Zhuojin Pipe Fitting Co., LtdDesign Optimization of Medium-Span Prefabricated Rectangular CFST Composite Truss Girder Bridge
Literature Overview
This paper by Liu Bin, Liu Yongjian, Zhou Xuhong, Li Zhou, and Wang Kangning, published in the Journal of Traffic and Transportation Engineering (2017, Vol. 17, No. 4, pp. 20-31), presents the design optimization of a novel prefabricated bridge structural system—the rectangular concrete-filled steel tube (CFST) composite truss girder bridge. The study covers overall design, main truss selection, cross-sectional design, deck slab selection, member selection, node selection and connection details, and includes finite element analysis of static performance, seismic response, effective width, and negative bending moment zone behavior. The research was supported by the National Natural Science Foundation of China (Grant 51378068), the National Key R&D Program (2016YFC0701202), and the Gansu Provincial Department of Transportation (Project 2016-34).
Design Optimization Framework
The paper proposes a systematic design optimization approach that addresses the following key aspects:
- Overall structural design: Selection of the continuous rigid frame system with a span arrangement suitable for medium spans.
- Main truss configuration: Optimization of the truss geometry to ensure primarily axial loading in members.
- Cross-sectional design: Selection of rectangular CFST members with appropriate dimensions and wall thicknesses.
- Deck slab design: Optimization of the concrete deck slab thickness and reinforcement.
- Member selection: Selection of member cross-sections based on load requirements.
- Node design and connection details: Development of efficient and constructible node connections.
The design philosophy emphasizes prefabrication and modular construction, which are key requirements for industrialized bridge construction in China.
Finite Element Analysis Results
The paper presents finite element analysis results for several critical aspects of the bridge design:
| Analysis Aspect | Key Finding |
|---|---|
| Static performance | Members primarily subjected to axial tension and compression |
| Seismic response | Initial input seismic force is 58.9% of equivalent prestressed concrete box girder bridge |
| Negative moment zone effective width coefficient | 0.899 |
| Partial composite connection effect | Deck slab axial tensile force reduced by 75.3% |
| Steel consumption ratio (vs. PC box girder) | 1.241 |
| Concrete consumption ratio (vs. PC box girder) | 0.485 |
| Superstructure mass ratio (vs. PC box girder) | 0.575 |
The seismic response finding is particularly noteworthy. The rectangular CFST composite truss girder bridge requires only 58.9% of the initial input seismic force compared to an equivalent prestressed concrete (PC) box girder bridge. This indicates that the truss system has a more favorable seismic response, likely due to the lighter superstructure mass and the ductile behavior of the steel members.
Partial Composite Technology for Negative Moment Zone
One of the most innovative aspects of this design is the application of partial composite technology in the negative bending moment zone of the deck slab. In a continuous rigid frame bridge, the negative moment zone at the interior supports experiences significant tensile forces in the top of the deck slab. Traditional design approaches require substantial reinforcement in this region, which can lead to cracking under service loads.
The partial composite connection design reduces the axial tensile force in the deck slab by 75.3%, which is a dramatic improvement. This is achieved by designing the shear connectors between the deck slab and the steel truss members to transfer only a portion of the shear force, allowing the deck slab to act partially independently. This reduces the restraint on the deck slab and minimizes cracking.
Welding and Fabrication Considerations
The prefabricated nature of the rectangular CFST composite truss girder bridge places significant demands on the welding and fabrication processes. The following welding-related considerations are critical:
- Longitudinal welds of rectangular tubes: The rectangular CFST members are fabricated by rolling steel plates into rectangular sections and welding the longitudinal seams. These welds must be designed and executed to ensure full penetration and adequate fatigue resistance. The weld design should account for the cyclic loading conditions in bridge applications.
- Circumferential welds at member ends: The truss members are connected at nodes through circumferential welds or bolted connections. The weld details at these locations are critical for fatigue performance, as they are subjected to stress concentrations.
- PBL stiffener welds: As discussed in the previous topic, the PBL stiffeners inside the rectangular tubes require careful welding to ensure proper shear transfer between the steel tube and the concrete core.
- Node weld details: The node connections are the most critical welding details in the truss structure. The weld design should minimize stress concentrations at the weld toes, and the weld geometry should be optimized for fatigue performance.
The prefabrication approach requires that all welding be performed in a controlled shop environment, which offers several advantages over field welding:
- Better control of welding parameters and environmental conditions.
- Easier implementation of NDT procedures.
- More consistent weld quality.
- Reduced weather-related defects.
However, the prefabrication approach also introduces challenges related to the transportation and erection of large prefabricated modules, which must be carefully planned to avoid damage during handling and installation.
Economic and Technical Comparison
The economic comparison presented in the paper is particularly valuable for decision-making. The rectangular CFST composite truss girder bridge uses 1.241 times the steel but only 0.485 times the concrete compared to an equivalent PC box girder bridge. The superstructure mass is reduced to 0.575 of the PC box girder bridge. This indicates that while the steel consumption is higher, the overall material efficiency is improved due to the significant reduction in concrete and superstructure mass.
The 41.5% reduction in superstructure mass also has implications for the substructure design, as lighter superstructures require smaller piers and foundations. This can lead to significant overall cost savings, particularly in areas with difficult foundation conditions.
Study Insights and Reflections
This paper presents a comprehensive design methodology for a novel prefabricated bridge structural system that combines the advantages of steel and concrete in an efficient and constructible manner. The application of partial composite technology in the negative moment zone is a creative solution to a common design challenge. The economic comparison demonstrates the competitive advantage of the rectangular CFST composite truss girder bridge in terms of material efficiency and constructability.
From a welding perspective, the prefabricated nature of the system offers the opportunity to achieve high weld quality in a controlled environment. However, the welding engineer must carefully consider the fatigue performance of the weld details, particularly at the node connections and the PBL stiffener welds. The use of appropriate welding processes, such as GTAW (gas tungsten arc welding) for root passes and FCAW (flux-cored arc welding) or SAW (submerged arc welding) for subsequent passes, along with rigorous NDT, is essential to ensure the long-term durability of the structure.
In summary, this paper provides a valuable contribution to the development of prefabricated bridge systems in China. The rectangular CFST composite truss girder bridge offers a compelling combination of structural efficiency, seismic performance, and constructability. The welding and fabrication aspects, while not the primary focus of the paper, are critical to the successful implementation of this structural system and should be given due attention in the design and construction phases.