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 from Shenyang University, published in the Journal of Shenyang University (Natural Science Edition) in 2022 (Volume 34, Issue 2, pages 126-133), investigates the fire resistance limit of hollow sandwich steel tube-concrete (CFST) short columns. The study was supported by the National Natural Science Foundation of China (52078306). Using the ABAQUS finite element software, the researchers analyzed the temperature transfer patterns and fire resistance behavior of hollow sandwich CFST columns under the ISO 834 standard fire curve, employing a sequentially coupled thermal-mechanical analysis approach. The results were validated against existing experimental data.

Core Technical Points

The hollow sandwich CFST column is a composite structural member that combines a steel tube with a hollow concrete core, creating a layered cross-section that offers potential advantages in weight reduction and fire resistance. The study systematically investigates the influence of four parameters on the fire resistance limit: cross-section shape, hollow ratio, load ratio, and core concrete compressive strength grade.

Thermal-Mechanical Analysis Framework

The analysis employs a sequentially coupled approach, where the thermal analysis is solved first to determine the temperature field distribution throughout the column under the ISO 834 standard fire curve, followed by a mechanical analysis using the temperature field as input to determine the structural response and ultimate failure condition. This approach requires appropriate thermal property models for both steel and concrete at elevated temperatures, as well as constitutive models that account for the degradation of material properties with temperature.

The temperature transfer in a hollow sandwich CFST column is more complex than in a solid CFST column due to the presence of the hollow cavity. The hollow space creates an additional thermal pathway that can either insulate the inner steel surface or create thermal gradients that induce additional stresses. The temperature distribution is non-uniform across the cross-section, with the outer steel tube experiencing higher temperatures than the inner concrete core, and the hollow cavity creating a thermal buffer zone.

Parametric Study Results

The load ratio emerges as the most influential parameter on the fire resistance limit. This is physically intuitive because the load ratio directly determines the stress level in the member under fire conditions; a higher load ratio means the member is closer to its load-carrying capacity at ambient temperature, leaving less margin for the degradation of material properties under fire exposure. The cross-section shape and hollow ratio also have significant effects, with the hollow ratio influencing both the thermal insulation capacity and the effective structural cross-section.

The relatively minor influence of the core concrete compressive strength grade is an important finding because it suggests that using higher-strength concrete in the core does not proportionally improve the fire resistance limit. This is because the fire resistance is governed more by the thermal behavior and the steel component's performance at elevated temperatures than by the concrete's compressive strength alone.

Engineering Practice Implications

For structural engineers designing hollow sandwich CFST columns for fire-exposed applications, the parametric study results provide clear guidance. The load ratio should be kept as low as practical to maximize the fire resistance margin. The cross-section geometry and hollow ratio should be optimized to balance structural efficiency with thermal insulation performance. The relatively minor role of concrete strength grade means that cost-effective concrete grades can be used without significantly compromising fire resistance.

For steel pipe manufacturers, this study highlights that the steel tube thickness and the quality of the steel-concrete interface are critical for fire resistance performance. Thicker steel tubes provide better thermal protection for the concrete core but may increase weight and cost. The manufacturing quality of the steel tube, including wall thickness uniformity and surface finish, directly affects the thermal-mechanical performance of the composite member under fire conditions.

Key Questions and Reflections

The study does not appear to address the effect of fire exposure duration beyond the ISO 834 standard curve, which could be important for structures in regions with different fire codes. Additionally, the behavior of the hollow cavity under fire conditions, including potential gas expansion and pressure effects, is not discussed. The long-term durability of the hollow sandwich CFST column after a fire exposure event, and its residual structural capacity, are also areas that warrant further investigation for practical post-fire assessment applications.

Study Insights and Implications

This research provides valuable engineering insight into the fire resistance behavior of hollow sandwich CFST columns, a composite structural system that offers potential advantages in both structural performance and fire resistance. The identification of the load ratio as the primary controlling parameter for fire resistance limit is a practically important finding that directly informs design decisions. The sequential thermal-mechanical FEA methodology, validated against experimental data, provides a reliable analytical tool for predicting the fire resistance of these members. Engineers can use these findings to optimize the design of hollow sandwich CFST columns for fire-exposed applications, balancing structural efficiency, thermal performance, and cost considerations.