Mechanical Behavior of Square Steel Tube Concrete Composite Columns Under Adjacent Two-Sided Fire

Literature Overview

This paper published in Building Structures (2025, Vol. 55, No. 14, pp. 115-121) by Yang Li and Bao Yanhong from Qinghai University investigates the mechanical properties of square steel tube concrete (STC) composite columns under adjacent two-sided fire conditions. Funded by the Qinghai Provincial Natural Science Foundation (2024-ZJ-745), the research uses ABAQUS finite element analysis to establish numerical models and analyze temperature field distribution, failure modes, internal force redistribution, stress-strain development, and fire resistance limit factors. The study also presents a simplified calculation formula for fire resistance limit based on six significant influencing factors.

Core Technical Findings

Temperature Field Distribution

The temperature field distribution in the cross-section of the STC composite column under adjacent two-sided fire exhibits single-axis symmetry. This symmetry is a direct consequence of the symmetric fire exposure on two adjacent faces, with the fire temperature gradient propagating inward from the exposed faces. The temperature distribution is governed by the thermal conductivity and heat capacity of the constituent materials, with the steel tube providing a relatively rapid heat transfer path while the concrete acts as a thermal barrier.

Failure Modes and Internal Force Redistribution

The failure of the STC composite column under adjacent two-sided fire involves several mechanisms:

• The steel tube on the fire-exposed faces experiences significant strength degradation due to temperature-induced reduction in yield strength and elastic modulus.
• The concrete core experiences thermal cracking and spalling, reducing its compressive capacity.
• Internal force redistribution occurs as the fire-exposed side loses capacity, transferring loads to the unexposed side.
• The column may fail by buckling when the combined axial load exceeds the reduced load capacity of the fire-damaged cross-section.

Key Influencing Factors

The parametric study identifies six factors with significant influence on the fire resistance limit:

Simplified Calculation Formula

The study presents a simplified calculation formula for the fire resistance limit that incorporates the six significant factors. The formula provides good agreement with finite element analysis results, offering a practical tool for engineering design.

Engineering Practice Insights

Fire Protection Design for STC Composite Columns

The findings of this study have direct implications for the fire protection design of steel tube concrete composite columns:

Steel Tube Material Selection:

• The fire resistance of the steel tube is governed by its temperature-dependent mechanical properties. According to Eurocode 4, the reduction factor for steel strength decreases significantly above 400°C, reaching approximately 0.6 at 600°C and 0.2 at 800°C.
• For fire-critical applications, steel grades with better high-temperature strength retention should be considered, such as fire-resistant steel grades (e.g., S355 Fire or equivalent).
• The steel tube thickness directly affects the thermal mass and the time to reach critical temperatures in the concrete core.

Concrete Protection:

• The concrete core serves as both a structural element and a fire protection layer for the steel tube.
• The fire resistance of the concrete depends on its thermal conductivity, which is affected by aggregate type, water-cement ratio, and curing conditions.
• Lightweight concrete with lower thermal conductivity may provide better fire protection for the steel tube but may have lower structural strength.

Welding and Connection Details:

• Welded connections in STC composite columns are vulnerable to fire damage due to the localized weakening of weld metal and HAZ at elevated temperatures.
• The welding process used for the steel tube seam and any internal welds should be selected to minimize residual stresses, as these can be exacerbated by thermal gradients during fire exposure.
• Post-weld heat treatment may be beneficial for reducing residual stresses that could contribute to premature failure under fire conditions.

Quality Control for Fire-Critical STC Columns

For STC composite columns designed for specific fire resistance ratings, the following quality control measures are essential:

Parametric Design Considerations

The parametric study results provide guidance for optimizing the fire resistance of STC composite columns:

1. Load Ratio Optimization: Design the column to maintain a load ratio below 0.6 to ensure adequate fire resistance margin. Higher load ratios significantly reduce fire resistance due to the accelerated degradation of structural capacity under fire conditions.
2. Slenderness Ratio Control: Limit the slenderness ratio to below 120 for fire-critical columns. Higher slenderness ratios increase buckling susceptibility, which is exacerbated by the asymmetric temperature distribution under two-sided fire.
3. Cross-Section Size Selection: Larger cross-section dimensions provide better fire resistance due to greater thermal mass and section modulus. However, this must be balanced against economic considerations and architectural constraints.
4. Steel Ratio Optimization: A steel ratio in the range of 20-40% provides a good balance between structural efficiency and fire resistance. Higher steel ratios increase the fire resistance but may not be economically justified.
5. Load Eccentricity Minimization: Design the column to minimize load eccentricity, as eccentric loading significantly reduces fire resistance by introducing combined bending and axial effects that accelerate failure.

Study Insights and Reflections

This research addresses an important but under-researched aspect of steel tube concrete composite column design: the behavior under asymmetric fire exposure. Most existing fire design codes and research focus on uniform fire exposure from all sides, which is a conservative assumption but does not reflect the actual fire scenarios in many real structures. The finding that the temperature field exhibits single-axis symmetry under adjacent two-sided fire is physically intuitive but important for accurate numerical modeling. The simplified calculation formula provided in the study is a valuable practical tool that bridges the gap between detailed finite element analysis and engineering design practice. However, I note that the study is based on numerical analysis rather than physical fire testing, which introduces uncertainties related to material property modeling, boundary condition assumptions, and the representation of complex fire phenomena such as concrete spalling and steel oxidation. Future research should include physical fire testing to validate the numerical models and refine the simplified calculation formula. For engineering practice, the key takeaway is that the fire resistance of STC composite columns is significantly affected by load ratio, slenderness ratio, and load eccentricity, and these factors should be carefully considered in fire-critical structural design. The steel tube material properties and welding quality are also critical, as any reduction in structural capacity due to material defects or welding imperfections would directly affect the fire resistance performance. This study contributes to the ongoing development of performance-based fire design methods for composite structural systems, which are increasingly used in modern construction.