Fire Resistance Performance of Axially Compressed Square Section CFST Composite Columns

Literature Overview

The study by Xiang Kai and Wang Guohui, published in the Journal of Southwest Jiaotong University in 2014, investigates the fire resistance performance of axially compressed square section concrete-filled steel tube (CFST) composite columns. Funded by the "Twelfth Five-Year Plan" National Science and Technology Support Program (2011BAK03B0104), this research combines experimental testing of four physical specimens with comprehensive finite element analysis to evaluate how fire exposure affects the structural integrity of CFST columns. The work was conducted at the Tianjin Fire Research Institute of the Ministry of Public Security, bringing both fire engineering expertise and structural engineering rigor to the investigation.

Experimental Program and Test Results

The experimental program involved four axially compressed square section CFST composite columns subjected to standard fire temperature-time curves. The specimens were designed to represent practical structural configurations found in modern high-rise buildings, where square CFST columns are increasingly used for their efficient load-carrying capacity and architectural flexibility. The key test parameters included fire load ratio, section tube ratio, and section side length, with each parameter varied systematically to isolate its effect on fire performance.

The experimental results revealed several important trends. When the fire load ratio decreased, the axial expansion deformation of the specimens increased while the axial compressive deformation decreased. This behavior is consistent with the fundamental physics of thermal expansion: at lower load levels, the column is less constrained and can expand more freely under heating. As the section side length increased, the internal temperature of the column decreased under the same heating curve and fire duration, which is attributed to the larger concrete mass absorbing more thermal energy and providing greater thermal inertia. The section tube ratio, which represents the proportion of the cross-section occupied by the steel tube, had a relatively minor effect on the internal temperature distribution.

Finite Element Analysis and Parametric Study

The finite element analysis extended the experimental findings by enabling a more comprehensive parametric study covering a wider range of design variables. The numerical model incorporated temperature-dependent material properties for both steel and concrete, including the degradation of steel yield strength at elevated temperatures and the spalling behavior of concrete under fire. The analysis identified three primary factors governing the fire resistance performance: the fire load ratio, the section side length, and the slenderness ratio. Secondary factors including the section tube ratio, longitudinal reinforcement ratio, and material strength showed comparatively limited influence on the overall fire performance.

The finite element results confirmed that the fire load ratio is the most critical parameter, as it directly determines the stress state of the column at the onset of fire exposure. Columns subjected to high load ratios at the time of fire ignition are more vulnerable to premature failure because the thermal weakening of the steel tube occurs under a high pre-existing stress state. The section side length influences the thermal gradient within the column, with larger sections exhibiting more uniform temperature distributions and better fire resistance. The slenderness ratio affects the buckling behavior under fire-induced thermal stresses, with slender columns being more susceptible to lateral instability.

Engineering Practice and Design Recommendations

For steel pipe engineers involved in the specification of CFST columns for fire-exposed structures, this study provides several actionable recommendations. First, the fire load ratio should be carefully evaluated during the design phase, as it has the most significant impact on fire resistance. Structures designed with conservative load assumptions may exhibit better fire performance than those designed at the limit state. Second, the section geometry should be optimized to provide adequate thermal mass, with larger section dimensions generally offering better fire resistance. Third, the section tube ratio should not be relied upon as a primary means of enhancing fire performance, as its influence is relatively minor compared to other parameters.

From a steel pipe manufacturing standpoint, the study highlights the importance of selecting steel grades with favorable high-temperature strength retention characteristics. Standard structural steels such as Q345 and Q355 experience significant strength degradation above 500 degrees Celsius, which can compromise the structural integrity of CFST columns during fire events. Consideration should be given to using fire-resistant steel grades or applying fire protection coatings to the steel tube surface to delay the onset of strength degradation.

Key Questions and Critical Reflection

A significant question arising from this study is the adequacy of current design codes for CFST columns in fire conditions. Many international codes, including Eurocode 4 and the Chinese code GB 51249, provide simplified methods for fire resistance design that may not fully capture the complex interaction between the steel tube and concrete core under thermal loading. The composite action that provides enhanced load capacity at ambient temperature may be partially or completely lost at elevated temperatures, particularly when the steel tube loses strength before the concrete core is significantly affected.

The study's finding that the section tube ratio has limited influence on fire performance is somewhat surprising given that the steel tube provides the primary structural confinement to the concrete core. However, this may be explained by the fact that the thermal degradation of steel strength is so severe at typical fire temperatures that the confinement benefit is largely negated regardless of the tube ratio. This insight has important implications for material selection: investing in thicker-walled steel tubes may not provide proportional improvements in fire resistance and should be balanced against cost considerations.

The research also raises questions about the long-term fire performance of CFST columns after a fire event, particularly regarding the residual capacity of the column following cooling. This aspect, while not directly addressed in the study, is critical for post-fire structural assessment and repair decisions, and warrants further investigation.