Zhuojin Pipe Fitting Co., LtdPure Bending Performance of Circular Hollow Steel Tube-Concrete Composite Members
Literature Overview
This paper by Ren Qingxin, Wei Qiuyu, and Ding Jinan from the School of Civil Engineering, Shenyang Jianzhu University, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) (2021, Vol. 37, No. 3), investigates the pure bending behavior of circular hollow steel tube-concrete composite members through finite element analysis using ABAQUS. The research is supported by the National Natural Science Foundation of China and other provincial funding programs. The study aims to provide a computational basis for the design of this relatively novel composite structural member type, which combines the advantages of concrete-filled steel tubes with the weight-reduction benefit of a hollow core.
Core Technical Points
Member Configuration and Design Rationale
The circular hollow steel tube-concrete composite member consists of an outer steel tube filled with concrete, but with a central hollow core that reduces self-weight. This configuration offers a favorable balance between structural efficiency and weight savings. The hollow rate (ratio of hollow diameter to outer diameter) is a critical design parameter that directly affects both the bending capacity and the weight of the member.
| Parameter | Values Investigated | Effect on Capacity |
|---|---|---|
| Steel tube outer diameter | 80 mm, 100 mm, 120 mm | Larger diameter increases capacity |
| Hollow rate | 0.137, 0.221, 0.325 | Higher hollow rate decreases capacity |
| Capacity reduction (diameter 80→100 mm) | Hollow rate 0.137→0.221 | 5.2% reduction |
| Capacity reduction (diameter 80→120 mm) | Hollow rate 0.137→0.325 | 19.5% reduction |
Finite Element Modeling and Validation
The authors validated their numerical model against existing experimental data before conducting parametric studies. Four characteristic points were defined on the moment-deflection curves to characterize the working mechanism of the member at different loading stages. The characteristic points likely correspond to the proportional limit, yield point, peak moment, and residual capacity, each representing a distinct phase of structural behavior.
The use of ABAQUS for three-dimensional finite element analysis is appropriate for capturing the complex stress states in composite members. The concrete constitutive model must account for confined concrete behavior, including the increased compressive strength and ductility due to the lateral confinement provided by the steel tube. The hollow core introduces a discontinuity in the confinement effect, which the model must accurately represent.
Working Mechanism at Characteristic Points
At the initial elastic stage, the steel tube and concrete act independently, each carrying a portion of the bending moment proportional to their respective flexural rigidities. As loading increases, the concrete approaches its compressive strength, and the steel tube begins to yield on the compression side. The hollow core reduces the effective concrete cross-section, which decreases the total compressive force available to resist the bending moment.
At the peak moment stage, the concrete in the compression zone has reached its ultimate strain, and the steel tube has yielded over a significant portion of the compression zone. The hollow core creates a region of reduced confinement, which may lead to localized concrete crushing. Beyond the peak, the member exhibits strain-hardening behavior in the steel tube, providing residual capacity.
Simplified Calculation Formula
The authors derived a simplified formula for the pure bending capacity using the superposition principle and limit equilibrium principle. The formula decomposes the total capacity into contributions from the steel tube, the concrete annulus, and the interaction effects between the two materials. The calculated results showed good agreement with experimental data, confirming the validity of the approach.
Engineering Practice Implications
For structural engineers designing buildings or bridges using hollow steel tube-concrete composite members, this study provides valuable guidance on the trade-offs between weight reduction and capacity loss. A hollow rate of 0.221 (diameter increase from 80 to 100 mm) results in only a 5.2% capacity reduction, which is an attractive option for weight-sensitive applications. However, increasing the hollow rate to 0.325 (diameter 120 mm) causes a 19.5% reduction, which may be unacceptable for critical structural elements.
From a welding and fabrication standpoint, the hollow steel tube-concrete composite member requires careful attention to the welding of the outer steel tube seams and any connection details. The welding procedure must be qualified for the specific steel grade used, and the heat input must be controlled to prevent excessive distortion of the thin-walled tube sections. The presence of the hollow core may also affect the welding sequence, as the tube must be fabricated, welded, and inspected before concrete placement.
The connection design between these composite members and other structural elements is another critical consideration. The hollow core reduces the effective section modulus at connection regions, and special attention must be paid to the moment transfer mechanism at beam-column joints.
Key Questions and Reflections
One significant question is the effect of the hollow core on the fire resistance of the composite member. The reduced concrete volume means less thermal mass and potentially reduced fire endurance. The hollow core also creates a void that could affect the fire behavior by allowing heat to penetrate to the inner surface of the steel tube, potentially accelerating steel temperature rise.
Another practical consideration is the construction methodology. How is the hollow core formed and maintained during concrete placement? The use of a central core formwork or a prefabricated inner tube must be carefully planned to ensure proper concrete placement without voids or honeycombing. The quality of the concrete-steel interface, particularly around the hollow core boundary, is critical for the composite action assumed in the design calculations.
Study Insights and Implications
This study demonstrates that circular hollow steel tube-concrete composite members offer a viable alternative to conventional concrete-filled steel tubes, with the added benefit of reduced self-weight. The simplified capacity formula provides a practical tool for preliminary design, while the finite element results offer detailed insight into the structural behavior. Engineers should carefully select the hollow rate based on the specific requirements of each application, balancing weight savings against capacity reduction. The study contributes meaningfully to the development of efficient composite structural systems for modern construction.