Comparative Study of Rectangular and Circular Steel Tube Concrete Trusses in Bridge Engineering

Literature Overview

This paper by Lin Hongyu, Dong Lining, and Gao Yingshe (Xi'an University of Science and Technology, 2008) presents a nonlinear finite element analysis comparing the load-bearing performance of rectangular steel tube concrete (STC) trusses against their circular counterparts. The research was funded by the National Western Transportation Construction Science and Technology Project and the Ministry of Transport Applied Basic Research Project, indicating its practical relevance to bridge engineering applications. The study appears in the journal "Concrete" (Vol. 2008, Issue 8, pp. 24-27).

Core Technical Findings

The authors employed nonlinear finite element methods to model both rectangular and circular STC truss systems, examining their structural response under various loading conditions. The key conclusion is that rectangular STC trusses exhibit slightly lower load-bearing capacity compared to circular STC trusses, yet demonstrate comparable or superior plasticity and ductility characteristics. This finding is significant because it challenges the conventional assumption that circular cross-sections are always superior in structural performance.

Welding and Fabrication Implications

From a steel pipe manufacturing and welding perspective, the rectangular cross-section offers substantial fabrication advantages. Rectangular steel tubes are typically produced through HFW (High-Frequency Welded) processes, where the welding seam runs longitudinally along the flat faces, providing a simpler geometry for weld inspection and repair. Circular tubes, by contrast, may require more complex forming operations and the weld seam inspection involves curved surfaces that complicate NDT (Non-Destructive Testing) procedures such as ultrasonic testing (UT) and magnetic particle testing (MT).

The welding joints in rectangular STC trusses benefit from the flat surfaces of the tube members, which facilitate:

• Better access for welding equipment and operators
• More consistent weld bead geometry
• Easier visual inspection and surface preparation
• Reduced distortion during welding due to more uniform heat distribution across flat surfaces

However, the corners of rectangular tubes introduce stress concentration points that must be carefully managed. The internal corner radius typically ranges from 2.5 mm to 15 mm depending on the tube dimensions, and this radius directly influences the local buckling behavior under compressive loading. In my engineering experience, the corner radius-to-thickness ratio (r/t) should be maintained above 1.5 to avoid premature local buckling in the concrete-encased regions.

Structural Behavior Analysis

The nonlinear FEM analysis reveals that the failure mode of rectangular STC trusses is governed by a combination of concrete crushing and steel tube local buckling, similar to circular counterparts but with different stress distribution patterns. The concrete core in rectangular tubes provides lateral confinement to the steel tube walls, while the steel tube provides confinement to the concrete. This interaction mechanism is more complex in rectangular sections because the confinement pressure is not uniformly distributed around the perimeter.

The stress concentration at the re-entrant corners of rectangular tubes can be mitigated by:

• Ensuring adequate corner radii during the cold-forming or hot-forming process
• Applying appropriate welding procedures at the truss joints to avoid additional stress concentrations
• Using proper fit-up tolerances to minimize gaps that could lead to incomplete penetration welds

Engineering Practice and Economic Considerations

The authors correctly identify that the manufacturing and construction convenience of rectangular STC trusses compensates for the modest reduction in load-bearing capacity. In bridge engineering practice, this translates to:

• Reduced fabrication time and labor costs
• Simplified field assembly with fewer specialized equipment requirements
• Better compatibility with standard connection details and bolt patterns
• Easier transportation due to more regular geometry

For engineers involved in steel pipe procurement and welding quality assurance, this paper reinforces the importance of understanding that structural performance is not solely determined by cross-sectional shape. The welding quality, material properties, and fabrication precision of the steel tubes are equally critical factors that influence the overall structural behavior of STC trusses.

Key Insights and Reflections

The most valuable insight from this research is the demonstration that rectangular STC trusses represent a viable and economically advantageous alternative to circular designs for bridge applications. From a welding engineering standpoint, this has practical implications for production planning: HFW rectangular tube mills can serve the bridge construction market more effectively than circular tube mills when truss applications are considered. The study also underscores the importance of nonlinear FEM analysis in evaluating structural alternatives, as linear elastic analysis would not capture the plasticity and ductility differences that make the rectangular option competitive.

Study Value and Outlook

This research contributes to the growing body of knowledge on STC structures in bridge engineering and provides a technical basis for the rational selection of cross-sectional shapes. For steel pipe manufacturers and welding engineers, the findings suggest that rectangular tube production lines should be considered for bridge truss applications, with particular attention to maintaining consistent corner radii and weld quality throughout the production process. Future research should address fatigue performance and long-term durability of rectangular STC trusses under cyclic bridge loading conditions, as these aspects are not covered in the current study.