Zhuojin Pipe Fitting Co., LtdPrestressed Concrete-Filled Rectangular Steel Tube Truss with Cantilever
Literature Overview
This experimental study by Cai Jian, Chen Guodong, Zuo Zhiliang, and Wu Yi (2009), published in the Journal of Jilin University (Engineering and Technology Edition), presents comparative tests on three large-scale truss specimens: a prestressed concrete-filled rectangular steel tube (RCFST) truss with cantilever, a prestressed rectangular steel tube truss, and a conventional RCFST truss. The research, funded by the National Basic Research Program of China (2004CA03300) and the Guangzhou Education Bureau Science and Technology Program (62063), addresses the structural performance of combined prestressing and concrete infill strategies in long-span truss systems.
Experimental Configuration and Test Results
Specimen Comparison
The three test specimens were designed at full-scale dimensions to represent practical engineering proportions. The key variables were the presence of prestressing and concrete infill in the compression chords:
| Specimen Type | Compression Chord Configuration | Prestressing Applied | Concrete Infill |
|---|---|---|---|
| Specimen A | Rectangular steel tube | Yes | No |
| Specimen B | Rectangular steel tube with concrete fill | No | Yes |
| Specimen C | Rectangular steel tube with concrete fill | Yes | Yes |
Performance Enhancement
The test results demonstrate a synergistic effect between prestressing and concrete infill:
- Deformation reduction: The combined prestressed and concrete-filled truss exhibited significantly reduced mid-span deflection compared to the prestressed steel tube truss alone. The concrete infill increases the effective flexural stiffness of the compression chords by providing lateral confinement to the steel walls and contributing additional compressive capacity.
- Ultimate load capacity: The ultimate bearing capacity of Specimen C was substantially higher than both Specimen A and Specimen B individually, confirming that the two strengthening measures are complementary rather than redundant.
- Local buckling prevention: Concrete infill effectively prevents local buckling of the rectangular steel tube compression chords. Without concrete, the thin-walled rectangular tubes are susceptible to web crippling and flange local buckling under concentrated compression, which can prematurely reduce the load-carrying capacity of the truss.
- Failure mode: The failure of the combined specimen was governed by material strength rather than instability, indicating that the structural system achieved a ductile, predictable failure mechanism.
Engineering Practice Integration
Design Considerations for Prestressed CFST Trusses
For engineers considering prestressed CFST truss systems for long-span applications such as airport terminals, exhibition halls, or railway station canopies, the following design principles emerge from this study:
- Prestress level optimization: The prestress force should be calibrated to offset a significant portion of the service load-induced compression in the chords, thereby reducing the net compressive stress and delaying local buckling.
- Concrete grade selection: The concrete infill should be designed to provide adequate confinement pressure to the steel tube walls. Higher concrete grades (C50 or above) are recommended for compression chords in prestressed configurations.
- Cantilever detailing: The cantilever region concentrates bending moments and shear forces, requiring careful connection design between the truss chords and the cantilever extension. The concrete infill in this region should be continuously poured to maintain structural continuity.
- Prestress loss accounting: Long-term prestress losses due to concrete creep, shrinkage, and relaxation must be accounted for in the design, as these losses directly affect the net compressive stress state in the chords.
Standards and Code References
The design of prestressed CFST trusses intersects multiple standard domains:
- GB 50017 (Code for Design of Steel Structures) for steel tube geometry and connection design.
- GB 50010 (Code for Design of Concrete Structures) for concrete infill and prestress design.
- JGJ/T 175 (Technical Specification for Concrete-Filled Steel Tubular Structures) for CFST member design provisions.
- JTG D60 for prestress loss calculations in transportation infrastructure applications.
Key Reflections
The synergistic strengthening effect observed in this study has broader implications for the design philosophy of hybrid structural systems. Rather than treating prestressing and concrete infill as independent strengthening measures, engineers should consider their combined effect on the stress state, buckling resistance, and ductility of structural members. The finding that concrete infill converts a stability-governed failure into a strength-governed failure is particularly valuable for seismic design, where ductile failure modes are preferred. This work provides experimental validation for the use of prestressed CFST trusses in demanding long-span applications where both stiffness and strength requirements are stringent.