Zhuojin Pipe Fitting Co., LtdStatic Performance Analysis of Rectangular Steel Tube Truss with Concrete-Filled Compression Chord Members
Literature Overview
This paper by Liu Yongjian, Li Yunxi, Liu Junping, and Yang Genjie from Chang'an University and Jiangsu Transportation Research Institute, published in Journal of Architecture and Civil Engineering in 2008 (Vol. 25, Issue 4, pp. 65-72), investigates the static performance of rectangular steel tube truss beams where the compression chord members are filled with concrete. Supported by national western transportation construction technology projects and ministry of education talent support programs, the research employs nonlinear finite element analysis considering both material and geometric nonlinearities to evaluate stress distribution, deformation patterns, plastic development, and failure modes.
Core Technical Content
Structural Configuration and Design Rationale
The rectangular steel tube truss is a widely used structural system in bridge construction, particularly for medium-span bridges (20-80 m) where the truss configuration provides efficient load distribution and lightweight construction. The compression chord members in a simply supported truss beam are typically the top chords, which carry compressive axial forces and are susceptible to buckling failure.
Filling the compression chord tubes with concrete transforms the structural behavior in several important ways:
- Increased flexural stiffness: The concrete infill significantly increases the second moment of area of the chord members.
- Enhanced buckling resistance: The concrete confinement prevents local buckling of the steel tube walls and increases the overall member stability.
- Improved ductility: The composite action between steel tube and concrete provides a more gradual failure mode compared to bare steel tube buckling.
- Reduced member size: The composite action allows for lighter steel sections while maintaining or exceeding the capacity of unfilled alternatives.
Nonlinear Finite Element Analysis
The analysis incorporates both material nonlinearity (elastic-plastic constitutive models for steel and concrete) and geometric nonlinearity (P-delta effects, large deformations), which are essential for accurately predicting the behavior of steel tube truss structures under load.
Key modeling considerations:
- Steel material model: Bilinear or multilinear elastic-plastic model per GB 50017 or ISO 6892-1.
- Concrete material model: Drucker-Prager or Willam-Warnke criterion for triaxial stress states.
- Steel-concrete interface: Frictional contact elements or bonded interface elements to capture shear transfer.
- Node modeling: Spring elements or rigid regions to represent the actual connection behavior.
Key Findings
| Analysis Aspect | Empty Chord Tube | Concrete-Filled Chord Tube |
|---|---|---|
| Stress concentration at nodes | Significant | Minimal |
| Bearing capacity | Lower (bottleneck) | Substantially improved |
| Deformation at nodes | Large | Small |
| Plastic development | Initiated at empty tube nodes | Distributed more uniformly |
| Failure mode | Excessive plastic deformation at empty tube nodes | Improved overall ductility |
| Ductility | Moderate | Good |
The critical finding is that the empty compression chord tube nodes represent the structural bottleneck, with pronounced stress concentration that governs the overall truss capacity. Filling these nodes with concrete dramatically reduces stress concentration and deformation, effectively eliminating this bottleneck and improving the truss's overall ductility.
Engineering Practice and Design Recommendations
Application in Bridge Engineering
Rectangular steel tube trusses are extensively used in Chinese highway bridge construction, particularly for:
- Medium-span highway bridges (30-80 m spans).
- Bridge approaches and transition spans.
- Modular and prefabricated bridge systems.
- Bridges in regions with seismic activity, where ductility is paramount.
The concrete-filled compression chord approach is particularly beneficial for:
- Long-span trusses where compression chord buckling is a critical design consideration.
- Bridges in corrosive environments where steel tube local buckling due to corrosion-induced wall thinning is a concern.
- Bridges subject to heavy traffic loading where fatigue resistance of compression members is critical.
- Bridges in seismic zones where energy dissipation and ductility are required.
Comparison of Chord Member Configurations
| Configuration | Relative Cost | Relative Weight | Relative Capacity | Constructability | Durability |
|---|---|---|---|---|---|
| Empty steel tube chord | 1.0 | 1.0 | 1.0 | Excellent | Moderate |
| Concrete-filled chord | 1.15-1.25 | 1.3-1.5 | 1.4-1.8 | Moderate | Good |
| Partially filled chord | 1.10-1.20 | 1.15-1.35 | 1.2-1.5 | Good | Good |
| Steel tube with internal stiffeners | 1.10-1.15 | 1.10-1.20 | 1.15-1.30 | Moderate | Good |
Quality Control Considerations
For concrete-filled steel tube chord members, the following quality control measures are essential:
- Concrete placement: Use of low-slump or self-consolidating concrete (SCC) to ensure complete filling without voids.
- Concrete density verification: Gamma-ray scanning or ultrasonic testing to detect voids or incomplete filling.
- Steel tube preparation: Cleaning and rust removal of the internal surface to ensure proper bond between steel and concrete.
- Curing: Adequate curing period before applying structural loads, typically 28 days for full strength development.
- Welding quality: Full penetration welds at chord-brace connections, verified by ultrasonic testing per GB/T 11345.
Study Insights and Reflections
The nonlinear finite element analysis presented in this paper provides valuable insights into the structural behavior of concrete-filled rectangular steel tube trusses, but the practical implementation requires careful attention to several factors that the numerical model may not fully capture.
The finding that empty compression chord tube nodes are the structural bottleneck is consistent with the well-known phenomenon of stress concentration at tubular joints in steel structures. In full-scale testing of steel tube trusses, plastic deformation typically initiates at the weld toes of chord-brace connections, where the combined effects of axial compression, bending, and local stress concentration create a critical stress state. The concrete infill effectively redistributes these stresses over a larger area, reducing peak stresses and promoting more uniform stress distribution.
However, engineers should be aware that concrete-filled chord members introduce new failure modes that do not exist in empty tube members:
- Concrete cracking under tensile stresses at the chord-brace junction.
- Steel-concrete interface debonding under cyclic loading.
- Concrete spalling if the steel tube is insufficiently thick to provide confinement.
- Differential shrinkage between concrete and steel during curing, potentially creating internal stresses.
The research contributes significantly to the understanding of composite steel tube truss behavior, but further investigation into fatigue performance, seismic response, and long-term durability of concrete-filled chord members would strengthen the engineering basis for widespread adoption. The nonlinear finite element methodology employed in this study provides a robust analytical framework that can be extended to address these additional performance aspects.
For practicing engineers, the key takeaway is that concrete-filled compression chord members represent a highly effective means of enhancing rectangular steel tube truss performance, particularly in terms of capacity and ductility. The approach is most beneficial when the empty tube configuration would require significantly larger members to meet capacity requirements, making the concrete infill an economically attractive solution despite the additional material and construction costs.