Cross-Sectional Temperature Field Analysis of Square Steel Tube Reinforced Concrete Columns Under Four-Sided Fire Exposure

Overview of the Study

The paper by Zhao Hainan, Zhao Tongfeng, and Sun Changzheng, published in Journal of Earthquake Resistance Engineering and Retrofitting (2011, Vol. 33, No. 5, pp. 32-36), investigates the fire resistance performance of square steel tube reinforced concrete (SRC) columns under four-sided fire exposure. The research was funded by the Ministry of Housing and Urban-Rural Development of China (project 2010-K3-54) and was conducted at Shenyang Jianzhu University. The study employs finite element analysis (FEA) using ABAQUS to determine the cross-sectional temperature field of SRC columns under standard fire conditions, and validates the numerical model against experimental data. The paper then investigates the influence of several key parameters on the temperature field, providing practical guidance for the fire design of composite columns.

Numerical Model and Validation

Finite Element Model

The numerical model was developed in ABAQUS, a widely used finite element software for structural analysis. The model accounts for the following physical phenomena:

1. Heat conduction — Heat transfer through the concrete, steel tube, and reinforcing steel by conduction.
2. Thermal boundary conditions — Standard fire curve (ISO 834 or equivalent) applied to all four sides of the column.
3. Temperature-dependent material properties — The thermal conductivity, specific heat, and thermal expansion of both concrete and steel are functions of temperature.
4. Steel-concrete interaction — The mechanical coupling between the steel tube and the concrete core, which affects the temperature distribution through the composite section.

The thermal properties of the materials were determined based on established standards and experimental data:

Model Validation

The numerical model was validated against experimental results from previous studies. The comparison showed good agreement between the predicted and measured temperature distributions, confirming the accuracy of the finite element model and the appropriateness of the material property formulations. This validation is essential because the temperature field directly determines the residual strength and deformation capacity of the column under fire conditions.

Parametric Study Results

Influence of Heating Time

The heating time is the most straightforward parameter: longer exposure to fire results in higher temperatures throughout the cross-section. The temperature gradient from the exposed surface to the center of the section increases with time, but the rate of temperature increase at the center decreases as the temperature approaches the fire temperature. This is because the concrete, with its low thermal conductivity, acts as an insulating layer that slows the heat transfer to the interior of the section.

Influence of Steel Ratio

The steel ratio (the ratio of steel cross-sectional area to total cross-sectional area) has a significant influence on the temperature field. A higher steel ratio means more steel is present in the section, and steel has a much higher thermal conductivity than concrete. This means that heat is transferred more rapidly to the interior of the section when the steel ratio is high. The practical implication is that columns with a higher steel ratio may require additional fire protection to maintain acceptable temperatures in the steel elements.

Influence of Section Size

The section size (the side length of the square section) has a direct influence on the temperature at the center of the section. Larger sections have a greater distance from the exposed surface to the center, and this additional distance provides more thermal insulation. The temperature at the center of a large section may be significantly lower than that of a small section exposed to the same fire for the same duration. This is a well-known phenomenon in fire engineering: the fire resistance of a member is directly related to its size and the distance from the exposed surface to the critical element.

Influence of Fire Protection

The presence and type of fire protection (such as concrete encasement, gypsum board, or intumescent paint) has a dramatic influence on the temperature field. Fire protection materials are designed to slow the heat transfer to the structural elements, maintaining lower temperatures in the steel and concrete for a longer duration. The paper compares several protection scenarios, including:

The results show that fire protection is the most effective means of controlling the temperature field in SRC columns under fire exposure. The choice of protection type and thickness should be based on the required fire resistance rating and the specific conditions of the application.

Technical Analysis and Engineering Practice

Temperature Field and Residual Strength

The temperature field is the primary determinant of the residual strength and deformation capacity of an SRC column under fire. The strength of both steel and concrete decreases with increasing temperature, but the rate of strength loss differs between the two materials:

• Steel — The yield strength of steel begins to decrease significantly above 400°C and is reduced by approximately 50% at 600°C. The modulus of elasticity also decreases with temperature, reducing the stiffness of the steel elements.
• Concrete — The compressive strength of concrete begins to decrease above 200°C and is significantly reduced above 400°C. The concrete also undergoes thermal cracking and spalling, which further reduces its load-bearing capacity.

The temperature field determines the distribution of residual strength throughout the cross-section. Elements at the exposed surface experience the highest temperatures and the greatest strength loss, while elements at the center experience lower temperatures and retain more of their original strength. The overall load-bearing capacity of the column is the integral of the residual strength over the entire cross-section.

Design Implications

The parametric study results have several important implications for the fire design of SRC columns:

1. Section size optimization — Larger sections provide better fire resistance due to the increased thermal insulation. However, larger sections are also more expensive and may not be practical in all applications. The engineer must balance fire resistance requirements with economic and spatial constraints.
2. Steel ratio management — A higher steel ratio increases the thermal conductivity of the section, which can increase the temperature at the center. However, a higher steel ratio also increases the structural strength. The engineer must balance these competing effects to achieve an optimal design.
3. Fire protection selection — Fire protection is the most effective means of controlling the temperature field, but it also adds cost and complexity to the design. The choice of protection type and thickness should be based on the required fire resistance rating and the specific conditions of the application.
4. Heating time consideration — The required fire resistance rating is typically expressed as a duration (e.g., 60 minutes, 90 minutes, 120 minutes). The temperature field at the end of the required duration determines the residual strength and the adequacy of the design.

Key Questions and Reflections

Several questions and observations arise from this study:

1. Four-sided versus one-sided fire — The study focuses on four-sided fire exposure, which is the most severe fire condition for a column. However, in practice, columns are often partially protected by adjacent structural elements or fire-resistant partitions. The results of this study should be considered as a worst-case scenario, and the actual fire resistance of a column in a building may be higher.
2. Mechanical coupling — The paper focuses on the thermal analysis, but the mechanical behavior of the column under fire is also important. The thermal expansion of the steel tube relative to the concrete core creates internal stresses that can affect the structural performance. A coupled thermo-mechanical analysis would provide a more complete picture of the fire performance.
3. Material variability — The study uses specific material property formulations, but the actual properties of concrete and steel can vary significantly between batches and manufacturers. The sensitivity of the temperature field to material property variability should be investigated.
4. Experimental validation — The paper validates the numerical model against previous experimental data, but the experimental data may not cover the full range of parameters studied in the parametric analysis. Direct experimental validation of the parametric trends would be valuable.

Study Insights and Implications

This study provides a comprehensive and practical analysis of the temperature field in square SRC columns under four-sided fire exposure. The parametric study results are directly applicable to the fire design of composite columns and provide engineers with clear guidance on the factors that influence fire performance.

The finding that heating time, section size, and fire protection are the primary parameters governing the temperature field is consistent with established fire engineering principles. The study confirms and quantifies these relationships for the specific case of SRC columns, providing a basis for the development of simplified design procedures.

The study also highlights the importance of numerical analysis in fire engineering. The finite element method allows for the efficient and accurate analysis of complex thermal fields that would be difficult or impossible to determine experimentally. The validation of the numerical model against experimental data provides confidence in the results and supports the use of numerical analysis as a design tool.

In conclusion, this study makes a valuable contribution to the understanding of the fire performance of SRC columns. The parametric study results provide practical guidance for the fire design of composite columns, and the numerical model can be used as a design tool for the analysis of specific structural configurations. The study should be extended to include coupled thermo-mechanical analysis and experimental validation to provide a more complete picture of the fire performance of SRC columns.