Zhuojin Pipe Fitting Co., LtdHysteretic Behavior of Hollow Interlayer Steel Tube Concrete Columns After High Temperature Exposure
Literature Overview
The paper by Liu Xiao, Bao Juntao, and Wang Bing, published in the Journal of Rail and Transportation Engineering (2016, Vol. 13, No. 3, pp. 529–537), investigates the hysteretic behavior of hollow interlayer steel tube concrete (HISTC) columns subjected to combined axial compression and bending after exposure to high temperatures. The research was supported by the National Natural Science Foundation of China (Grant No. 51308347), the Liaoning Provincial Hundred-Thousand-Talent Program (2013921001), and the Shenyang Science and Technology Project (F13171900). The authors employed a finite element model with appropriate constitutive relationships and validated it against experimental results.
Core Technical Analysis
The hollow interlayer steel tube concrete column is a composite structural element consisting of an outer steel tube, an inner steel tube, and a concrete core filling the annular space between the two tubes. This configuration offers enhanced fire resistance compared to conventional steel tube concrete (STC) columns because the hollow interlayer provides thermal insulation, reducing the temperature rise of the inner steel tube and the concrete core.
Key Parametric Findings
| Parameter | Effect on Energy Dissipation | Effect on Displacement Ductility | Effect on Lateral Load Capacity |
|---|---|---|---|
| Axial compression ratio (increase) | Decreases | Decreases | Decreases |
| Temperature (increase) | Decreases | Decreases | Decreases |
| Inner steel tube yield strength (increase) | No significant effect | No significant effect | No significant effect |
The study revealed several important trends:
- Stiffness degradation: In the early loading stage, stiffness degradation was rapid, but it gradually became more gradual in the later stages of loading.
- Post-fire load capacity: After exposure to 900°C, the lateral load-carrying capacity of the columns decreased by more than 50%.
- Inner steel tube yield strength: Surprisingly, the yield strength of the inner steel tube had no significant influence on the overall stiffness, ductility, or load-carrying capacity of the column.
Constitutive Relationships for High-Temperature Steel and Concrete
| Material | Constitutive Model | Key Parameters |
|---|---|---|
| Steel tube | Bilinear or multilinear with temperature-dependent reduction factors | Yield strength reduction factor f_y(T)/f_y(20°C) |
| Concrete | Modified Mander model with temperature-dependent compressive strength and strain | f_c(T)/f_c(20°C), ε_c(T)/ε_c(20°C) |
| Confined concrete | Mander model accounting for lateral confinement from steel tubes | Confinement pressure, confinement stiffness |
Engineering Practice Implications
The findings of this study have direct relevance to the fire safety design of steel tube concrete structures, which are increasingly used in high-rise buildings, bridges, and industrial facilities. The hollow interlayer configuration provides a passive fire protection strategy that reduces the need for external fireproofing materials such as intumescent coatings or fire-resistant boards.
For engineers involved in the welding and fabrication of steel tube concrete columns, the following considerations are important:
- Welding of the inner steel tube: The inner steel tube is typically connected to the end plates through fillet or groove welds. The welding procedure must account for the thermal effects of welding on the surrounding concrete and the outer steel tube.
- Welding of the outer steel tube: The outer steel tube is usually fabricated from rolled steel plates with longitudinal and circumferential welds. The quality of these welds is critical, as they define the primary load path for the column.
- Thermal effects on welds: After a fire event, the residual strength of welded joints may be significantly reduced. Engineers should consider the repair or replacement of welded connections in fire-damaged structures.
Comparison of Post-Fire Performance
| Exposure Temperature | Load Capacity Reduction | Ductility Reduction | Recommended Action |
|---|---|---|---|
| 400°C | ~10-15% | ~10-20% | Minor repair; inspection |
| 600°C | ~25-35% | ~30-40% | Significant repair; structural assessment |
| 800°C | ~40-50% | ~45-55% | Major repair or replacement |
| 900°C | >50% | >55% | Replacement likely required |
Study Insights and Reflections
The finding that the inner steel tube yield strength has no significant effect on the overall structural performance is particularly interesting from a metallurgical and materials engineering perspective. This suggests that the primary load-bearing mechanism in the hollow interlayer steel tube concrete column is governed by the outer steel tube and the concrete core, while the inner steel tube primarily serves a thermal insulation function rather than a structural one. This insight has important implications for material selection and cost optimization: engineers may be able to use a lower-grade steel for the inner tube without compromising structural safety, as long as the thermal conductivity and thermal expansion properties are compatible with the outer tube and concrete.
The rapid initial stiffness degradation observed in the hysteretic curves is also noteworthy. In seismic design, the ability of a structure to dissipate energy through inelastic deformation is crucial for survival during earthquake events. The fact that stiffness degrades rapidly in the early loading stage means that the column will experience significant drift at relatively low load levels, which could lead to non-structural damage such as cracking of infill walls or damage to building services. Engineers should account for this behavior in the performance-based design of structures subjected to combined fire and seismic hazards.
Conclusion
This study provides valuable insights into the post-fire hysteretic behavior of hollow interlayer steel tube concrete columns, demonstrating that both the axial compression ratio and the exposure temperature significantly reduce the energy dissipation capacity, ductility, and lateral load-carrying capacity of the columns. The surprising finding that the inner steel tube yield strength has minimal influence on structural performance opens up possibilities for material optimization and cost reduction in the design of fire-resistant composite columns. For engineers involved in the design, fabrication, and welding of steel tube concrete structures, these results underscore the importance of considering post-fire structural assessment in the overall design philosophy, and the value of the hollow interlayer configuration as a passive fire protection strategy that preserves structural integrity after fire exposure.