Prestressed concrete continuous box girder bridges have become a preferred solution in modern bridge engineering due to their excellent load-bearing capacity and structural efficiency. This study focuses on the structural design of such bridges, aiming to explore how to achieve safety, economy, aesthetics, and efficient construction of bridges by optimizing span selection, cross-sectional configuration, prestressing tendon arrangement, and support system design. 

1. General Design Principles

The design of prestressed concrete continuous box girder bridges shall comply with a series of general principles to ensure structural safety, economy, aesthetics, and constructability. These principles form the cornerstone of bridge design and guide the advancement of the entire design process. 

(1) Safety: The primary principle is to ensure the safety of the bridge structure. During design, various possible load conditions (including dead load, live load, temperature load, wind load, seismic load, etc.) shall be fully considered. Through accurate calculation and analysis, it shall be ensured that the bridge meets the requirements for load-bearing capacity and stability both under normal service conditions and extreme load actions. In addition, the durability of the structure shall be considered, and appropriate anti-corrosion and anti-rust measures shall be adopted to extend the service life of the bridge. 

(2) Economy: On the premise of ensuring safety, the design shall focus on economy. The project cost shall be reduced through reasonable measures such as span selection, cross-section optimization, and material conservation. At the same time, construction convenience shall also be taken into account to reduce construction difficulty and costs. 

(3) Aesthetics: As public infrastructure, the aesthetics of bridges is also an important factor that cannot be ignored. During design, attention shall be paid to the coordination between the bridge and the surrounding environment, and techniques such as concise and smooth lines and harmonious color matching shall be adopted to make the bridge a beautiful scenic spot. 

(4) Constructability: The design shall fully consider the feasibility of construction conditions and construction technologies. A reasonable structural design can not only reduce construction difficulty and improve construction efficiency but also ensure construction quality. Therefore, full communication with the construction unit shall be conducted during the design process to ensure the implementability of the design scheme. 

2. Span and Cross-Section Design 

(1) Selection of Span Ratio The span ratio is a key parameter in bridge design, which directly affects the mechanical performance and economic benefits of the bridge. The selection of the span ratio shall comprehensively consider factors such as topography, traffic flow, navigation requirements, and construction conditions. Generally speaking, in areas with relatively flat terrain and large traffic flow, the span ratio can be appropriately increased to reduce the number of piers and improve driving comfort; while in areas with complex terrain and high navigation requirements, the span ratio shall be appropriately reduced to meet the requirements for navigation clearance. The reasonable range of the span ratio shall be determined based on specific project conditions through calculation, analysis, and comparison. 

(2) Cross-Section Design Cross-section design is a crucial part of bridge structural design, which is directly related to the load-bearing capacity and stability of the bridge. The key elements of cross-section design include beam height, top slab thickness, bottom slab thickness, and web thickness. - The selection of beam height shall meet the requirements for strength and stiffness, while considering construction convenience and economy. - The top slab thickness shall be sufficient to bear the transmission of the deck pavement layer and vehicle load. - The bottom slab thickness shall meet the requirement for uniform distribution of compressive stress. - The web thickness shall consider shear transmission and local stability. During design, the recommended range and design method can be provided based on code requirements and calculation analysis results. 

3. Support and Support System Design 

(1) Support Setting Principles and Type Selection Supports are important force-transmitting components in bridge structures, which connect the bridge superstructure and piers/abutments and transmit the loads on the bridge superstructure to the piers/abutments. The setting of supports shall follow the principles of reasonable force transmission and clear load path. When selecting the support type, factors such as structural form, load characteristics, temperature changes, and seismic actions shall be comprehensively considered. Common support types include fixed supports, sliding supports, and rolling supports, which shall be selected according to specific conditions during design. 

(2) Influence of Supports on Bridge Force and Stability The performance of supports directly affects the mechanical performance and stability of the bridge. Reasonable support setting can reduce the deformation and displacement of the bridge superstructure and improve the overall stiffness of the structure; while unreasonable support setting may lead to uneven force on the structure, excessive stress concentration and deformation, and even structural damage. Therefore, the influence of supports on bridge force and stability shall be fully considered in support design. 

 (3) Design Ideas and Methods of Support Systems The design of support systems shall follow the overall design principle to ensure the coordination and consistency between supports and the bridge structure. During design, factors such as the arrangement position, quantity, and type of supports, as well as the connection method with piers/abutments and the bridge superstructure, shall be considered. At the same time, necessary mechanical analysis and verification of the supports shall be conducted to ensure their safety and reliability under various working conditions.

  4. Arrangement of Prestressing Tendons 

 (1) Principles and Methods of Prestressing Tendon Arrangement The arrangement of prestressing tendons is a key link in the design of prestressed concrete bridges. Reasonable arrangement of prestressing tendons can effectively improve the load-bearing capacity and crack resistance of the bridge. When arranging prestressing tendons, the principles of clear force transmission and uniform distribution shall be followed to ensure that the prestress can be transmitted to the concrete in accordance with the design requirements. The arrangement forms of prestressing tendons include longitudinal prestressing tendons, transverse prestressing tendons, and vertical prestressing tendons: - Longitudinal prestressing tendons are mainly used to improve the longitudinal load-bearing capacity and crack resistance of the bridge superstructure. - Transverse prestressing tendons are used to improve the transverse stiffness and stability of the structure. - Vertical prestressing tendons are used to control stress concentration and deformation in local areas. 

(2) Friction Loss and Anchorage Efficiency of Prestressing Tendons During the tensioning process of prestressing tendons, they will be affected by various resistances, resulting in friction loss, which will affect the actual tensioning effect and anchorage efficiency of the prestressing tendons. To reduce friction loss and improve anchorage efficiency, corresponding measures shall be taken during design, such as selecting tensioning equipment with a low friction coefficient, optimizing the arrangement form of prestressing tendons, and strengthening the structural treatment of the anchorage zone. 

5. Arrangement of Diaphragms 

 (1) Functions of Diaphragms Diaphragms are important transverse connecting components in bridge structures, whose main functions include improving the overall stiffness of the bridge, restricting distortional deformation, and transmitting transverse shear force. Through reasonable arrangement of diaphragms, the integrity and stability of the box girder structure can be effectively enhanced, the load transmission path can be optimized, and stress concentration can be reduced, thereby improving the load-bearing capacity and durability of the bridge. 

(2) Arrangement Positions of Diaphragms 

1. At Supports: Diaphragms are usually arranged at each support of the box girder to enhance the transverse stiffness of the structure near the supports and restrict the generation of distortional stress. For curved box girders, when the inner radius is within a small range (e.g., less than 240m), additional diaphragms shall be arranged between spans to further improve the overall performance of the structure. 

2. At Mid-Span and Other Necessary Positions: In addition to the support positions, diaphragms may also need to be arranged at mid-span or other necessary positions according to the span and width of the bridge. The arrangement of these diaphragms shall be determined through detailed structural analysis to ensure the stability and safety of the overall bridge structure. 

(3) Thickness Requirements of Diaphragms 

The thickness of diaphragms shall meet the force requirements and consider construction convenience and economy. Generally speaking: 

• The thickness of diaphragms at side supports usually ranges from 0.8m to 1.2m.

• The thickness of diaphragms at middle supports shall be determined through calculation based on force conditions such as the position of the supports, generally reaching more than 1.2m, and shall also meet the requirements of structural details.

For prestressed concrete continuous box girder bridges, the thickness of diaphragms shall also consider the arrangement and anchorage requirements of prestressing tendons to ensure that the prestress can be effectively transmitted and exerted. In addition, the arrangement of diaphragms shall also consider the coordination and integrity with other structural components. During the design process, coordination with components such as the main girder and supports shall be conducted to ensure that the entire bridge structure has reasonable force transmission, clear load paths, and good stability.

Conclusion

In summary, this study conducts a comprehensive and in-depth discussion on the structural design of prestressed concrete continuous box girder bridges. Through span optimization, refined cross-section design, scientific arrangement of prestressing tendons, and innovative design of support systems, the load-bearing capacity, durability, and construction efficiency of the bridge are significantly improved. These research results not only enrich the design theory of prestressed concrete bridges but also provide strong support for practical engineering applications, indicating that this technology will play a more important role in the field of bridge construction in the future.