Bridge construction has evolved significantly over the years, and one of the most innovative techniques in modern bridge engineering is the balanced cantilever method. This method is particularly suitable for constructing bridges with spans ranging from 50 to 250 meters. In this article, we will explore the key features, benefits, and procedures of the balanced cantilever method, including the differences between cast-in-place and precast segments.
I. Introduction to the Balanced Cantilever Method
The balanced cantilever method of bridge construction involves the use of cantilever segments that are constructed alternately on opposite ends of piers. This technique is adaptable to a variety of bridge designs, including cast-in-place and precast options. Its primary advantage lies in its ability to build long-span bridges without requiring auxiliary supports such as scaffolding or falsework. This method is especially suitable for projects in challenging environments such as congested urban areas, rough terrain, water bodies, and areas with environmental sensitivities. Additionally, it is a preferred method for cable-stayed bridges due to its inherent ability to support long spans with minimal external assistance.
II. Concept of the Balanced Cantilever Method
The balanced cantilever method involves constructing segments on cantilevers that extend from opposite sides of the piers, creating a balanced structure. This method allows for flexibility in dealing with irregular terrain, varying span lengths, and challenging construction sites. The segments are placed one at a time, either in a cast-in-place or precast manner, and as each segment is added, it is supported by new stays (in the case of cable-stayed bridges). No external scaffolding or temporary supports are necessary, which makes it both cost-effective and efficient for building long-span bridges in difficult conditions.
The balanced cantilever method is especially advantageous in locations with limited access or environmental restrictions. It eliminates the need for large cranes or temporary supports in sensitive ecosystems or busy urban environments.
III. Types of Cantilever Methods
There are two primary approaches for using the cantilever method in bridge construction: cast-in-situ segments and precast segments. Each method has its own advantages and is chosen based on the project’s requirements, such as time constraints, span length, and site conditions.
1. Cast-in-situ Segment Construction
In the cast-in-situ method, segments are poured and constructed directly on-site. This method is typically employed when large, heavy segments are required, as it allows for greater flexibility in segment size and structure.
The construction process begins with the preparation of the lower infrastructure and the erection of the piers, which act as the foundation for the bridge. Special formwork is positioned to begin casting the pier segments, which then serve as platforms for the form travelers that will carry out the casting of the bridge segments. The form travelers move along the cantilever arms, casting segments as they go. Soffit shuttering, web shuttering, and deck shuttering are all installed to ensure that the segments take their correct shape.
Once each segment is formed, the concrete is poured on both sides of the pier. Cast-in-situ segments typically range in length from 3 to 5 meters and are cast every five days per form traveler. The construction continues until the midpoint of the bridge is reached, where the cantilevers meet and close the gap. The final section is then constructed, completing the bridge.
2. Precast Segment Construction
In the precast method, segments are made off-site in a controlled environment and transported to the bridge construction site. The procedure begins with the construction of the foundation and pier shafts for the permanent piers, which will support the precast segments.
Once the piers are ready, a lifting frame is used to transport and place the precast segments onto the piers. These segments are then joined with wet joints, which are cured to ensure the proper bonding between segments. This process is repeated for each pair of segments until the bridge is completed.
One key advantage of the precast method is the faster construction timeline. Precasting in a controlled environment leads to higher-quality segments, and the faster rate of production allows the bridge to be completed more quickly than with cast-in-situ methods.
IV. Procedure for Balanced Cantilever Construction
1. Cast-in-situ Bridge Construction Procedure
The process for constructing a bridge using the cast-in-situ balanced cantilever method is as follows:
- Step 1: The construction begins with the preparation of the lower infrastructure, including the foundation and piers (Fig. 3).
- Step 2: Special formwork is positioned on the piers to begin casting the first pier segments (Fig. 4). These segments act as platforms for form travelers, which will be used to construct the cantilever segments.
- Step 3: Once the pier segments are in place, soffit shuttering, web shuttering, and deck shuttering are installed (Figs. 5 and 6). This ensures that the segments are formed accurately and efficiently.
- Step 4: Concrete is poured on both sides of the pier, and each segment is cast over a period of approximately five days per form traveler (Figs. 7 and 8).
- Step 5: The segments are continued until the midpoint is reached, where a balanced pair is joined (Fig. 9). The bridge is then closed at the final section (Fig. 10).
2. Precast Bridge Construction Procedure
The process for constructing a bridge with precast segments involves the following steps:
- Step 1: Construct the foundations and pier shafts for all permanent piers (Fig. 11).
- Step 2: Use the piers as an erection platform for placing the precast segments.
- Step 3: Install a lifting frame to handle the placement of the precast segments (Fig. 12).
- Step 4: Place the first pair of precast segments and construct wet joints. Cure the joints for proper bonding.
- Step 5: Repeat the process for the next pair of precast segments, curing each joint as necessary. Continue this until the bridge is completed.
- Step 6: Finally, remove the lifting frame once the bridge is fully constructed (Fig. 13).
V. Segment Casting Methods
There are two primary methods for casting segments: short line method and long line method.
- Short Line Method: In this slower method, segments are cast three to four at a time, typically requiring more time for production. This is ideal for smaller projects or where space constraints are an issue (Fig. 14).
- Long Line Method: The long line method is faster, with segments equal to one span being cast at a time, allowing for quicker production and reduced overall construction time (Fig. 15).
VI. Cast-in-Place vs Precast Segments
Both cast-in-place and precast segments offer unique advantages:
- Cast-in-place Construction: Ideal for large, heavy segments, cast-in-place construction offers the flexibility to handle complex designs. Alignment corrections are easier to accommodate, but the concrete is usually younger when loaded, which may require more adjustments during construction.
- Precast Construction: Precast segments offer faster production and are ideal for situations where precision and speed are essential. However, they require transportation to the site and precise alignment during installation.
VII. Conclusion
The balanced cantilever method is an efficient and cost-effective technique for constructing long-span bridges. Whether using cast-in-place or precast segments, this method provides the flexibility to work in challenging environments without requiring large-scale temporary supports. With its ability to accommodate irregular terrain and complex design requirements, the balanced cantilever method continues to be a preferred choice for engineers around the world. Whether for cable-stayed bridges or other types of long-span structures, this technique proves to be a vital tool in modern bridge construction.