Zhuojin Pipe Fitting Co., LtdDynamic Response of Double Tube Concrete Filled Steel Tube Columns Under Coupled High Temperature and Impact Loading
Literature Overview
The paper by Kong Xiangqing, Zhang Huiling, Zhang Wenping, Fu Ying, and Zhang Wenjiao, published in Journal of Vibration and Shock in 2023 (Vol. 42, Issue 10, pp. 92-102), presents a finite element study on the dynamic response of concrete-filled double-tube (CFDT) columns under the coupled action of high temperature and lateral impact. The research is supported by the National Natural Science Foundation of China (Grant No. 51479168) and the Liaoning Provincial Department of Education Basic Research Project (LJK20626). The authors develop a finite element model using ABAQUS that accounts for material temperature softening, strain rate strengthening, and axial force effects, validated against fire test and lateral impact test data. Parametric studies investigate the influence of cross-section geometry, axial compression ratio, impact velocity, and concrete strength on the impact resistance performance at elevated temperatures.
Core Technical Content
Finite Element Model Development
The finite element model captures the complex mechanical behavior of CFDT columns through several key material models:
- Steel tube: A temperature-dependent elastic-plastic model with strain rate strengthening based on the Cowper-Symonds model. The yield strength decreases with temperature according to established relationships, with significant softening above 500°C.
- Core concrete: A temperature-dependent constitutive model that accounts for the reduction in compressive strength, elastic modulus, and tensile strength with increasing temperature. The model also incorporates strain rate effects using the HPS (Holmquist-Johnson-Cook) damage model.
- Outer steel tube: Similar to the inner tube but with potentially different material properties and thermal exposure.
The model is validated in two stages: first against fire test data to verify the thermal and structural behavior under elevated temperatures, and then against lateral impact test data at ambient temperature to confirm the dynamic response prediction capability.
Dynamic Response Analysis
The study investigates the impact response through several key metrics:
| Response Parameter | Description | Key Finding |
|---|---|---|
| Impact force | Peak and plateau force during impact | Decreases with increasing temperature |
| Mid-span displacement | Maximum deflection at impact point | Increases with temperature |
| Load distribution | Spatial distribution of impact force | Concentrates near impact point |
| Damage evolution | Progressive damage accumulation | Accelerates at high temperatures |
The research reveals that CFDT columns exhibit bending-dominated failure under lateral impact, with the failure mode remaining consistent across the temperature range studied. The fire resistance limit of CFDT columns is found to be 2.1 times that of conventional single-tube CFST columns, demonstrating the significant benefit of the double-tube configuration.
Parametric Study Results
The parametric analysis provides valuable quantitative insights:
| Parameter | Variation Range | Effect on Impact Force at 800°C |
|---|---|---|
| Axial compression ratio | 0 to 0.6 | 32.4% decrease in plateau force |
| Concrete strength | 40 to 60 MPa | 22.8% increase in plateau force |
| Impact velocity | Various | Higher velocity reduces energy absorption per unit displacement |
| Cross-section geometry | Various | Outer tube dimensions have greater influence than inner tube |
The finding that axial force has a detrimental effect on impact resistance is particularly significant for engineering practice. Columns subjected to high axial loads, such as those in multi-story buildings or heavy industrial structures, are more vulnerable to impact damage at elevated temperatures. The 32.4 percent reduction in impact force plateau value when the axial compression ratio increases from 0 to 0.6 at 800°C represents a substantial degradation that must be accounted for in design.
The positive effect of concrete strength is also noteworthy, with a 22.8 percent improvement in impact force plateau value when concrete strength increases from 40 to 60 MPa at 800°C. This suggests that using higher-strength concrete in CFDT columns can partially compensate for the temperature-induced degradation of impact resistance.
Welding and Fabrication Considerations
The CFDT column configuration involves two concentric steel tubes with concrete filling the annular space between them. This configuration presents specific challenges for fabrication and welding:
- Concentricity control: The inner and outer tubes must be maintained in precise concentric alignment during fabrication. Any eccentricity creates non-uniform concrete thickness, leading to uneven confinement and potential weak zones.
- Tie welds: The inner and outer tubes are typically connected by transverse tie welds at regular intervals to maintain concentricity and ensure composite action. These welds must be designed and executed to avoid creating stress concentrations.
- Concrete placement: The annular space must be completely filled with concrete without voids. The placement process requires specialized techniques such as centrifugal casting or pressure injection to ensure uniform filling.
- Weld quality verification: Ultrasonic testing (UT) is essential for verifying the quality of tie welds and any longitudinal welds in the tubes. Time-of-flight diffraction (TOFD) or phased array ultrasonic testing (PAUT) may be required for complex weld geometries.
The welding procedure must account for the thermal effects of welding on the already temperature-damaged steel tubes. In fire-exposed scenarios, the residual strength of the steel tubes after fire cooling must be assessed before any repair welding is performed.
Engineering Practice Implications
The research provides critical insights for the design of CFDT columns in applications where fire and impact resistance are both required, such as in:
- Petrochemical facilities where process vessels and piping may be exposed to both fire and explosion risks.
- Nuclear power plants where containment structures must withstand both fire and seismic impact.
- Military and security applications where blast and fire resistance are design requirements.
The finding that CFDT columns offer 2.1 times the fire resistance of conventional CFST columns is a compelling argument for their use in fire-prone environments. The double-tube configuration provides additional thermal insulation through the air gap or concrete layer between the tubes, slowing the temperature rise in the inner tube and preserving structural integrity for longer durations.
The parametric results guide design optimization:
- Minimize axial compression ratio where possible to preserve impact resistance.
- Use higher-strength concrete to enhance impact performance at elevated temperatures.
- Optimize cross-section geometry to maximize the benefit of the outer tube while maintaining practical fabrication considerations.
Key Questions and Reflections
Several important questions emerge from this research. First, the study focuses on lateral impact, but real-world scenarios may involve oblique or multi-directional impacts that require further investigation. The interaction between impact direction and the double-tube configuration may produce different damage patterns that affect structural integrity differently.
Second, the study assumes a uniform temperature distribution across the column cross-section, which may not be realistic in practical fire scenarios where temperature gradients develop due to differential heat exposure. The thermal gradient effects on the steel tubes and concrete may create additional stresses that compound the impact damage.
Third, the long-term behavior of CFDT columns after fire exposure and impact damage is not addressed. The residual strength, fatigue life, and corrosion resistance of damaged columns are critical considerations for post-event assessment and repair decisions.
Another consideration is the scalability of the findings. The study focuses on specific column dimensions and material properties, and the applicability to larger-scale columns or different material combinations requires further investigation. The strain rate effects may vary with scale due to different strain rate distributions in larger cross-sections.
Study Insights and Implications
This research makes a significant contribution to the understanding of CFDT column behavior under coupled fire and impact loading, providing quantitative data that can directly inform engineering design. The finite element model, validated against experimental data, offers a reliable predictive tool for assessing the impact resistance of CFDT columns at elevated temperatures. The parametric study provides actionable design guidance, highlighting the importance of axial compression ratio control and concrete strength optimization. For engineers involved in steel pipe manufacturing and welding, the implications are that the quality of fabrication and welding directly affects the fire and impact performance of CFDT columns, and that the double-tube configuration, while offering superior fire resistance, requires careful attention to concentricity control and tie weld quality. The research also underscores the need for continued investigation into the long-term behavior of fire-damaged CFDT columns and the development of repair and retrofit strategies for post-event structural assessment.