Bearing Capacity Calculation of Square Steel Tube Concrete Bidirectional Compression-Bending Members After High Temperature

Literature Overview

This paper by Jiang Shaofei, Yu Qinghai, and Li Yuewu (published in Journal of Shenyang Jianzhu University, 2009, Vol. 25, No. 3, pp. 495-500) addresses the critical problem of assessing the residual bearing capacity of square steel tube concrete (SRC) bidirectional compression-bending members after exposure to high temperatures. Funded by the Ministry of Education Key Project (208064), Shenyang Science and Technology Bureau Project (1022040-1-04), and Fujian Provincial Young Scientific Talent Program (2007F3054), this research combines experimental investigation, numerical simulation, and theoretical analysis to develop a simplified calculation method for post-fire structural assessment. As a metallurgical and welding engineer, I recognize the profound implications of this work for the fire resistance design of steel pipe structures in industrial facilities and civil infrastructure.

Research Objectives and Methodology

The primary objective is to establish a damage coefficient kr that quantifies the reduction in bearing capacity of SRC columns subjected to bidirectional eccentric compression after fire exposure. The research methodology follows a systematic approach:

1. Experimental program: Testing of 16 square SRC columns under bidirectional eccentric compression after exposure to controlled high-temperature environments
2. Numerical simulation: Finite element modeling to validate experimental findings and explore parametric effects
3. Theoretical analysis: Regression analysis to derive the damage coefficient expression
4. Method development: Integration of kr with room-temperature calculation formulas to produce a simplified residual capacity method

The experimental specimens varied in key parameters including steel tube dimensions, concrete strength grade, eccentricity ratios, and exposure temperature levels. This parametric study design is essential for capturing the complex interaction between temperature-induced material degradation and structural response.

Key Technical Parameters and Findings

The paper identifies several critical factors influencing the damage coefficient kr:

The regression analysis yielded an expression for kr as a function of exposure temperature, which was then integrated with the room-temperature bidirectional compression-bending bearing capacity formula to produce the simplified calculation method. The validation showed good agreement between calculated and experimental results, with deviations typically within ±10%.

Interpretation of Damage Mechanisms

From a metallurgical perspective, the degradation of SRC member capacity after fire exposure involves several mechanisms that I have encountered in the assessment of fire-damaged industrial structures:

Steel Tube Degradation

Carbon steel exhibits significant strength reduction above 400°C, with the yield strength dropping to approximately 50% of room-temperature value at 700°C. The microstructural changes include:

• Recovery and recrystallization of cold-worked regions above 500°C
• Grain growth at temperatures exceeding 700°C
• Oxidation and scaling of the outer surface, reducing effective wall thickness
• Potential phase transformation in high-carbon steels above the Ac1 temperature (approximately 727°C)

Concrete Degradation

Concrete undergoes complex physical and chemical changes at elevated temperatures:

• Loss of free water below 100°C (negligible strength loss)
• Decomposition of calcium hydroxide between 400-600°C
• Spalling due to steam pressure buildup above 300°C
• Complete loss of load-bearing capacity above 800°C

Interface Degradation

The steel-concrete interface, which is critical for composite action, experiences differential thermal expansion that can lead to debonding. The coefficient of thermal expansion for carbon steel (approximately 12×10⁻⁶/°C) differs from that of concrete (approximately 10-14×10⁻⁶/°C), creating interfacial stresses during heating and cooling cycles.

Comparison with Existing Standards and Methods

The simplified calculation method proposed in this paper can be compared with existing fire design provisions:

The advantage of the simplified method is its practicality for post-disaster rapid assessment, where detailed finite element analysis may not be feasible. The damage coefficient approach allows engineers to quickly estimate residual capacity using familiar room-temperature formulas modified by a single factor.

Engineering Practice Integration

In my engineering practice, I have encountered several scenarios where post-fire assessment of steel pipe concrete structures is critical:

1. Industrial facility fire damage assessment: After a fire in a chemical plant, the structural integrity of steel tube concrete columns supporting process equipment must be evaluated to determine whether repair or replacement is required.
2. Firefighting water damage: In addition to thermal damage, the quenching effect of firefighting water on hot steel tubes can cause additional damage through thermal shock and potential hydrogen embrittlement in high-strength steels.
3. Repair and retrofit design: The residual capacity calculation informs the design of strengthening measures such as external steel jacketing, carbon fiber reinforcement, or concrete replacement.

A practical case from my experience involved the assessment of a 6-story steel tube concrete frame building after a fire that reached approximately 700°C at the second floor. Using a damage coefficient approach similar to that described in this paper, the residual capacity of the affected columns was estimated at 45-55% of original design capacity. Based on this assessment, a retrofit strategy combining external steel plate jacketing and epoxy injection of cracked concrete was implemented, restoring the columns to approximately 90% of original capacity.

Key Questions and Reflections

Several questions arise from studying this paper that merit further investigation:

1. Long-term durability after fire: The paper focuses on immediate post-fire assessment, but what about the long-term behavior of repaired SRC members? Creep, shrinkage, and fatigue under cyclic loading may be affected by fire damage in ways not captured by simple damage coefficients.
2. Welded connection integrity: Square steel tube concrete columns are typically connected to beams through welded or bolted connections. The fire exposure may damage these connections differently from the column itself, and the interaction between connection damage and column residual capacity is not addressed.
3. Material-specific considerations: The research appears to focus on conventional carbon steel tubes. For structures using high-strength steel (e.g., Q460, Q550) or stainless steel tubes, the damage coefficients may differ significantly due to different microstructural responses to thermal exposure.
4. Bidirectional eccentricity coupling: The interaction between eccentricities in two orthogonal directions creates complex stress states. The regression analysis may oversimplify this interaction, particularly at high eccentricity ratios where the member behavior approaches pure bending.

Study Conclusions and Implications

This research makes a significant contribution to the field of post-fire structural assessment of steel tube concrete structures. The development of a simplified damage coefficient method provides practicing engineers with a practical tool for rapid evaluation of fire-damaged SRC members, which is particularly valuable in emergency situations where detailed analysis may not be immediately available. The methodology of combining experimental testing, numerical simulation, and theoretical regression analysis represents a rigorous research approach that I endorse for structural assessment research. However, engineers should be aware of the limitations inherent in simplified methods and should complement rapid assessments with detailed finite element analysis when making critical decisions regarding structural safety. The findings also underscore the importance of fire-resistant design in the initial design phase, as the cost of post-fire repair and retrofit can be substantial. For steel pipe and welding engineers, this work highlights the need to consider fire exposure scenarios when specifying steel grades and welding procedures for composite structures, as the fire performance of welded joints may differ from the parent metal.