Zhuojin Pipe Fitting Co., LtdExperimental Study on Axial Compression Members of Different Cross-Sections After High Temperature
Literature Overview
This paper, published in the Journal of Northeastern University (Natural Science Edition) in 2002, presents experimental research on the mechanical properties of circular, square, and rectangular cross-section concrete filled steel tube (CFST) axial compression members after exposure to high temperatures. The study was conducted by researchers from Northeastern University and Shenyang Institute of Construction Engineering, supported by the Liaoning Provincial Department of Education and the Ministry of Construction Science and Technology Development Project. The research addresses a critical engineering concern: the residual load-bearing capacity of CFST columns after fire exposure, which is essential for post-fire structural assessment and rehabilitation decisions.
Test Methodology and Specimen Configuration
The experimental program included specimens with three different cross-sectional geometries: circular, square, and rectangular. Each specimen was subjected to controlled heating to simulate fire exposure, followed by cooling and subsequent axial compression testing to measure residual load-bearing capacity. The test methodology followed established fire testing protocols, with temperature control achieved through furnace heating and thermocouple monitoring at critical locations on the steel tube surface and within the concrete core.
| Cross-Section Type | Residual Capacity Loss | Relative Performance |
|---|---|---|
| Circular | Maximum loss | Worst performance |
| Rectangular | Moderate loss | Intermediate performance |
| Square | Minimum loss | Best performance |
The test results revealed a clear hierarchy in residual capacity performance: circular section CFST members experienced the greatest reduction in residual load-bearing capacity after high temperature exposure, followed by rectangular sections, with square sections showing the least reduction. This finding is counterintuitive given that circular sections are generally considered to provide the most uniform confinement to core concrete under normal loading conditions.
Residual Capacity Analysis and Reduction Coefficients
The paper proposes methods for calculating the residual load-bearing capacity and discusses the capacity reduction coefficients for different cross-sectional types. The capacity reduction coefficient is defined as the ratio of residual capacity to original capacity at room temperature, and it varies with both the maximum exposure temperature and the cross-sectional geometry. The analysis reveals that the reduction coefficients are not constant but depend on the specific thermal history and the interaction between steel tube and concrete core during heating and cooling.
The differential performance of cross-sectional types can be attributed to several factors. Circular sections, while providing uniform confinement, are more susceptible to global buckling after thermal weakening of the steel tube. Square sections benefit from the corner confinement effect, which maintains structural integrity even after significant thermal degradation. Rectangular sections, with their varying wall aspect ratios, exhibit intermediate behavior with performance depending on the specific aspect ratio.
Post-Fire Structural Assessment Implications
The findings of this research have direct implications for post-fire structural assessment of buildings and industrial facilities containing CFST columns. Engineers conducting post-fire evaluations can use the capacity reduction coefficients to estimate the remaining load-bearing capacity of CFST columns, which is essential for determining whether the structure can be safely reoccupied or requires repair or replacement. The cross-sectional type is a critical parameter in this assessment, and the research provides quantitative data to support decision-making.
From a fire protection design perspective, the research highlights the importance of considering cross-sectional geometry in fire resistance design. Square CFST columns may offer superior fire performance compared to circular sections, suggesting that cross-sectional selection should be a consideration in fire-prone applications. However, the practical implications must be balanced against other design factors such as material efficiency, fabrication cost, and architectural requirements.
Key Reflections and Study Insights
The finding that square sections outperform circular sections in post-fire residual capacity challenges conventional wisdom in CFST design, where circular sections are typically preferred for their superior confinement characteristics under normal loading. This research suggests that the design priorities shift significantly after fire exposure, and engineers must consider the entire lifecycle of the structure, including post-fire performance. The capacity reduction coefficients proposed in this research provide a practical tool for post-fire assessment, but engineers should be aware that these coefficients are based on specific test conditions and may require adjustment for different fire scenarios, concrete grades, and steel tube specifications. The research underscores the importance of continued experimental investigation into the post-fire behavior of composite structures to improve the safety and resilience of the built environment.