Fire Resistance Analysis of CFST Composite Column RC Beam Spatial Joint

Research Background and Motivation

The paper by Zhang Bo, Qin Xiaoxiao, Xu Guangpeng, and Ren Qingxin, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) in 2024, addresses the fire resistance performance of a spatial joint connecting a concrete-filled steel tube composite column with reinforced concrete (RC) beams. This research was supported by the National Natural Science Foundation of China (Grant No. 51808351) and Shenyang Science and Technology Plan Project, reflecting the growing importance of fire safety in composite structural systems.

The composite column studied in this paper combines the advantages of CFST construction with RC beam elements, creating a hybrid structural system that is increasingly popular in modern building construction. The spatial joint, where multiple beams intersect with the column, represents a critical structural and fire safety node. Understanding the fire behavior of such joints is essential for ensuring that the entire structural system maintains its integrity during a fire event.

Methodology and Analytical Framework

The authors employed ABAQUS finite element software to establish both a temperature field model and a mechanical model under ISO 834 standard fire conditions. The ISO 834 fire curve, which represents a standard time-temperature relationship for structural fire testing, provides a well-defined thermal loading scenario that allows for systematic comparison with experimental results and other numerical studies.

The analysis follows a two-step approach: first, the temperature field distribution within the joint region is computed under fire exposure, and second, the mechanical response is evaluated using the temperature-dependent material properties obtained from the thermal analysis. This sequential coupling approach is standard practice in fire engineering analysis and captures the essential physics of structural fire behavior.

The model was validated against experimental test data, and the authors report good agreement between simulated and tested results, confirming the credibility of the numerical methodology.

Key Findings on Temperature Distribution and Structural Response

A particularly important finding is that the joint region temperature is significantly lower than the non-joint region due to the protective effect of the intersecting beams and slabs. This thermal shielding effect is a natural consequence of the geometry of the spatial joint and provides an inherent fire protection advantage that should be recognized in design.

The data above shows that increasing the number of beams from 2 to 3 or 4 reduces the fire resistance limit by approximately 41-44%. This counterintuitive result occurs because more beams create additional thermal paths that allow heat to penetrate into the joint region more effectively, despite the apparent protective geometry.

Another critical finding concerns the load transfer mechanism. At room temperature, the internal CFST core carries 43.27% of the total internal force. Under the combined action of high temperature and axial load, this proportion increases to 52.9% at 180 minutes. This shift in load distribution has important implications for the design of composite columns, as the CFST core must be designed to carry an increasing proportion of the load as temperatures rise.

Working Mechanism Analysis

The authors provide a detailed analysis of the working mechanism of the spatial joint under fire conditions. The temperature field distribution reveals a complex pattern influenced by the geometry of the intersecting members, the thermal properties of the steel and concrete materials, and the boundary conditions imposed by the surrounding structure.

The deformation and internal force distribution under fire loading shows that the joint region undergoes significant thermal expansion and contraction, leading to complex stress states that differ substantially from ambient temperature conditions. The interaction between the steel tube, the concrete core, and the RC beams creates a coupled structural system where the behavior of each component influences the others.

Engineering Practice Implications

For practical engineering applications, the findings of this study suggest several important design considerations. First, the protective effect of beam and slab elements on the joint region should be explicitly accounted for in fire resistance calculations, potentially allowing for reduced fire protection material requirements at the joint. Second, the load redistribution from the composite column to the internal CFST core under fire conditions means that the CFST core must have adequate fire resistance design margins.

Third, the reduction in fire resistance limit with increasing beam count highlights the need for careful attention to the number of intersecting members at a joint. In buildings with complex floor plans where many beams converge at a single column, additional fire protection measures may be required at the joint region.

Reflections on Material Behavior at Elevated Temperatures

From a materials engineering perspective, the behavior of steel and concrete at elevated temperatures is the fundamental driver of the structural response observed in this study. Steel loses strength and stiffness rapidly above approximately 400 degrees Celsius, while concrete experiences progressive degradation of its mechanical properties and potential spalling above 300 degrees Celsius. The composite action between steel and concrete in the CFST column provides a degree of mutual protection, but this protection diminishes as temperatures rise.

The welding joints within the composite column, if any, must also be considered in the fire analysis. Welded connections may have different thermal properties and strength degradation characteristics compared to the parent material, and the heat-affected zone of a weld may be particularly vulnerable to high temperature exposure.

Concluding Remarks

This study provides valuable insights into the fire resistance behavior of CFST composite column-RC beam spatial joints, demonstrating that these joints possess good fire resistance performance capable of meeting practical engineering requirements. The systematic analysis of temperature field distribution, failure modes, deformation patterns, and internal force redistribution offers engineers a comprehensive understanding of the joint behavior under fire conditions. The findings regarding the influence of beam count and load redistribution under fire loading have direct practical implications for the fire safety design of composite structural systems.