Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Hollow Double-Skin Steel Tube Concrete Columns After High Temperature Exposure
Literature Overview
The research by Liu Xiao and colleagues from Shenyang University and Tsinghua University, published in the Journal of Railways and Road Engineering in 2014, investigates the axial compression behavior of hollow double-skin filled with concrete (CFDST) columns after exposure to elevated temperatures. CFDST columns represent an innovative structural system that combines the benefits of double-skin steel tube construction with concrete infill, offering enhanced fire resistance, improved confinement effects, and reduced self-weight through the hollow core. The study combines experimental validation with numerical simulation to dissect the post-fire mechanical behavior and identify the key parameters influencing residual load-bearing capacity.
Core Technical Findings
The investigation examined the effects of exposure temperature, hollow rate, nominal steel ratio, inner and outer steel tube yield strengths, and concrete compressive strength on the post-fire axial compression capacity. The key findings reveal that the outer steel tube yield strength has a significant positive effect on residual load-bearing capacity, while concrete strength has a major influence at ambient temperature but diminishing effect at elevated temperatures. The hollow rate and inner steel tube yield strength show minimal influence on post-fire capacity.
| Parameter | Effect on Post-Fire Capacity | Temperature Sensitivity |
|---|---|---|
| Exposure Temperature | Decreasing trend with increasing temperature | Primary degradation factor |
| Outer Steel Tube Yield Strength | Significant positive effect | Maintains influence at elevated temperatures |
| Concrete Compressive Strength | Major effect at ambient temperature | Influence decreases with temperature |
| Hollow Rate | Minimal effect | Consistent across temperature range |
| Inner Steel Tube Yield Strength | Minimal effect | Consistent across temperature range |
| Nominal Steel Ratio | Moderate effect | Interaction with temperature |
Mechanism Analysis
The numerical simulation results provide insight into the failure mechanisms of CFDST columns after fire exposure. At ambient temperature, the concrete core contributes substantially to the load-bearing capacity, and the double-skin steel tubes provide effective confinement that enhances the concrete's compressive strength through triaxial stress state. However, after exposure to elevated temperatures, the concrete's mechanical properties degrade significantly due to the breakdown of calcium silicate hydrate (C-S-H) gel, the dehydration of ettringite, and the microcracking that develops at temperatures above 400°C.
The outer steel tube, being directly exposed to fire, experiences temperature-dependent strength loss following the Eurocode or ASTM E1131 temperature-strength relationships. However, the inner steel tube, shielded by the concrete core and the air gap in the hollow section, experiences a lower temperature rise and retains a higher proportion of its original strength. This differential temperature exposure explains why the outer tube yield strength has a more significant influence on post-fire capacity than the inner tube strength.
Engineering Practice Implications
For steel pipe manufacturers, this research highlights the importance of selecting appropriate steel grades for CFDST columns intended for fire-exposed applications. The outer tube should be designed with a higher yield strength to compensate for post-fire strength degradation, while the inner tube can use a more economical grade since its contribution to post-fire capacity is limited. This has direct implications for material selection and cost optimization in fire-resistant structural design.
From a welding perspective, the connections between the inner and outer steel tubes, as well as the connection to the concrete core through shear connectors or dowels, must be designed to maintain integrity under fire exposure. The differential thermal expansion between the steel tubes and concrete can induce significant interfacial stresses during heating and subsequent cooling, potentially leading to debonding or connector failure. Welded shear connectors should be designed with adequate ductility to accommodate these thermal strains.
Key Questions and Reflections
The finding that the hollow rate has minimal effect on post-fire capacity is somewhat counterintuitive, given that the hollow core reduces the concrete confinement volume. This may be explained by the fact that at elevated temperatures, the concrete's contribution to load-bearing capacity is already significantly degraded, making the confinement effect less critical. However, this conclusion should be interpreted with caution, as the study may not have covered the full range of hollow rates that are practically relevant.
The diminishing influence of concrete strength at elevated temperatures has important implications for material selection. In fire-prone structures, investing in higher-strength concrete may not provide proportional benefits in terms of post-fire performance, and the design effort should focus more on the steel tube properties and protective measures.
Study Insights and Outlook
This study contributes valuable insights into the post-fire behavior of CFDST columns, supporting the development of fire-resistant structural systems that combine steel tube and concrete. The identification of the outer steel tube yield strength as the primary design parameter for post-fire capacity provides clear guidance for engineers. However, the research is limited to axial compression loading, and the behavior under combined axial and lateral loading, which is more representative of real structural conditions, requires further investigation. Future work should also consider the cooling phase behavior, which may introduce additional thermal stresses and potential damage mechanisms not captured in the current study.