Zhuojin Pipe Fitting Co., LtdConstitutive Relationships of Steel and Concrete in Square CFST after High Temperature Exposure
Literature Overview
This paper by Jiang Shaofei, Li Ming, Bian Zhongbao, and Xu Piyuan, published in the Journal of Liaoning Technical University (Natural Science Edition) in 2005 (Vol. 24, No. 5, pp. 677-679), addresses a critical but often overlooked aspect of fire-resistant design for concrete-filled steel tubular (CFST) structures: the constitutive behavior of both steel and concrete components after exposure to elevated temperatures. Funded by the Ministry of Construction Science and Technology Research Fund (2000[034]) and the Shenyang Science and Technology Bureau (1022040-1-04), this study provides a theoretical framework that remains relevant to modern fire engineering practice.
Core Technical Concepts
The study begins with a systematic analysis of the stress-strain behavior of steel and concrete in square CFST members at ambient temperature, summarizing commonly used constitutive models. The authors then extend this analysis to post-fire conditions, reviewing existing constitutive models for both materials after high-temperature exposure. The central contribution is the proposal of a new constitutive model that accounts for the lateral confinement effect of the steel tube on the concrete core after constant high-temperature exposure.
The key technical challenge addressed is that fire exposure fundamentally alters the mechanical properties of both constituent materials. Steel loses yield strength progressively with temperature, with significant degradation occurring above 600°C. Concrete undergoes more complex changes, including strength recovery at moderate temperatures (200-400°C) due to continued hydration, followed by progressive degradation at higher temperatures due to dehydration and microcracking. The interaction between these two materials in a confined state creates a constitutive relationship that cannot be simply derived from individual material behavior.
Proposed Constitutive Model and Validation
The proposed model incorporates the lateral constraint effect of the steel tube on the concrete core under post-fire conditions. This is a critical refinement because the confinement pressure from the steel tube modifies the concrete's compressive strength and ductility, and this modification changes with temperature as the steel tube's own mechanical properties degrade.
| Material Component | Ambient Temperature Model | Post-Fire Model | Key Modification |
|---|---|---|---|
| Steel Tube | Standard stress-strain curve | Temperature-dependent degradation | Yield strength and modulus reduction with temperature |
| Concrete Core | Confined concrete model | Temperature-dependent confined model | Confinement effect modulated by steel tube degradation |
| Composite Behavior | Interaction model | Post-fire interaction model | Coupled degradation of confinement and core strength |
Experimental validation confirmed good agreement between theoretical predictions and test results, demonstrating the model's validity and rationality. The authors emphasize that the model captures the essential physics of the post-fire behavior while maintaining computational tractability for structural analysis.
Engineering Practice Implications
From a practical engineering standpoint, this research has several important implications. First, it provides a quantitative basis for evaluating the residual structural capacity of CFST members after fire exposure, which is essential for post-fire structural assessment and repair decision-making. Second, the model can be incorporated into finite element analysis software to predict the post-fire behavior of CFST structures under residual loading conditions.
The study also highlights the importance of the steel tube's confinement effect in maintaining structural integrity after fire. Even when the concrete has suffered significant strength loss, the steel tube's confinement can maintain a portion of the load-bearing capacity. This insight has direct implications for fire-resistant design philosophy, suggesting that the steel tube itself serves as a critical safety element in post-fire scenarios.
The constitutive model proposed in this study represents an important step toward more accurate post-fire structural assessment methods for CFST structures. By explicitly accounting for the coupled degradation of steel and concrete properties and their interaction through confinement, the model provides a more realistic representation of post-fire behavior than models that treat the materials independently. Engineers involved in fire safety design, post-fire structural assessment, and the development of fire-resistant CFST design codes would find this research particularly valuable as it bridges the gap between material-level fire behavior research and structural-level performance prediction.