Fire Resistance Performance of Composite Steel Tube-Concrete Columns with Concrete Spalling Analysis

Literature Overview

This paper by Xiang Kai, Pan Yanchong, and Wang Guohui from the Tianjin Fire Research Institute of the Ministry of Public Security investigates the fire resistance behavior of composite steel tube-concrete columns, specifically examining the influence of concrete spalling under fire exposure. Funded by the National Natural Science Foundation of China (Grant No. 51408134), the study was published in Fire Science and Technology in 2017 (Volume 36, Issue 9, pp. 1182-1186). The research employs a validated finite element model to analyze temperature fields, axial deformation-time curves, sectional internal forces, and failure modes for both spalled and non-spalled conditions.

Core Technical Findings

The study reveals several critical observations regarding concrete spalling effects on fire resistance performance:

• Concrete temperature within the cross-section increases significantly when spalling occurs; the greater the spalling depth, the higher the internal concrete temperature becomes.
• Three spalling parameters—spalling area ratio, spalling depth, and spalling location—all demonstrably influence the fire resistance performance of composite columns.
• After spalling, both the core concrete and the steel tube assume a substantially increased load share, while the reinforced concrete structure external to the steel tube experiences a marked reduction in load-bearing contribution.
• The failure mode at fire resistance limit may fundamentally change for columns that have experienced concrete spalling compared to those that have not.

Temperature Field Behavior

The temperature distribution within composite columns is highly sensitive to spalling events. In non-spalled conditions, the concrete acts as a thermal barrier, moderating the temperature rise in the steel tube and core concrete. Once spalling initiates, this protective layer is compromised, exposing the steel tube to more direct thermal exposure. The finite element model captures the non-linear temperature gradients that develop across the cross-section, revealing that spalling depth correlates directly with peak internal temperatures.

Engineering Practice Implications

From a design perspective, this research underscores the critical importance of accounting for concrete spalling in fire resistance calculations for composite columns. Traditional fire design approaches often assume uniform thermal exposure without considering localized spalling events. The findings suggest that:

1. Fire protection coatings or refractory linings should be designed with sufficient thickness to prevent spalling initiation during the design fire exposure duration.
2. The load redistribution mechanism post-spalling means that steel tube capacity becomes the governing factor—designers must ensure adequate steel tube fire resistance margins.
3. The location of spalling is not merely a geometric parameter but a critical variable; spalling at compression zones versus tension zones will produce different structural responses.

Study Insights and Reflections

This research contributes valuable data to the fire engineering community regarding composite column behavior. The validated finite element approach provides a practical tool for parametric studies that would be prohibitively expensive through physical testing alone. However, the study raises important questions about the predictability of spalling initiation—the exact moment and location of spalling in real fires depend on numerous factors including concrete mix design, curing history, and fire loading rate, which are difficult to capture in simplified models. Engineers should consider incorporating spalling probability assessments into fire safety evaluations for composite structures, particularly in high-rise buildings where composite columns are prevalent.