Fire Resistance Performance of Circular Steel Tube Concrete Members with Internal Steel Sections

Literature Overview

Han Yi, Wang Jingxuan, and Wang Wenda from Lanzhou University of Technology published this study in the Journal of Natural Disasters (2015, Vol. 24, No. 2), investigating the fire resistance of circular steel tube concrete columns with internal steel sections. The research was supported by the National Natural Science Foundation (51468037) and Gansu Provincial Higher Education Research Project (2013A-032). Using ABAQUS finite element software, the authors established a thermal-mechanical coupled numerical model and conducted comprehensive parametric analysis.

Technical Background and Design Rationale

Steel tube concrete (CFST) columns are widely used in high-rise buildings, bridges, and industrial structures due to their high load-bearing capacity, ductility, and construction efficiency. However, under fire conditions, the steel tube loses strength rapidly at elevated temperatures, potentially leading to premature structural failure. The introduction of internal steel sections (such as H-sections or cross-sections) within the concrete core provides additional load-bearing capacity and improves the overall fire resistance of the composite member.

The thermal-mechanical coupled analysis accounts for the temperature-dependent material properties of steel, concrete, and the interaction between different components. Under standard fire conditions (ISO 834 or equivalent), the temperature rises to 1000 degrees Celsius over approximately 2 hours, creating complex stress redistributions within the composite cross-section.

Numerical Model and Validation

The ABAQUS model incorporates:

Temperature-dependent constitutive models for steel (yield strength reduction, elastic modulus degradation)
Concrete thermal expansion and spalling criteria
Contact elements between steel tube and concrete, and between concrete and internal steel section
Standard ISO 834 fire curve for thermal loading
Thermal-mechanical coupling with appropriate element types (S4R for steel, C3D8R for concrete)

The model was validated against experimental data from existing research, confirming its accuracy in predicting displacement-time curves and failure modes.

Failure Stages and Key Findings

Stage	Temperature Range	Behavior	Dominant Mechanism
Initial expansion	0-300 degrees C	Column shortening, thermal expansion	Thermal strain, thermal stress buildup
Axial compression	300-600 degrees C	Progressive load redistribution	Steel strength loss, concrete softening
Failure	600-800 degrees C	Rapid displacement increase, collapse	Steel tube yielding, concrete spalling

The parametric analysis revealed several important relationships:

Contact stress between steel tube and concrete decreases with increasing temperature due to differential thermal expansion and material softening.
Contact stress between concrete and internal steel section also decreases with temperature but at a different rate than the tube-concrete interface.
Fire load ratio, slenderness ratio, steel tube steel ratio, and fire protection thickness are the most significant parameters affecting fire resistance limit.

Parameter	Effect on Fire Resistance Limit	Sensitivity
Fire load ratio	Higher load ratio reduces fire resistance	High
Slenderness ratio	Higher slenderness reduces fire resistance	High
Steel tube steel ratio	Higher steel ratio improves fire resistance	Moderate-High
Fire protection thickness	Thicker protection improves fire resistance	High
Internal steel section size	Larger section improves fire resistance	Moderate

Engineering Practice Considerations

For steel tube manufacturing and fabrication, the fire resistance performance of CFST columns with internal sections has several implications. The steel tube used for such columns must be manufactured to maintain dimensional accuracy under thermal cycling conditions. The weld joints connecting the steel tube segments (if applicable) must be designed to maintain structural integrity at elevated temperatures. For welded tubes, the longitudinal weld seam represents a potential weak point under thermal loading.

The internal steel section must be properly positioned within the concrete core to ensure uniform concrete cover and effective composite action. The fabrication process must ensure that the steel section is centered and that the concrete is placed without voids around the section. Welding of the internal steel section to the steel tube (if required for composite action) must use procedures suitable for elevated temperature performance, avoiding excessive residual stresses.

Study Insights and Reflections

This research provides valuable insights into the fire behavior of hybrid CFST columns with internal steel reinforcement. The finding that contact stresses decrease with temperature is critical for understanding the progressive loss of composite action during fire exposure. Engineers designing such columns for fire-prone applications should consider the thermal compatibility of different materials and the potential for differential expansion to create gaps at interfaces. The parametric study provides a basis for optimizing the design of internal steel sections and fire protection systems to achieve target fire resistance ratings. The numerical approach demonstrated here enables efficient evaluation of design alternatives without the need for extensive full-scale fire testing, which is prohibitively expensive.