Fire Hysteresis Performance of Rectangular Steel Tube Concrete Flanged Beam-Column Joints

Literature Overview

This paper by Xue Jinghong, Dong Xiaoyao, Qi Xingbo, and Wang Xiang (Northeast Petroleum University, 2019) investigates the hysteresis performance of rectangular steel tube concrete flanged beam-column joints under fire conditions. The study was funded by the National Natural Science Foundation of China (Grant No. 51178087) and the Heilongjiang Provincial Department of Education Research Project (2016YSFX-02). Published in the Journal of Shihezi University (Natural Science Edition), Vol. 37, No. 2, pp. 170-175, this research addresses an important gap in the understanding of fire-resistant behavior of steel tube concrete composite structures.

Core Technical Content

The study employs thermal-mechanical coupled numerical simulation to analyze the hysteresis performance of rectangular steel tube concrete flanged I-beam to column joints under standard fire conditions defined by ISO 834. The comparison between the composite joint and a pure steel joint provides valuable insights into the fire performance advantages of the SRC composite approach.

The ISO 834 standard fire curve defines a temperature-time relationship that is widely used in fire resistance testing and analysis. The temperature rises at a rate of approximately 10°C per minute initially, reaching 1000°C in about 1 hour. The thermal-mechanical coupled analysis accounts for the temperature-dependent degradation of material properties, including the reduction in steel yield strength, elastic modulus, and concrete compressive strength.

The hysteresis curves describe the relationship between the joint moment and rotation under cyclic loading at various temperatures. The study found that as temperature increases, the hysteresis curves become progressively flatter, indicating reduced stiffness and energy dissipation capacity. The onset of plastic deformation occurs earlier at higher temperatures, which is consistent with the known degradation of steel properties under fire conditions.

Thermal-Mechanical Analysis and Technical Points

The thermal-mechanical coupled analysis involves solving two coupled problems: the thermal problem, which determines the temperature distribution in the joint under fire exposure, and the mechanical problem, which determines the stress and deformation response of the joint at the elevated temperatures. The coupling is bidirectional: the temperature affects the mechanical properties, and the mechanical deformation can affect the thermal response through changes in the geometry and contact conditions.

The temperature-dependent material models are critical for the accuracy of the analysis. For steel, the reduction in yield strength and elastic modulus follows well-established relationships, such as those provided in Eurocode 3 Part 1-2. For concrete, the degradation is more complex and depends on the temperature history, with potential spalling at high temperatures. The rectangular steel tube concrete flanged beam combines the thermal mass of the concrete with the structural efficiency of the steel tube, providing inherent fire resistance advantages.

The hysteresis performance comparison between the composite joint and the pure steel joint reveals several important findings:

• The composite joint maintains better hysteresis performance at elevated temperatures compared to the pure steel joint, indicating superior energy dissipation capacity under fire conditions.
• The concrete core provides thermal insulation to the steel tube, delaying the temperature rise in the steel and thereby preserving the structural capacity for a longer period.
• The plastic deformation onset is delayed in the composite joint compared to the pure steel joint, providing additional safety margin under fire conditions.

Engineering Practice Implications

For steel pipe manufacturing and welding quality control, this study has several important implications:

• The welding quality of the steel tube connections in the joint is critical for fire performance. Any weld defects can create weak points that may fail prematurely under fire-induced thermal stresses and mechanical loads.
• The dimensional accuracy of the rectangular steel tubes affects the fit-up quality during joint assembly, which in turn affects the welding quality and the overall joint integrity.
• The surface preparation of the steel tubes before concrete filling is important for ensuring proper steel-concrete bond, which contributes to the composite action and fire performance.

The study also highlights the importance of fire protection measures for steel tube concrete structures. While the concrete core provides inherent fire resistance, the steel tube itself may still be vulnerable to high temperatures, particularly at the upper flange where heat accumulates. The study recommends strengthening the fire protection methods for the upper flange steel tube, which is an important practical recommendation for design and construction.

Key Questions and Reflections

The study raises the important question of how the fire performance of the joint is affected by the quality of the steel tube manufacturing and welding. Any defects in the steel tube, such as incomplete weld fusion, porosity, or dimensional deviations, can create weak points that may initiate failure under fire conditions. The thermal-mechanical coupled analysis assumes ideal material properties, and any deviation from these assumptions in practice can significantly affect the fire performance.

Another reflection is the role of the concrete core in providing fire protection to the steel tube. The concrete acts as a thermal insulator, but its effectiveness depends on the thickness of the concrete cover, the quality of the concrete-steel bond, and the absence of voids or delamination at the interface. Any degradation in the concrete quality or the steel-concrete bond can reduce the fire protection effectiveness.

The study also raises the question of how the fire performance of the joint is affected by the seismic loading that may occur simultaneously with fire. The combination of seismic loads and fire-induced material degradation can create a more severe loading scenario than either load acting alone. The hysteresis performance under this combined loading is an important area for future research.

Study Insights and Implications

This paper provides valuable insights into the fire hysteresis performance of rectangular steel tube concrete flanged beam-column joints. The thermal-mechanical coupled numerical simulation approach is a rigorous and effective method for analyzing the fire behavior of complex structural components. The comparison between the composite joint and the pure steel joint demonstrates the fire performance advantages of the SRC composite approach.

For steel pipe manufacturers and welding engineers, the key insight is that the fire performance of SRC joints is directly related to the quality of the steel tube components and their connections. The welding process must be carefully controlled to ensure full fusion, avoid defects, and minimize residual stresses that can be exacerbated by thermal cycling during fire exposure. Quality control procedures should include comprehensive non-destructive testing of all critical welds, dimensional inspection of steel tubes, and verification of material properties.

The study reinforces the importance of considering fire performance in the design and construction of SRC structures, particularly in high-seismic regions where the combination of seismic and fire loads can create critical failure scenarios. The recommendation to strengthen the fire protection of the upper flange steel tube is a practical and important design consideration that should be incorporated into future SRC joint designs.