External Steel Jacket Reinforcement of Fire-Damaged Steel Tube Concrete Composite Short Columns Axial Compression Performance

Literature Overview

This paper by Zhang Zhengtao and colleagues from Shenyang Jianzhu University investigates the axial compression behavior of steel tube concrete (STC) composite short columns that have been subjected to fire exposure and subsequently reinforced with external steel jackets. The research combines experimental testing with finite element analysis (FEA) to evaluate how fire duration, lacing bar spacing, and external steel jacket thickness influence the load-bearing capacity and stiffness of repaired specimens. The work is funded by the National Natural Science Foundation of China (Youth Science Fund, 51208035) and the Liaoning Province "Xingliao Talent Plan" (XLYC1902027), and was published in Industrial Construction in 2020.

Core Technical Findings

The study reveals several critical engineering insights regarding the repair of fire-damaged STC columns through external steel jacketing:

External steel jacket reinforcement significantly improves both load-bearing capacity and stiffness of fire-damaged STC columns, with improvement levels that can even exceed those of undamaged ambient-temperature specimens.
As fire exposure duration increases, the reduction in load-bearing capacity follows a non-linear pattern — initially decreasing more rapidly, then the rate of decline diminishes. Concurrently, the repair effectiveness of the external steel jacket progressively weakens with longer fire exposure times.
Increasing lacing bar spacing gradually reduces load-bearing capacity, though the reduction magnitude is relatively modest. However, lacing bar spacing has a substantially greater impact on specimen stiffness.
Increasing the thickness of the external steel jacket produces a dramatic improvement in load-bearing capacity, with an even more pronounced effect on stiffness recovery.

Parameter Influence Summary

Parameter	Effect on Load Capacity	Effect on Stiffness	Critical Observation
Fire duration increase	Decreases (non-linear)	Decreases	Repair effectiveness of jacket diminishes with longer exposure
Lacing bar spacing increase	Slight decrease	Significant decrease	Stiffness more sensitive than capacity
External steel jacket thickness increase	Large increase	Even larger increase	Most effective repair parameter

Engineering Practice Implications

From a practical standpoint, this research provides valuable guidance for structural engineers dealing with post-fire repair of composite columns in industrial buildings, commercial complexes, and infrastructure. The finding that external steel jacket reinforcement can restore — and even exceed — original load capacity is particularly encouraging for retrofits where column replacement is impractical or prohibitively expensive.

However, several engineering considerations must be addressed:

The diminishing repair effectiveness with increasing fire duration suggests that severely fire-damaged columns may require more aggressive intervention strategies beyond simple external jacketing, such as internal core replacement or complete column replacement.
The significant sensitivity of stiffness to lacing bar spacing implies that in seismic retrofit applications, lacing bar configuration should be optimized to ensure adequate lateral restraint and prevent premature buckling.
The strong correlation between jacket thickness and performance improvement provides a clear design lever — engineers can adjust jacket thickness to achieve target performance levels, subject to constructability and cost constraints.

Key Questions and Reflections

The study raises important questions about the long-term durability of externally jacketed fire-damaged columns. Fire exposure can alter the microstructure of both the steel tube and the concrete core, potentially affecting the bond interface between the jacket and the damaged substrate. The paper does not extensively address issues such as corrosion resistance of the jacket-concrete interface over time, which is critical for service life assessment.

Additionally, while the study focuses on axial compression, real-world columns often experience combined loading conditions including bending, shear, and torsion. The applicability of the jacket reinforcement approach under multi-axial loading scenarios warrants further investigation. From a welding and fabrication perspective, the connection details between the external steel jacket and existing structural elements represent a critical quality control point, where weld integrity and fit-up tolerances directly influence load transfer efficiency.

Study Insights

This research contributes meaningfully to the growing body of knowledge on post-fire structural repair of composite members. The combination of experimental validation with numerical modeling provides a robust framework for understanding failure mechanisms and predicting performance. For practicing engineers, the key takeaway is that external steel jacketing is a viable and effective repair strategy for fire-damaged STC columns, particularly when jacket thickness is appropriately designed and lacing bar spacing is optimized for stiffness recovery.