Design Optimization of Prefabricated Rectangular CFST Composite Truss Girder Bridge for Medium Span

Literature Overview and Design Philosophy

This paper by Liu Bin, Liu Yongjian, Zhou Xuhong, Li Zhou, and Wang Kangning, published in Journal of Traffic and Transportation Engineering (2017, Vol. 17, No. 4, pp. 20-31), presents a comprehensive design methodology for a prefabricated rectangular steel tube concrete composite truss girder bridge targeting medium-span applications. The design optimizes the traditional concrete box girder by replacing the web and bottom slab with a rectangular CFST truss system, enabling prefabricated construction and rapid assembly. The study covers overall design, main truss selection, cross-section selection, bridge deck selection, member selection, and joint selection with connection details. Finite element analysis was conducted to evaluate static performance, seismic response, deck effective width, and negative moment region behavior. The research was supported by the National Natural Science Foundation of China (51378068), the National Key R&D Program (2016YFC0701202), and the Gansu Provincial Department of Transportation Research Project (2016-34).

Structural Design Optimization Process

The design optimization follows a systematic approach that begins with the recognition that traditional concrete box girders suffer from heavy self-weight, long construction time, and limited prefabrication potential. The proposed rectangular CFST composite truss girder bridge addresses these limitations by utilizing the high strength-to-weight ratio of steel tubes and the composite action with concrete to create a lightweight, rapidly constructible structural system.

The main truss members are designed to carry primarily axial tension and compression loads, which is an efficient structural form that maximizes material utilization. The rectangular CFST members are sized to optimize the balance between structural capacity, material economy, and fabrication practicality. The cross-section design considers the interaction between the main trusses, the bridge deck, and the transverse distribution beams to ensure efficient load transfer.

Partial Composite Technology in Negative Moment Region

One of the most significant innovations presented in this paper is the application of partial composite technology in the negative moment region of the bridge deck. In the negative moment zone, typically located above the supports, the bridge deck experiences tensile stresses in the top fibers. By incorporating steel reinforcement or composite action in this region, the axial tension in the bridge deck is reduced by 75.3%, significantly improving the crack resistance of the concrete deck.

From a steel pipe and welding perspective, the partial composite technology in the negative moment region requires careful attention to the welding of shear connectors and reinforcement details. The shear connectors, typically stud welders, must be welded to the top flange of the rectangular steel tube with adequate weld strength to transfer the longitudinal shear forces between the steel tube and the concrete deck. The welding of stud shear connectors to the top flange of rectangular tubes introduces local thermal effects that can cause distortion of the tube flange, particularly near the longitudinal welds where the material has already experienced welding thermal cycles.

The negative moment region design also requires consideration of the fatigue performance of the welded connections. The bridge deck is subjected to repeated traffic loading that induces cyclic stresses in the shear connectors and the top flange welds. The weld details must be designed to minimize stress concentration, with smooth transitions and full-penetration welds where possible. Fatigue assessment per EN 1993-2 or GB 50017 should be conducted for all welded connections in the negative moment region.

Seismic Performance and Steel Tube Design

The finding that the rectangular CFST composite truss bridge has an initial seismic input force of only 58.9% of an equivalent prestressed concrete box girder bridge is a compelling argument for the seismic resilience of this bridge type. The reduced superstructure mass (57.5% of the box girder) directly reduces the seismic inertial forces, while the ductile nature of steel tubes provides additional energy dissipation capacity during seismic events.

For the steel pipe manufacturer, the seismic design requirements impose additional constraints on the tube fabrication and welding processes. The steel tubes must exhibit adequate ductility, with elongation values meeting or exceeding the specified minimum (typically 20% for Q345 steel and 18% for Q390 steel). The weld joints must be designed to be at least as ductile as the base metal, requiring full-penetration butt welds with proper weld preparation, adequate heat input, and post-weld inspection. The welding procedure specifications must be qualified for seismic applications, with attention to weld toughness, hydrogen control, and residual stress management.

Economic and Technical Comparison

The economic analysis presented in the paper demonstrates that the rectangular CFST composite truss bridge offers significant advantages over traditional prestressed concrete box girders. The steel consumption is 24.1% higher, but the concrete consumption is 51.5% lower and the superstructure mass is 42.5% lower. The reduced mass translates to lower foundation costs, faster construction, and reduced seismic forces. The prefabrication capability enables factory production of the steel tube members with PBL stiffeners, followed by on-site assembly and concrete casting, significantly reducing construction time.

The prefabrication approach has important implications for the steel pipe manufacturing industry. Factory fabrication of rectangular CFST members allows for controlled welding environments, consistent quality control, and efficient use of welding equipment and inspection resources. The modular design enables standardized tube dimensions and connection details, facilitating mass production and reducing unit costs. However, the prefabrication approach also requires careful planning of the transportation and erection logistics, as the prefabricated members must be designed to survive transport stresses and to be easily connected on site.

Engineering Practice and Quality Control

The design methodology presented in this paper should be accompanied by a rigorous quality control program for the steel tube fabrication and welding processes. The following quality control measures are recommended:

1. Steel plate procurement: verify mill certificates for chemical composition, mechanical properties, and non-metallic inclusions; perform supplementary impact testing for seismic applications
2. Tube fabrication: maintain dimensional tolerances per the design specifications; inspect all welds by RT and UT; measure residual stresses on representative samples
3. PBL stiffener installation: verify stiffener plate quality and welding; inspect PBL welds by MT and spot UT; verify concrete key formation after casting
4. Prefabricated member inspection: complete dimensional survey, weld inspection, and hydrostatic testing before shipment
5. On-site connection welding: qualify welding procedures for field conditions; inspect all field welds by RT, UT, and MT; verify weld quality before concrete casting
6. Post-casting inspection: verify concrete fill quality; inspect tube surfaces for any damage during construction; document as-built condition

Study Insights and Reflections

This design optimization study demonstrates that the rectangular CFST composite truss girder bridge is a technically viable and economically competitive alternative to traditional prestressed concrete box girders for medium-span applications. The combination of prefabrication, partial composite technology, and the inherent advantages of steel tubes in seismic performance creates a compelling case for the widespread adoption of this bridge type. For the steel pipe and welding industry, this work signals a significant market opportunity in the production of high-quality rectangular steel tubes with integrated PBL stiffener systems. The industry must invest in fabrication capabilities, welding expertise, and quality control systems to meet the demanding requirements of this emerging bridge technology. The success of these bridges will ultimately depend on the reliability and consistency of the steel tube fabrication and welding processes, making quality assurance a non-negotiable component of the supply chain.