Temperature Field Experimental Study and Analysis of Ribbed Thin-Walled Square Steel Tube Concrete Columns Under Fire

Literature Overview

This paper by Wang Weiwei, Mi Zhenwei, Lv Xuetao, and Zhang Yuzhuo from Liaoning Technical University, published in Fire Science (Chinese and English) in 2017, investigates the temperature distribution within ribbed thin-walled square steel tube concrete columns subjected to standard fire conditions. Three full-scale specimens were tested under fire loading, and a finite element temperature field model was developed and validated against experimental results. The study examines the effects of rib spacing, steel ratio, section dimensions, fire duration, and fire exposure conditions on the temperature field distribution. The research was supported by the Liaoning Provincial Department of Education General Project (Grant No. LJYL033).

Core Technical Findings

The experimental results demonstrate that under four-sided fire exposure, the cross-sectional temperature field of the ribbed thin-walled square steel tube concrete column exhibits biaxial symmetry. The corner regions experience the highest temperatures due to the combined heat flux from two adjacent fire-exposed faces, while the mid-span of each face has lower temperatures. This corner temperature amplification effect is a critical design consideration because the highest temperatures govern the degradation of material properties and the potential for premature failure.

The finite element model accurately reproduced the experimental temperature distributions, with good agreement between predicted and measured temperatures at various locations within the cross-section. The model captures the heat transfer mechanisms including conduction through the concrete core, convection at the fire-exposed steel tube surface, and the thermal contact resistance between the steel tube and concrete interface.

An important finding is that the stiffening ribs have a localized influence on the temperature field only in the immediate vicinity of the weld connection points. Away from these localized zones, the ribs do not significantly alter the overall heat transfer pattern. This means that rib spacing and steel ratio are not critical parameters for fire resistance design of the temperature field, simplifying the design process.

Engineering Practice Implications

The findings of this study have several important implications for the fire design of steel tube concrete columns:

1. Section size optimization: The section side length is identified as a primary factor influencing the temperature field. Larger sections have greater thermal mass and longer heat conduction paths, resulting in lower core temperatures for the same fire exposure duration. This supports the use of larger sections for fire-critical structural elements.
2. Fire exposure scenarios: The distinction between one-sided, two-sided, and four-sided fire exposure is critical. The temperature field under four-sided exposure is the most severe, and design should be based on the worst-case exposure scenario appropriate for the building's fire compartment configuration.
3. Rib design simplification: The finding that ribs have negligible effect on the overall temperature field means that rib configuration can be optimized for structural performance (stability and load-bearing capacity) without concern for fire resistance implications. This decoupling simplifies the design process.
4. Fire duration considerations: The temperature penetration depth increases with fire duration, and the peak core temperature eventually stabilizes as thermal equilibrium is approached. Design fire duration should be selected based on the required structural performance duration and the building's fire safety objectives.

Key Questions and Reflections

One important limitation of the study is the use of only three full-scale specimens, which limits the statistical reliability of the experimental findings. The parametric study was conducted primarily through finite element analysis, which, while validated against limited experimental data, relies on assumed material property degradation models that may not fully capture the complex behavior of steel and concrete under fire conditions.

Another consideration is the effect of fire-induced spalling on the temperature field. The study focuses on temperature distribution under standard fire conditions but does not address the potential for concrete spalling, which can expose the steel tube to direct fire exposure and dramatically alter the temperature field. In practice, the risk of spalling depends on the concrete mix design, aggregate type, and moisture content, and should be considered in conjunction with temperature field analysis.

Additionally, the study does not address the structural response to the temperature field, including thermal stresses, creep under sustained elevated temperatures, and the progressive loss of load-bearing capacity. A complete fire design requires coupling the temperature field analysis with structural analysis to predict the residual capacity at the end of the design fire duration.

Summary

This study provides valuable experimental and analytical data for the fire design of ribbed thin-walled square steel tube concrete columns. The identification of section size, fire duration, and fire exposure condition as primary factors influencing the temperature field offers clear design guidance. The finding that rib configuration has negligible effect on the overall temperature field simplifies the design process by decoupling structural and fire design considerations. Engineers designing steel tube concrete columns for fire resistance should use these findings to select appropriate section dimensions, verify fire exposure scenarios, and develop efficient rib configurations that meet both structural and fire performance requirements.