Bridge construction has evolved significantly over the years, with a variety of methods designed to improve efficiency, reduce construction time, and enhance structural integrity. One such method gaining popularity is the precast method, which involves manufacturing bridge components offsite and then assembling them onsite. This approach not only speeds up construction but also ensures a high level of quality control. The precast method is particularly beneficial in projects where time is of the essence or when working with difficult terrain. Let’s explore the key precast techniques used in bridge construction, including precast beams, precast decks, and precast segmental decks.
I. Precast Beams
Precast beams are typically used for short-span bridges, generally ranging from 5 meters to 50 meters in length. These bridges are often found in railway or motorway applications, where quick construction is crucial. Standard inverted tee beams or M-beams are the preferred choices for such projects.
The process of erecting precast beams is straightforward but requires precision. Once the beams are manufactured in a controlled environment, they are transported to the construction site, where they are positioned by crane. This technique allows for rapid assembly, and construction teams can set up up to four beams per day.
In many cases, a cast-in-situ slab top deck is used in conjunction with the beams. The rate of construction for the slab is typically one span per week. This combination of precast beams and a cast-in-situ slab top deck helps maintain a steady rate of progress without compromising the quality of the structure.
II. Precast Decks
For larger, long-span bridges and viaducts, precast decks offer an efficient solution. This method is especially advantageous when construction schedules are tight, and there’s a need to complete the bridge quickly. Precast decks are manufactured offsite and then positioned on-site using cranes or a purpose-built gantry system.
Using this method, a complete precast deck for a long viaduct can be placed quickly, significantly reducing construction time. Typically, the construction rate is about two spans per day when a gantry system is in use. If this pace is maintained, a full kilometer of deck can be placed in just three weeks.
However, this method requires careful planning to ensure that the precast decks are delivered on time. The engineering team must create a detailed construction schedule that coordinates the delivery of the decks with the contractor’s ability to place and store them. Delays in production or delivery can slow down the entire project, so effective communication and organization are key to success.
III. Precast Segmental Decks
The precast segmental deck method is commonly used for larger bridges, especially when the deck depth is too challenging for cast-in-situ construction. Box girder segments are often used, as they can be as deep as 2 meters or less, between 2.5 meters and 4 meters long, and can support decks up to 15 meters wide.
In this method, the deck segments are prestressed either internally or externally, with internal tendons being protected from moisture. The use of prestressing allows for the segments to be placed efficiently and securely. The construction process is highly repetitive, making it ideal for modern placement techniques, such as balanced or free cantilever about a pier.
A crane or self-launching gantry system can place up to six segments per day. The rate of construction can vary depending on whether internal or external prestressing is used. Internally prestressed segments typically follow a construction rate of one span per week, while externally prestressed segments allow for a faster pace of up to three spans per week. This accelerated rate of construction ensures that even large, complex bridges can be completed within a reasonable timeframe.
IV. Conclusion
The precast method of bridge construction offers numerous benefits, including faster completion times, enhanced quality control, and reduced on-site construction activity. Whether using precast beams for shorter spans, precast decks for long viaducts, or precast segmental decks for large bridges, each technique provides a unique set of advantages that can be tailored to suit the specific needs of a project.