Experimental Study of Large-Span Rectangular CFST Trusses with Cantilevers

Research Overview and Engineering Context

This paper by Yang Chun, Cai Jian, Chen Guodong, Luo Qizhi, and Zuo Zhiliang from South China University of Technology and Foshan University of Science and Technology, published in the "Journal of South China University of Technology (Natural Science Edition)" (2008, Vol. 36, No. 3, pp. 128–133), presents experimental research on large-span rectangular concrete-filled steel tube (CFST) trusses with cantilevers. This research addresses a practical structural engineering challenge: the design and performance evaluation of large-span truss systems that incorporate cantilever extensions, which are common in stadium roofs, airport terminals, and industrial buildings.

Rectangular CFST trusses represent an efficient structural system that fully exploits the load-bearing advantages of concrete-filled steel tubes while maintaining the lightweight and long-span capabilities of truss configurations. The addition of cantilever extensions extends the functional coverage of the truss without requiring additional support columns, which is particularly valuable in applications where clear-span requirements are strict.

Experimental Program and Specimen Configuration

The experimental program included two large-scale models: a rectangular CFST truss with cantilever and a comparable rectangular steel tube truss without concrete infill. This comparative approach allows direct evaluation of the contribution of concrete infill to the structural performance of the truss system.

Both specimens were subjected to monotonic loading to characterize their load-bearing capacity, deformation behavior, and failure modes. The large scale of the specimens ensured that the test results are representative of actual structural behavior rather than being dominated by small-scale effects.

Test Results and Performance Comparison

The experimental results demonstrate clear advantages of the CFST truss configuration over the conventional steel tube truss:

The most significant finding is that concrete infill effectively prevents local buckling of the steel tube walls, which is a common failure mode in thin-walled steel tube trusses. In the steel tube truss, local buckling of the chord and web members reduces the effective cross-sectional area and can lead to premature structural failure. The concrete infill provides lateral restraint to the steel tube walls, enabling them to maintain their geometric integrity throughout the loading process and reach their full material capacity.

Failure Mode Analysis

The failure of the CFST truss specimens was governed by material strength rather than stability failure. This is a fundamentally different failure mechanism from the steel tube truss, where stability failure (local buckling) governs the ultimate capacity. The material-strength-controlled failure mode of the CFST truss provides a more predictable and reliable structural response, as the failure progression is more gradual and the pre-failure deformation provides warning signs.

The failure typically initiates at the most critically stressed member, which is usually the bottom chord at mid-span under gravity loading. The concrete crushing in the compressed chord members is accompanied by steel tube yielding, and the failure propagates through the truss as adjacent members reach their capacity.

Welded Joint Behavior and Design Considerations

A critical finding from this research is that the end moments at the chord and web member ends, at the welded joint locations, are significant and cannot be neglected in the design of welded steel tube and CFST trusses. This finding has direct implications for joint design and weld detailing.

In welded truss connections, the chord and web members are directly welded to form rigid joints. The continuity of the chord member across the joint creates bending moments at the connection that must be resisted by the joint configuration and weld details. For CFST trusses, the presence of concrete infill at the joint region adds complexity to the joint behavior, as the concrete must be accommodated within the joint geometry without interfering with the weld access and quality.

The research emphasizes that the joint end moments must be explicitly considered in the structural analysis and design of welded trusses. Neglecting these moments can lead to inadequate joint design, potential weld failures, and reduced structural safety margins. Design codes should incorporate provisions for joint moment resistance in welded truss connections.

Engineering Practice Implications

For truss manufacturers and structural engineers, this research provides validated design guidance for rectangular CFST trusses with cantilevers. The demonstrated performance advantages of CFST members over hollow steel tube members justify the additional cost of concrete infill in applications where structural efficiency and long-term performance are prioritized.

From a manufacturing perspective, the fabrication of rectangular CFST trusses requires careful sequencing of operations: steel tube fabrication and welding, concrete infill, and final assembly. The concrete infill must be placed before the truss is assembled into its final configuration, requiring either vertical or horizontal placement methods that ensure complete filling without voids. The welding of truss joints must be planned to accommodate the concrete-filled members, with appropriate access provisions for weld preparation and inspection.

The cantilever extension of the truss introduces additional design considerations related to the moment and shear forces at the cantilever root. The CFST configuration provides enhanced resistance to these forces through the composite action of steel and concrete, but the joint details at the cantilever transition must be carefully designed to ensure adequate load transfer.

Study Insights and Practical Value

This research provides experimental validation of the structural performance of rectangular CFST trusses with cantilevers, addressing a practical engineering need for large-span structural systems with extended functional coverage. The comparative testing approach clearly demonstrates the benefits of concrete infill in preventing local buckling and improving overall structural performance. The identification of significant joint end moments as a critical design consideration provides essential guidance for the proper design of welded truss connections.

The research findings support the adoption of rectangular CFST trusses in large-span applications where the combination of high bearing capacity, improved stiffness, and enhanced local buckling resistance provides significant structural advantages. The practical implications extend to material selection, fabrication procedures, welding specifications, and quality control protocols that ensure the reliable performance of these structural systems in service. Future research should address the fatigue performance of welded CFST truss joints under cyclic loading, the long-term durability of concrete infill in different environmental conditions, and the development of simplified design methods that can be incorporated into structural design standards for practical engineering application.