Fire Resistance of Square CFST Columns Under Opposite Two-Sided Fire Exposure

Literature Overview

The paper by Lv Xuetao, Yang Hua, and Zhang Sumei, published in Industrial Construction (Volume 42, Issue 6, 2012, pp. 148–152), addresses a critical gap in fire engineering for composite steel-concrete structures. Most existing codes and research assume uniform four-sided fire exposure for square steel tube concrete (CFST) columns, yet real fire scenarios frequently produce asymmetric thermal loading. This study investigates the fire resistance behavior of square CFST columns under a specific non-uniform condition: opposite two-sided fire exposure. The work was supported by the National Natural Science Foundation of China (Grant No. 50708028) and the Central Universities Basic Research Business Fee Special Fund (HIT.NSRIF.2010015).

Core Technical Points

The fundamental insight of this research is that when only two opposite faces of a square CFST column are exposed to fire, the temperature gradient across the section becomes highly asymmetric. Unlike the uniform four-sided case where the steel tube and core concrete heat up nearly simultaneously and uniformly, two-sided exposure creates a pronounced thermal differential between the heated faces and the unheated faces. This differential induces secondary bending moments and thermal stresses that fundamentally alter the load-bearing mechanism.

The authors conducted extensive parametric analyses to characterize:

• The temperature field distribution within the steel tube wall and core concrete under two-sided fire conditions
• The evolution of residual load-bearing capacity over time
• The influence of key geometric and material parameters on fire resistance duration

Key Technical Parameters and Findings

Simplified Capacity Reduction Formula

The authors proposed a simplified formula for the load-bearing capacity influence coefficient under two-sided fire exposure. This coefficient, denoted as the capacity reduction factor, accounts for:

1. The thermal degradation of steel yield strength as a function of temperature
2. The loss of concrete compressive strength at elevated temperatures
3. The additional bending effects caused by asymmetric temperature distribution
4. The thermal expansion mismatch between the steel tube and concrete core

The formula is designed to be practical enough for engineering assessment while capturing the essential physics of non-uniform fire exposure.

Interpretation and Engineering Practice Integration

From a steel pipe manufacturing and welding perspective, this research has several important implications:

Steel Tube Selection and Fabrication

The study confirms that the steel tube acts as both a structural element and a passive fire protection layer for the concrete core. In manufacturing practice, this means:

• Wall thickness tolerances become more critical for fire-exposed applications, as thinner sections will allow faster heat transfer to the core
• Surface quality and coating uniformity matter because localized thin spots can create thermal weak points
• The choice of steel grade (e.g., Q235, Q345, Q390) affects the temperature at which significant strength loss begins

Welding Considerations for CFST Members

For welded square CFST columns, the weld integrity at elevated temperatures is a concern. The following welding quality aspects become relevant:

• Heat-affected zone (HAZ) properties at high temperatures: the HAZ may soften at lower temperatures than the base metal, creating potential weak links under fire
• Residual stresses from welding can interact with thermal stresses during fire exposure, potentially accelerating failure
• Weld geometry and penetration quality affect the thermal conductivity path from the fire zone to the concrete core

Comparison with Existing Codes

The gap between code provisions and actual fire scenarios is significant. Engineers relying solely on standard code provisions may overestimate the fire resistance of CFST columns in asymmetric fire conditions, leading to potential safety deficiencies.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation:

1. How does the two-sided fire exposure condition interact with the connection details at column ends? The paper focuses on the column itself, but in practice, connections may govern the overall system behavior.
2. The study assumes the ISO 834 standard fire curve. Real fires, especially in industrial settings, may have different temperature-time profiles. How sensitive are the results to the fire curve selection?
3. The simplified formula is derived from parametric analysis. Validation against full-scale fire tests would strengthen confidence in its application to critical infrastructure.
4. For welded square CFST columns, the weld metal properties at elevated temperatures deserve dedicated study. The thermal cycling experienced during fire exposure may cause additional degradation in weld quality.

Study Insights and Implications

This research represents an important step toward more realistic fire design of CFST structures. The recognition that non-uniform fire exposure conditions require dedicated analysis methods, rather than relying on simplified uniform exposure assumptions, is a maturation of fire engineering practice. For steel pipe manufacturers and welding engineers, the key takeaway is that fire performance is an integral design requirement that influences material selection, wall thickness specifications, and welding quality control criteria. Future work should extend to three-sided fire exposure, consider the interaction between fire and seismic loading (post-fire seismic capacity), and develop weld-specific fire design guidelines for CFST members.