approach is consistent with modern design philosophies that recognize the multi-physics nature of structural behavior. For CFST members in regions with large seasonal temperature ranges (e.g., Northern China, where annual temperature swings can exceed 40°C), the thermal stress component can represent 15-25% of the total stress range, which is non-negligible for fatigue design.
Engineering Practice Considerations
In practical applications, the following measures can mitigate thermal stress concerns in CFST members:
- Thermal insulation: Applying insulation to the exterior of the steel tube to reduce the temperature gradient between the steel and concrete.
- Expansion joint design: Incorporating movement joints in long continuous CFST members to accommodate thermal expansion.
- Material selection: Using steel grades with lower thermal expansion coefficients, such as certain austenitic stainless steels (α ≈ 17×10⁻⁶/°C for 304 stainless steel, though this is actually higher than carbon steel, the improved creep resistance at elevated temperatures may be beneficial).
- Concrete mix design: Selecting concrete with thermal expansion coefficient matched to the steel tube material, minimizing the differential expansion.
Key Questions and Reflections
A critical question that remains unaddressed in this study is the effect of thermal cycling on the long-term fatigue life of the steel tube. Repeated seasonal heating and cooling, combined with the differential expansion between steel and concrete, creates a cyclic stress that could contribute to low-cycle fatigue damage at the steel-concrete interface. For CFST members in critical infrastructure such as bridges, high-rise buildings, and offshore platforms, fatigue assessment under thermal cycling should be incorporated into the design.
Another reflection concerns the applicability of linear thermoelastic theory to the actual behavior of CFST members. The linear assumption is valid for small temperature differences and elastic stress states, but for large temperature gradients or when the steel tube enters the plastic range, nonlinear effects become significant. The FEA model used in this study should be extended to include material nonlinearity and geometric nonlinearity for a more complete analysis.
Study Insights and Implications
This paper makes a valuable contribution to the understanding of thermal effects in CFST members and should be considered in the design of CFST structures in regions with significant seasonal temperature variations. The derived analytical equations provide a useful tool for preliminary design and sanity checking of FEA results. For steel pipe manufacturers, the key implication is that the quality and consistency of the steel tube geometry, weld quality, and surface condition directly affect the thermal performance of the CFST member. Tight manufacturing tolerances and rigorous quality control are not merely structural requirements but also thermal management requirements.