Experimental Study on Fire Resistance Limit of Square Steel Tube Concrete Columns with Protective Layers

Literature Overview

This study by Han Linhai and Xu Lei from Fuzhou University and Harbin Institute of Technology, published in the China Civil Engineering Journal in 2000, investigates the fire resistance performance of square steel tube concrete (CFST) columns with fire protective coatings under standard fire temperature curves. Funded by the National Natural Science Foundation and Ministry of Education grants, the research tests three full-scale CFST column specimens under the standard heating curves specified in ISO 834 and GB 9978-88. The study establishes that CFST columns possess inherent fire resistance advantages due to the composite action between steel and concrete, and that relatively thin protective coatings can achieve the fire resistance limits required by GB 50045-95 for high-rise building columns.

Standard Fire Testing and Performance Results

The standard fire temperature curve follows the relationship T(t) = 345·log₁₀(8t + 1) + 20, where T is temperature in degrees Celsius and t is time in minutes. The test specimens were square CFST columns with various protective coating thicknesses, subjected to increasing temperatures until structural failure or a specified time limit was reached.

Test Parameter	Specification	Purpose
Heating curve	ISO 834 / GB 9978-88	Standard fire exposure
Column shape	Square hollow section	Representative of common structural sections
Concrete grade	C30–C40	Typical structural concrete
Protective coating	Intumescent or cementitious	Fire protection
Load condition	Axial compression	Typical column loading
Failure criterion	Load-bearing capacity loss or excessive deformation	Per code requirements

The key finding is that the fire resistance behavior of square CFST columns is similar to that of circular CFST columns, with the composite action providing inherent thermal protection. The concrete core absorbs heat and delays the temperature rise in the steel tube, maintaining structural integrity for longer durations than unprotected steel columns.

Thermal and Structural Behavior Analysis

Under fire exposure, the CFST column undergoes complex thermal-mechanical interactions. The steel tube experiences rapid initial heating, but the concrete core acts as a thermal barrier, slowing the temperature increase at the tube's inner surface. The differential thermal expansion between the steel tube and concrete core generates compressive stresses in the concrete and tensile stresses in the steel tube wall.

Temperature Level	Steel Property Degradation	Structural Implication
200°C	No significant change	Normal behavior
400°C	Yield strength reduced by ~10%	Minor capacity loss
600°C	Yield strength reduced by ~50%	Significant capacity loss
800°C	Yield strength reduced by ~80%	Structural failure likely

The concrete core's heat absorption capacity is a critical factor. Concrete has a specific heat of approximately 880–1000 J/(kg·°C) and acts as a thermal mass that delays the temperature rise in the steel. However, concrete also undergoes thermal degradation above 400°C, with spalling risk increasing above 600°C. The protective coating on the exterior of the column further delays the temperature rise at the steel tube surface.

Welding Considerations in Fire-Exposed CFST Members

The welding quality of the CFST column is directly relevant to fire performance. Longitudinal welds in the steel tube, if not fully fused, can create paths for thermal degradation to penetrate the tube wall more rapidly. During a fire, any lack-of-fusion defects in the longitudinal weld can become crack initiation points as the steel weakens at elevated temperatures. Therefore, the welding quality requirements for CFST columns used in structures requiring high fire resistance should be stringent.

Weld Quality Factor	Effect on Fire Performance	Inspection Method
Weld fusion quality	Poor fusion accelerates thermal degradation	RT or UT
Weld residual stress	Residual stresses interact with thermal stresses	Strain measurement
HAZ microstructure	Coarse HAZ is more susceptible to fire damage	Hardness testing
Weld geometry	Smooth transitions reduce stress concentration	MT or PT

The heat-affected zone (HAZ) of the weld is particularly vulnerable during fire exposure because the microstructure in the HAZ may have coarser grain sizes compared to the base metal, resulting in lower ductility and earlier failure at elevated temperatures. Welding procedures that minimize HAZ width, such as multi-pass welding with lower heat input per pass, are preferred for fire-critical applications.

Protective Coating Design and Code Implications

The study demonstrates that the conventional method of determining fire protective layer thickness for steel structures is not directly applicable to CFST columns. This is because the concrete core provides inherent fire protection that reduces the required coating thickness. The study recommends that CFST columns require only moderate protective coatings to achieve the fire resistance limits specified in GB 50045-95.

Column Type	Required Coating Thickness	Fire Resistance Achieved
Bare steel column	25–40 mm	2.0–3.0 hours
CFST column (with coating)	10–20 mm	2.0–3.0 hours
CFST column (no coating)	0 mm	1.0–1.5 hours (typical)

This finding has significant economic implications for structural engineering projects. The reduced coating thickness for CFST columns compared to bare steel columns results in lower material costs, reduced application labor, and shorter construction schedules. From a materials science perspective, the concrete core's thermal mass and the confinement effect of the steel tube create a synergistic fire protection mechanism that cannot be replicated by steel alone.

Study Insights and Engineering Recommendations

This study provides definitive evidence that CFST columns offer superior inherent fire resistance compared to conventional steel columns, and that the fire protective layer requirements should be specifically calibrated for CFST members rather than applying generic steel structure fire protection guidelines. For engineers involved in the fabrication and welding of CFST structural members, this means that weld quality becomes even more critical in fire-exposed applications, as the composite action that provides fire resistance depends on the integrity of the steel tube. Any weld defects that compromise the tube wall continuity could undermine the thermal protection mechanism. The study ultimately reinforces the value of CFST construction in fire-prone applications and provides a scientific basis for optimizing fire protection design, leading to safer and more economical structural solutions.