Finite Element Analysis of Circular Composite Double Steel Tubular Concrete Short Columns Under Axial Compression

Literature Overview

The paper by Ci Junchang, Yan Weiming, and Jia Hong, published in the Journal of Beijing University of Technology in 2020, presents a finite element analysis of circular concrete-filled double steel tubular (CFDST) short columns under axial compression. The study was supported by the National Natural Science Foundation of China (Grants 51978021, 51878017) and the National Key R&D Program (Grants 2017YFC1500604, 2017YFC1500603). The CFDST column, featuring both an outer and inner steel tube surrounding a concrete core, represents an advanced composite structural system with enhanced confinement and load-bearing capacity.

Core Technical Content

The researchers developed a finite element model using ABAQUS software and addressed the unique challenge of concrete confined by both inner and outer steel tubes simultaneously. Two constitutive models for the confined core concrete were adopted and validated against existing experimental data. The validated models were then used to expand the parametric study range beyond what experimental testing alone could achieve.

Constitutive Model Comparison

Aspect	Model 1	Model 2
Confinement consideration	Accounts for dual-tube confinement	Simplified confinement approach
Validation accuracy	Good agreement with test data	Reasonable agreement with test data
Applicability	Sandwich concrete, core concrete, and steel tubes	Core concrete primarily
Recommended use	Full CFDST system modeling	Preliminary design estimation

Parametric Study Findings

The parametric study examined the effects of various geometric and material parameters on the axial compression behavior:

Inner tube diameter ratio: Increasing the inner tube diameter ratio enhances confinement effectiveness and ultimate strength.
Outer tube diameter ratio: Larger outer tubes provide greater confinement but may lead to increased material usage.
Wall thickness ratio: The relative wall thickness of inner and outer tubes influences the confinement pressure distribution and failure mode.
Concrete strength: Higher concrete strength increases ultimate capacity but may reduce ductility if not properly confined.
Steel tube material grade: Higher-grade steel tubes improve load capacity but require careful consideration of weldability and fracture toughness.

Engineering Practice Integration

From a steel pipe fabrication and welding quality control standpoint, several critical aspects emerge:

Dual-tube manufacturing precision: The CFDST column requires two concentric steel tubes with precise dimensional tolerances. The gap between the inner and outer tubes must be uniformly controlled to ensure proper concrete filling and confinement effectiveness.
Welding quality for tube connections: Any longitudinal or circumferential welds on the steel tubes must meet stringent quality requirements, as weld defects could initiate premature failure under axial compression.
Material compatibility: The steel tubes, concrete, and any reinforcing elements must be compatible in terms of thermal expansion, corrosion resistance, and mechanical property matching.

Quality Control Considerations

Quality Aspect	Control Method	Acceptance Criteria
Tube dimensional accuracy	CMM measurement	Per ASTM A500 or GB/T 8163
Weld integrity	RT/UT inspection	No裂纹, no incomplete fusion
Surface condition	Visual + PT	No scale, dents, or surface defects
Material certification	MTC + tensile test	Conformance to specified grade
Concentricity	On-site assembly check	Gap variation within tolerance

Study Insights and Reflections

This study demonstrates the power of validated finite element analysis in extending research beyond experimental limitations. The dual-tube confinement concept offers superior load-bearing capacity compared to single-tube CFST columns, making it attractive for high-rise building cores, bridge piers, and offshore structures. However, the increased fabrication complexity and cost must be weighed against the structural benefits.

For pipe fabricators, the CFDST column represents a demanding application requiring high precision in tube manufacturing, welding, and assembly. The inner tube must be manufactured to tight tolerances to ensure proper positioning within the outer tube, and the concrete filling process must be carefully controlled to avoid voids that would compromise confinement effectiveness. The study's parametric findings provide valuable guidance for optimizing tube dimensions and material selections in future projects.

The validated constitutive models serve as a foundation for more advanced analyses, including seismic response, fire resistance, and long-term durability assessment, all of which are critical for the safe and economical design of CFDST structures.