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

Literature Overview

This paper by Xiong Mingxiang, Hu Qidong, Liu Boyuan, and Lin Jing (2022, Engineering Mechanics, Vol. 39, No. 11, pp. 177-185) investigates the fire resistance performance of high-strength hollow sandwich steel tube concrete (HS-STC) columns. The study was funded by the Guangdong Provincial Natural Science Foundation (Grant 2021A1515012390) and the Guangzhou Science and Technology Program (Grant 20210201407). Standard fire resistance tests were conducted on axially loaded columns with varying cross-sectional shapes, boundary conditions, coating thicknesses, and load ratios. The authors proposed a ductility indicator for characterizing fire resistance performance and compared the behavior of high-strength columns with conventional columns.

Core Technical Findings

The fire resistance behavior of high-strength hollow sandwich SRC columns exhibits a distinctive two-stage deformation pattern. In the early stage of fire exposure, the column expands due to thermal expansion of the steel tube and the concrete core. As the fire progresses and material properties degrade, compressive deformation increases significantly until the fire resistance limit is reached. This behavior is similar to that of conventional columns but with important quantitative differences.

The finding that high-strength steel reduces the fire resistance limit is counterintuitive but physically explainable. High-strength steels typically have lower temperature-dependent strength retention compared to conventional steels, particularly above 600°C, where the yield strength drops more rapidly. This is related to the microstructural characteristics of high-strength steels, which often rely on precipitation hardening and grain refinement mechanisms that are thermally unstable at elevated temperatures.

Temperature Field and Material Behavior

The temperature field distribution within the hollow sandwich SRC column is governed by heat conduction through the steel tube, the air gap (if present), and the concrete core. The air gap in a hollow sandwich section acts as a thermal insulator, reducing the heat transfer rate to the inner steel tube and the concrete core. However, the effectiveness of this insulation depends on the air gap dimensions, the ambient temperature, and the presence of any filler material.

The fire resistance limit is determined by the point at which the column's load-bearing capacity, as calculated using high-temperature material properties, falls below the applied load. The authors employed the European Code 4 (EN 1993-1-2) calculation models for both axially loaded columns and beam-columns, incorporating temperature-dependent material parameters for both steel and concrete. The calculated fire resistance limits showed good agreement with experimental results.

Ductility Indicator and Design Implications

The proposed ductility indicator provides a quantitative measure of the column's deformation capacity under fire conditions. This is particularly important for the design of fire-resistant structures, where ductile behavior allows for redistribution of internal forces and provides warning before catastrophic failure. The ductility indicator can be used to compare different column configurations and to optimize the design for both fire resistance and structural performance.

From a fire protection design perspective, the study highlights the importance of fire-resistant coatings in protecting high-strength steel columns. The coating thickness required to achieve a target fire resistance rating depends on the steel grade, the concrete strength, and the load ratio. For high-strength steel columns, thicker coatings may be required to compensate for the lower temperature-dependent strength retention.

Key Questions and Reflections

A critical question is how the hollow sandwich configuration affects the fire resistance compared to a solid SRC column of equivalent cross-sectional area. The air gap provides thermal insulation but also reduces the overall section modulus and moment of inertia, which may be detrimental in terms of buckling resistance. The optimal air gap dimension that balances thermal insulation and structural efficiency warrants further investigation.

Another consideration is the long-term fire performance under repeated fire exposure, which is not addressed in this study but is relevant for structures in high-risk fire environments. The degradation of the steel-concrete interface bond under thermal cycling may reduce the composite action and, consequently, the fire resistance.

Summary and Conclusions

This paper provides valuable experimental and analytical insights into the fire resistance performance of high-strength hollow sandwich SRC columns. The identification of the two-stage deformation pattern and the proposal of a ductility indicator contribute to a more comprehensive understanding of fire behavior in advanced composite columns. The finding that ultra-high-strength concrete enhances fire resistance while high-strength steel reduces it has important implications for material selection in fire-prone environments. The validated calculation method based on European Code 4 models provides a practical tool for fire resistance design of HS-STC columns, and the study contributes to the ongoing development of fire design codes for advanced composite structures.