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

Literature Overview

This study by Jiang Shaofei and colleagues from Shenyang Jianzhu University, published in 2004 in the journal of Shenyang Jianzhu University (Natural Science Edition), presents experimental research on the residual mechanical behavior of concrete-filled steel tube (CFST) members subjected to double eccentric compression after exposure to elevated temperatures. The research was supported by the Ministry of Construction Science and Technology Program and the Shenyang Science and Technology Bureau. Fifteen test specimens were investigated using a purpose-built double eccentric compression loading apparatus. The work is significant because it addresses a critical gap in the post-fire assessment of CFST structural members, particularly those subjected to biaxial eccentric loading conditions common in frame columns.

Core Technical Content

The study examines how CFST members retain their load-bearing capacity and ductility after being subjected to constant high temperatures. The authors designed a specialized loading device capable of applying biaxial eccentric compressive loads, which is non-trivial because most standard testing rigs only accommodate uniaxial eccentricity. The key finding is that CFST members maintain relatively high residual bearing capacity and good ductility even after significant thermal exposure, with temperature being the dominant factor influencing residual strength.

Key Experimental Parameters

Interpretation of Findings

From a steel pipe manufacturing and structural engineering perspective, the retained bearing capacity of CFST members after fire exposure has profound implications for post-disaster structural assessment. The steel tube constrains the concrete core, and this confinement effect partially persists even after thermal degradation. The concrete, while losing compressive strength at elevated temperatures, continues to provide lateral support to the steel tube. The interaction between steel and concrete at elevated temperatures is complex because the thermal expansion coefficients of the two materials differ significantly.

Engineering Practice Implications

In practical fire engineering, the assessment of CFST columns after a fire event requires a systematic approach. The following framework can be applied:

1. Temperature measurement: Determine the maximum temperature reached at various locations along the column, accounting for the thermal gradient between the outer steel surface and the concrete core.
2. Residual strength estimation: Apply the relationships established in this study to estimate the remaining axial and biaxial eccentric capacity.
3. Ductility evaluation: Assess whether the member retains sufficient deformation capacity to withstand secondary loading events such as aftershocks.
4. Repair or replacement decision: Based on the residual capacity assessment, determine whether the member can be retained, requires repair, or must be replaced.

The finding that temperature is the primary influence factor aligns with fundamental material science principles. At temperatures below approximately 400 degrees Celsius, carbon structural steel retains most of its yield strength. Between 400 and 600 degrees Celsius, significant strength degradation occurs. Above 600 degrees Celsius, the steel approaches its proportional limit at room temperature. The concrete core experiences progressive strength loss starting at approximately 100 degrees Celsius due to moisture evaporation, with more severe degradation above 400 degrees Celsius.

Connection to Steel Pipe Quality and Welding

From the perspective of steel pipe manufacturing, this research highlights the importance of steel grade selection for CFST applications in fire-prone environments. Pipes made from higher-grade steels (such as Q345 or Q390 per GB/T 1591) will exhibit better residual strength after fire exposure compared to lower-grade steels (Q235 per GB/T 700). Additionally, the welding quality of any longitudinal or circumferential welds in the steel tube becomes critical because weld HAZ regions may have different thermal response characteristics than the base metal.

Key Questions and Reflections

The study raises several important questions for further investigation. First, how does the heating and cooling rate affect the residual mechanical properties? In real fire scenarios, the heating rate varies significantly depending on the fire load, compartment geometry, and ventilation conditions. Second, what is the effect of repeated thermal cycling, which may occur in industrial applications? Third, how do different concrete mix designs and aggregate types influence the residual behavior?

The design of a double eccentric compression testing apparatus is itself a notable engineering achievement. Applying precise biaxial eccentric loads requires careful alignment and load application mechanisms to avoid unintended bending moments. This apparatus could serve as a model for future testing of other structural members under complex loading conditions.

Study Insights and Implications

The practical value of this research lies in its contribution to the development of post-fire assessment standards for CFST structures. As CFST construction becomes increasingly common in high-rise buildings, bridges, and industrial facilities, the ability to evaluate member performance after fire exposure is essential for ensuring public safety and minimizing unnecessary replacement costs. The finding that CFST members retain good residual ductility is particularly encouraging, as ductility is a key indicator of structural safety margin. Engineers should consider these findings when developing fire-resistant design strategies and post-fire inspection protocols for structures incorporating CFST members.