Flexural Performance of T-Shaped Steel Tube Concrete Composite Members

Literature Overview

This paper by Xu Lihua, Liu Shengbing, Wen Fang, and Du Guofeng from Wuhan University and Yangtze University was published in the Journal of Huazhong University of Science and Technology (Natural Science Edition) in 2009, Volume 37, Issue 2, pages 117–120. Funded by the Hubei Provincial Department of Construction Science and Technology Project (Grant K200513), the study investigates a novel T-shaped steel tube concrete (CFT) composite member formed by directly welding two rectangular steel tubes together. The research combines experimental testing with finite element analysis to characterize the flexural performance of this new composite section.

Novel Composite Section Design

The key innovation in this study is the creation of a T-shaped CFT composite member by welding two rectangular steel tubes in a T-configuration. This approach differs from traditional T-shaped CFT members, which typically use a single steel tube or fabricated steel section. The novel design leverages the inherent composite action of steel tube concrete while achieving a T-shaped cross-section through a relatively simple fabrication process.

Design Parameter	Novel T-Shaped Member	Traditional T-Shaped Member
Steel tube configuration	Two rectangular tubes welded	Single tube or fabricated section
Concrete fill	Both tubes filled independently	Single continuous fill
Welding requirement	Butt weld at T-junction	Varies by fabrication method
Fabrication complexity	Moderate	Higher
Steel-to-concrete ratio	Higher (two steel tubes)	Lower

Experimental Program

The authors conducted flexural tests on 8 groups totaling 16 T-shaped CFT composite members. The experimental parameters investigated include:

Parameter	Variable Range	Number of Levels
Steel ratio (steel area / total area)	Multiple levels	3–4
Shear span ratio	Multiple levels	3–4
Section dimensions	Multiple levels	3–4

The tests characterized the moment-deflection behavior, failure modes, and ultimate flexural capacity of the composite members. The results were compared with traditional T-shaped CFT members to evaluate the advantages of the novel design.

Finite Element Analysis and Validation

ABAQUS software was used to perform full-process analysis of the moment-deflection curves for the T-shaped CFT composite members. The numerical analysis results showed good agreement with experimental results, with bearing capacity differences within 5% between the two methods. This level of agreement validates the finite element model for parametric studies and design applications.

The finite element model likely incorporated:

Concrete damage plasticity model for the core concrete behavior.
Bilinear or multilinear kinematic hardening model for the steel tube material.
Detailed mesh refinement at the weld junction where stress concentrations occur.
Contact elements to capture potential separation between the two tubes under bending.

Capacity Formula and Verification

The authors proposed a calculation formula for the ultimate flexural bearing capacity of the novel T-shaped CFT composite member. The formula was verified against 5 groups of test specimens, with maximum calculation errors of 14% between computed and experimental results. This level of accuracy is acceptable for engineering design purposes and provides a practical design tool.

Engineering Implications for Pipe Fabrication and Welding

The novel T-shaped CFT composite member has several implications for steel pipe manufacturing and welding practice:

The butt weld at the T-junction between two rectangular tubes is a critical structural element that must be qualified to full-penetration standards.
Weld quality directly affects the composite member's flexural performance, as the weld junction is a potential location for stress concentration and crack initiation.
The two-tube configuration requires careful dimensional control during tube fabrication to ensure proper fit-up at the weld junction.

From a welding process perspective, the following considerations are relevant:

Welding Consideration	Recommendation	Standard Reference
Weld process	GTAW + SAW or FCAW	ASME B31.3, GB/T 985
Weld preparation	V-groove or U-groove	GB/T 985.1
Preheat temperature	Based on steel grade and thickness	AWS D1.1
Post-weld treatment	Stress relief if required	ASME BPVC Section VIII
NDT requirements	RT or UT for full-penetration verification	GB/T 3323, GB/T 11345

Study Insights and Reflections

The research demonstrates that combining two rectangular steel tubes into a T-shaped configuration creates a structurally efficient composite member with competitive flexural performance compared to traditional designs. The direct welding approach simplifies fabrication while maintaining structural integrity, provided that weld quality is rigorously controlled.

The 5% agreement between numerical and experimental results validates the finite element approach for this type of composite member analysis. The proposed capacity formula, with a maximum error of 14%, provides a practical design tool that balances accuracy with simplicity.

The study also highlights the importance of steel ratio and shear span ratio as key design parameters, findings that are consistent with established CFT design principles. The comparison with traditional T-shaped members suggests that the novel design offers advantages in terms of fabrication simplicity and potentially improved structural efficiency.

Conclusion

This study presents a practical and innovative approach to creating T-shaped CFT composite members through the welding of two rectangular steel tubes. The combination of experimental testing and validated finite element analysis provides reliable design data and a practical capacity formula. Engineers involved in steel pipe fabrication and structural design should consider this approach for applications where T-shaped CFT sections are required, while ensuring rigorous weld quality control at the critical junction.