Zhuojin Pipe Fitting Co., LtdImpact Resistance Calculation Method for Circular Filled Double-Skin Steel Tubular Columns with Stainless Steel Outer Tube
Literature Overview
This study by An Guoqing, Zhao Hui, Wang Rui, and Li Tieying from Taiyuan University of Technology, published in Engineering Mechanics in 2021 (Vol. 38, No. 6, pp. 227-236), addresses a significant gap in the structural engineering literature. Circular Filled Double-Skin Steel Tubular (CFDST) columns, particularly those incorporating a stainless steel outer tube, have attracted increasing attention for their superior corrosion resistance and ductility in aggressive environments. However, their impact resistance under coupled axial compression and lateral impact loading remains poorly understood. The authors developed a finite element model using ABAQUS to simulate the dynamic response of these composite columns, building upon prior experimental work, and proposed a dynamic amplification factor (DIF) formula for predicting the impact force plateau value.
Core Technical Points and Analysis
The fundamental innovation lies in recognizing that the plastic deformation of the outer stainless steel tube constitutes the primary energy dissipation mechanism during lateral impact. This finding has direct implications for the selection of stainless steel grades and wall thicknesses in impact-prone applications such as bridge piers, offshore platforms, and blast-resistant structures. The study systematically examined the influence of several critical parameters on impact performance.
Influence of Axial Compression Ratio
The axial compression ratio emerges as the most detrimental parameter. When the axial compression ratio exceeds 0.5, the degradation in impact resistance becomes markedly pronounced. This threshold aligns with observations in conventional reinforced concrete and solid concrete-filled steel tube (CFST) columns, yet the transition is sharper in CFDST configurations due to the confined concrete core's limited capacity to compensate for the increased axial stress state. Engineers should treat an axial compression ratio above 0.5 as a design limit for impact-critical CFDST members.
Influence of Nominal Steel Ratio and Other Parameters
The nominal steel ratio, the strength of both the outer stainless steel tube and the inner concrete, the impact velocity, and the outer diameter of the cross-section were all identified as having substantial effects on impact performance. Higher nominal steel ratios generally improve the overall stiffness and energy absorption capacity, while increased impact velocity amplifies the dynamic effects and reduces the effective material strength through strain rate sensitivity. The outer diameter influences the geometric stability and the extent of local buckling during impact.
| Parameter | Effect on Impact Performance | Engineering Recommendation |
|---|---|---|
| Axial compression ratio | Degrades impact resistance; effect intensifies above 0.5 | Limit to 0.5 or below for impact-critical members |
| Nominal steel ratio | Higher ratio improves energy absorption | Optimize for economic efficiency while maintaining ductility |
| Outer tube strength | Higher strength increases stiffness but may reduce ductility | Balance strength and ductility requirements |
| Impact velocity | Higher velocity amplifies dynamic effects | Design for expected maximum impact velocity with safety margin |
| Outer diameter | Larger diameter improves geometric stability | Coordinate with span and loading conditions |
Dynamic Amplification Factor Formula
The proposed DIF formula provides a practical tool for estimating the impact force plateau value under coupled axial-impact loading. The formula was validated against finite element results and demonstrated good predictive accuracy. This represents a meaningful step toward incorporating CFDST columns into impact design codes, which currently lack specific provisions for double-skin configurations.
Integration with Engineering Practice
From a metallurgical and welding perspective, the use of stainless steel as the outer tube in CFDST columns introduces unique fabrication challenges. Stainless steel grades such as duplex 2205 or austenitic 316L are commonly specified for their corrosion resistance, but their welding behavior differs significantly from carbon steel. The heat-affected zone (HAZ) of austenitic stainless steel is susceptible to sensitization and intergranular corrosion, while duplex stainless steels require careful thermal input control to maintain the ferrite-austenite balance. When fabricating CFDST columns, the outer stainless tube must be joined to the inner steel tube and the concrete core through appropriate construction sequences that minimize distortion and residual stress.
The impact resistance findings have direct relevance to the selection of connection details. Welded connections between CFDST columns and beam elements must be designed to accommodate the large plastic deformations observed in the outer tube during impact events. Full-penetration groove welds with appropriate preheat and interpass temperature control are essential to prevent cold cracking in stainless steel welds. Post-weld heat treatment may be necessary for higher-strength stainless grades to relieve residual stresses that could initiate cracking under cyclic or impact loading.
Key Reflections and Study Insights
This research contributes meaningfully to the understanding of CFDST column behavior under dynamic loading, yet several questions remain open. The finite element model relies on material constitutive models for stainless steel that may not fully capture the strain rate sensitivity observed in high-strength duplex grades. Future work should incorporate rate-dependent material models and validate the DIF formula against experimental impact tests at higher velocities. Furthermore, the interaction between the stainless outer tube and the concrete core under repeated or multi-directional impact loading deserves further investigation. For engineers currently specifying CFDST columns in impact-critical applications, the findings suggest that the axial compression ratio should be carefully controlled, and the outer tube geometry and material grade should be selected to ensure adequate plastic deformation capacity. The proposed DIF formula offers a valuable starting point for preliminary design, but detailed finite element analysis remains essential for final design verification.