Axial Compression Behavior of Square Steel Tube Concrete Columns After Sustained High-Temperature Exposure

Literature Overview

This paper by Yang Hua and Han Linhai, published in the Journal of Harbin University of Architecture in 2001, addresses a critical engineering concern: the residual mechanical performance of square steel tube concrete (SRC) short columns subjected to sustained elevated temperatures ranging from 20°C to 900°C. The study is particularly relevant for fire engineering applications, where structural members may experience prolonged thermal loading before fire suppression or natural cooling occurs. The research is supported by the National Natural Science Foundation of China (Grant No. 5958007), the Fok Ying Tung Education Foundation, and the Ministry of Education Young Teachers Fund, reflecting its recognized significance in structural engineering research during that period.

Core Experimental Framework and Parameters

The investigation employed 10 square steel tube concrete short specimens subjected to sustained high-temperature exposure followed by axial compression loading. The experimental matrix varied the exposure temperature T from ambient (20°C) up to 900°C, providing a comprehensive thermal spectrum that encompasses typical fire scenarios from early-stage heating to full-scale structural fire conditions.

The experimental protocol followed a two-stage approach: first, specimens were heated to target temperatures and held at those temperatures for a sustained period to ensure thermal equilibrium throughout the cross-section; second, after cooling to ambient conditions, the specimens were subjected to axial compression tests to capture the load-deformation response and ultimate bearing capacity.

Core Concrete Constitutive Model Development

A key contribution of this study is the development of a constitutive model for the confined concrete core after sustained high-temperature exposure. The authors built upon the well-established ambient-temperature confined concrete model (based on the Han-Lin model framework) and extended it to account for the degradation effects of thermal exposure. This approach is methodologically sound because the confinement effect provided by the steel tube interacts with the thermally degraded concrete in a complex manner that cannot be simply treated as a linear superposition of ambient-temperature confinement and thermal degradation.

The model accounts for:

• Reduction in concrete compressive strength due to thermal exposure, which follows a nonlinear degradation pattern with temperature.
• Changes in the concrete stress-strain curve shape, including shifts in peak strain and post-peak softening behavior.
• The interaction between the thermally weakened concrete and the steel tube, where the steel tube's own strength and stiffness are also affected by the temperature history.

Key Findings and Technical Insights

The experimental results reveal several important technical observations:

1. Residual deformation capacity: Even after exposure to temperatures as high as 900°C, the SRC columns retained relatively good deformation resistance and maintained a substantial level of post-peak bearing capacity. This is attributed to the composite action between the steel tube and the concrete core, where the steel tube continues to provide lateral confinement even when the concrete has undergone significant thermal degradation.
2. Strength and modulus degradation: Both the residual compressive strength and the residual elastic modulus of the SRC columns decreased monotonically with increasing exposure temperature. The degradation rate is not uniform across the temperature spectrum; rather, it accelerates at intermediate temperatures (approximately 400°C to 700°C) where concrete undergoes significant microstructural changes including dehydration of calcium silicate hydrates, decomposition of calcium hydroxide, and spalling of the cement paste matrix.
3. Numerical-analytical correlation: The numerical analysis results showed good agreement with the experimental data, validating the proposed constitutive model. Simplified calculation formulas for both the residual axial compression bearing capacity and the residual axial compression modulus were derived, providing practical tools for engineering assessment of thermally damaged SRC structures.

Engineering Practice Integration

From the perspective of steel pipe manufacturing and structural engineering practice, this research has several direct implications:

• Fire-resistant design of steel tube concrete structures: The simplified formulas enable engineers to estimate the residual capacity of SRC members after a fire event, which is essential for post-fire structural assessment and repair decision-making.
• Steel tube material selection: The study implicitly highlights the importance of selecting steel grades for the confining tube that maintain adequate strength at elevated temperatures, as the steel tube's residual strength directly influences the composite member's post-fire performance.
• Quality control considerations: For SRC structures in fire-prone environments, the manufacturing quality of the steel tube (wall thickness uniformity, weld integrity for welded tubes, surface finish) becomes even more critical because any pre-existing defects can be exacerbated by thermal exposure.

Reflections and Key Questions

Several aspects of this study merit further consideration in practical engineering contexts:

• The study focuses on square cross-sections, which is common in building structures but less common in pipeline applications. The confinement efficiency of square sections differs from circular sections due to the non-uniform lateral pressure distribution, particularly at the corners where stress concentrations develop.
• The sustained temperature exposure condition differs from the transient heating condition typical of structural fires, where the temperature rises and falls over time. The residual performance under transient fire exposure may differ from that under sustained exposure, and this distinction is important for realistic fire engineering assessments.
• The 900°C exposure temperature exceeds the melting point of many structural steels, meaning that at the upper end of the temperature range, the steel tube may have undergone significant phase transformations and potential melting, which fundamentally alters the composite mechanism.

Summary

This study provides valuable experimental and analytical data on the post-fire performance of square SRC columns, establishing a foundation for fire-resistant design and post-fire assessment of steel tube concrete structures. The proposed constitutive model and simplified formulas offer practical tools that bridge the gap between fundamental research and engineering application. For steel pipe manufacturers and structural engineers involved in fire-prone construction environments, the findings underscore the importance of composite structural action and the need for material specifications that account for elevated-temperature performance.