Bridge construction has evolved significantly over the years, with advancements in materials, engineering techniques, and technology. Among the various methods used today, the Span-by-Span Casting method stands out as one of the most economic, rapid, and efficient techniques for building long bridges and viaducts. This method has proven especially effective for individual spans of up to 60 meters, making it a preferred choice for many modern infrastructural projects.
What is the Span-by-Span Casting Method?
The Span by Span Casting method is a bridge construction technique that involves constructing the deck of a bridge incrementally, segment by segment, starting from one abutment and continuing toward the other end. This method was historically associated with cantilever construction but has since evolved with advancements in external prestressing. Today, it is widely regarded as one of the most efficient methods for building bridges and viaducts.
Unlike traditional methods, the Span by Span Casting method allows for the construction of bridges in a continuous fashion, using pre-cast segments that are placed incrementally and precisely. The economic and rapid nature of this method makes it a favored choice for constructing long-span bridges that require minimal interruption to traffic and the surrounding environment.
Key Components of the Span by Span Casting Method
The Span by Span method incorporates several essential components that make the construction process feasible and effective. These include the construction of the deck, supporting systems such as temporary stay masts or assembly trusses, and the transportation and placement of segments.
Deck Construction
The process begins at one abutment, where the first deck segment is placed. From there, additional segments are placed continuously, extending towards the other end of the bridge. Each segment is carefully positioned to form a continuous deck, with each unit contributing to the final structure.
Temporary Stay Mast System vs. Assembly Truss
In order to position and support the segments, two main systems are used: the temporary stay mast system and the more commonly used assembly truss. The assembly truss is a crucial part of the process. It is used to transport the segments, and once positioned, it holds the segments in place while additional segments are added. These systems ensure that the segments are correctly aligned and securely positioned before they are connected to form a complete deck.
Segment Transport
Before placement, the pre-cast segments are transported to the construction site. Depending on the location and the accessibility of the site, segments may be transported by truck (lorry) or barge. The transportation system is crucial as it ensures the timely and safe delivery of the segments to the span under construction.
Step-by-Step Process of Span by Span Casting
The Span by Span method follows a systematic, step-by-step process that begins with preparation and ends with the completion of each span.
Preparation Stage
Before the segments are placed, the assembly truss, equipped with sliding pads, is braced over two piers. This truss system is crucial for supporting the segments as they are positioned and moved into place. Once the truss is securely in place, the segments are ready for transport.
Segment Placement
Once the truss is in position, segments are delivered to the site and placed onto the sliding pads. These pads allow the segments to be slid into their exact positions with minimal effort and maximum precision. The placement process continues until the entire span is complete. After all segments are in position, the final pier segment is placed to complete the span.
Finalization of the Span
The final steps involve running longitudinal prestressing tendons through ducts in the segments and prestressing the entire span. Prestressing adds strength to the structure, ensuring its durability and safety. After this, deck joints are cast, sealed, and grouted, ensuring the continuity and stability of the deck.
The Assembly Truss
The assembly truss plays a key role throughout the process. It is used to hold and support the segments as they are moved into place. After the completion of one span, the truss is lowered and moved to the next span, where the entire construction cycle begins anew. This continuous movement of the truss allows for the rapid completion of multiple spans, ultimately completing the bridge.
Advantages of the Span by Span Casting Method
The Span by Span method offers several advantages, making it a popular choice in modern bridge construction.
Economic and Cost-Effective
One of the primary reasons for the method’s popularity is its cost-effectiveness. The continuous construction of segments allows for a steady, efficient workflow, reducing the need for complex equipment and labor. This results in lower overall costs compared to traditional construction methods.
Speed and Efficiency
Because the method involves incremental construction, it allows for faster completion times. The ability to transport and place segments quickly, combined with the assembly truss system, helps to speed up the entire process, reducing the overall construction timeline.
Flexibility
The Span by Span Casting method can be applied in various locations and conditions, including remote or challenging sites where other methods may not be feasible. The use of transport systems like lorries and barges also makes it adaptable to different environments and terrains.
Improved Structural Integrity
The use of external prestressing in the Span by Span method results in a more robust and stable structure. The method’s ability to control the placement and prestressing of each segment ensures that the final bridge is strong, durable, and able to withstand the stresses it will encounter over its lifespan.
Challenges and Considerations
While the Span by Span Casting method offers numerous benefits, it does come with its own set of challenges.
Segment Transport and Placement
The transportation of pre-cast segments can pose logistical challenges, especially in remote locations or areas with limited access. The precise placement of each segment requires careful planning and execution, which can increase the complexity of the project.
Environmental and Logistical Considerations
In some cases, environmental factors such as weather conditions and geographical obstacles may hinder the construction process. Additionally, coordinating the delivery of materials and ensuring that the necessary equipment is available can pose logistical challenges.
Conclusion
The Span by Span Casting method has become a cornerstone of modern bridge construction. Its ability to create long, continuous spans efficiently and economically has made it a go-to choice for building bridges and viaducts. As infrastructure demands continue to grow, the method’s flexibility, speed, and cost-effectiveness will likely play a significant role in the construction of future bridges. With ongoing advancements in engineering and technology, it’s clear that the Span by Span Casting method will remain an essential technique in the development of large-scale bridge projects around the world.