Finite Element Analysis of Steel Tube Coal Gangue Concrete Compression-Bending Members

Literature Overview

This paper, published in the Journal of Shenyang Jianzhu University (2010, Vol. 26, No. 6), presents a finite element analysis of steel tube concrete (CFST) members filled with coal gangue concrete (CGC) under cyclic compression-bending loads. The authors—Li Guochang, Shu Zheng, and Zhang Chunyu—utilized ABAQUS to simulate the hysteretic behavior of these members and compared computational results with experimental data. The study investigates the influence of axial compression ratio on load-displacement hysteretic curves, stiffness degradation, ductility, and energy dissipation capacity. The work was supported by the Ministry of Housing and Urban-Rural Development Science and Technology Project (2008-k2-26) and Liaoning Provincial University Research Project (2008583).

Technical Background and Motivation

Coal gangue concrete represents an environmentally motivated alternative to conventional concrete, utilizing coal gangue—a byproduct of coal mining and processing—as a replacement for natural aggregate. From a steel pipe and structural engineering perspective, the integration of CGC into CFST members raises several important questions regarding material compatibility, interface behavior, and long-term structural performance:

• Aggregate replacement effects: Coal gangue has different mechanical properties, porosity, and surface texture compared to natural aggregate, which affects the concrete's compressive strength, elastic modulus, and bond characteristics.
• Interface behavior: The steel tube-concrete interface in CGC-filled tubes may differ from conventional CFST due to altered concrete surface properties and potential chemical interactions.
• Cyclic loading response: The hysteretic behavior under reversed loading is critical for seismic performance, and the CGC's unique properties may influence energy dissipation and ductility.
• Axial compression ratio effects: The interaction between axial load and lateral cyclic loading creates complex stress states that must be accurately modeled.

Finite Element Modeling Approach

Material Modeling

The accurate simulation of steel tube coal gangue concrete compression-bending members requires careful material modeling of all constituent materials:

The Concrete Damage Plasticity model in ABAQUS is particularly suitable for this application because it accounts for:

1. Tensile cracking and compressive crushing through separate damage variables
2. Nonlinear unloading behavior through degraded stiffness
3. Inelastic volumetric strain under compression
4. Different stress-strain responses in tension and compression

Mesh and Element Selection

For cyclic loading simulations, the mesh quality and element type are critical for capturing the progressive damage accumulation and stiffness degradation:

• Solid elements: 8-node reduced integration (C3D8R) elements are typically used for both steel tube and concrete core
• Mesh refinement: Critical regions including the steel tube walls, concrete core, and potential failure zones require finer mesh density
• Contact definition: The steel-concrete interface requires careful contact definition to capture potential slip and separation under cyclic loading
• Geometric imperfections: Initial geometric imperfections (typically based on fabrication tolerances) should be included to trigger realistic buckling modes

Loading Protocol

The cyclic loading protocol simulated in this study likely follows standard seismic testing procedures:

1. Initial axial load: Applied and held constant to simulate gravity loading
2. Cyclic lateral displacement: Imposed in incremental amplitudes, typically doubling at each step
3. Displacement control: Ensures consistent strain levels regardless of stiffness degradation
4. Multiple cycles per amplitude: Typically 2-3 cycles at each displacement level to capture cyclic degradation

Key Findings and Technical Analysis

Hysteretic Behavior

The paper reports that the computed hysteretic curves show good agreement with experimental data and exhibit a full spindle shape, indicating good energy dissipation capacity and ductility. This is significant because:

• The spindle shape indicates that the steel tube effectively confines the coal gangue concrete, preventing premature concrete crushing and maintaining load-carrying capacity
• The core CGC and steel tube work synergistically, with the steel tube providing lateral confinement and the CGC providing core support
• The good agreement between FEA and experimental results validates the modeling approach

Axial Compression Ratio Effects

The influence of axial compression ratio on member performance is a critical finding:

The paper specifically notes that when horizontal displacement is small (Δ/Δy < 3), stiffness degradation is very pronounced. As displacement continues to increase, the stiffness degradation gradually becomes more gradual. This observation is consistent with the progressive yielding and buckling of the steel tube walls under cyclic compression-bending.

Stiffness Degradation Mechanism

The stiffness degradation in CFST members under cyclic loading occurs through several mechanisms:

1. Initial stage (elastic): Minimal stiffness loss; linear load-displacement relationship
2. Steel yielding stage: Progressive yielding of steel tube walls causes initial stiffness reduction
3. Concrete cracking stage: Tensile cracking in the CGC core reduces effective area and stiffness
4. Buckling stage: Local buckling of steel tube walls causes significant stiffness loss
5. Post-buckling stage: Large inelastic deformations; stiffness degradation rate decreases as the member reaches a more stable deformed configuration

The observation that stiffness degradation becomes more gradual at larger displacements is mechanically intuitive—once the steel tube has buckled and the concrete has extensively cracked, the remaining stiffness is primarily from the confined concrete core and the residual strength of the buckled steel walls.

Welding and Fabrication Considerations

From a steel pipe fabrication and welding perspective, the manufacturing of steel tube members for CGC-filled applications raises several important considerations:

• Tube geometry: The steel tubes must be manufactured to tight dimensional tolerances to ensure uniform concrete fill and consistent confinement pressure
• Welded joints: If the steel tubes require field welding for assembly, the weld quality directly affects the member's seismic performance
• HAZ effects: Welded joints create localized changes in material properties that may affect buckling initiation under cyclic loading
• Residual stresses: Welding-induced residual stresses can interact with applied loads to reduce the member's effective load capacity

The paper does not extensively discuss fabrication and welding aspects, but from my experience in steel pipe manufacturing, these factors are critical for ensuring that the theoretical performance predicted by finite element analysis is achieved in practice.

Study Insights and Reflections

This study makes a valuable contribution to the understanding of CFST members using environmentally friendly concrete alternatives. The validation of the finite element model against experimental data provides confidence in using computational methods for parametric studies and design optimization.

However, I note several areas that deserve further investigation:

1. Long-term durability: The interaction between coal gangue and the steel tube over time, particularly in the presence of moisture and aggressive agents, is not addressed. Coal gangue may contain sulfur compounds that could accelerate corrosion of the steel tube.
2. Scale effects: The study likely focuses on specimen-scale testing. The transition from specimen behavior to structural member behavior involves scale effects that can significantly influence ductility and energy dissipation.
3. Welded connection details: In practical applications, CFST columns are connected to beams through welded or bolted connections. The behavior of these connections under cyclic loading, particularly when the column is filled with CGC, warrants investigation.
4. Concrete fill quality: The quality of concrete placement inside the steel tube—particularly the compaction and absence of voids—directly affects the member's performance. This is a practical fabrication challenge that finite element models often idealize.

The synergy between the steel tube and coal gangue concrete, as demonstrated by the full spindle-shaped hysteretic curves, is encouraging from a seismic design perspective. The steel tube provides effective confinement, while the CGC provides a sustainable concrete alternative. However, the full realization of this potential requires attention to fabrication quality, particularly in welding and concrete placement, to ensure that the theoretical advantages translate into practical structural performance.