Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Circular High-Strength Hollow Sandwich CFST Members
Literature Overview
This 2021 paper by Wang Cancan et al., published in the Journal of Xi'an University of Architecture and Technology, investigates the axial compression performance of circular high-strength hollow sandwich concrete-filled steel tube (CFST) members. These members are composed of Q690 high-strength steel tubes and C100 (C120) ultra-high-strength concrete, with a hollow core between the inner and outer steel tubes. The research was conducted at Xi'an University of Architecture and Technology with funding from the National Natural Science Foundation of China. Six specimens were tested with concrete strength, section hollow ratio, and section steel ratio as the primary variables, and the results were supplemented by ABAQUS finite element analysis.
Structural Configuration and Material Properties
The hollow sandwich CFST members consist of an outer steel tube, an inner steel tube, and a concrete layer between them, with a hollow core in the center. This configuration combines the high strength of Q690 steel and C100/C120 concrete to achieve exceptional load-bearing capacity while maintaining a relatively lightweight section.
| Material | Grade | Typical Properties |
|---|---|---|
| Steel | Q690 | Yield strength 690 MPa, ultimate strength ~760 MPa |
| Concrete | C100 | Compressive strength 100 MPa |
| Concrete | C120 | Compressive strength 120 MPa |
| Hollow ratio | Variable | Primary test parameter |
| Steel ratio | Variable | Primary test parameter |
Experimental Results and Failure Modes
Load-Carrying Capacity Trends
The experimental results show clear trends in the load-carrying capacity of the specimens:
- Increasing concrete strength leads to increased load-carrying capacity, as expected from the higher compressive strength of the infill material.
- Increasing the section hollow ratio leads to decreased load-carrying capacity, as the hollow core reduces the effective cross-sectional area and the confinement effect on the concrete.
- Increasing the section steel ratio leads to decreased load-carrying capacity, which is counter-intuitive but can be explained by the reduced concrete volume and the associated reduction in the confinement-enhanced concrete contribution.
Failure Modes
Two distinct failure modes were identified:
| Failure Mode | Description | Post-Peak Behavior | Ductility |
|---|---|---|---|
| Bulging type | Local outward buckling of outer tube | High post-peak load capacity | Good |
| Shear type | Shear failure at critical section | Rapid post-peak load drop | Relatively low |
The bulging-type failure is characterized by the outward deformation of the outer steel tube, creating a visible bulge in the member. This mode is associated with higher post-peak load capacity and better ductility, making it the preferred failure mode from a seismic design perspective. The shear-type failure occurs when the concrete layer between the inner and outer tubes fails in shear, leading to a sudden loss of load-carrying capacity.
Finite Element Analysis and Parametric Study
The ABAQUS finite element model was constructed using the concrete damaged plasticity model for concrete and bilinear kinematic hardening for steel. The hollow core was modeled as a void, and the interface between the steel tubes and concrete was modeled using appropriate contact conditions.
| FE Parameter | Description |
|---|---|
| Concrete model | Concrete damaged plasticity |
| Steel model | Bilinear kinematic hardening |
| Contact type | Penalty-based |
| Element type | C3D8R (solid) |
| Mesh size | 20-30 mm |
| Boundary conditions | Axial compression, symmetric |
The parametric study using finite element analysis examined the effects of hollow ratio, concrete strength, and inner/outer steel tube diameter-to-thickness ratios on the axial compression performance. The results confirmed the experimental trends and provided additional insight into the stress distribution within the member.
Proposed Calculation Formulas
The paper proposes analytical formulas for the axial compression load-carrying capacity and stiffness of circular high-strength hollow sandwich CFST members. The formulas incorporate the contributions of the outer steel tube, inner steel tube, and confined concrete layer, with appropriate reduction factors for the hollow core effect. The comparison between calculated and experimental values showed good agreement, validating the proposed formulas.
Engineering Practice Implications
The hollow sandwich CFST configuration offers several advantages for engineering applications:
- High strength-to-weight ratio: The combination of high-strength steel and ultra-high-strength concrete with a hollow core provides excellent load-bearing capacity with reduced self-weight.
- Design flexibility: The hollow core can be used for utility routing, reducing the need for additional service ducts in building structures.
- Thermal insulation: The hollow core provides natural thermal insulation, which is beneficial for fire resistance and energy efficiency.
- Application potential: Suitable for high-rise building columns, bridge piers, and offshore platform structures where high strength and lightweight are required.
However, the shear-type failure mode poses a concern for seismic design. Engineers should ensure that the member geometry and material properties are selected to promote the bulging-type failure mode, which offers better ductility and post-peak behavior. The diameter-to-thickness ratios of both the inner and outer steel tubes should be carefully controlled to prevent local buckling that could trigger shear-type failure.
Study Insights and Reflections
This research contributes to the growing body of knowledge on high-strength composite concrete-filled steel tube structures, which are increasingly being considered for applications requiring exceptional strength and lightweight. The identification of two distinct failure modes and their associated ductility characteristics is particularly valuable for design practitioners, as it provides clear guidance on the geometric and material parameters that influence failure mode.
The proposed analytical formulas represent a practical tool for designers, enabling preliminary sizing and verification of hollow sandwich CFST members without the need for extensive finite element analysis. However, the formulas should be validated against a broader database of experimental results before being incorporated into design codes. Future research should address the cyclic loading behavior of these members, their fire resistance, and the long-term durability of the C100/C120 concrete in aggressive environments. The combination of Q690 steel and ultra-high-strength concrete also raises questions about the weldability and formability of the steel tubes, which should be investigated in conjunction with manufacturing process studies.