Fire Resistance Limit of Rectangular Steel Tube Reinforced Concrete Columns Under Adjacent Two-Sided Fire

Literature Overview

This paper by Lv Xuetao and colleagues from Liaoning Technical University investigates the fire resistance limit of rectangular hollow steel tube (RHST) reinforced concrete (RC) columns subjected to fire on two adjacent faces. The study employs ABAQUS finite element software to establish a thermal-mechanical coupled analysis model and performs a parametric study on factors including perimeter, aspect ratio, slenderness ratio, reinforcement ratio, load ratio, and material strength. The theoretical results are validated against existing experimental data from domestic and international sources, demonstrating good agreement.

Core Technical Points

Fire Exposure Condition and Modeling Approach

The critical distinction in this work is the fire exposure scenario: adjacent two-sided fire rather than the conventional four-sided fire. This condition is more realistic for interior columns in multi-story buildings where fire typically originates from one or two adjacent rooms. The thermal boundary conditions differ significantly from uniform four-sided exposure, leading to asymmetric temperature gradients across the cross-section and a more complex stress redistribution pattern.

The ABAQUS model couples heat transfer analysis with structural analysis. The thermal module computes the temperature field distribution within the steel tube, concrete core, and reinforcement, while the structural module evaluates the load-bearing capacity degradation over time under the elevated temperature field. Material constitutive models for steel and concrete at elevated temperatures follow Eurocode 4 and Chinese code GB 51249 provisions, incorporating strength reduction factors and thermal expansion coefficients.

Parametric Study Results

Theoretical Formula

The paper derives a theoretical formula for the fire resistance limit that incorporates the load ratio, section dimensions, and slenderness ratio as the dominant variables. The formula provides a practical estimation tool for engineers during preliminary design stages, though it should be validated against project-specific conditions.

Process and Standards Analysis

From a steel pipe manufacturing and welding perspective, several points merit attention:

1. Weld quality impact on fire performance: The longitudinal and transverse welds in rectangular steel tubes (typically formed by ERW, HFW, or LSAW processes) represent potential weak points under fire exposure. Weld HAZ regions may experience accelerated strength degradation due to microstructural changes induced by the welding thermal cycle, compounded by the subsequent fire exposure.
2. Material selection for fire-prone applications: For columns likely to experience fire exposure, the steel grade selection should consider not only room-temperature mechanical properties but also high-temperature strength retention. Q345 and Q390 steels show different degradation behaviors at 400-800°C, which affects the overall fire resistance.
3. Weld residual stress interaction: Residual stresses from the pipe forming and welding processes interact with thermally induced stresses during fire. The pre-existing residual stress field can accelerate buckling initiation at elevated temperatures, potentially reducing the fire resistance below predictions based on room-temperature residual stress patterns.

Integration with Engineering Practice

In practical engineering, the findings of this study have direct implications for the design of steel tube reinforced concrete columns in commercial and residential buildings:

• Interior columns in open-plan office buildings frequently experience two-sided fire exposure, making this study particularly relevant.
• The load ratio is the most controllable design parameter; reducing the applied load relative to the design capacity provides the most effective improvement in fire resistance.
• Section geometry optimization (increasing side length, reducing slenderness ratio) offers structural efficiency gains that are easily implemented during the design phase.
• For existing structures undergoing retrofit, the theoretical formula can serve as a screening tool to identify columns requiring fire protection enhancement.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation:

1. How do the specific welding processes used for rectangular tube fabrication (HFW versus LSAW) affect the fire resistance performance? The weld HAZ behavior at elevated temperatures deserves dedicated experimental study.
2. The study focuses on rectangular tubes; how would the results differ for circular tubes with similar cross-sectional areas?
3. The thermal analysis assumes uniform fire exposure on the two faces; real fires exhibit non-uniform temperature distributions that could be more severe at certain locations.
4. The long-term interaction between fire damage and subsequent structural loading (post-fire assessment) is not addressed.

Study Insights and Implications

This paper provides a valuable theoretical framework for assessing the fire resistance of rectangular steel tube RC columns under realistic fire exposure conditions. The identification of load ratio, section side length, and slenderness ratio as the three primary parameters offers clear design guidance. For steel pipe manufacturers, the study underscores the importance of producing tubes with consistent geometric tolerances, as section dimensions directly influence fire performance. The theoretical formula derived in the paper represents a useful tool for preliminary design verification, though engineers should exercise caution when applying it outside the parameter ranges studied. The work contributes to the growing body of knowledge on composite steel-concrete structures and supports the development of performance-based fire design methodologies.