Zhuojin Pipe Fitting Co., LtdHysteretic Performance of Square Hollow Sandwich CFST Flexural-Compression Members via Finite Element Analysis
Literature Overview
This 2010 paper by Huang Hong, Huang Cheng, and Chen Mengcheng from East China Jiaotong University, published in Railway Construction (Volume 50, Issue 7, pages 57-61), investigates the hysteretic behavior of square hollow sandwich concrete-filled steel tube (CFST) flexural-compression members. The research is funded by the National Natural Science Foundation of China (Grant No. 50968006) and the Jiangxi Provincial Natural Science Foundation (2009GZC0023). The authors developed finite element models using ABAQUS that incorporate concrete damage and stiffness degradation under cyclic loading, as well as the Bauschinger effect in steel materials.
Core Technical Content
The square hollow sandwich CFST member is a composite structural element consisting of an outer square steel tube, an inner hollow core, and a concrete layer sandwiched between the outer and inner tubes. This configuration differs from conventional solid-core CFST members in that the concrete is confined between two steel tubes rather than filling a single tube. The hollow inner core reduces material usage while maintaining structural integrity.
Modeling Approach and Key Considerations
The finite element model accounts for several critical nonlinear phenomena:
- Concrete damage and degradation: Under cyclic loading, concrete undergoes progressive cracking and crushing. The model incorporates a damage constitutive model that captures the reduction in stiffness and strength with increasing strain amplitude and number of loading cycles.
- Steel Bauschinger effect: When steel is loaded in tension after prior compression (or vice versa), the yield strength in the reverse direction is reduced. The model captures this through appropriate multiaxial yield criteria and kinematic hardening rules.
- Full load-displacement response: The analysis traces the complete hysteretic loops from initial elastic loading through progressive yielding, strain hardening, and eventual degradation.
Stress Distribution Analysis
The authors analyzed longitudinal stress distributions in both the steel tube and the concrete core through contour plots at various stages of the loading history. These visualizations reveal how load is transferred between the steel and concrete components throughout the cyclic loading process.
Process and Standards Analysis
The accuracy of the finite element results relative to experimental data is confirmed as being in good agreement, validating the modeling approach. For engineers working with sandwich CFST members, several practical considerations arise:
| Modeling Parameter | Implementation Detail | Engineering Significance |
|---|---|---|
| Concrete damage model | Stiffness and strength degradation under cyclic loading | Captures progressive cracking and crushing |
| Steel Bauschinger effect | Multiaxial yield criterion with kinematic hardening | Reflects real material behavior under reversed loading |
| Interface modeling | Bond-slip behavior between steel and concrete | Critical for composite action assessment |
| Mesh density | Refined near critical zones (corners, mid-span) | Ensures accurate stress capture at high-gradient regions |
From a steel pipe manufacturing standpoint, the fabrication of sandwich CFST members requires precise control of the gap between the outer and inner tubes to ensure uniform concrete placement. The welding of the outer square tube must achieve full penetration at all seams, as any defect would compromise the confinement effect on the sandwich concrete layer. The inner tube, while not directly bearing loads in the same manner, provides additional confinement and helps maintain the integrity of the concrete during construction.
Integration with Engineering Practice
The hysteretic performance data derived from this study provides essential input for elastoplastic dynamic response analysis of structures incorporating sandwich CFST members. Railway and bridge engineering applications, where such members may be used as piers or support columns, benefit from accurate predictions of energy dissipation capacity, residual deformation, and degradation trends under seismic or dynamic loading.
The stress distribution contour plots offer engineers a visual understanding of how the composite section behaves under load. Areas of high stress concentration in the steel tube — typically at the corners of the square section and at the interface with the concrete — indicate potential locations for local buckling or weld failure. These insights can inform detailed design decisions regarding reinforcement plates, welding procedures, and quality control requirements.
Study Insights and Reflections
The value of this study lies in its rigorous treatment of material nonlinearities under cyclic loading. The inclusion of both concrete damage and steel Bauschinger effects in a single finite element framework demonstrates the complexity of modeling composite CFST behavior. For practitioners, the key takeaway is that the hollow sandwich configuration does not inherently compromise hysteretic performance, provided that the steel-concrete interface is properly modeled and the concrete confinement is maintained throughout the loading history. The good agreement between numerical and experimental results gives confidence in using such models for design verification, reducing the need for extensive physical testing while still capturing the essential mechanics of the system.