Parametric Analysis of Axial Compression Performance of High-Strength Concrete-Filled Steel Tube Columns After Fire Exposure

Literature Context and Research Motivation

The paper by Wang Xiaochu, Yang Yuqi, Liu Xiao, Hui Yanchuan, Cui Youxuan, Yuan Lihao, Zhu Jun, and Hou Dongxu from Shenyang University (Journal of Shenyang University, Natural Science Edition, ISSN 2095-5456, Vol. 36, Issue 1, 2024, pp. 61-67) addresses the critical issue of structural performance degradation in high-strength concrete-filled steel tube (CFST) columns following fire exposure. Funded by the National Natural Science Foundation of China (52078306), the Liaoning Provincial Department of Education Basic Science Research Project (LJKQZ2021165), and the Shenyang Science and Technology Program (21-108-9-21), this research contributes to the growing body of knowledge on fire-resistance design of composite structures.

Research Methodology

The study employs ABAQUS finite element software to simulate the axial compression behavior of high-strength CFST columns after fire exposure. The parametric analysis focuses on three primary variables:

The load-displacement curves are analyzed for each parameter combination, and typical curves are examined to understand the failure mechanisms and load-carrying behavior.

Key Technical Findings

Effect of Steel Tube Yield Strength

The results demonstrate that increasing the steel tube yield strength leads to increased ultimate load-bearing capacity after fire exposure. This finding is consistent with the general principle that higher-strength materials retain greater absolute strength at elevated temperatures, although the relative strength retention ratio may be lower than for lower-strength grades.

Effect of Concrete Strength

Higher concrete strength similarly contributes to greater ultimate bearing capacity of the post-fire column. However, the relationship is more complex because concrete experiences significant strength degradation at elevated temperatures, with the degradation rate depending on the specific concrete composition and maximum temperature reached.

Effect of Fire Exposure Time

The most critical finding is that increasing fire exposure time leads to a decreasing trend in ultimate bearing capacity. This is attributed to:

1. Greater temperature penetration into the column cross-section with longer exposure
2. More severe degradation of concrete strength and steel properties at higher temperatures
3. Potential for spalling of the concrete cover in longer-duration fires

Load Distribution Analysis

A particularly important finding is that at fire exposure times of 60, 90, and 120 minutes, the concrete within the column carries the primary load. This observation has significant implications for:

• The design philosophy of post-fire structural assessment
• The relative importance of concrete quality versus steel tube quality in fire-resistance design
• The development of fire-resistance enhancement strategies

Steel Tube Material Considerations for Fire Applications

From a metallurgical perspective, the behavior of steel tubes during and after fire exposure involves several complex phenomena:

• Strength degradation follows a characteristic curve with a minimum around 550-600°C before partial recovery at higher temperatures
• Creep deformation becomes significant above 400°C, affecting the residual stress state
• Phase transformations may occur in certain alloy grades at extreme temperatures
• The thermal gradient through the tube wall creates bending moments that can lead to local buckling

The selection of steel grade for fire-exposed CFST columns should consider not only ambient temperature properties but also the strength retention at expected fire temperatures. Low-alloy high-strength steels generally show better fire performance than carbon steels of equivalent ambient strength due to their more gradual strength degradation.

Engineering Practice and Design Implications

The research findings have direct implications for structural design and fire safety engineering:

1. Post-fire structural assessment should account for the progressive nature of strength degradation with exposure time
2. The concrete component may be more critical than the steel tube in determining post-fire capacity for moderate fire durations
3. Fire protection strategies should focus on limiting the temperature reached by the concrete core
4. High-strength steel tubes provide benefits but the advantage diminishes at higher fire temperatures

For welding operations on CFST columns in fire-exposed structures, the welding procedure must ensure adequate toughness at both ambient and elevated temperatures. Post-weld heat treatment may be necessary to restore mechanical properties in the heat-affected zone, particularly for high-strength grades.

Study Insights and Outlook

This parametric study provides valuable quantitative insights into the fire behavior of high-strength CFST columns. The finding that concrete carries the primary load at typical fire exposure times challenges the conventional emphasis on steel tube properties in fire-resistance design. Future research should investigate the effects of fire cooling rates on residual strength, the behavior of columns with different cross-sectional geometries, and the development of simplified design expressions for post-fire capacity assessment. The integration of experimental fire testing with numerical simulation will be essential to validate and refine the parametric trends identified in this study.