Fire Resistance of Cross-Shaped Composite Columns with Embedded Steel Tube High-Strength Concrete Core

Literature Overview

This study, published in Industrial Construction (Vol. 46, No. 2, 2016), investigates the fire resistance performance of cross-shaped composite columns reinforced with an embedded steel tube high-strength concrete (HSC) core. The research was conducted by Yang Yong and colleagues from Xi'an University of Architecture and Technology and the State Key Laboratory of Subtropical Building Science. The work addresses a practical structural engineering challenge: improving the load-bearing and fire-resistance capacity of cross-shaped special-shaped columns commonly used in high-rise buildings and industrial structures.

Core Technical Approach

The researchers designed two full-scale cross-shaped composite column specimens subjected to ISO 834 standard fire curves and one specimen tested under ambient axial compression. The key design variables investigated were:

The composite column concept involves nesting a steel tube filled with high-strength concrete within the conventional reinforced concrete cross-shaped column cross-section. This hybrid approach leverages the superior compressive strength and thermal stability of steel tube concrete (SRC) to protect the internal reinforcement and core concrete from high-temperature degradation.

Key Findings and Technical Interpretation

The experimental results reveal several important engineering insights:

1. The embedded steel tube high-strength concrete core significantly enhances the fire resistance limit of the cross-shaped column. The steel tube acts as a protective jacket, delaying spalling of the core concrete and maintaining structural integrity at elevated temperatures.
2. Hoop spacing has a relatively minor influence on the fire resistance limit of the composite column. This finding suggests that the dominant fire-resistance mechanism is provided by the embedded steel tube-HSC core rather than the conventional transverse reinforcement.
3. The failure mode at high temperature involves progressive concrete spalling from the outer cross-section arms, while the embedded core remains relatively intact longer, maintaining residual load-carrying capacity.

Engineering Implications for Steel Pipe Applications

From a steel pipe and welding engineering perspective, this study highlights the critical role of steel tubes as fire-protection elements in composite structures. The steel tube must be fabricated with appropriate weld quality to ensure that:

• Weld seams do not become weak points under thermal gradient loading.
• The tube-to-concrete bond interface maintains integrity during fire exposure.
• Material selection for the steel tube should account for high-temperature strength retention (e.g., Q345 or Q390 grade structural steel).

The study also raises important questions about the weld integrity of the steel tube ends and any potential T-joint or lap welds that might be present at the interface between the embedded tube and the surrounding concrete. Under ISO 834 fire conditions, differential thermal expansion between steel and concrete can induce additional stresses at welded connections.

Study Insights and Reflections

This research contributes valuable data to the design of composite special-shaped columns in fire-prone environments. For engineers involved in steel pipe fabrication and structural detailing, the key takeaway is that steel tubes embedded in concrete can serve dual purposes: enhancing mechanical performance and providing inherent fire protection. However, the study does not extensively address the long-term durability of the steel tube under cyclic thermal loading or the potential for corrosion at the steel-concrete interface after fire exposure. Future work should examine the post-fire residual strength and repairability of such composite columns, as well as the effect of steel tube material grade and wall thickness on fire performance. The finding that hoop spacing is less critical than the embedded core is particularly useful for optimizing construction cost and detailing complexity in practical applications.