Fire Resistance Limit of Hollow Sandwich Steel Tube Concrete Short Columns

Literature Overview

The paper by Liu Xiao, Lu Mengchao, Wang Bing, and Hou Dongxu (Shenyang University, 2022), published in the Journal of Shenyang University (Natural Science Edition) (Vol. 34, No. 2, pp. 126-133), investigates the fire resistance limit of hollow sandwich steel tube concrete (HSC) short columns using finite element analysis with ABAQUS software. Funded by the National Natural Science Foundation of China (Grant 52078306), the study employs the ISO-834 standard fire curve and a sequential thermal-mechanical coupled analysis to examine the temperature distribution and structural performance of HSC columns under fire loading. The research parameterizes the fire resistance limit with respect to section shape, hollow ratio, load ratio, and core concrete compressive strength grade.

Analytical Methodology

The study employs a two-step sequential analysis approach in ABAQUS: first, a thermal analysis to determine the temperature distribution within the HSC column under the ISO-834 standard fire curve; second, a mechanical analysis using the temperature field results to evaluate the structural response and determine the fire resistance limit. This sequential approach, while less computationally demanding than a fully coupled analysis, is widely accepted for fire engineering applications when the thermal-mechanical coupling effects are not dominant.

Material Models and Thermal Parameters

Component	Thermal Model	Mechanical Model
Steel tube	Temperature-dependent thermal conductivity and specific heat	Temperature-dependent elastic-plastic constitutive model with strength reduction
Core concrete	Temperature-dependent thermal conductivity, specific heat, and density	Temperature-dependent confined concrete model with damage evolution
Interface	Thermal contact resistance	Bond-slip contact model

The ISO-834 standard fire curve is defined by the relationship T = 20 + 345 * log10(8t + 1), where T is the gas temperature in degrees Celsius and t is the time in minutes. This curve reaches approximately 900°C at 2 hours and approximately 1000°C at 4 hours, representing a severe fire scenario.

Parametric Study Results

The study identified four parameters that influence the fire resistance limit of HSC short columns:

Parameter	Influence Level	Trend
Load ratio (N/Nu)	Primary factor	Higher load ratio significantly reduces fire resistance limit
Section shape	Significant factor	Shape affects thermal gradient and confinement effectiveness
Hollow ratio	Significant factor	Higher hollow ratio reduces fire resistance by decreasing thermal mass and confinement
Core concrete strength grade	Minor factor	Higher strength grade has limited effect on fire resistance limit

The finding that the load ratio is the dominant parameter is consistent with fire engineering principles, as the fire resistance limit is fundamentally determined by the ratio of the applied load to the residual load-bearing capacity at elevated temperatures. The load ratio directly determines the stress level in the steel tube at any given temperature, and therefore the rate at which the steel approaches its yield and failure temperatures.

Engineering Implications for Steel Pipe Applications

The hollow sandwich steel tube concrete column concept has implications for the design of steel pipe structures in fire-exposed environments. The hollow core serves as a thermal insulation layer, reducing the rate of heat transfer to the inner steel surface and the concrete core. However, this benefit is offset by the reduction in thermal mass and the potential for accelerated heating of the inner steel surface. From a welding and fabrication perspective, the HSC column requires precise alignment and welding of the inner and outer steel tubes, and the quality of these welds is critical to the structural integrity under both normal and fire loading conditions. The welds must maintain their mechanical properties at elevated temperatures, which places requirements on the welding consumables and post-weld heat treatment.

Welding Considerations for HSC Columns

The inner and outer steel tubes are typically connected by transverse stiffeners or welds at regular intervals.
Welding consumables should be selected to maintain adequate strength at elevated temperatures (up to 800°C for short durations).
Post-weld residual stresses should be minimized to prevent premature yielding under combined thermal and mechanical loading.
Non-destructive testing (NDT) of welds should include examination for lack of fusion and porosity, which can be exacerbated by the confined geometry of the HSC section.

Study Insights and Reflections

This research provides valuable quantitative data on the fire resistance behavior of a novel composite section type. The finding that the load ratio is the dominant parameter reinforces the importance of accurate load assessment in fire design, as overestimation of the applied load can lead to excessive conservatism, while underestimation can result in inadequate fire protection. The relatively minor effect of concrete strength grade on fire resistance is somewhat counterintuitive but can be explained by the fact that concrete strength at elevated temperatures is governed primarily by the thermal degradation of the cement paste and aggregate bonding, which is less sensitive to the ambient-temperature strength grade. The study also highlights the potential of hollow sandwich sections as an efficient structural solution that combines the thermal insulation benefit of the hollow core with the structural efficiency of the steel tube-concrete composite action. Future research should extend the analysis to include the effect of fire duration, the interaction between fire and seismic loading, and the behavior of HSC columns under eccentric loading conditions. The parametric study approach used here is directly transferable to the optimization of steel pipe fire protection designs, where the selection of fire-resistant coatings, insulation thickness, and steel grade can be systematically evaluated using similar finite element methodologies.