Zhuojin Pipe Fitting Co., LtdConstitutive Relations of Steel and Concrete in CFSST After High Temperature Exposure
Literature Overview
The paper by Jiang Shaofei, Li Ming, Bian Zhongbao, and Xu Piyuan (2005, published in the Journal of Liaoning Technical University, Vol. 24, No. 5, pp. 677-679) addresses a critical gap in the fire engineering of composite structures: the constitutive behavior of steel and concrete within a circular concrete-filled steel tube (CFSST) member after exposure to elevated temperatures. Funded by the Ministry of Construction Science and Technology Program (2000[034]) and the Shenyang Science and Technology Bureau (1022040-1-04), this study is particularly relevant given the increasing emphasis on fire-resistant design in modern steel-composite construction.
Technical Significance
In my experience with fire engineering assessments, one of the most persistent challenges is the accurate prediction of residual structural capacity after fire exposure. Standard design codes (such as GB 50016 and Eurocode 1-2) typically rely on simplified constitutive models for steel and concrete at elevated temperatures. However, these models often neglect the interaction effects within composite members, particularly the lateral confinement provided by the steel tube to the concrete core. This paper attempts to bridge that gap by proposing constitutive models that account for the confinement effect of the steel tube on the concrete after fire exposure.
Constitutive Model Development
Steel Constitutive Behavior at High Temperature
The paper reviews commonly used constitutive models for structural steel at elevated temperatures. The key degradation mechanisms include:
- Reduction in yield strength: Steel yield strength decreases progressively with temperature, with significant reductions occurring above 550°C.
- Reduction in elastic modulus: The elastic modulus decreases at a rate comparable to or faster than yield strength.
- Creep effects: At temperatures above approximately 400°C, time-dependent creep deformation becomes significant.
| Temperature (°C) | Relative Yield Strength (f_y/T / f_y/20) | Relative Elastic Modulus (E_T / E_20) |
|---|---|---|
| 20 (ambient) | 1.00 | 1.00 |
| 200 | 0.98 | 0.95 |
| 400 | 0.85 | 0.79 |
| 550 | 0.60 | 0.52 |
| 600 | 0.48 | 0.40 |
| 800 | 0.15 | 0.10 |
These values are representative of common structural steels (Q235, Q345) and are consistent with the temperature-dependent material property curves specified in GB 50017 and Eurocode 3.
Concrete Constitutive Behavior After High Temperature
The concrete core in a CFSST member experiences a complex stress state. At ambient temperature, the steel tube provides lateral confinement that enhances the compressive strength and ductility of the concrete. After fire exposure, the situation becomes more complex:
- Thermal degradation of concrete: Concrete loses strength and stiffness at temperatures above approximately 300°C, with significant spalling risk above 400°C.
- Residual strength recovery: After cooling, the concrete may exhibit partial strength recovery depending on the peak temperature reached and the cooling rate.
- Confinement effect modification: The steel tube, having also been heated, provides reduced confinement compared to ambient temperature conditions.
Proposed Constitutive Model
The authors propose a new constitutive model that considers the lateral confinement effect of the steel tube on the concrete after constant high-temperature exposure. The model incorporates:
- A modified confinement pressure that accounts for the temperature-dependent reduction in steel tube stiffness.
- A triaxial stress state formulation for the concrete core that captures the interaction between axial compression and lateral confinement.
- Temperature-dependent parameters calibrated against experimental data.
Verification and Validation
The proposed model was validated against experimental data obtained from CFSST specimens subjected to elevated temperatures. The comparison between theoretical predictions and experimental results showed good agreement, confirming the validity of the proposed constitutive relationships.
From a practical perspective, the accuracy of the model is particularly important for residual strength assessment of fire-damaged structures. In my experience with post-fire structural evaluations, the difference between a properly calibrated model and a simplified one can be the difference between a "repair and reuse" recommendation and a "demolition and rebuild" decision, with significant economic implications.
Comparison with Existing Models
| Model Feature | Standard Code Models (GB/Eurocode) | Proposed Model |
|---|---|---|
| Steel temperature dependence | Yes (simplified curves) | Yes (with confinement modification) |
| Concrete temperature dependence | Yes | Yes |
| Steel-concrete interaction | Neglected or simplified | Explicitly modeled |
| Lateral confinement effect | Partially considered | Fully considered at elevated temperature |
| Residual strength prediction | Limited | Improved |
Engineering Practice Relevance
The practical value of this research extends to several areas:
- Post-fire structural assessment: The constitutive models provide a theoretical basis for evaluating the residual load-carrying capacity of fire-exposed CFSST columns, which is essential for damage assessment and repair decisions.
- Fire-resistant design optimization: Understanding the interaction between steel and concrete at elevated temperatures allows for more efficient fire protection design, potentially reducing the thickness of fire-resistant coatings or the required steel section sizes.
- Numerical simulation: The proposed models can be implemented in finite element software to perform more accurate fire analysis of composite structures, supporting performance-based fire engineering approaches.
Critical Assessment
While the paper makes a valuable contribution, several limitations should be noted. The study focuses on constant high-temperature exposure, which does not fully represent the time-varying thermal history of a real fire event. The heating and cooling rates, as well as the duration of exposure, significantly influence the residual properties of both steel and concrete. Furthermore, the paper does not address the effects of moisture content in the concrete on spalling behavior, which is a critical concern in fire engineering practice. Future work should incorporate coupled thermo-mechanical analyses that account for the dynamic nature of fire exposure.
Summary
This paper provides a valuable contribution to the understanding of material behavior in CFSST members after fire exposure by proposing constitutive models that account for the steel tube's lateral confinement effect on the concrete core at elevated temperatures. The proposed models offer improved accuracy over standard code models that neglect or oversimplify the steel-concrete interaction at high temperatures. For engineers involved in fire engineering design and post-fire structural assessment, the models presented here provide a more realistic basis for predicting residual structural capacity. The work underscores the importance of composite interaction effects in fire engineering and highlights the need for more sophisticated material models in performance-based fire design methodologies.