Residual Mechanical Performance of Concrete-Filled Steel Tube Members Under Biaxial Eccentric Compression After High Temperature Exposure

Literature Overview

This paper by Jiang Shaofei, Li Ming, Fu Chun, and Liu Deqing from Shenyang Jianzhu University presents the results of an experimental investigation on the residual mechanical properties of concrete-filled steel tube (CFST) members subjected to biaxial eccentric compression after exposure to sustained high temperatures. The study was funded by the Ministry of Construction Science and Technology Program and the Shenyang Science and Technology Bureau. Fifteen specimens were tested using a specially designed biaxial eccentric compression loading device. The research aims to accumulate data for developing identification standards for the residual mechanical performance of CFST members after fire exposure. Published in the Journal of Shenyang Jianzhu University (Natural Science) in 2004, this work addresses a critical gap in post-fire structural assessment methodology.

Core Technical Content and Experimental Approach

The authors designed a dedicated biaxial eccentric compression loading apparatus capable of applying controlled eccentricities in two orthogonal directions simultaneously. This is a significant engineering challenge, as conventional uniaxial loading frames cannot replicate the complex stress states encountered in real structural members after fire damage. The experimental matrix included fifteen specimens varying in temperature exposure levels and eccentricity ratios. The primary research objective was to evaluate how much load-bearing capacity and ductility remain after thermal degradation.

The key findings indicate that even after high-temperature exposure, CFST members under biaxial eccentric compression retain a relatively high load-bearing capacity and exhibit good ductility. The dominant factor influencing residual capacity is the exposure temperature. This observation carries profound implications for post-fire structural assessment: rather than automatically condemning fire-exposed CFST members, engineers can potentially utilize quantitative residual strength evaluation to determine whether repair or continued service is warranted.

Engineering Practice Implications

From a metallurgical and structural engineering perspective, this research is particularly valuable because CFST members are increasingly used in high-rise buildings, bridges, and industrial structures where fire resistance is a critical design consideration. The composite action between the steel tube and the confined concrete provides inherent fire resistance advantages compared to unprotected steel sections, but the degree of residual capacity after a fire event remains uncertain.

The finding that temperature is the primary factor influencing residual performance aligns with metallurgical principles. At elevated temperatures, steel loses yield strength progressively—typically retaining approximately 65% of ambient-temperature yield strength at 600°C. Concrete, meanwhile, undergoes progressive micro-cracking and dehydration, losing compressive strength at a rate that depends on the heating rate and concrete composition. The confinement effect of the steel tube helps mitigate concrete spalling and maintains a degree of structural integrity even at elevated temperatures.

For post-fire assessment practice, this research supports the development of a systematic evaluation framework. Engineers can measure or estimate the maximum exposure temperature experienced by a CFST member and then apply appropriate reduction factors to the original design capacity. The biaxial eccentric compression data is especially relevant because real structural members rarely experience purely axial loading, and the interaction between axial force and biaxial bending moments significantly affects failure modes.

Key Questions and Study Reflections

One important question arising from this study is the interaction between the steel tube and concrete at elevated temperatures. The confinement effect that gives CFST members their superior performance at ambient temperature may be partially compromised at high temperatures due to differential thermal expansion between steel and concrete. The authors' observation of retained ductility suggests that the composite action is not completely lost, but the extent of this retention as a function of temperature deserves further investigation.

Another consideration is the practical applicability of the biaxial eccentric loading device. The complexity of this apparatus means that replicating these tests in routine quality control or post-fire assessment is challenging. Future work should focus on developing simplified analytical models that can predict residual capacity based on measurable parameters such as temperature history, member geometry, and material grades.

The study also raises questions about the influence of heating rate and cooling conditions on residual properties. The experiments appear to have used sustained high-temperature exposure, but real fire events involve rapid heating followed by potentially rapid cooling. The thermal cycling effects on the steel-concrete interface and on the microstructure of both materials are not fully addressed in this paper.

Reference Value and Outlook

This research provides essential experimental data for developing post-fire assessment standards for CFST structures. The systematic study of biaxial eccentric compression behavior after thermal exposure fills a notable gap in the technical literature, as most prior research focused on uniaxial or pure compression cases. For structural engineers involved in post-fire damage assessment, the finding that significant residual capacity and ductility can be retained offers a rational basis for making repair-versus-replacement decisions.

The work also highlights the need for standardized test methods and evaluation procedures. As CFST construction becomes more prevalent in seismic and fire-prone regions, the ability to quantitatively assess post-fire residual performance becomes increasingly important. Future research should integrate thermal-mechanical finite element modeling with experimental validation to develop comprehensive prediction tools that can guide post-fire structural decisions with confidence.