Axial Compression Performance of Stiffened Square CFST Columns After Fire Exposure

Literature Overview

The research by Li Xuefeng, Wang Huifeng, and Liu Zhenhui, published in the Journal of Shenyang Jianzhu University (Natural Science Edition), Volume 37, Issue 4, 2021, investigates the axial compression performance of stiffened square steel tube concrete (CFST) columns after fire exposure. Using ABAQUS finite element analysis validated against existing experimental data, the study examines the effects of fire duration, stiffener rib thickness, and steel yield strength on post-fire bearing capacity. The research addresses an important structural safety concern: the residual load-carrying capacity of composite columns after fire exposure.

Core Technical Findings

The study reveals that under ambient temperature conditions, both ordinary CFST columns and stiffened CFST columns exhibit mid-span barrel bulging under axial loading, with local buckling of stiffener ribs. The concrete load-sharing ratio during the loading process follows a pattern of first increasing and then decreasing, with a maximum value of 40.11%. When subjected to ISO-834 standard fire curve heating, the bearing capacity of specimens decreases significantly. Changes in stiffener rib thickness and steel strength have relatively smaller effects on bearing capacity.

Fire-Induced Material Degradation Mechanisms

The significant reduction in bearing capacity after fire exposure can be attributed to the thermal degradation of constituent materials:

Steel degradation:

• Yield strength reduction begins above 300°C, with significant degradation above 550°C
• At 600°C, steel retains approximately 50% of ambient temperature yield strength
• Elastic modulus reduction follows a similar pattern to strength reduction
• Thermal expansion causes additional compressive stresses in the concrete core

Concrete degradation:

• Moisture loss above 100°C reduces concrete strength and bond with steel
• Free water evaporation above 200°C creates internal microcracking
• Decomposition of calcium hydroxide above 400°C leads to significant strength loss
• Spalling risk increases with rapid heating rates and high water content

Interface degradation:

• Thermal incompatibility between steel and concrete creates interfacial stresses
• Differential thermal expansion may cause debonding at the steel-concrete interface
• Reduced bond strength affects the composite action between steel tube and concrete core

Stiffener Rib Function and Performance

The stiffener ribs in square CFST columns serve to increase the local buckling resistance of the tube walls. Under axial compression, the tube walls experience compressive stresses that can lead to local buckling between stiffeners. The presence of stiffeners:

1. Reduces the effective buckling length of the tube walls
2. Provides additional load-carrying cross-section
3. Enhances the confinement effect on the concrete core
4. Distributes stress concentrations from connection details

After fire exposure, the stiffener ribs may undergo local buckling at reduced load levels due to thermal degradation of steel properties. The finding that stiffener thickness has a relatively small effect on post-fire performance suggests that the fire-induced degradation of steel properties dominates over geometric strengthening effects.

Post-Fire Assessment and Repair Considerations

For structural engineers involved in post-fire assessment of CFST columns, the following considerations are critical:

The ISO 834 fire curve represents a standard fire exposure scenario, but actual fires may exhibit different temperature-time profiles. The post-fire residual capacity depends on the specific fire exposure history, including peak temperature, duration at elevated temperature, and cooling rate. Rapid cooling may introduce additional thermal stresses that further reduce capacity.

Engineering Design Implications

The research findings have several important implications for structural design practice:

• Fire-resistant design of CFST columns: The composite action between steel tube and concrete provides inherent fire resistance, but stiffened configurations require specific consideration of stiffener performance at elevated temperatures.
• Post-fire structural assessment: The significant capacity reduction after fire exposure necessitates rigorous assessment before returning structures to service. The stiffened configuration may provide additional safety reserves compared to unstiffened columns.
• Repair and rehabilitation: Post-fire repair may involve replacement of severely degraded stiffener ribs, concrete core replacement, or complete column replacement depending on the extent of damage.
• Safety margin verification: The conclusion that stiffened CFST columns retain adequate safety reserves after fire exposure provides valuable confidence for post-fire assessment decisions.

The load-sharing ratio behavior—concrete load contribution first increasing then decreasing—reflects the progressive transfer of load from steel to concrete as steel degrades under fire, followed by concrete degradation at higher temperatures. The maximum concrete contribution of 40.11% indicates that even at peak load transfer, the steel tube continues to carry the majority of the load.

Study Insights and Reflections

This research contributes to the understanding of post-fire structural performance of composite columns, an area of increasing importance as fire safety becomes a more prominent consideration in structural design. The systematic parametric analysis provides design guidance for selecting appropriate stiffener configurations and steel grades for fire-exposed applications.

The finding that fire duration has the most significant effect on post-fire performance underscores the importance of fire protection design in preventing excessive temperature exposure. The relatively minor effects of stiffener thickness and steel strength on post-fire performance suggest that fire protection measures (such as intumescent coatings or fire-resistant encasements) are more effective than geometric or material modifications for maintaining post-fire capacity.

For future research, the investigation of different fire exposure scenarios (including flashover conditions, localized fires, and rapid cooling) would enhance the applicability of the findings. Additionally, the long-term performance of repaired post-fire columns under cyclic loading would provide important information for structural rehabilitation decisions. The work represents a valuable contribution to the fire safety design and assessment of composite steel-concrete structures.