Three-Sided Fire Exposure Mechanical Performance of Square CFST Composite Columns

Literature Overview

The study by Bao YanHong and Yang Li from Qinghai University, published in Advances in Steel Construction (2023, Vol. 25, No. 11, pp. 43-53), investigates the mechanical behavior of square cross-section concrete-filled steel tube composite columns under three-sided fire exposure. Funded by the Qinghai Provincial Natural Science Foundation (2019-ZJ-971Q), this research addresses a gap in fire engineering literature where most studies focus on four-sided uniform fire exposure. As a metallurgist specializing in steel pipe fabrication and welding, I find the thermal-mechanical coupling analysis particularly relevant to understanding how fire affects the steel tube material properties and the composite action between steel and concrete.

Finite Element Modeling and Verification

The authors employ ABAQUS finite element software to establish a detailed thermal-mechanical model of the square CFST composite column under three-sided fire exposure. The model is validated against existing experimental data from the literature, confirming the accuracy of the numerical approach. The temperature field distribution, failure modes, axial displacement-time relationships, internal force redistribution, stress, and strain behaviors are all analyzed systematically.

The thermal analysis accounts for the ISO 834 standard fire curve applied to three faces of the square column, creating an asymmetric thermal gradient. This asymmetry induces significant bending moments in addition to the axial compressive forces, which is a fundamentally different failure mechanism from the symmetric case. The steel tube material properties are modeled with temperature-dependent constitutive relationships, which is critical for accurate prediction of structural response.

Key Findings on Fire Resistance Performance

The research reveals that the cross-sectional temperature field distribution under three-sided fire exposure exhibits single-axis symmetry, with the unexposed face serving as a thermal shield. The authors identify five primary factors influencing the fire resistance limit: load ratio, slenderness ratio, column section dimensions, external concrete compressive strength, and section steel ratio. Based on systematic parametric analysis, a simplified calculation formula for the fire resistance limit is proposed.

The steel tube in a CFST column serves a dual role: it provides structural strength and acts as a formwork during concrete placement. Under fire conditions, the steel tube's yield strength degrades significantly above 600°C, and the protective concrete cover becomes the primary thermal barrier. The asymmetric fire exposure creates differential thermal expansion between the heated and unheated faces, generating bending moments that can cause premature failure.

Welding and Fabrication Relevance

From a fabrication perspective, the square CFST composite columns typically involve welded square steel tubes with longitudinal and circumferential welds. The quality of these welds directly affects the column's fire performance, as weld defects can act as preferential sites for crack initiation under thermal cycling. The longitudinal weld in a square tube is particularly critical because it is often oriented parallel to the column axis, meaning that any weld deficiency creates a continuous weak line along the member length.

For welded square tubes used in CFST columns, the following quality measures are essential: longitudinal welds should be inspected using radiographic testing (RT) or ultrasonic testing (UT) per GB/T 3323 or GB/T 11345; circumferential welds at column splices should receive full volumetric inspection; and the weld HAZ should be evaluated for potential softening during fire exposure. The thermal history of the weld during fire exposure can further reduce the already-compromised properties of the HAZ, making initial weld quality even more critical.

Simplified Calculation Formula and Design Implications

The proposed simplified formula for fire resistance limit provides practical guidance for engineers designing CFST columns for fire resistance. The formula incorporates the identified influential parameters in a form suitable for preliminary design calculations. However, engineers should note that such simplified formulas are typically valid only within the parameter ranges studied and should be verified against detailed finite element analysis for critical applications.

The study's emphasis on three-sided fire exposure is practically significant because in real building fires, columns near walls or corners often experience non-uniform heat exposure. The current fire design codes, including GB 50016 and EN 1993-1-2, primarily address uniform fire exposure, creating a design gap that this research helps to bridge. For structural engineers, this means that CFST columns in corner locations or near interior walls may require additional fire protection measures beyond what the uniform exposure design would suggest.

Summary and Engineering Insights

This research contributes meaningfully to the fire engineering community by addressing a realistic but under-studied fire exposure scenario for CFST columns. The parametric analysis and simplified formula provide practical tools for engineers. From a metallurgical and welding perspective, the study reinforces the importance of weld quality in fire-exposed structures, as the thermal degradation of steel properties during fire exposure amplifies any pre-existing material weaknesses at weld interfaces. Engineers should ensure that welded CFST columns are fabricated to the highest achievable quality standards, with particular attention to longitudinal weld integrity, and that fire protection strategies account for the non-uniform thermal exposure conditions that are common in real building configurations.