Experimental Study on Axially Compressed Concrete-Filled Steel Tube Members with Different Cross-Sections After High Temperature

Literature Overview

This paper by Jiang Shaofei and Liu Ming (2002), published in the Journal of Northeastern University, presents experimental research on the mechanical properties of axially compressed CFST members with different cross-section shapes (circular, square, and rectangular) after exposure to high temperatures. The research was funded by the Liaoning Provincial Department of Education (992721678) and the Ministry of Construction Science and Technology Development Project (2000[034]). The study investigates the residual bearing capacity and mechanical behavior of CFST columns after fire exposure, providing essential data for fire-resistant design of composite structures.

Research Significance and Background

Fire safety is a critical consideration in the design of steel and composite structures. CFST columns are widely used in high-rise buildings, bridges, and industrial structures due to their high strength-to-weight ratio and ductility. However, the performance of these members after fire exposure is complex due to the interaction between the steel tube and concrete core under elevated temperatures. Understanding the residual capacity of CFST members after fire is essential for:

• Post-fire structural assessment and repair decisions
• Fire-resistant design optimization
• Insurance and risk assessment
• Emergency response planning

Experimental Program

The study examines three cross-section shapes: circular, square, and rectangular. The specimens were subjected to controlled heating at various temperatures and then tested for residual axial compression capacity. The experimental variables include:

Key Experimental Findings

The research reveals a clear hierarchy in the residual bearing capacity reduction among different cross-section shapes:

This finding is counterintuitive at first glance, as circular sections are generally considered the most efficient cross-section for CFST columns. However, the explanation lies in the differential thermal expansion and confinement behavior of different shapes under fire conditions.

Technical Interpretation

The differential performance of cross-section shapes after high temperature exposure can be explained by several mechanisms:

1. Thermal expansion effects: Square and rectangular sections have corners that provide geometric restraint to thermal expansion of the steel tube. This restraint helps maintain the confinement effectiveness even at elevated temperatures. Circular sections, lacking corners, are more susceptible to outward expansion and loss of confinement.
2. Confinement distribution: In square and rectangular sections, the concrete at the corners experiences higher triaxial stress states due to the geometric configuration. This enhanced confinement provides a reserve capacity that is less affected by temperature-induced degradation.
3. Buckling behavior: Circular tubes are more prone to ovalization under thermal loading, which reduces their ability to confine the concrete core. Square tubes maintain their shape better under thermal expansion due to the corner restraint.
4. Concrete-steel interaction: The bond between concrete and steel tube is affected differently in different cross-section shapes. The corners of square sections provide additional mechanical interlock that helps maintain the composite action after cooling.

Residual Bearing Capacity Calculation

The paper discusses methods for calculating the residual bearing capacity and proposes reduction factors for design purposes. The reduction factors account for:

• Loss of steel strength at elevated temperature
• Degradation of concrete properties (strength, stiffness, bond)
• Thermal residual stresses after cooling
• Geometric imperfections introduced by thermal deformation

The proposed calculation method provides a practical tool for post-fire assessment of CFST columns. The method should be applied with caution, as the actual residual capacity depends on the specific fire exposure history, which may be difficult to reconstruct in real-world scenarios.

Engineering Practice Considerations

The findings have several practical implications for engineering practice:

1. Cross-section selection: For fire-prone applications, square CFST columns may be preferred over circular ones, despite the slightly lower efficiency of square sections under normal service conditions.
2. Fire protection design: The differential performance of cross-section shapes should inform the design of fire protection systems. Circular CFST columns may require additional fire protection measures.
3. Post-fire assessment: The residual capacity reduction factors can be used in post-fire structural assessment to determine whether a column can remain in service, requires repair, or must be replaced.
4. Quality control: The manufacturing quality of CFST columns, particularly the quality of the concrete-steel bond, is critical for maintaining residual capacity after fire exposure. Proper concrete placement and curing are essential.

From a welding perspective, the connections between CFST columns and other structural elements must also be considered in fire design. Welded connections may experience different thermal stress states than the column itself, potentially leading to premature connection failure before the column reaches its residual capacity limit.

Study Insights and Reflections

This research provides valuable experimental data for the fire-resistant design of CFST structures. The finding that square sections outperform circular sections in residual capacity retention is particularly significant for design practice. It suggests that the conventional wisdom favoring circular CFST columns for all applications may need to be reconsidered for fire-prone structures.

The research also highlights the complexity of fire behavior in composite structures. The interaction between steel and concrete under thermal loading is highly nonlinear and depends on numerous factors including heating rate, temperature duration, and cooling conditions. Further research is needed to develop comprehensive design guidelines that account for these complexities.

The practical implication for engineers is that fire-resistant design of CFST structures requires careful consideration of cross-section shape, fire protection strategy, and post-fire assessment procedures. The research provides a foundation for developing more refined design methods that can optimize the balance between structural efficiency and fire safety.

Concluding Remarks

These five studies collectively represent significant contributions to the understanding and design of steel pipe and composite structural systems. The research on ring-shaped steel core CFST columns and constraint tie rod-reinforced CFST columns demonstrates innovative approaches to enhancing structural performance through internal and external reinforcement. The soil pressure distribution study provides critical insights for the design of large-diameter buried steel pipes in hydropower applications. The seamless tube rolling simulation software addresses the need for process optimization in modern steel tube manufacturing. Finally, the high-temperature performance study of CFST members provides essential data for fire-resistant design and post-fire assessment. Together, these studies reflect the multidisciplinary nature of modern steel pipe engineering, encompassing structural mechanics, materials science, manufacturing technology, and geotechnical engineering. Engineers working in this field must maintain a broad knowledge base and continuously update their understanding of both theoretical developments and practical applications to deliver safe, efficient, and economical solutions.