Pure Bending Behaviour of Rectangular Hollow Sandwich Concrete-Filled Steel Tube Members

Literature Overview

The paper by Xie, Chen, and Huang (2012), published in Railway Engineering (Vol. 52, No. 12, pp. 155-158), presents an experimental and numerical study on the pure bending behaviour of rectangular hollow sandwich concrete-filled steel tube (HSCFST) members. Funded by the National Natural Science Foundation of China (Grants No. 51008122 and 50968006) and the Jiangxi Provincial Department of Education (Grant No. GJJ10709), the research investigates a novel structural system in which concrete is confined between two concentrically placed steel tubes. The study designed three rectangular HSCFST specimens with varying hollow ratios and one rectangular solid CFST specimen for comparison, and conducted pure bending tests to evaluate the flexural mechanical properties. The results demonstrate that HSCFST members exhibit significantly higher flexural load capacity compared to solid CFST members of similar dimensions, while maintaining good ductility.

Core Technical Points

The hollow sandwich CFST (HSCFST) concept involves placing concrete between two concentric steel tubes, creating a sandwich structure with steel on both the inner and outer surfaces. This configuration offers several theoretical advantages over conventional solid CFST members:

1. Enhanced confinement of the concrete core due to the presence of both inner and outer steel tubes.
2. Improved flexural stiffness and load capacity due to the dual steel tube configuration.
3. Potential for reduced material usage compared to a solid CFST member of equivalent flexural capacity.
4. Better protection of the concrete core from environmental degradation due to the outer steel tube.

The experimental programme involved four specimens: three HSCFST specimens with different hollow ratios and one solid CFST specimen. The hollow ratio, defined as the ratio of the inner tube outer diameter to the outer tube inner diameter, was the primary variable. The following table summarises the key experimental findings:

The experimental results clearly show that the flexural load capacity of HSCFST members increases with the hollow ratio, with the highest hollow ratio specimen exhibiting the greatest flexural capacity. This is attributed to the increased confinement effect of the inner tube on the concrete core, which enhances the concrete's compressive strength and ductility under bending. All specimens exhibited good plastic deformation capacity, with no brittle failure observed.

Welding and Fabrication Challenges

The fabrication of HSCFST members involves several welding operations that are more complex than those for conventional CFST members. The inner tube must be positioned concentrically within the outer tube, and the two tubes must be connected at regular intervals to ensure composite action. This connection is typically achieved through welding of spacer bars, shear connectors, or welding rings between the inner and outer tubes.

The following welding challenges are specific to HSCFST fabrication:

• The inner tube is typically smaller in diameter than the outer tube, which may require different welding procedures and parameters for the inner tube welds compared to the outer tube welds.
• The connection between the inner and outer tubes must be welded from the inside of the outer tube, which can be a confined and difficult welding environment, particularly for larger diameter tubes.
• The welding sequence must be carefully planned to minimise distortion of both tubes, as distortion of the inner tube relative to the outer tube can affect the concentricity and the concrete fill quality.
• The welding of shear connectors or spacer bars between the two tubes introduces additional welds that must be inspected and tested for quality.

From a quality control perspective, the following NDT procedures are recommended for HSCFST fabrication:

Finite Element Modelling and Validation

The finite element analysis of the HSCFST specimens was performed to validate the experimental results and to investigate the structural behaviour in more detail. The modelling approach employed shell elements for the steel tubes and solid elements for the concrete, with appropriate contact definitions between the steel tubes and the concrete core. The customised uniaxial constitutive models for steel and concrete were calibrated against experimental stress-strain data.

The finite element results showed good agreement with the experimental moment-deflection curves, confirming the validity of the modelling approach. The key finding from the numerical analysis is that the inner tube provides additional confinement to the concrete core, which enhances the concrete's compressive strength and ductility under bending. This confinement effect is more pronounced in the compression zone of the cross-section, where the concrete is subjected to the highest compressive stresses.

Engineering Practice Implications

For engineers considering the use of HSCFST members in structural applications, this study provides several practical insights:

1. HSCFST members offer a viable alternative to solid CFST members for flexural applications, with the potential for improved load capacity and material efficiency.
2. The hollow ratio should be optimised based on the specific structural requirements, as the flexural load capacity increases with hollow ratio but the cost of fabrication also increases.
3. The welding quality of the inner-outer tube connections is critical to the composite action of the HSCFST member, and rigorous quality assurance procedures must be implemented.
4. The concrete fill process for HSCFST members requires careful planning to ensure that concrete is placed between the two tubes without disturbing the inner tube position.

Study Insights and Outlook

This study makes a valuable contribution to the understanding of HSCFST member behaviour under pure bending. The demonstration that HSCFST members can achieve higher flexural load capacity than solid CFST members of similar dimensions is a significant finding that could lead to material savings in structural applications. For pipe fabrication engineers, the key challenge is the fabrication of the inner-outer tube connection, which requires careful planning of the welding sequence, precise control of welding parameters, and comprehensive NDT to ensure weld quality. Future research should investigate the combined loading behaviour of HSCFST members under axial compression and bending, as well as the seismic performance of HSCFST columns and beams. The integration of HSCFST technology with modern welding and fabrication techniques, such as automated welding and robotic welding, could further improve the quality and efficiency of HSCFST member production.