Fire Resistance Performance of High-Strength Steel Tube Concrete Columns

Overview and Research Context

This paper by Zhang, Wang, and Zhang (2018), published in Progress in Steel Building Structures, investigates the fire resistance performance of concrete-filled steel tubular (CFST) columns using high-strength structural steels (Q460 and Q690). The research addresses an important practical concern: as the steel industry develops higher strength grades to reduce material usage and improve structural efficiency, it is essential to understand how these materials perform under fire conditions. The study employs three-dimensional finite element analysis to conduct a comprehensive parametric investigation of the fire resistance behavior of high-strength CFST columns.

The use of high-strength steels in CFST columns offers significant advantages in terms of structural efficiency and material economy. However, the fire resistance performance of high-strength steels is of particular concern because the strength degradation of high-strength steels at elevated temperatures may differ from that of conventional carbon steels. Understanding this behavior is critical for the safe and economical design of fire-exposed CFST columns.

Finite Element Model and Validation

A three-dimensional finite element model was developed to simulate the fire resistance behavior of CFST columns under constant axial load. The model incorporates the following key features:

• Thermal analysis coupled with nonlinear stress analysis to capture temperature-dependent material behavior.
• Concrete-steel interface behavior modeled to account for bond degradation and slip at elevated temperatures.
• Initial geometric imperfections included to represent realistic column behavior.
• Heat conduction analysis performed to determine temperature distribution within the column cross-section.

The model was validated against existing experimental results, confirming its accuracy for predicting the fire resistance behavior of CFST columns.

Parametric Study Results

The parametric study investigated the following variables:

Key findings from the parametric study include:

• Concrete diameter and strength have a greater influence on fire resistance than steel tube strength.
• At the same load ratio, fire resistance generally decreases with increasing steel strength but increases with increasing concrete strength.
• At the same absolute load, higher steel strength grades significantly improve fire resistance because the higher load-bearing capacity at room temperature allows for lower load ratios.

Engineering Practice Implications

The findings have significant implications for the design of high-strength CFST columns in fire-exposed environments:

1. When designing CFST columns for fire resistance, the concrete properties (strength and diameter) should be given equal or greater attention than the steel grade selection.
2. The use of high-strength steel (Q460 or Q690) is beneficial for fire resistance when the absolute load is fixed, as it allows for lower load ratios and thus greater reserve strength at elevated temperatures.
3. The concrete-steel interface behavior is a critical factor in fire resistance prediction, and its degradation at elevated temperatures should be properly accounted for in analytical models.
4. Concrete moisture content can significantly affect fire resistance through spalling mechanisms, and appropriate measures should be taken to control moisture content in the concrete mix.

Study Insights and Reflections

The parametric study provides a comprehensive understanding of how different design parameters interact to influence the fire resistance of high-strength CFST columns. The finding that concrete properties dominate over steel strength in determining fire resistance is particularly important for design optimization. This suggests that in fire-exposed applications, investing in higher strength concrete may provide better fire resistance returns than upgrading the steel grade alone.

The distinction between analyzing fire resistance at a fixed load ratio versus a fixed absolute load is also important. At a fixed load ratio, higher strength steel reduces fire resistance because the strength margin is consumed more rapidly. However, at a fixed absolute load, higher strength steel improves fire resistance because it allows for a lower load ratio. This dual perspective is essential for practical design, where engineers must consider both the structural demand (absolute load) and the utilization ratio (load ratio) when selecting materials.

The finite element approach used in this study is a powerful tool for investigating fire resistance behavior, particularly for parametric studies that would be impractical to conduct experimentally. However, the accuracy of finite element predictions depends on the proper calibration of temperature-dependent material models and interface behavior, which remains an area requiring continued research and refinement.