Zhuojin Pipe Fitting Co., LtdComparative Analysis of Mechanical Properties of Circular and Square Steel Tube High-Strength Concrete Columns After High Temperature
Literature Overview
This paper by Chen Zongping, Chen Jianjia, and Zhang Yaqi from Guangxi University investigates the post-fire axial compression behavior of concrete-filled steel tube (CFST) short columns with circular and square cross-sections, considering the effects of exposure temperature and concrete strength grade. Funded by the National Natural Science Foundation of China (51578163), the Guangxi Natural Science Foundation (2016GXNSFDA380032), and other Guangxi provincial programs, the study was published in the Journal of Guangxi University (Natural Science Edition) in 2018. Fire resistance is a critical design consideration for CFST columns, and understanding how different cross-section shapes perform after fire exposure is essential for safe structural design.
Experimental Program
The experimental campaign comprised 30 CFST short column specimens, with three variables: cross-section shape (circular and square), exposure temperature, and concrete strength grade. The specimens were heated to specified temperatures and then subjected to axial compression testing after cooling. The test matrix allows for systematic comparison of how cross-section shape, temperature, and concrete strength interact to influence post-fire mechanical properties.
| Variable | Levels | Rationale |
|---|---|---|
| Cross-section shape | Circular, Square | Common CFST shapes in practice |
| Exposure temperature | Multiple levels | Simulate fire exposure conditions |
| Concrete strength grade | Multiple grades | Cover typical structural concrete range |
| Specimen type | Short columns | Isolate material degradation from stability effects |
The short column configuration is appropriate for studying material degradation, as it eliminates the influence of column slenderness and buckling, which would confound the assessment of temperature-induced property changes.
Key Findings and Technical Analysis
The primary finding is that both circular and square CFST columns exhibit similar degradation trends in peak load, peak displacement, axial stiffness, ductility coefficient, and energy dissipation capacity as exposure temperature and concrete strength increase. However, the circular CFST specimens are more sensitive to changes in temperature and concrete strength than the square CFST specimens. This means that the circular cross-section, while providing better confinement due to its uniform geometry, is more vulnerable to the adverse effects of high temperature and high-strength concrete.
The mass loss ratio comparison between circular and square specimens provides insight into the fire exposure behavior. The mass loss is attributed to moisture evaporation from the concrete, decomposition of the cement paste, and potential oxidation of the steel tube. The differences in mass loss between circular and square specimens reflect differences in heat transfer efficiency: the circular cross-section has a more uniform heat distribution, while the square cross-section has corners that may experience higher temperature gradients and more severe concrete degradation.
The load-displacement curves of the post-fire specimens show the characteristic behavior of CFST columns: an initial linear elastic phase, a nonlinear hardening phase, and a post-peak softening phase. The degradation of these curves with increasing temperature reflects the loss of concrete strength, steel strength, and bond strength between the steel tube and concrete. The ductility coefficient and energy dissipation capacity, which are critical for seismic performance, are also affected by fire exposure, and the paper quantifies these effects.
| Performance Parameter | Circular CFST | Square CFST | Observation |
|---|---|---|---|
| Peak load degradation | More sensitive to temperature | Less sensitive | Circular degrades faster |
| Peak displacement | More affected by temperature | Less affected | Circular more vulnerable |
| Axial stiffness | More affected by temperature | Less affected | Circular loses stiffness faster |
| Ductility coefficient | More affected by temperature and strength | Less affected | Circular less robust post-fire |
| Energy dissipation | More affected | Less affected | Circular less energy-absorbing post-fire |
| Mass loss ratio | Compared between shapes | Compared between shapes | Reflects heat transfer differences |
Engineering Practice Implications
From a fire engineering and structural design perspective, several practical insights emerge:
- Cross-section selection for fire resistance: The findings suggest that square CFST columns may offer better post-fire performance than circular columns, particularly in terms of ductility and energy dissipation retention. This is counterintuitive, as circular sections are generally preferred for their superior confinement effect under normal conditions. However, the more uniform heat distribution in circular sections may lead to more uniform but more complete degradation of the concrete and steel, while the square section's corner effects may create a more heterogeneous degradation pattern that preserves some load-carrying capacity.
- High-strength concrete in fire-exposed structures: The use of high-strength concrete in CFST columns that may be exposed to fire requires careful consideration. While high-strength concrete provides higher load capacity under normal conditions, its post-fire performance may be more severely degraded, particularly in circular sections. The design should account for the residual strength and ductility after fire exposure, not just the pre-fire strength.
- Fire protection design: The findings support the need for fire protection measures (such as fire-resistant coatings, fire boards, or increased section dimensions) for CFST columns, particularly those with circular cross-sections and high-strength concrete. The fire protection design should be based on the expected fire exposure temperature and duration, and should account for the specific cross-section shape and concrete strength.
- Residual strength assessment: After a fire event, the residual strength and ductility of CFST columns must be assessed to determine whether the structure can be repaired or must be replaced. The findings in this paper provide a basis for developing residual strength assessment methods, although the specific degradation relationships depend on the fire exposure history and material properties.
- Design code implications: Current design codes for fire design of CFST columns may not adequately distinguish between circular and square cross-sections. The findings suggest that cross-section-specific design provisions or reduction factors should be considered in fire design codes.
Reflections and Limitations
The study provides valuable experimental data on the post-fire behavior of CFST columns, but several limitations should be noted. First, the exposure temperatures and durations are not specified in the abstract, and the results may be specific to the particular fire exposure conditions used. Second, the study focuses on axial compression and does not consider eccentric loading, which is the more common condition in real structures and may be more sensitive to fire-induced degradation. Third, the study does not address the behavior of CFST columns under cyclic loading after fire exposure, which is relevant for post-fire seismic assessment. Finally, the study does not consider the long-term effects of fire exposure, such as delayed spalling or progressive corrosion of the steel tube after fire damage.
Summary
The comparative study of circular and square CFST short columns after high temperature exposure demonstrates that both cross-section shapes exhibit similar degradation trends in mechanical properties, but circular columns are more sensitive to temperature and concrete strength changes. This finding has important implications for the fire design of CFST columns, suggesting that square cross-sections may offer better post-fire robustness. The study contributes to the development of more accurate fire design methods for CFST columns and highlights the need for cross-section-specific considerations in fire engineering practice. Future research should extend to eccentric loading, cyclic loading, and long-term post-fire performance to provide a more comprehensive basis for fire design and post-fire assessment of CFST structures.