Zhuojin Pipe Fitting Co., LtdInelastic Buckling Load of Steel Tube Coal Gangue Concrete Axially Compressed Intermediate Columns
Literature Overview
This paper by Li Guochang, Long Haibo, and Wang Zhaoqiang (2004, Journal of Shenyang Jianzhu University (Natural Science Edition), Vol. 20, No. 4, pp. 291–293) investigates the inelastic buckling behavior of concrete-filled steel tubes (CFST) filled with coal gangue concrete. The research was supported by the Liaoning Provincial Natural Science Foundation (Grant No. 9910300201) and conducted at Shenyang University.
The study addresses a resource efficiency challenge: coal gangue, a waste byproduct of coal mining, is used as a partial replacement for conventional aggregates in concrete. When this coal gangue concrete is filled into steel tubes, the resulting composite member can serve as a vertical load-bearing structural element. The paper derives a theoretical formula for the inelastic buckling load using tangent modulus theory, conducts experimental validation, and determines the limiting slenderness ratios that distinguish short, intermediate, and long column behavior.
Core Technical Approach
The tangent modulus theory, a classical approach to inelastic buckling analysis, was adapted for the composite CFST member with coal gangue concrete. The key innovation lies in the determination of the effective tangent modulus of the composite material, which accounts for the interaction between the steel tube and the coal gangue concrete core under compressive loading.
The theoretical derivation proceeds as follows:
- Establish the stress-strain relationship for the composite material under axial compression.
- Determine the tangent modulus of the composite at the buckling stress level.
- Apply the Euler buckling formula with the tangent modulus replacing the elastic modulus.
- Derive the limiting slenderness ratios that define the transition between elastic buckling, inelastic buckling, and material yielding.
| Column Category | Slenderness Range | Governing Failure Mode | Design Approach |
|---|---|---|---|
| Short column | λ < λ₁ | Material yielding | Based on compressive strength |
| Intermediate column | λ₁ < λ < λ₂ | Inelastic buckling | Tangent modulus theory |
| Long column | λ > λ₂ | Elastic buckling | Euler formula |
The paper determines specific values for λ₁ and λ₂ for the coal gangue concrete-filled steel tube members, which differ from conventional CFST members due to the altered material properties of the coal gangue concrete.
Technical Analysis of Coal Gangue Concrete Effects
Coal gangue, when properly processed and incorporated into concrete, exhibits different mechanical properties compared to conventional concrete:
- Lower compressive strength due to the lower strength of gangue aggregates
- Potentially lower elastic modulus
- Different Poisson's ratio, affecting the confinement mechanism
- Variable durability characteristics depending on gangue composition
These property differences directly influence the buckling behavior of the CFST members. The reduced confinement effect of coal gangue concrete on the steel tube wall means that the composite action is weaker compared to conventional CFST, potentially leading to earlier local buckling of the steel tube.
From a welding and fabrication standpoint, the use of coal gangue concrete does not directly affect the welding process of the steel tube. However, the concrete mix design must be carefully controlled to ensure adequate workability for tube filling. The gangue aggregate gradation and water demand must be optimized to achieve complete filling without voids, which would compromise the composite action and potentially lead to premature failure.
Experimental Validation and Results
The experimental program included axial compression tests on CFST specimens with varying slenderness ratios, allowing the determination of the inelastic buckling load and the limiting slenderness ratios. The theoretical predictions based on the tangent modulus theory were compared with experimental results, demonstrating reasonable agreement.
The key finding is that the limiting slenderness ratios for coal gangue concrete-filled steel tubes are lower than those for conventional CFST members, reflecting the reduced composite action and lower effective stiffness of the coal gangue concrete. This means that coal gangue CFST members transition from short-column behavior to intermediate-column behavior at lower slenderness ratios, requiring earlier consideration of buckling effects in design.
Engineering Practice Considerations
The use of coal gangue concrete in CFST members offers significant environmental benefits by utilizing a waste material that would otherwise require disposal. However, engineers must be aware of the following considerations:
- Material characterization: The mechanical properties of coal gangue concrete must be thoroughly characterized through laboratory testing, as the properties can vary significantly depending on the gangue source and processing.
- Durability assessment: Coal gangue may contain reactive components (sulfides, alkalis) that could lead to long-term degradation. Accelerated durability testing is recommended before structural application.
- Quality control: The concrete mix design must be tightly controlled, with batch testing for slump, air content, and compressive strength. The gangue aggregate should be washed and screened to remove deleterious components.
- Welding considerations: While the welding process itself is unaffected, the welding residual stresses in the steel tube may interact with the lower confinement pressure from the coal gangue concrete, potentially affecting the local buckling behavior.
Key Questions and Reflections
The paper raises important questions about the long-term performance of coal gangue concrete in structural applications. Does the gangue aggregate exhibit different creep and shrinkage characteristics compared to conventional aggregates? How does the confinement mechanism evolve over time as the concrete ages? These questions are critical for the long-term serviceability assessment of CFST members with coal gangue concrete cores.
Additionally, the paper does not address the seismic performance of coal gangue CFST members, which is a significant omission given that ductility and energy dissipation capacity are essential for seismic-resistant design. The reduced confinement effect of coal gangue concrete may adversely affect the ductility of the composite member under cyclic loading, warranting further investigation.
Study Insights and Conclusions
This paper makes a valuable contribution to the field of sustainable construction by demonstrating the feasibility of using coal gangue concrete in CFST structural members. The tangent modulus theory approach for inelastic buckling analysis is well-established and provides a sound theoretical basis for design. The determination of limiting slenderness ratios specific to coal gangue CFST members is a practical contribution that can be directly applied in design. However, the limited scope of the study — primarily axial compression without cyclic loading — suggests that further research is needed before widespread structural application, particularly in seismic regions. The welding and fabrication community should note that while the basic fabrication process is similar to conventional CFST members, the concrete quality control requirements are more stringent due to the variability of gangue aggregates. Overall, this work represents a meaningful step toward resource-efficient construction, though comprehensive performance data, particularly for seismic and long-term durability, is still needed.