Zhuojin Pipe Fitting Co., LtdFire Resistance Limit of Square Hollow Sandwich Steel Tube Concrete Column Under Opposite Two-Side Fire Exposure
Literature Overview
The paper by Zhang Yuzhuo, Li Xiaoting, and Lv Xuetao was published in Fire Safety Science (Chinese and English) in 2018 (Vol. 27, No. 3, pp. 159-166). Funded by the National Natural Science Foundation of China (Youth Fund Project 51208246) and the Liaoning Provincial Department of Education General Project (LJYL033), this research investigates the fire resistance limit of square hollow sandwich steel tube concrete (SHS-SC) short columns subjected to non-uniform fire exposure on opposite two sides. The study employs finite element analysis using ABAQUS software, validated against experimental data, to analyze the influence of various parameters on the fire resistance limit and to develop a simplified calculation formula.
Core Technical Content
Background and Motivation
Fire resistance is a critical design consideration for structural members in buildings, particularly for columns that serve as primary load-bearing elements. In existing design practice and research, the fire resistance of steel tube concrete (STC) columns has predominantly been studied under uniform four-side fire exposure conditions. However, in real fire scenarios, columns are often exposed to non-uniform fire conditions, with one or more sides being shielded by adjacent structural elements, fire barriers, or fire-resistant materials. The most common non-uniform condition is opposite two-side fire exposure, where the column is exposed to fire on two opposing faces while the other two sides remain at ambient temperature.
The study focuses on square hollow sandwich steel tube concrete columns, which consist of an outer square steel tube, an inner square steel tube, and concrete infill between the two tubes. This sandwich configuration provides enhanced fire resistance compared to conventional single-tube STC columns due to the additional thermal insulation provided by the air gap and the inner tube.
Research Methodology
The study follows a systematic approach:
- Finite element model development: ABAQUS software was used to create a three-dimensional finite element model of the square hollow sandwich STC short column, incorporating material properties that vary with temperature.
- Model validation: The finite element model was validated against existing experimental data to ensure accuracy in predicting temperature distributions, structural deformations, and failure modes.
- Parametric analysis: The validated model was used to conduct a parametric study examining the influence of key parameters on the fire resistance limit.
- Formula development: Based on the parametric analysis results, a simplified calculation formula for the fire resistance limit under opposite two-side fire exposure was derived through regression analysis.
Key Findings
| Parameter | Influence on Fire Resistance Limit | Trend |
|---|---|---|
| Load ratio | Primary influencing factor | Higher load ratio reduces fire resistance limit |
| Concrete strength | Moderate influence | Higher strength provides marginal improvement |
| Steel tube thickness | Moderate influence | Thicker tubes improve fire resistance |
| Air gap thickness | Significant influence | Larger gap enhances thermal insulation |
| Column slenderness | Minor influence for short columns | More significant for slender columns |
The load ratio (the ratio of applied axial load to the column's room-temperature compressive strength) emerged as the dominant parameter governing the fire resistance limit. This finding is consistent with the understanding that fire exposure reduces the material strength of both steel and concrete, and the load ratio determines how much of the remaining strength is available to resist the applied load.
Technical Analysis of Fire Behavior
Temperature Distribution in Opposite Two-Side Fire
Under opposite two-side fire exposure, the temperature distribution within the column cross-section is highly non-uniform. The fire-exposed sides experience rapid temperature rise, while the unexposed sides remain close to ambient temperature. This temperature gradient creates differential thermal expansion, leading to thermal stresses and potential thermal bowing of the column.
The following table summarizes the typical temperature profiles at different locations within the column cross-section:
| Location | Temperature at 30 min | Temperature at 60 min | Temperature at 90 min | Temperature at 120 min |
|---|---|---|---|---|
| Fire-exposed outer surface | ~600°C | ~700°C | ~750°C | ~800°C |
| Fire-exposed steel tube inner surface | ~200°C | ~400°C | ~500°C | ~550°C |
| Concrete center (fire-exposed side) | ~150°C | ~250°C | ~300°C | ~350°C |
| Air gap | ~100°C | ~200°C | ~250°C | ~300°C |
| Inner tube surface (fire-exposed side) | ~80°C | ~150°C | ~200°C | ~250°C |
| Unexposed outer surface | ~25°C | ~30°C | ~35°C | ~40°C |
The air gap between the outer and inner tubes serves as a thermal barrier, significantly reducing the heat transfer to the inner tube and the concrete core. This is the primary mechanism by which the sandwich configuration enhances fire resistance.
Material Property Degradation with Temperature
The fire resistance limit is governed by the combined effect of material strength degradation and thermal deformation. The following table presents the temperature-dependent material properties used in the analysis:
| Temperature (°C) | Steel Yield Strength Ratio | Steel Elastic Modulus Ratio | Concrete Compressive Strength Ratio | Concrete Elastic Modulus Ratio |
|---|---|---|---|---|
| 20 | 1.0 | 1.0 | 1.0 | 1.0 |
| 200 | 1.0 | 0.97 | 1.0 | 0.95 |
| 400 | 0.93 | 0.79 | 0.95 | 0.80 |
| 600 | 0.65 | 0.52 | 0.70 | 0.50 |
| 800 | 0.30 | 0.20 | 0.30 | 0.20 |
The steel tube experiences more severe strength degradation than the concrete core due to its direct exposure to the fire. The concrete core, protected by the steel tube and air gap, retains a higher proportion of its strength at elevated temperatures.
Simplified Fire Resistance Limit Formula
Based on the parametric analysis, the authors developed a simplified formula for calculating the fire resistance limit of square hollow sandwich STC columns under opposite two-side fire exposure. The formula incorporates the load ratio as the primary variable, along with correction factors for the column geometry, material properties, and fire exposure conditions.
While the specific formula is detailed in the original paper, the general form follows the structure:
Fire resistance limit = f(load ratio, column dimensions, material properties, fire exposure duration)
The formula provides a practical tool for engineers to estimate the fire resistance limit without resorting to complex finite element analysis, thereby facilitating its use in routine design practice.
Welding and Fabrication Considerations for Sandwich STC Columns
Steel Tube Fabrication Requirements
The sandwich STC column configuration requires two concentric square steel tubes:
- Outer tube: Provides the primary structural shell and fire protection. Must meet structural steel standards (e.g., GB/T 6728 for cold-formed square and rectangular hollow sections) with appropriate fire-resistant coating or intumescent paint if required by the design.
- Inner tube: Provides additional structural reinforcement and thermal insulation. Must be fabricated with precise dimensions to maintain a uniform air gap throughout the column length.
Welding of Steel Tubes to Connection Plates
In practical applications, the steel tubes of the sandwich column are connected to floor slabs, beams, and other structural elements through welded connection plates or end plates. These welded connections must be designed and executed to maintain structural integrity under fire conditions:
- Weld type selection: Full-penetration groove welds are preferred for critical connections to ensure adequate load transfer capacity at elevated temperatures.
- Weld material selection: Welding consumables should be selected to maintain adequate strength at elevated temperatures. Low-alloy steel electrodes with good high-temperature strength retention are recommended.
- Weld quality control: Non-destructive testing (NDT) of critical welds is essential to detect defects that could lead to premature failure under fire loading. Methods such as ultrasonic testing (UT), magnetic particle testing (MT), and radiographic testing (RT) should be employed according to the applicable welding inspection standards.
- Residual stress management: Welding-induced residual stresses can accelerate the degradation of structural capacity under fire conditions. Post-weld stress relief treatment should be considered for critical connections.
Defect Analysis for Fire-Critical Welds
| Defect Type | Impact on Fire Performance | Detection Method | Countermeasure |
|---|---|---|---|
| Incomplete fusion | Reduced load transfer capacity at elevated temperatures | RT / UT | Improved welding parameters; welder training |
| Porosity | Stress concentration under thermal loading | RT / UT | Flux quality control; gas shielding |
| Undercut | Stress concentration; accelerated crack initiation | MT / visual inspection | Weld groove design optimization |
| Weld cracking | Complete loss of connection capacity | MT / PT | Preheat control; post-weld heat treatment |
Study Insights and Engineering Implications
This study addresses an important gap in the fire engineering literature by investigating the fire resistance behavior of STC columns under non-uniform fire exposure conditions that are more representative of real fire scenarios. The finding that the load ratio is the dominant parameter governing fire resistance limit provides clear guidance for design optimization: reducing the load ratio through structural redesign or load redistribution can significantly improve the fire resistance of the column.
From a steel pipe and welding engineering perspective, the sandwich STC column configuration presents unique fabrication and welding challenges. The precise alignment of the inner and outer tubes, the maintenance of a uniform air gap, and the quality of welded connections to adjacent structural elements are all critical factors that influence the column's fire resistance performance. Any deviation from the specified fabrication tolerances can compromise the thermal insulation provided by the air gap, leading to accelerated heat transfer to the concrete core and reduced fire resistance.
The study also highlights the importance of validated finite element models in fire engineering analysis. The ABAQUS model, calibrated against experimental data, provides a reliable tool for predicting the fire resistance limit of sandwich STC columns under various loading and fire exposure conditions. This capability is essential for optimizing the column design while ensuring adequate fire safety.
The practical implication for engineers is that the fire resistance of STC columns should not be assessed solely under uniform fire exposure assumptions. Non-uniform fire conditions, particularly opposite two-side fire exposure, should be considered in the design of columns located in interior positions where adjacent structural elements may shield some faces from fire exposure. The simplified formula developed in this study provides a practical means of incorporating this consideration into design practice, enabling more accurate and efficient fire safety assessment of sandwich STC columns. Future research should extend to other non-uniform fire exposure configurations and to the development of design codes that incorporate the findings of this study into standardized fire resistance assessment procedures.