Hysteretic Performance of Rectangular Steel Tube Concrete Flange Beam-Column Joints Under Fire

Literature Overview

This paper by Xue Jinghong, Dong Xiaoyao, Qi Xingbo, and Wang Xiang from Northeast Petroleum University, published in 2019 in the Journal of Shihezi University (Natural Science Edition), investigates the hysteretic performance of rectangular steel tube concrete (SRC) flange beam-column joints under fire conditions. The study employs thermal-mechanical coupled numerical simulation following the ISO 834 standard fire curve and compares the performance of SRC flange composite joints with pure steel joints. The research was supported by the National Natural Science Foundation of China (51178087) and the Heilongjiang Provincial Department of Education Research Project (2016YSFX-02).

Core Technical Approach

Thermal-Mechanical Coupled Analysis

The analysis methodology involves:

1. Thermal analysis — Heat transfer analysis following the ISO 834 standard fire curve (T = 345 × log₁₀(8t + 1) + 20, where T is temperature in °C and t is time in minutes) to determine the temperature distribution within the joint components.
2. Material property degradation — Temperature-dependent mechanical properties of steel and concrete are incorporated, including:

• Steel yield strength reduction: f_y(T) = f_y(20°C) × α(T), where α(T) is the strength retention factor
• Concrete compressive strength degradation: f_c(T) = f_c(20°C) × β(T)
• Elastic modulus reduction for both materials

1. Mechanical analysis — Cyclic loading analysis under seismic conditions to evaluate hysteretic behavior at elevated temperatures.

Temperature-Dependent Material Properties

Key Findings

Hysteretic Behavior Evolution

The study reveals several critical findings regarding the hysteretic performance of SRC flange beam-column joints under fire:

1. Progressive degradation of hysteretic loops — As temperature increases, the hysteretic curves become increasingly flat, indicating reduced energy dissipation capacity. The area enclosed by the hysteretic loop, which represents energy dissipation, decreases significantly with temperature.
2. Earlier onset of plasticity — At elevated temperatures, yielding occurs at lower stress levels due to material strength degradation. The transition from elastic to plastic behavior is accelerated, reducing the elastic range of the joint.
3. Superior performance of SRC joints — The rectangular flange SRC composite beam joints demonstrate better hysteretic performance compared to pure steel joints throughout the heating process. The concrete core provides additional confinement and load-bearing capacity that degrades more gradually than steel.

Comparative Performance

Connection with Steel Pipe and Welding Engineering

The fire performance of SRC joints has direct implications for steel pipe and welding engineering:

• Weld performance under fire — The welded connections in SRC joints (beam-to-column welds, tube-to-plate welds) are critical under fire conditions. The heat-affected zone (HAZ) of welds may experience accelerated strength degradation at elevated temperatures. The microstructural changes in the HAZ, including grain coarsening and phase transformations, can reduce the temperature resistance of welded connections.
• Steel tube material selection — The choice of steel grade for the rectangular steel tubes in the flange beams affects fire performance. Higher-grade steels (e.g., Q345, Q390) have higher initial strength but may experience more significant relative strength loss at elevated temperatures compared to lower-grade steels (e.g., Q235). The temperature at which steel loses 50% of its strength is approximately 550-600°C for most structural steels.
• Fire protection of steel tubes — The paper specifically recommends enhanced fire protection for the upper flange steel tube, which is most exposed to fire. Fire-resistant coatings, intumescent paints, or fireproof boards should be applied to the exposed surfaces of steel tubes to delay temperature rise and maintain structural integrity.
• Residual stress effects — Welding residual stresses in the steel tubes and welded connections can interact with thermal stresses during fire, potentially accelerating failure. The superposition of residual stresses and thermal stresses can create local stress concentrations that exceed the temperature-degraded yield strength.

Key Questions and Reflections

The study raises important questions about the design philosophy for fire-resistant SRC structures in seismic zones:

1. Performance-based fire design — Should SRC joints be designed for a specific fire resistance rating (e.g., 60 min, 90 min, 120 min) or for a performance-based target (e.g., maintaining load-bearing capacity for a specified duration)?
2. Weld inspection under fire conditions — How should weld quality assessment be modified for structures exposed to fire? Post-fire weld inspection using NDT methods (UT, MT, PT) should be mandatory to assess the extent of damage.
3. Fire-resistant welding consumables — Should welding consumables be selected specifically for fire resistance? The chemical composition of the weld metal and its HAZ can influence temperature-dependent mechanical properties.

The recommendation to use rectangular flange SRC composite beams in high-seismic zones with fire hazards is well-founded, but the paper acknowledges that further research on fire protection methods for the upper flange steel tube is needed. This is a critical area for future investigation, as the upper flange is typically the most fire-exposed component and its failure can trigger progressive collapse.

Reference Value and Outlook

This study provides essential insights into the seismic-fire coupled behavior of SRC joints, which is increasingly relevant as buildings become taller and more complex. The thermal-mechanical coupled analysis methodology can be extended to other joint configurations and structural systems. For steel pipe and welding engineers, the key message is that fire performance is not solely a function of material selection and fire protection but is also significantly influenced by welding quality and residual stress management.