Fire Resistance Performance of Reinforced Stainless Steel Pipe Concrete Columns with Embedded Cross Steel

Literature Overview

This study by Liu Yanzhi, Deng Jiqian, and Tan Qinghua from Hunan University and the National University of Defense Technology investigates the fire resistance behavior of a novel composite column system: reinforced stainless steel pipe concrete (R-SSPC) columns with embedded cross-section steel shapes. Published in the Journal of Architecture and Civil Engineering (Volume 36, Issue 3, 2019, pages 66-73), this work was supported by the National Natural Science Foundation of China (Grant 51408212) and the Hunan Provincial Natural Science Foundation (Grant 2018JJ3058).

Structural System Description and Design Rationale

The R-SSPC column concept addresses a fundamental challenge in stainless steel structural engineering: the high material cost of stainless steel relative to carbon steel. Stainless steel offers superior corrosion resistance and excellent ductility at elevated temperatures, making it attractive for fire-prone structural applications. However, the high cost limits its widespread use in full-section columns. The proposed solution embeds a cross-shaped steel profile within a square stainless steel pipe concrete column, creating a hybrid system that reduces stainless steel usage while maintaining structural integrity.

The column cross-section consists of a square stainless steel tube filled with concrete, with a cruciform steel section (typically Q235 or Q345 carbon steel) embedded at the center. This configuration creates a composite action between the stainless steel shell, the concrete core, and the embedded steel shape, each contributing differently to load-bearing capacity under fire conditions.

Finite Element Modeling and Validation Approach

The study employed finite element analysis (FEA) using validated models to simulate the fire resistance behavior of the R-SSPC columns. The modeling approach followed a rigorous verification procedure:

The temperature-dependent material models are critical for accurate fire simulation. Stainless steel retains a higher proportion of its yield strength at elevated temperatures compared to carbon steel, with the yield strength ratio at 600°C being approximately 0.50-0.55 for common austenitic grades versus 0.28 for S355 carbon steel. The embedded carbon steel shape, however, loses strength more rapidly at high temperatures, necessitating careful analysis of load redistribution between the components.

Fire Resistance Performance Analysis

The study examined the temperature history, axial displacement-time relationships, typical failure modes, and internal force redistribution under various load ratios. The key findings reveal important structural behavior:

1. The embedded steel shape enhances both axial and lateral stiffness at elevated temperatures, delaying the onset of large deformations.
2. The failure mode of R-SSPC columns is characterized by axial compression failure, indicating that the column system maintains its load-carrying capacity through the fire exposure duration.
3. Under low fire load ratios, the embedded steel shape absorbs part of the load transferred from the stainless steel shell as its temperature-dependent material properties degrade, thereby improving overall fire resistance.
4. The improvement effect is more pronounced at lower load ratios, suggesting that the system performs best under service-level loading conditions during fire events.

Comparison with Conventional Stainless Steel Pipe Concrete Columns

When compared with conventional stainless steel pipe concrete (SSPC) columns of equivalent steel content, the R-SSPC system demonstrates superior fire resistance performance. The mechanism is rooted in the differential temperature sensitivity of the two steel types. As the stainless steel shell loses strength at high temperatures, the embedded carbon steel shape—which is positioned at the column core where temperatures are lower due to the thermal mass of surrounding concrete—continues to carry load. This creates an internal load-sharing mechanism that delays overall column failure.

The fire load ratio (the ratio of applied axial load to the column's ambient-temperature capacity) is a critical design parameter. At low load ratios (typically below 0.3), the R-SSPC system achieves the most significant improvement in fire resistance duration. At higher load ratios, the benefit diminishes as the column approaches its ultimate capacity earlier in the fire exposure.

Engineering Design Implications

For practical engineering applications, several design considerations emerge from this study. The embedded cross steel should be positioned to maximize its thermal protection from the surrounding concrete, typically at the geometric center of the section. The connection between the embedded steel and the stainless steel shell—whether through welding, bolted connections, or mechanical interlocking—must be designed to ensure composite action throughout the fire exposure. Fire protection design codes such as GB 50016 and Eurocode 1-2 provide frameworks for assessing fire resistance, and the findings of this study can inform the development of simplified design procedures for R-SSPC columns.

Study Insights and Practical Implications

This research presents a practical solution to the economic barrier of stainless steel structural applications. By combining the high-temperature strength retention of stainless steel with the cost-effectiveness of carbon steel in a composite configuration, the R-SSPC column offers a viable pathway for cost-conscious yet fire-resilient structural design. The numerical methodology employed—validated FEA with temperature-dependent material models—provides a reliable tool for parametric studies that would be prohibitively expensive through physical fire testing alone. Engineers designing stainless steel structures should consider this hybrid approach for applications where both corrosion resistance and fire safety are critical requirements, particularly in offshore platforms, chemical processing facilities, and high-rise building cores where stainless steel is already specified for durability reasons.