Zhuojin Pipe Fitting Co., LtdComparative Analysis of Fire Resistance Performance of Steel Pipe Concrete Composite Columns
Literature Overview
This 2023 paper by Wang Weihua, Lu Yabiao, Zuo Shuai, Zhang Zhiyong, and Gao Xian, published in the Journal of Huaqiao University (Natural Science Edition), presents a comprehensive nonlinear finite element analysis of four types of steel pipe concrete composite columns under fire conditions. The study uses ABAQUS to simulate temperature field development, displacement-time curves, and internal force redistribution for square steel pipe concrete columns, double-skin steel pipe concrete columns, cross-steel embedded columns, and four-chamber steel pipe concrete columns.
Fire Behavior Analysis Framework
Fire resistance design of steel pipe concrete columns involves understanding the coupled thermal-mechanical behavior under elevated temperatures. The study considers different load ratios (axial load divided by room-temperature ultimate capacity) to evaluate how pre-existing mechanical demand affects fire performance.
| Column Type | Configuration | Relative Fire Resistance | Dominant Failure Mode |
|---|---|---|---|
| Square steel pipe concrete | Single square section | Baseline | Compressive buckling |
| Double-skin steel pipe concrete | Inner + outer steel tubes | Moderate improvement | Inner tube yielding |
| Cross-steel embedded | Cross-shaped steel inside square tube | Significant improvement | Progressive yielding |
| Four-chamber steel pipe concrete | Four-cell composite section | Highest improvement | Chamber wall buckling |
Temperature-Dependent Material Degradation
The fire resistance performance of steel pipe concrete columns is governed by the temperature-dependent degradation of both steel and concrete properties. For structural steel, the yield strength retains approximately 90% at 300°C, 75% at 500°C, and drops to 50% at 600°C. The concrete infill provides thermal insulation to the inner steel surfaces, creating a temperature gradient that can be leveraged through internal steel reinforcement.
The key finding that lower load ratios allow greater temperature-induced material degradation before failure is consistent with the concept of internal force redistribution. At low axial loads, the internal steel elements (cross-steel or inner tubes) can continue to carry load even after the outer square tube has experienced significant strength loss, effectively creating a staged degradation mechanism.
Engineering Practice and Welding Implications
From a steel pipe fabrication standpoint, the various column configurations require different manufacturing approaches:
- Square steel pipes: Typically produced by HFW (high-frequency welding) per GB/T 6728 or hot-rolled square tubes per GB/T 6725. Weld seam quality is critical for fire resistance, as incomplete welds create preferential failure paths under thermal cycling.
- Double-skin configurations: Require concentricity control between inner and outer tubes, typically achieved through welding connection details at regular intervals. The welding process must minimize HAZ extent to preserve fire-resistant capacity.
- Cross-steel embedded columns: The cross-shaped steel section is usually fabricated from rolled sections welded together, requiring full-penetration butt welds with 100% ultrasonic inspection per NB/T 47013.
Quality Control Considerations for Fire-Exposed Components
For steel pipes intended for fire-exposed structural applications, additional quality control measures are recommended:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| UT (Ultrasonic Testing) | Detect internal defects in weld seams | No level II indications per GB/T 11345 |
| MT (Magnetic Particle Testing) | Detect surface cracks at weld connections | No level I indications per GB/T 26951 |
| Hydrostatic testing | Verify weld integrity of square tubes | 1.5× design pressure, 30 s hold |
| Hardness testing | Verify HAZ hardness in connection welds | ≤350 HV for Q345/Q355 grade steel |
Key Findings and Design Implications
The study demonstrates that internal steel reinforcement (whether in the form of cross-steel or inner tubes) significantly enhances fire resistance, with the improvement being most pronounced at lower load ratios. This finding has direct implications for structural design optimization: columns designed with lower utilization ratios at ambient temperature will demonstrate superior fire performance, potentially reducing the need for additional fire protection measures such as intumescent coatings.
Summary and Reflections
This research provides valuable comparative data for selecting appropriate steel pipe concrete column configurations based on fire resistance requirements. The parametric analysis methodology and the consideration of internal force redistribution mechanisms offer practical guidance for engineers designing fire-resistant composite structures. For steel pipe manufacturers, the study underscores the importance of consistent material properties across the full temperature range expected in fire scenarios, which requires careful control of chemical composition, particularly carbon and sulfur content, to maintain adequate ductility at elevated temperatures.