Fire Resistance Performance of High-Strength Hollow Sandwich Steel Tube Concrete Column

Literature Overview

This paper published in Engineering Mechanics in 2022 (Vol. 39, No. 11, pp. 177-185) presents experimental and analytical studies on the fire resistance performance of hollow sandwich steel tube concrete columns incorporating high-strength steel and ultra-high strength concrete. The research was conducted at the Center for Protective Engineering Research, School of Civil Engineering, Guangzhou University, supported by the Guangdong Provincial Natural Science Foundation (Grant No. 2021A1515012390) and the Guangzhou Science and Technology Program (Grant No. 20210201407). The study addresses the relatively unexplored area of fire performance for advanced material combinations in composite structural columns.

Experimental Program and Test Configuration

The standard fire resistance tests were conducted on axially loaded columns under ISO 834 standard fire curve conditions. The experimental matrix included variations in cross-sectional shape, boundary conditions, fireproof coating thickness, and load ratio. The test parameters were designed to systematically investigate the influence of each variable on the fire resistance limit of the columns.

Fire Behavior and Thermal Response Characteristics

The experimental results revealed that the fire exposure time-displacement response of high-strength hollow sandwich steel tube concrete columns follows a pattern similar to that of ordinary columns. During the early fire exposure phase, the column exhibits thermal expansion, followed by progressive compressive deformation as temperatures increase, ultimately reaching the fire resistance limit. This deformation behavior is consistent with the expected thermal-mechanical response of steel-concrete composite systems under fire loading.

A critical finding is that high-strength steel hollow sandwich columns exhibit lower fire resistance limits compared to ordinary steel columns under identical conditions. This counterintuitive result can be attributed to the temperature-dependent strength degradation characteristics of high-strength steel grades. High-strength steels, which typically have higher carbon and alloy content, tend to lose a greater proportion of their yield strength at elevated temperatures compared to ordinary structural steels. The yield strength reduction factor for high-strength steels at 600 degrees Celsius is typically lower than that for ordinary carbon steels, leading to earlier structural failure under fire conditions.

Conversely, ultra-high strength concrete hollow sandwich columns exhibit higher fire resistance limits compared to ordinary or high-strength concrete columns. This is attributed to the superior thermal stability and residual strength retention of ultra-high strength concrete, which benefits from its dense microstructure and reduced porosity.

Analytical Model and Code Compliance Assessment

The study proposes a ductility index to characterize the fire resistance performance of the columns and compares the fire behavior of high-strength columns with ordinary columns. Analytical calculations were performed based on Eurocode 4 models for axially loaded columns and beam-column members at ambient temperature, incorporating high-temperature mechanical parameters for the constituent materials. The calculated high-temperature buckling bearing capacity and fire resistance limits showed good agreement with experimental results.

From a welding and fabrication quality perspective, the fire resistance of hollow sandwich steel tube concrete columns is also influenced by the quality of the steel tube fabrication and any welded connections. The steel tube manufacturing process (seamless, ERW, or welded) affects the material homogeneity and residual stress distribution, which in turn influences the thermal-mechanical response under fire exposure. Welded steel tubes may exhibit different fire behavior at the weld zones compared to the parent material, particularly if the weld heat-affected zone has altered microstructural properties.

Study Insights and Engineering Implications

This study provides important experimental data for the fire design of high-strength steel and ultra-high strength concrete composite columns. The key engineering insight is that the selection of high-strength steel for fire-exposed structural elements requires careful consideration, as the fire resistance performance may be inferior to that of ordinary steel despite the superior ambient temperature strength. For practicing engineers, the practical implication is that fire protection measures such as intumescent coatings or fire-resistant encasements may need to be more generously specified for high-strength steel composite columns. The proposed analytical approach based on Eurocode 4 with modified high-temperature parameters provides a practical design methodology that can be implemented in engineering practice. The findings also highlight the need for further research on the long-term fire exposure behavior of these advanced material combinations, particularly regarding spalling of ultra-high strength concrete at high temperatures.