Zhuojin Pipe Fitting Co., LtdDynamic Response of Double-Tube CFST Columns Under Coupled High Temperature and Impact Loading
Literature Overview
The paper by Kong Xiangqing, Zhang Huiling, Zhang Wenping, Fu Ying, and Zhang Wenjiao (2023), published in Vibration and Shock, investigates the dynamic response of concrete-filled double-tube (CFDT) columns under coupled high temperature and impact loading conditions using ABAQUS finite element analysis. Funded by the National Natural Science Foundation of China (Grant No. 51479168) and the Liaoning Provincial Department of Education (Grant No. LJK20626), this research addresses a critical safety scenario: the impact resistance of structural columns after exposure to fire.
Problem Context
Concrete-filled double-tube (CFDT) columns, also known as double-skin concrete-filled steel tube columns, consist of an inner steel tube, an outer steel tube, and concrete filling the space between them. This configuration provides enhanced fire resistance, impact resistance, and overall structural performance compared to conventional single-tube CFST columns. However, the combined effect of fire exposure followed by impact loading (such as vehicle collision, blast, or debris impact) represents a severe limit state that requires careful analysis.
Methodology
Finite Element Model
The authors developed a finite element model in ABAQUS that considers:
- Temperature softening: Material property degradation with temperature for both steel and concrete
- Strain rate strengthening: The dynamic enhancement of material strength under high strain rate conditions
- Axial force effects: The influence of pre-existing axial compression on impact response
- Segmented validation: The model was validated through staged comparison with existing fire tests and room-temperature lateral impact tests of CFDT columns
Material Models
| Material Property | Ambient Temperature Model | Elevated Temperature Model |
|---|---|---|
| Steel yield strength | σ_y = constant | σ_y(T) = f(T) with reduction factor |
| Steel elastic modulus | E = 206 GPa | E(T) = E × reduction factor |
| Concrete compressive strength | f_c = design value | f_c(T) = f_c × temperature reduction |
| Concrete tensile strength | f_t ≈ 0.1f_c | f_t(T) = significant reduction |
| Strain rate effect | Not applicable | Dynamic increase factor (DIF) |
Key Results and Findings
Temperature Effects on Impact Performance
| Temperature (°C) | Impact Force Platform Value | Relative to Ambient | Failure Mode |
|---|---|---|---|
| 20 (ambient) | Baseline | 100% | Bending with local buckling |
| 400 | Moderate reduction | ~85% | Bending with increased deformation |
| 600 | Significant reduction | ~70% | Severe bending with concrete spalling |
| 800 | Substantial reduction | ~55% | Progressive collapse mechanism |
Axial Force Influence
The research demonstrates that axial force has a detrimental effect on impact performance. When the axial compression ratio increases from 0 to 0.6, the impact force platform value at 800°C decreases by 32.4%. This is attributed to:
- Pre-existing compressive stresses reduce the available ductility
- Higher axial loads promote earlier local buckling of the steel tubes
- The combination of axial compression and bending creates unfavorable stress states
Concrete Strength Influence
Concrete strength significantly affects impact performance at elevated temperatures. At 800°C, increasing the concrete strength from 40 MPa to 60 MPa increases the impact force platform value by 22.8%. This finding has direct implications for material selection in fire-prone environments.
Fire Resistance Comparison
A particularly significant finding is that the fire resistance limit of CFDT columns is 2.1 times that of conventional single-tube CFST columns. This demonstrates the inherent advantage of the double-tube configuration for fire-impact coupled scenarios.
Failure Mode Analysis
Bending Failure Dominance
Under coupled high temperature and impact loading, CFDT columns primarily exhibit bending failure. The damage evolution analysis reveals:
- Initial stage: Local deformation at the impact point with elastic response
- Plastic stage: Yielding of the outer tube at the impact zone with progressive concrete damage
- Post-peak stage: Concrete crushing and spalling, inner tube deformation, and potential progressive collapse
Damage Evolution
The finite element analysis tracks damage parameters including:
- Concrete damage (tensile and compressive cracking)
- Steel plastic strain accumulation
- Interface debonding between steel tubes and concrete
- Progressive loss of load-bearing capacity
Engineering Practice Implications
Design Considerations for Fire-Impact Coupled Scenarios
- Material selection: Higher-strength concrete (60 MPa or above) provides significantly better impact resistance at elevated temperatures. This supports the use of high-performance concrete in critical infrastructure.
- Axial load management: Limiting the axial compression ratio to below 0.4 is recommended to maintain adequate impact resistance at elevated temperatures.
- Double-tube configuration: The 2.1 times fire resistance improvement justifies the additional cost of double-tube construction for critical applications.
Quality Control Requirements
For CFDT columns designed for fire-impact coupled scenarios:
- Welding quality: All welds must meet full penetration requirements with appropriate heat-affected zone (HAZ) control. Poor welds create preferential failure paths under impact.
- Concrete placement: The annular concrete between tubes requires specialized placement methods to ensure full compaction without voids.
- Tube-to-tube spacing: The gap between inner and outer tubes must be controlled to ensure uniform concrete distribution and adequate bond.
- Fire protection: While the double-tube configuration provides inherent fire resistance, additional fire protection may be required for extreme fire scenarios.
Connection with Steel Pipe and Welding Engineering
The manufacturing and welding aspects of CFDT columns are critical to their performance under coupled loading:
Steel Tube Manufacturing
- Inner tube: Typically seamless or ERW, requiring precise dimensional control for proper concrete placement
- Outer tube: May be HFW, LSAW, or spiral welded depending on diameter and application
- Surface preparation: Both tubes require clean, rust-free surfaces for optimal concrete bond
Welding Considerations
- Connection welds: End connections must accommodate thermal expansion during fire exposure and impact deformation
- HAZ properties: Welded joints may exhibit reduced strength at elevated temperatures compared to base metal
- Residual stresses: Manufacturing residual stresses interact with thermal and impact stresses, potentially initiating failure
Key Questions and Reflections
The research raises several important questions:
- How does the impact performance of CFDT columns vary with impact angle (normal, oblique, grazing)?
- What is the effect of repeated fire-impact cycles on cumulative damage?
- How do different concrete types (normal, high-performance, ultra-high-performance) perform under coupled loading?
- What are the appropriate performance-based design criteria for fire-impact coupled scenarios?
The segmented validation approach (separate validation against fire tests and impact tests before combined analysis) provides confidence in the model but also highlights the need for dedicated experimental data on coupled fire-impact scenarios.
Study Insights
This research provides critical insights for the design of structural columns that must withstand both fire and impact events. The quantitative demonstration that CFDT columns offer 2.1 times the fire resistance of conventional CFST columns, combined with the detailed analysis of temperature, axial force, and concrete strength effects, provides engineers with actionable design guidance. The finding that concrete strength improvement from 40 to 60 MPa yields a 22.8% increase in impact performance at 800°C supports the use of high-performance concrete in critical infrastructure applications. The detrimental effect of high axial compression ratios on impact performance at elevated temperatures underscores the importance of integrated design that considers multiple limit states simultaneously. Engineers involved in the design of bridges, tunnels, and critical facilities should incorporate these findings into their performance-based design approaches for fire-impact coupled scenarios.