Zhuojin Pipe Fitting Co., LtdFinite Element Analysis of Steel Tube Coal Gangue Concrete Compression-Bending Members
Literature Overview
The paper published in Journal of Shenyang Jianzhu University (2010, Vol. 26, No. 6) by Li Guochang, Shu Zheng, and Zhang Chunyu presents finite element analysis of steel tube concrete (STC) members filled with coal gangue concrete (CGC) under cyclic compression-bending loads. The study uses ABAQUS software to simulate the hysteretic behavior of these members and investigates the influence of axial compression ratio on load-displacement hysteresis curves, energy dissipation capacity, and ductility.
Material Properties and Constitutive Models
Coal gangue concrete is a sustainable alternative to conventional concrete, utilizing coal gangue (a waste product from coal mining) as a partial or full replacement for natural aggregate. The mechanical properties of CGC differ significantly from ordinary concrete, requiring specialized constitutive models for accurate finite element simulation.
| Property | Ordinary Concrete (C40) | Coal Gangue Concrete |
|---|---|---|
| Compressive strength (MPa) | ~40 | 20-35 (depending on mix) |
| Elastic modulus (GPa) | ~30 | 18-28 |
| Tensile strength (MPa) | ~2.5 | 1.5-2.2 |
| Poisson's ratio | 0.2 | 0.18-0.22 |
| Peak strain | 0.003 | 0.002-0.0025 |
| Post-peak behavior | Gradual degradation | More brittle |
Constitutive Model Implementation
The finite element model requires appropriate constitutive models for both the steel tube and the concrete core:
- Steel tube: Multilinear kinematic hardening model (Chaboche model) to capture cyclic Bauschinger effect and ratcheting behavior.
- Coal gangue concrete: Concrete Damage Plasticity (CDP) model with modified compression and tension envelopes reflecting the unique stress-strain behavior of CGC.
Hysteresis Curve Analysis
The simulated load-displacement hysteresis curves exhibit a characteristic spindle shape (纺锤形), indicating good energy dissipation capacity and ductility. This shape is consistent with experimental observations and demonstrates the complementary working mechanism between the steel tube and the coal gangue concrete core.
Axial Compression Ratio Effects
| Axial Compression Ratio | Peak Load | Peak Displacement | Ductility Index | Energy Dissipation |
|---|---|---|---|---|
| 0.2 | Baseline | Moderate | High | Moderate |
| 0.4 | Increased | Reduced | Moderate | High (optimal) |
| 0.6 | Further increased | Significantly reduced | Low | Reduced |
The study reveals that the axial compression ratio has a substantial influence on both load-bearing capacity and ductility. At low axial compression ratios, the member exhibits significant stiffness degradation when horizontal displacement is small (Δ/Δy < 3), with the degradation gradually leveling off as displacement increases.
Key Findings and Technical Insights
Stiffness Degradation Pattern
The stiffness degradation behavior follows a distinctive pattern:
- Initial loading: Linear elastic response with full composite action between steel tube and CGC.
- Cracking phase: Gradual stiffness reduction as microcracks develop in the CGC core.
- Yielding phase: Significant stiffness degradation as the steel tube begins to yield and CGC enters the post-peak softening region.
- Stabilization phase: Stiffness degradation rate decreases as the steel tube provides increasing confinement to the damaged CGC.
Complementary Working Mechanism
The steel tube and coal gangue concrete core work in a complementary manner:
- The CGC core provides compressive strength and reduces the slenderness ratio of the steel tube.
- The steel tube provides lateral confinement, preventing spalling of the CGC and enhancing its ductility.
- Under cyclic loading, the steel tube absorbs energy through plastic deformation while the CGC provides additional resistance through friction and interlocking.
Engineering Practice Considerations
For practical application of coal gangue concrete-filled steel tubes in seismic regions, the following considerations are important:
| Design Parameter | Recommended Range | Rationale |
|---|---|---|
| Axial compression ratio | 0.2-0.4 | Optimal balance of strength and ductility |
| Steel tube yield strength | ≥ 345 MPa | Adequate energy dissipation capacity |
| CGC compressive strength | ≥ 25 MPa | Sufficient confinement effectiveness |
| D/Dt ratio | 30-60 | Prevent local buckling while maintaining efficiency |
| Concrete cover to interface | 20-30 mm | Accommodate thermal expansion and construction tolerance |
Reflections and Implications
The research demonstrates that coal gangue concrete, despite having lower mechanical properties than ordinary concrete, can achieve satisfactory seismic performance when confined within a steel tube. This finding has significant implications for sustainable construction, as it provides a viable pathway for utilizing mining waste in structural applications. The finite element modeling approach validated against experimental data provides a reliable tool for parametric studies and design optimization, reducing the need for costly physical testing. Engineers designing STC members for seismic applications should consider the specific constitutive behavior of CGC, particularly its more brittle post-peak response, when selecting appropriate design parameters and detailing requirements.