Zhuojin Pipe Fitting Co., LtdImpact Resistance Calculation of Circumferentially Filled Double Steel Tube Columns with Stainless Steel Outer Tube
Literature Overview
The paper by An Guoqing, Zhao Hui, Wang Rui, and Li Tieying from Taiyuan University of Technology, published in Engineering Mechanics (Vol. 38, No. 6, 2021, pp. 227-236), addresses a significant gap in structural engineering: the impact resistance of Circumferentially Filled Double Steel Tube (CFDST) columns with a stainless steel outer tube. The research was funded by the China Postdoctoral Science Foundation (Project No. 2020M670656). While reinforced concrete and conventional solid CFST members have well-established impact design methods, the CFDST configuration with stainless steel cladding remains under-researched, particularly under coupled axial compression and lateral impact loading conditions.
Core Technical Content and Mechanism Analysis
The authors employed ABAQUS finite element analysis to simulate the behavior of CFDST columns subjected to simultaneous axial force and lateral impact. The structural configuration consists of an outer stainless steel tube, a hollow annular gap, and an inner concrete-filled steel tube. This hybrid geometry offers distinct advantages over solid CFST: reduced self-weight, improved fire resistance due to the hollow gap acting as a thermal buffer, and enhanced corrosion protection through the stainless steel outer layer.
The primary energy dissipation mechanism identified is the plastic deformation of the outer steel tube. Under lateral impact, the outer tube undergoes progressive local buckling and plastic hinge formation, which absorbs the majority of the kinetic energy imparted by the impacting object. The inner concrete-filled tube contributes to the overall load-bearing capacity but plays a secondary role in impact energy absorption.
Key Parameter Influences
| Parameter | Effect on Impact Resistance | Critical Threshold |
|---|---|---|
| Axial compression ratio | Weakens impact performance | >0.5 shows pronounced degradation |
| Nominal steel ratio | Positive correlation with impact capacity | Higher ratio improves energy absorption |
| Outer tube and concrete strength | Significant influence on peak impact force | Directly affects plateau force value |
| Impact velocity | Higher velocity increases dynamic amplification | DIF increases with velocity |
| Section outer diameter | Larger diameter provides greater stiffness | Affects both strength and ductility |
Dynamic Amplification Factor Calculation
A notable contribution of this work is the proposed calculation formula for the Dynamic Amplification Factor (DIF) of the impact force plateau value under coupled axial-impact loading. The DIF accounts for the reduction in impact capacity caused by pre-existing axial stress, which effectively reduces the cross-sectional area available for plastic deformation. When the axial compression ratio exceeds 0.5, the available plastic reserve is significantly diminished, leading to a nonlinear increase in DIF and a corresponding sharp decline in the member's ability to withstand subsequent impacts.
Engineering Practice Implications
From a steel pipe manufacturing perspective, this research has direct relevance to the specification and production of stainless steel outer tubes for CFDST columns. The stainless steel grade selection (typically SUS304 or SUS316L per ASTM A270 or GB/T 12771) must balance corrosion resistance with adequate ductility to ensure sufficient plastic deformation capacity under impact. The outer tube wall thickness and diameter-to-thickness ratio (D/t) are critical manufacturing parameters that directly influence the local buckling mode and energy absorption capacity.
The finding that axial compression ratio above 0.5 significantly degrades impact performance has important design implications. In multi-story structures where CFDST columns are subjected to high axial loads from gravity, the impact resistance margin must be carefully evaluated. Engineers should consider either reducing the design axial compression ratio or incorporating additional impact protection measures such as external armor or sacrificial impact-absorbing layers.
Quality Control Considerations
The manufacturing quality of the CFDST column, particularly the weld integrity between the inner tube and the outer tube (if connected), and the uniformity of concrete filling in the annular space, are critical for achieving the predicted impact performance. Non-destructive testing (NDT) protocols should include ultrasonic testing (UT) of the concrete fill quality and magnetic particle testing (MT) of any weld connections. The hollow gap must be maintained free of debris and moisture ingress to prevent corrosion of the inner tube over the service life.
Key Reflections
This research bridges an important gap between conventional CFST design methodology and the emerging CFDST configuration. The proposed DIF formula provides a practical tool for preliminary design, though engineers should be aware that the finite element model calibration relies on specific boundary conditions and material models that may not directly transfer to all CFDST configurations. Future work should validate these findings through full-scale impact tests, particularly at higher axial compression ratios where the model predictions may become less conservative. The stainless steel outer tube also introduces material-specific considerations: the work-hardening behavior of austenitic stainless steel differs significantly from carbon steel, and the strain rate sensitivity under impact loading should be carefully characterized in material testing per ASTM A370.