A box girder bridge is a type of bridge where the main beams consist of girders shaped like hollow boxes. These girders are often constructed from materials like prestressed concrete, structural steel, or a composite of steel and reinforced concrete. The closed-cell structure of a box girder provides enhanced torsional stiffness and strength, which is one of the main reasons for its widespread use in bridge construction. In this article, we will explore the design features, construction methods, applications, and benefits of box girder bridges.
Design and Structure
A box girder is formed by two vertical web plates joined by a common flange at both the top and the bottom, creating a closed cell. This unique design significantly improves torsional stiffness, offering superior strength compared to open-section beams. In addition to its strength, the design of the box girder often incorporates material reduction to minimize the dead load. By removing unnecessary material from the section, the box girder structure becomes more efficient, which is especially important for larger spans.
Box girders are typically used when longer spans are needed, as they offer an effective solution for structures with considerable loads. This design also allows for the optimization of material use, making box girder bridges an attractive choice for engineers focused on both strength and economy.
Advantages of Box Girder Bridges
One of the primary advantages of box girder bridges is their ability to span longer distances compared to other types of bridges, such as T-beam girder bridges. A box girder bridge typically requires fewer piers, reducing the overall cost and increasing the span range. This allows for the construction of more economical bridges with fewer support points.
Moreover, box girders are ideal for larger spans and wider decks. They are highly adaptable, with the ability to evolve over time into more efficient designs. For instance, cantilever designs on top flanges and inclined webs in external cells enhance the aesthetic appeal and structural efficiency of the bridge. The flexibility of the box girder design means it can be used for spans up to 150 meters, depending on the construction method.
Construction Methods
There are several methods used in the construction of box girder bridges, each chosen based on the project requirements, location, and span length.
- Cast-in-Place Construction Box girders can be constructed on-site using falsework. This method involves placing temporary supports (falsework) to hold the girders in place while the concrete is cast. Once the structure is complete, the falsework is removed. This method is particularly useful for bridges that are difficult to access or for situations where prefabricated elements are not feasible.
- Prefabricated Box Girders In some cases, box girders are fabricated in a dedicated fabrication yard and then transported to the construction site. Cranes are used to install the pre-made sections, which can speed up the construction process and reduce the on-site labor required.
- Incremental Launching Method The incremental launching method involves the use of gantry cranes to incrementally place new segments of the box girder onto the completed portions of the bridge. This method is often used for bridges over large distances or in locations with difficult access, as it allows for precise placement of the sections.
- Cantilever Construction Method In cantilever construction, the bridge is built one segment at a time, with sections extending outward from the supports. This method is particularly advantageous for bridges over deep or wide valleys and is used for long-span bridges. Images of cantilever and incremental launching methods can illustrate how these techniques work in practice.
Specifications
Box girder bridges can cover a wide range of spans, from 25 meters for smaller structures up to 300 meters for larger non-suspended concrete decks. For spans longer than 50 meters, box girders are often the most feasible and cost-effective choice. Single-cell box girders are commonly used for spans ranging from 40 meters to 270 meters, providing both strength and an aesthetically pleasing appearance.
The design of box girders can be tailored to provide a slender, elegant profile. The web of the box girder gives it a sleek, minimalist look when combined with a slim parapet profile. Additionally, box girders are highly effective for both longitudinal and transverse load distribution, making them suitable for a variety of medium and long-span structures.
Construction techniques vary depending on the length of the bridge, with span-by-span construction or balanced cantilever methods often employed for bridges in the 40m to 55m range.
Applications of Box Girder Bridges
Box girder bridges are particularly useful for modern elevated structures, such as those found in light rail transport systems. They are also commonly used for cable-supported bridges, curved bridges, and footbridges. While box girders are rarely used in building construction, they can be an ideal solution when loads are carried eccentrically to the beam axis.
In cable-supported bridges, the box girder provides the necessary strength and torsional rigidity to handle the complex loading conditions. Similarly, curved bridges benefit from the resistance to torsion offered by the box girder’s closed-cell structure, making them the preferred option for bridges that need to follow non-linear paths.
Benefits of Box Girder Bridges
- Economical and Aesthetic Solution
Box girders offer a cost-effective and visually appealing solution for overpasses, underpasses, and grade separation structures in modern highway systems. The streamlined design can be customized for various aesthetic preferences, while the structural efficiency keeps costs manageable. - Large Span Capability
Box girders allow for wider decks, which are ideal for accommodating prestressing cables and heavy traffic loads. This makes box girder bridges particularly useful for long-span projects. - Versatility in Use
The internal space of box girders can be used to house utilities such as gas pipes and water mains, providing a dual function for the bridge structure. - Easy Maintenance
The interior space of box girders is easily accessible for maintenance without the need for scaffolding, which reduces costs and improves the efficiency of repairs. - High Structural Efficiency and Torsional Resistance
Box girders have exceptional resistance to torsion, making them particularly well-suited for curved bridges. Their long-span capabilities reduce the need for multiple support points, further contributing to cost savings.
Conclusion
Box girder bridges are a modern and efficient solution for a variety of bridge construction projects, from highway overpasses to rail transit systems. Their unique design offers significant structural advantages, including high torsional stiffness, reduced material use, and the ability to handle long spans. With various construction methods available, such as incremental launching and cantilever construction, box girder bridges can be adapted to suit a wide range of project requirements. Furthermore, their aesthetic appeal, economic benefits, and ease of maintenance make them an ideal choice for engineers and designers working on large infrastructure projects.