An integral bridge is a unique type of bridge construction in which there are no joints between the spans or between the span and the abutments. Instead, the superstructure and the abutments act as a single, unified structural element. This design eliminates the need for expansion joints and bearings, which are typically used in traditional bridges to accommodate thermal movements and other stresses. In integral bridges, the entire structure is monolithic, which contributes to better performance and reduced maintenance over time.
Types of Integral Bridges
Integral bridges can be classified based on the types of abutments they use. These include:
- Integral Bridge with Frame Abutments
- Integral Bridge with Flexible Support Abutments
- Integral Bridge with Bank Pad Abutments
- Integral Bridge with Semi-Integral End Screen Abutments
Each type has its own set of design characteristics and suitability for different construction conditions.
Detailed Explanation of Each Type
1. Integral Bridge with Frame Abutments
In an integral bridge with frame abutments, the structure behaves like a portal frame. The moments, shear forces, and axial loads are transferred directly from the bridge deck to the supporting structure. The frame abutments also serve as retaining walls, keeping backfill in place. This type of abutment is typically founded on spread footings or embedded wall footings to provide stability.
One of the key challenges in this design is dealing with thermal expansion and contraction. As the bridge expands or contracts due to temperature changes, the beams of the deck move horizontally. To accommodate this displacement without damaging the foundation, the abutments need to be flexible. The use of reinforced concrete abutments is not ideal here; instead, steel sheet piles are often employed to allow the necessary flexibility.
2. Integral Bridge with Flexible Support Abutments
Flexible support abutments are designed to allow the piles to move horizontally during thermal expansion or contraction. The key feature of this design is the post holes created around the piles. These holes provide enough space for the piles to move without being restricted by the surrounding soil or foundation. The post holes are usually lined with materials such as precast concrete rings for larger piles or UPVC and polythene piping for smaller piles.
This design eliminates soil interaction with the pile, ensuring that the pile can move freely. Additionally, a duct is installed for inspecting the durability of the pile inside the post hole. The movement of the pile is further facilitated by ensuring that the holes around the piles are not filled with any material and that they do not come into contact with the end screen wall.
3. Integral Bridge with Bank Pad Abutments
Bank pad abutments are a variation where the end supports are fully integral with the deck beams. However, unlike other designs, these supports are not fixed into the ground. Instead, they are allowed to slide and rotate, accommodating movements caused by thermal expansion or contraction. The ability to slide and rotate allows the abutments to adjust to the forces acting on them, but it can affect the bearing capacity of the soil. To mitigate this, the bearing pressure of the soil at the serviceability limit state is kept lower than typical static values.
The bank pad abutments include an end screen wall behind which backfill is placed. The width of the end screen wall is designed to match the width of the pavement above it. This design is especially beneficial when pile foundations are used, as it minimizes soil interaction with the structure.
4. Integral Bridge with Semi-Integral End Screen Abutments
In this type of bridge, the end screen wall and deck beams are integrated, but the end screen wall does not directly support the deck beams. Instead, bearings that can accommodate horizontal displacement are used to support the deck. This design significantly reduces the soil-substructure interaction, making it particularly suitable in areas where such interaction might cause problems.
This type of bridge is ideal when there are concerns about the soil’s ability to provide stable support or when minimizing interaction between the soil and substructure is a priority.
Advantages of Integral Bridges
Integral bridges offer several significant advantages over conventional bridges:
- Lower Construction and Maintenance Costs: Integral bridges are simpler to construct, as they don’t require expansion joints or bearings. This results in reduced material and labor costs, as well as lower maintenance costs over the lifespan of the bridge.
- Rapid Construction: The elimination of joints and bearings speeds up the construction process. The simplicity of the design allows for a quicker and more efficient building timeline.
- Reduced Tolerance Issues: Without expansion joints and bearings, integral bridges have fewer concerns regarding tolerance restrictions. The structure’s monolithic design ensures smoother operation.
- Use of Existing Foundations: When integral bridges are constructed to replace an older bridge, the foundation of the previous bridge can often be reused. This reduces the overall project cost.
- Prevention of Water Leakage: Water leakage is a major concern for conventional bridges, but integral bridges can be designed with drainage layers behind the abutments to prevent leakage onto critical structural elements.
- Improved Riding Quality: The absence of expansion joints leads to a smoother, more comfortable ride for vehicles crossing the bridge. The continuous deck surface offers a more uniform ride quality.
Limitations of Integral Bridges
Despite their many advantages, integral bridges come with certain limitations:
- Limited Suitability in Certain Climate Zones: Integral bridges are not ideal in regions where temperature variations cause expansions or contractions greater than 51mm. The design may not be able to accommodate such large movements without risking structural damage.
- Challenges with Poor Soil Strength: These bridges are less suitable for areas with weak or poor-quality subsoil or embankments. The performance of the bridge can be compromised if the foundation soil is not stable or strong enough to support the structure.
- Dependence on Geometry and Materials: The geometry of the bridge and the materials used for construction play a critical role in the displacement effects that occur during temperature changes. Careful consideration of these factors is essential to avoid excessive movement.
- Risk of Plastic Hinges in Piles: The expansion and contraction forces may result in the formation of plastic hinges in the piles, which can reduce the axial load capacity. This issue requires careful foundation design to prevent structural failure.
- Length Limitations: Integral bridges are generally most effective for shorter spans. For steel girder bridges, the recommended maximum length is around 40 meters, while concrete girder bridges can typically extend up to 50 meters.
Conclusion
Integral bridges offer a cost-effective, efficient, and robust solution for many types of bridge construction projects. With the elimination of joints and bearings, they provide smoother operations, reduced maintenance, and simplified construction processes. However, it is crucial to consider environmental conditions, soil quality, and design specifics to ensure that an integral bridge is the best choice for a given project. By carefully balancing these factors, engineers can maximize the benefits of integral bridges while minimizing their limitations.