Zhuojin Pipe Fitting Co., LtdAxial Compression Mechanism Analysis of Hollow Sandwich High-Strength Steel Tube Concrete Columns After Fire Exposure
Literature Overview
The study by Liu Xiao, Wang Yi, and Wang Bing, published in Industrial Construction (2019, Vol. 49, No. 5, pp. 146-153), investigates the post-fire axial compression behavior of hollow sandwich high-strength steel tube concrete columns (CFDST). Funded by the National Natural Science Foundation of China (Project 51308347) and the Shenyang Science and Technology Plan Project (18-013-0-16), this research addresses an important structural engineering challenge: understanding the residual load-bearing capacity of advanced composite columns after fire exposure.
The authors, from Shenyang University's School of Building and Civil Engineering, employed ABAQUS finite element software to establish computational models validated by experimental data, and conducted parametric studies to identify key design parameters affecting post-fire performance.
Core Technical Content and Methodology
The hollow sandwich steel tube concrete column (CFDST) is an innovative structural system consisting of an outer steel tube, an inner steel tube, and a concrete core sandwiched between them. This configuration provides enhanced load-bearing capacity and ductility compared to conventional steel tube concrete columns, but its post-fire behavior is complex due to the interaction between multiple components at elevated temperatures.
Constitutive Model Selection
A critical aspect of this study is the selection of appropriate constitutive relationships for the sandwich concrete at elevated temperatures. The authors selected a reasonable core concrete constitutive model that accounts for:
- Temperature-dependent compressive strength degradation
- Strain softening behavior at high temperatures
- Confinement effect from both inner and outer steel tubes
- Differential thermal expansion between steel and concrete
Finite Element Model Validation
The ABAQUS model was validated against experimental results, demonstrating good agreement between simulated and measured load-displacement curves. This validation confirms the model's reliability for parametric analysis and mechanism investigation.
Parametric Study Results
The parametric study examined the influence of four key variables on post-fire axial compression performance:
| Parameter | Variation Range | Effect on Residual Capacity |
|---|---|---|
| Fire exposure duration | 30-180 min | Decreasing with increasing duration |
| Hollow ratio (inner tube) | 0.2-0.6 | Decreasing with increasing hollow ratio |
| Outer tube yield strength | Q345 to Q460 | Significant influence on residual capacity |
| Concrete compressive strength | 30-60 MPa | Moderate influence on residual capacity |
Load Contribution Analysis
A particularly important finding is the shift in load contribution proportions between components when the outer tube yield strength is increased:
| Component | Q345 Outer Tube | Q460 Outer Tube | Change |
|---|---|---|---|
| Core concrete | 60% | 30% | -30% |
| Outer steel tube | 25% | 50% | +25% |
| Inner steel tube | 15% | 20% | +5% |
This significant redistribution demonstrates that high-strength outer tubes assume a dominant role in post-fire load bearing, particularly as the concrete core degrades with fire exposure.
Mechanism Analysis
The study provides detailed mechanism analysis of the interaction between inner and outer steel tubes and the concrete core under post-fire axial compression:
- Initial elastic stage: All components deform elastically, with the concrete core carrying the majority of the load due to its higher stiffness.
- Concrete degradation stage: As fire exposure progresses, concrete strength decreases significantly, leading to reduced load contribution from the core. The steel tubes, which retain higher strength ratios at elevated temperatures, progressively assume greater load share.
- Post-fire compression stage: After cooling, the column exhibits reduced residual capacity compared to unexposed specimens. The load-displacement curve shows modified stiffness and reduced ductility, with the failure mode shifting from concrete crushing to steel tube local buckling.
- Interaction force evolution: The interfacial interaction forces between steel tubes and concrete change significantly during and after fire exposure, affecting the composite action efficiency.
Engineering Practice Implications
The findings have direct implications for the design and assessment of steel tube concrete columns in fire-prone environments:
- High-strength steel tube recommendation: The use of Q460 or higher grade outer tubes is strongly recommended for applications requiring significant post-fire residual capacity, as these tubes maintain higher strength ratios at elevated temperatures.
- Hollow ratio optimization: The hollow ratio should be carefully optimized to balance weight efficiency with post-fire residual capacity, with values exceeding 0.5 generally not recommended for critical structures.
- Fire exposure duration consideration: Design should account for realistic fire exposure durations based on occupancy type and fire protection provisions, with residual capacity assessments performed for the expected exposure duration.
- Post-fire assessment methodology: The parametric relationships established in this study can inform rapid post-fire structural assessment procedures for damaged buildings.
Key Questions and Reflections
Several important questions arise from this research:
- How does the post-fire behavior change with cyclic fire exposure, such as repeated heating and cooling events?
- What is the effect of fire protection systems (such as intumescent coatings or water mist systems) on the residual capacity of CFDST columns?
- Can the constitutive model developed for concrete be extended to account for moisture effects and spalling phenomena?
The study's focus on axial compression is appropriate for column assessment, but practical structures may experience combined loading conditions that require additional investigation.
Study Insights and Implications
This literature provides valuable insights into the post-fire behavior of advanced composite column systems. The significant shift in load contribution proportions with outer tube strength highlights the importance of material selection in fire-resistant design. For engineers involved in structural fire engineering, the parametric relationships established here offer a foundation for performance-based fire design of steel tube concrete structures. The validated finite element model serves as a powerful tool for investigating additional parameters and loading conditions not covered in this study.