Load-Bearing Capacity of Circular and Square CFST Columns After Fire Exposure

Literature Overview

This paper by Han Linhai and Huo Jingsi, published in China Civil Engineering Journal (2002, Vol. 35, No. 4), investigates the post-fire load-bearing capacity of circular and square concrete-filled steel tube (CFST) columns. The research addresses a critical and under-explored area of structural engineering: the residual strength of CFST members after exposure to fire. The paper establishes stress-strain models for steel and concrete at elevated temperatures and uses numerical analysis to predict the post-fire load-deformation behavior of CFST columns under ISO 834 standard fire curve conditions.

Technical Background

Fire exposure represents one of the most severe environmental threats to structural integrity. In a fire, steel and concrete experience significant degradation of their mechanical properties:

Steel strength degradation: Steel loses strength progressively with increasing temperature. At 400°C, steel retains approximately 75% of its room-temperature strength; at 600°C, approximately 40%; and at 800°C, less than 20%.
Concrete strength degradation: Concrete also loses strength with temperature, but the degradation is more complex due to the dehydration of calcium hydroxide and other chemical reactions. At 400°C, concrete retains approximately 50% of its strength; at 600°C, approximately 25%; and above 700°C, significant spalling may occur.
Thermal expansion mismatch: Steel and concrete have different coefficients of thermal expansion, leading to thermal stresses at the steel-concrete interface during fire exposure and subsequent cooling.
Spalling risk: In CFST members, the steel tube can constrain the lateral expansion of concrete during heating, potentially leading to concrete spalling if the confining pressure exceeds the concrete's tensile capacity.

The post-fire residual strength of CFST columns is critical for structural assessment after a fire event. Engineers need to determine whether the structure can be safely repaired and reused or must be demolished and rebuilt. Without reliable prediction methods, the conservative approach is to assume complete loss of strength, which can lead to unnecessary demolition and significant economic loss.

Methodology

The paper's methodology consisted of two main components:

Material models at elevated temperatures: The authors established stress-strain relationships for both steel and concrete at elevated temperatures. For steel, the models accounted for the progressive reduction in yield strength and elastic modulus with temperature, as well as the potential for creep and stress relaxation at sustained elevated temperatures. For concrete, the models captured the nonlinear stress-strain behavior at elevated temperatures, including the reduced peak strength, reduced peak strain, and modified post-peak behavior.

Numerical analysis framework: Using the established material models, the authors performed numerical analysis of circular and square CFST columns under the ISO 834 standard fire curve. The ISO 834 curve defines a temperature-time relationship for standard fire testing: T = 345 × log₁₀(8t + 1) + 20, where T is temperature in °C and t is time in minutes. The numerical analysis computed the load-deformation relationship of CFST columns after fire exposure, considering the residual material properties and any damage accumulated during heating.

Key Findings

Validation with experimental data: The numerical analysis results were compared with available experimental data, and the agreement was found to be generally good. This validation is important because it confirms that the numerical framework can be reliably used for predicting post-fire behavior under conditions not directly tested experimentally.

Influencing factors: The study systematically analyzed the effects of the following parameters on post-fire load-bearing capacity:

Parameter	Effect on Post-Fire Capacity	Mechanism
Fire duration	Negative	Longer exposure leads to higher temperature and greater strength degradation
Steel ratio	Positive	Higher steel ratio provides more structural continuity and confinement
Steel grade	Complex	Higher grade steel may have different high-temperature behavior
Concrete strength	Moderate	Higher strength concrete may have more pronounced spalling risk
Eccentricity ratio	Negative	Higher eccentricity increases flexural demand and reduces capacity
Section size	Positive	Larger sections have better thermal inertia and slower temperature rise

Circular vs. square sections: The comparison between circular and square CFST columns revealed differences in post-fire behavior. Circular sections generally performed better because:

The uniform geometry provides more uniform thermal exposure and stress distribution.
The continuous curvature provides better confinement of the core concrete.
The absence of corners eliminates stress concentration points that can initiate spalling.

Square sections, on the other hand, experienced higher stress concentrations at the corners during heating, which could lead to earlier concrete spalling and greater capacity loss.

Eccentric loading: The effect of eccentricity was found to be particularly significant. Under eccentric loading, the post-fire capacity reduction was more pronounced than under concentric loading. This is because the flexural component of the loading introduces additional stress concentrations that are more sensitive to material degradation.

Engineering Practice Implications

Post-fire structural assessment: The paper provides a framework for assessing the residual strength of CFST columns after fire exposure. Engineers can use the numerical analysis approach to estimate the remaining load-bearing capacity based on the fire exposure history (temperature-time curve) and