Zhuojin Pipe Fitting Co., LtdFire Resistance Performance of Hollow Steel Tube Concrete Composite Long Columns Under Three-Sided Fire Exposure with Axial Compression
Literature Overview
Published in 2023 in the "Journal of Shenyang Jianzhu University" by Zhang Bo, Xu Guangpeng, Ren Qingxin, and Qin Xiaoxiao, this paper investigates the fire resistance performance of hollow steel tube concrete (HSTC) composite long columns subjected to three-sided fire exposure under axial compression. The research was supported by the National Natural Science Foundation (Project 51808351) and the Shenyang Science and Technology Plan (Project 21-108-9-34). Using ABAQUS finite element analysis, the authors examined temperature field distribution, failure modes, deformation characteristics, and internal force distribution, while proposing a simplified fire resistance limit calculation formula.
Modeling Approach and Analysis Parameters
The finite element models were established in ABAQUS to simulate both the temperature field and mechanical response of the HSTC composite long columns. The three-sided fire exposure condition represents a realistic fire scenario where one face of the column remains protected, which is common in interior column fire scenarios.
| Parameter | Variation Range | Engineering Context |
|---|---|---|
| Hollow Ratio | Multiple levels | Affects thermal and mechanical response |
| Section Side Length | 300 mm to 1200 mm | Covers typical column sizes |
| Load Ratio | 0.4 to 0.7 | Represents service load conditions |
| Slenderness Ratio | 22 to 66 | Covers short to long column behavior |
Key Results and Performance Trends
The analysis revealed that internal force redistribution occurs in HSTC composite long columns under high temperature, indicating the development of complex stress states as temperatures rise. The column side length shows a strong positive correlation with fire resistance limit:
| Side Length Increase | Fire Resistance Limit Improvement |
|---|---|
| 300 mm to 600 mm | 131.83% |
| 300 mm to 900 mm | 221.81% |
| 300 mm to 1200 mm | 312.02% |
The slenderness ratio exhibits a negative correlation with fire resistance limit:
| Slenderness Ratio Increase | Fire Resistance Limit Reduction |
|---|---|
| 22 to 44 | 25.46% |
| 22 to 66 | 62.77% |
The load ratio also significantly reduces fire resistance limit:
| Load Ratio Increase | Fire Resistance Limit Reduction |
|---|---|
| 0.4 to 0.5 | 39.03% |
| 0.4 to 0.6 | 65.71% |
| 0.4 to 0.7 | 85.54% |
The most significant finding is that the fire resistance performance of HSTC composite columns is superior to that of hollow reinforced concrete columns. This advantage stems from the steel tube's ability to maintain structural integrity at elevated temperatures and provide confinement to the concrete core.
Technical Analysis of Fire Behavior
The fire resistance behavior of HSTC composite columns involves several interacting mechanisms:
- Thermal expansion mismatch: The steel tube and concrete core expand at different rates, creating internal stresses that can lead to interface debonding.
- Concrete spalling: The hollow section creates a different thermal gradient pattern compared to solid sections, potentially affecting spalling behavior.
- Steel strength degradation: The steel tube loses strength progressively with increasing temperature, reducing its load-bearing capacity and confinement effect.
- Buckling behavior: The slenderness ratio significantly affects the column's susceptibility to thermal buckling, with longer columns experiencing earlier failure.
The three-sided fire exposure creates an asymmetric temperature distribution, leading to bending moments in addition to axial compression. This combined stress state accelerates failure compared to uniform fire exposure.
Engineering Practice Considerations
For steel pipe manufacturers and structural engineers, this research provides several practical insights:
- Steel grade selection: The steel tube material must maintain adequate strength and stiffness at fire exposure temperatures. Structural steel grades with good high-temperature performance (such as those conforming to GB/T 1591 or similar) should be specified.
- Wall thickness design: Thicker steel tube walls provide better fire resistance through increased thermal mass and delayed temperature rise in the core. However, this must be balanced against weight and cost considerations.
- Weld quality: Welded joints in the steel tube must maintain integrity under thermal cycling. Welding procedures should ensure adequate weld metal toughness and resistance to thermal fatigue.
- Column geometry: The strong influence of section size and slenderness ratio on fire resistance should guide column dimension selection. Larger sections and shorter columns provide inherently better fire performance.
Simplified Calculation Formula Validation
The proposed simplified fire resistance limit calculation formula showed good agreement with finite element model results. This formula provides a practical design tool for engineers to estimate fire resistance without performing detailed finite element analysis. The formula accounts for the key parameters identified in the study: section geometry, hollow ratio, load ratio, and slenderness ratio.
Study Insights and Reflections
The research demonstrates that the hollow steel tube concrete composite system offers inherent fire resistance advantages over conventional reinforced concrete columns. The steel tube provides structural continuity during fire exposure, maintaining load-bearing capacity even as concrete properties degrade. This finding supports the use of HSTC composite columns in fire-critical applications.
The significant influence of slenderness ratio highlights the importance of structural stability considerations in fire design. Long columns are particularly vulnerable to thermal buckling, and the fire resistance limit decreases rapidly with increasing slenderness. Design codes should incorporate appropriate stability checks for fire conditions.
The load ratio sensitivity indicates that columns operating at higher service loads have significantly reduced fire resistance margins. This suggests that fire design should consider the actual service load level rather than using a fixed design load, potentially allowing for more efficient structural design.
The hollow section creates unique thermal behavior compared to solid sections. The hollow space acts as a thermal buffer, slowing the temperature rise in the steel tube and concrete core. However, it also creates a cavity that may promote concrete spalling or facilitate fire spread within the column.
Summary
This study provides comprehensive insights into the fire resistance behavior of hollow steel tube concrete composite long columns under three-sided fire exposure. The findings on the influence of section geometry, slenderness ratio, and load ratio offer practical guidance for fire-resistant structural design. The proposed simplified calculation formula provides a convenient design tool, while the demonstrated superiority of HSTC columns over hollow reinforced concrete columns supports the adoption of steel tube concrete technology in fire-critical applications. Engineers should integrate these findings into structural design practices, ensuring that fire resistance requirements are met through appropriate material selection, geometric design, and connection detailing.