Zhuojin Pipe Fitting Co., LtdBending Performance of Conical Hollow Sandwich Steel Tube Concrete Members
Literature Overview
This paper by Liu Han, Shi Yanli, Fan Jiahao, and Wang Wenda from Lanzhou University of Technology, published in Progress in Steel Building Structures (Vol. 23, No. 5, 2021, pp. 9-17), investigates the pure bending behavior of conical hollow sandwich concrete-filled steel tube (CHS-CFST) members. The research was supported by the National Natural Science Foundation of China (Grant 51768038) and the Gansu Provincial Construction Science and Technology Program (JK2021-42). The study focuses on members with large hollow ratios and analyzes the effects of conical angle, hollow ratio, slenderness ratio, steel tube thickness-to-diameter ratio, steel strength, and concrete strength on the bending capacity and deformation characteristics.
Structural Configuration and Innovation
The conical hollow sandwich CFST member represents an innovative structural element that combines several design features:
- Conical geometry: The member has a varying cross-section along its length, with the outer steel tube diameter changing from a larger end to a smaller end. This tapering allows for efficient material utilization by matching the section properties to the bending moment distribution.
- Hollow sandwich construction: The space between the outer steel tube and an inner steel tube is filled with concrete, creating a sandwich structure. The hollow ratio (defined as the ratio of the inner steel tube area to the total cross-sectional area) can be varied to optimize weight and performance.
- Dual steel tube system: Both the outer and inner steel tubes contribute to the structural performance, with the outer tube providing bending resistance and the inner tube serving as a formwork and additional load-bearing element.
This configuration offers potential advantages in applications where weight reduction is critical, such as long-span structures, offshore platforms, and large-diameter pipe columns, while maintaining adequate bending capacity through the concrete core.
Numerical Modeling and Validation
The authors developed a validated finite element model based on existing experimental data. The modeling approach includes:
| Modeling Aspect | Approach | Justification |
|---|---|---|
| Outer steel tube | Shell elements with elastic-plastic material | Captures local buckling and yielding |
| Inner steel tube | Shell elements with elastic-plastic material | Accounts for inner tube contribution |
| Concrete core | Solid elements with concrete damage plasticity | Models cracking and crushing |
| Steel-concrete interface | Frictional contact or ties | Simulates bond and slip |
| Boundary conditions | Pinned-pinned with moment application | Represents pure bending test setup |
The model was validated against experimental data showing good agreement in peak load, deformation at peak load, and failure mode.
Key Findings
Moment-Deflection Behavior
The moment-deflection relationship of the conical hollow sandwich CFST members exhibits three distinct stages:
- Elastic stage: Linear relationship between moment and deflection, with all materials behaving elastically.
- Elastoplastic stage: Progressive yielding of steel tubes and cracking of concrete, leading to a gradual reduction in stiffness.
- Plastic stage: Formation of plastic hinges, with significant moment redistribution and eventual failure.
Effect of Conical Angle
As the conical angle (the angle of taper between the larger and smaller ends) increases, the failure location progressively shifts toward the smaller cross-section end. This is expected, as the smaller section has lower section modulus and therefore lower bending capacity. The conical angle directly influences the distribution of bending stresses along the member length and determines the critical section for design.
Effect of Hollow Ratio
The hollow ratio has a relatively minor effect on the deformation curve shape but a more significant influence on the overall stiffness. Higher hollow ratios reduce the concrete core volume, which decreases the composite action between steel and concrete, leading to lower initial stiffness and earlier onset of plastic behavior.
Parameter Sensitivity Analysis
The following table summarizes the relative influence of various parameters on bending capacity:
| Parameter | Influence on Bending Capacity | Sensitivity Level |
|---|---|---|
| Conical angle | Significant | High |
| Hollow ratio | Significant | High |
| Outer steel tube strength | Significant | High |
| Inner steel tube strength | Significant | Moderate |
| Outer steel tube thickness-to-diameter ratio | Moderate | Moderate |
| Concrete compressive strength | Minor | Low |
The finding that concrete strength has a relatively minor influence on bending capacity is notable. This suggests that in bending-critical applications, the steel tube properties dominate the structural response, and increasing concrete strength beyond a certain level provides diminishing returns.
Proposed Design Formula
The authors propose a calculation method for the bending capacity of conical hollow sandwich CFST members. The formula accounts for the contributions of the outer steel tube, inner steel tube, and concrete core, with appropriate interaction terms. The proposed method shows good agreement with numerical simulation results, with deviations generally within ±10%.
Engineering Applications and Considerations
The conical hollow sandwich CFST member has potential applications in several structural scenarios:
- Pipe columns in multi-story buildings: The tapered geometry can be designed to match the bending moment distribution, reducing material usage at the top of the column where moments are lower.
- Long-span bridge piers: The hollow construction reduces self-weight while maintaining adequate bending resistance through the composite action.
- Offshore platform legs: The dual steel tube system provides redundancy and damage tolerance, which is critical for offshore structures.
- Energy absorption members: The progressive yielding and large plastic deformation capacity make these members suitable for use as energy-dissipating elements in seismic isolation systems.
However, several practical challenges must be addressed:
- Fabrication complexity: The conical geometry requires specialized forming techniques for the steel tubes, increasing manufacturing costs.
- Concrete placement: Filling the annular space between two conical steel tubes with concrete is technically challenging and requires careful control of vibration and compaction.
- Quality inspection: Non-destructive testing of the concrete-concrete interface and the internal concrete core is more difficult than for conventional CFST members.
- Connection design: The varying cross-section complicates the design of connections at both ends of the member, requiring specialized connection details.
Critical Assessment
The research provides valuable insights into the bending behavior of an innovative structural member type. However, the analysis is limited to pure bending, which is a simplified loading condition. In practice, these members will be subjected to combined bending, axial compression, and shear, which may significantly alter the failure mode and capacity.
The numerical model, while validated against existing experimental data, relies on constitutive models that have known limitations in capturing the complex behavior of concrete under multiaxial stress states. The concrete damage plasticity model in particular may overestimate the post-cracking stiffness and underestimate the fracture energy dissipation.
Furthermore, the study does not address the effects of long-term loading, including creep and shrinkage of the concrete core, which can induce additional stresses in the steel tubes and potentially accelerate fatigue damage.
The proposed design formula should be further validated against additional experimental data, particularly for members with extreme parameter combinations (very high hollow ratios, large conical angles, and high-strength materials). The formula's applicability to members with different aspect ratios and boundary conditions should also be investigated.
This research represents a meaningful step forward in the development of innovative composite structural members. The conical hollow sandwich CFST concept offers a promising combination of weight efficiency and structural performance, and further research and standardization efforts are warranted to bring this technology into mainstream structural engineering practice.