Prestressed Concrete-Filled Rectangular Steel Tube Truss with Cantilever

Overview and Structural Innovation

The paper by Cai Jian, Chen Guodong, Zuo Zhiliang, and Wu Yi (2009, Journal of Jilin University, Vol. 39, No. 2) presents a comparative experimental study of three large-scale truss models: a prestressed rectangular CFST truss with cantilever, a prestressed rectangular steel tube truss, and a rectangular CFST truss without prestressing. Funded by the National Basic Research Program (2004CA03300) and Guangzhou Municipal Education Bureau (62063), this work addresses the combined effect of prestressing and concrete infill on the performance of large-span truss structures.

Experimental Program and Test Results

Three full-scale models were tested under gravity loading to evaluate failure modes, deformation characteristics, and load-carrying capacity. The test matrix is summarized below:

Key Findings

• The prestressed concrete-filled truss exhibited significantly reduced deflection compared to the non-prestressed counterparts, demonstrating that prestressing effectively counteracts the dead-load deformation component.
• The ultimate load-carrying capacity was markedly improved when both prestressing and concrete infill were applied simultaneously. This synergistic effect exceeds the simple sum of individual contributions.
• Concrete infill effectively prevents local buckling of the compression steel tube walls. This is a critical finding because local buckling of thin-walled rectangular tubes is often the governing failure mode in compression chords.
• The failure mode of the best-performing specimen was governed by material strength rather than instability, indicating that the composite action between steel and concrete was fully mobilized.

Technical Analysis and Engineering Practice

Local Buckling Prevention

From a steel pipe and welding perspective, the prevention of local buckling by concrete infill is highly significant. Rectangular steel tubes used in truss chords typically have wall thicknesses ranging from 6 mm to 12 mm, with width-to-thickness ratios (b/t) that can exceed the limits specified in EN 1993-1-1 for unstiffened compression elements. The concrete infill provides lateral restraint that effectively increases the critical buckling stress of the tube walls. This means that thinner-walled tubes can be used in composite trusses without sacrificing stability, leading to material savings and reduced fabrication complexity.

Welding Implications

The fabrication of rectangular CFST trusses involves critical weld joints at the chord-diagonal intersections and at the cantilever connections. The prestressing system introduces additional weld details for anchorages and stressing hardware. The test results confirming that failure is material-strength governed rather than weld-governed suggest that the weld design was adequate, but this also means that weld quality remains a critical factor in achieving the full composite capacity. In practice, full-penetration butt welds with 100% UT inspection should be specified for all chord-diagonal junctions in prestressed CFST trusses.

Prestressing System Design

The prestressing arrangement described in this paper is analogous to prestressed concrete design but applied to a steel-concrete composite truss. The prestress force is introduced through tendons anchored at the compression chords. The key design parameters include:

• Prestress level: typically 60-70% of the tendon's characteristic tensile strength
• Loss estimation: accounting for elastic shortening, friction, anchorage slip, and long-term relaxation
• Compatibility between steel tube dimensions and tendon anchorage details

Study Insights

This research demonstrates a powerful structural concept: combining prestressing with concrete infill in steel tube trusses creates a synergistic system where each component compensates for the weakness of the other. The steel tube provides tensile capacity and ductility, the concrete provides compressive capacity and lateral restraint, and the prestressing system pre-compresses the structure to counteract service loads. For engineers involved in steel pipe fabrication and welding, the key takeaway is that the composite action must be designed and fabricated with equal attention to all three elements — the steel tube geometry, the concrete quality, and the prestressing system integrity.

Conclusion

The experimental evidence clearly supports the use of prestressed concrete-filled rectangular steel tube trusses for large-span applications with cantilevers. The combined approach yields superior deflection control, higher ultimate capacity, and improved resistance to local buckling. Engineers should consider this composite-prestressed concept as a viable alternative to conventional steel trusses, particularly where span-to-depth ratios are large and material efficiency is a design priority.