A balanced cantilever bridge is mostly constructed for cast-in-place and precast segmental bridges. While cast-in-place segmental bridges are constructed for shorter bridges with longer spans, precast segmental bridges are constructed for large-scale bridges with a shorter span (50-120 m).
There are two technologies used to construct the cast-in-place balanced cantilever bridge called form travelers and self-launching suspension movable scaffolding system, which work on the free erection sequences and linear movement erection process, respectively.
Ground cranes, lifting frames, and self-launching gantries are used to construct the balanced cantilever bridge using precast segments. Self-launching gantries work on the principle of linear movement erection from one abutment to another. In contrast, ground cranes and lifting frames work on the free erection sequence process.
Generally, box girder sections are used for the construction of a balanced cantilever bridge. If the depth of the span is constant, then precast segments can be used for the span of 50-70 m. However, if the depth of the span is varying, then the demand for flexural capacity of the span would be more. In that case, the use of the precast segment becomes difficult due to its size. Therefore, the lifting and transportation of heavy segments becomes difficult. In such cases, cast-in-place segments are used.
In this article, we discuss the most common erection technologies used for precast and cast-in-place segments for balanced cantilever bridges.
1. Ground Cranes
Good access is required for the erection throughout the length of the balanced cantilever bridge. Cranes normally give the easiest and quickest erection systems with the least investment and different segments can be erected at once. The primary objective for the erection of a bridge using ground cranes is to provide access to the site and height of the piers as a balanced cantilever bridge is usually constructed in inaccessible terrains.
2. Deck-Supported Lifting Frames
Deck-supported lifting frames are used to construct balanced cantilever bridges for long and curved spans, tall piers, and spans located over water. In such cases, exceptional lifters can deal with heavier sections and canal boat conveyance can help minimize geometry and weight limitations.
Lifting frames are also used in the construction of cable-stayed bridges and are considered a standard solution for erection whenever the site and time factors restrain the use of in-place casting. Despite the interruptions while moving to the next pier, the lifting frames can tackle erection conditions that are inconsistent with ground cranes and self-launching gantries.
3. Fixed Lifting Frames
The connection of the fixed lifting frames is secured to the tip of the cantilever and has a restricted load-carrying capacity. Derricks with turning arms can lift from behind or horizontally, and they are utilized distinctly in balanced cantilever bridges because of the twist that they transfer to the deck.
The precast segments are lifted along the cantilever noses on either side by the wheeled straddle carriers. The weight of such lifters is very less and doesn’t need any counterweight. Thus, the placement of segments on the pier table becomes easy, but the anchoring of segments to the deck becomes necessary during the operation.
4. Self-Launching Gantries
Self-launching gantries are used to accomplish a quick erection process and limit ground interruption. The gantries can directly erect the supports from one abutment to another and the precast segments can directly be deployed over the finished surface of the bridge.
Construction of two bridges located nearby can be done simultaneously by moving the gantry from one bridge to another. Therefore, a faster construction rate can be achieved, which may arrive at 2+2 segments per support per day.
A couple of derricks are used to lift and move the precast segments into position. If the precast segments are transported to the bridge, then the derrick picks them up at the backside end of the gantry. However, if the segments are stored at the ground, then the cranes are used to raise them to the deck level.
The old gantries had the same length as of the span to be erected. Their length was just adequate for self-launching. A small length of span and gantries resulted in closer supports. In the case of shorter gantries, the front end of the cantilever bridge is subjected to more loading. Thus, the erection of the pier table becomes more complex.
The new-age gantries are twice the length of the bridge span and are supported by the middle span. Thus, the need for extra support reduces during operation. A longer span truss can be constructed with new age gantries and the cost of erection also comes down as the continuous pre-stressing throughout the length decreases the quantity of reinforcement.
Self-launching gantries are more widely used as no ground cranes are required during operation, less requirement of labors, and the launching and placement of pier tables is quite simple.
5. Form Travelers
When the span of the bridge is long, but the overall length of the bridge is too short for segmental precasting, then the form travelers are used to erect the bridges by in-place casting. Also, in-place casting is required for the erection of a curved bridge as it becomes very difficult to produce consistently curved segments. The length of the casting cell for the form traveler is 5 m and can be designed for a carrying capacity of up to 500 tons.
Change in the geometry of the segment during operation can be achieved by suspending the casting cell from the hanger bars. Working platforms are developed around the form traveler for creating more working space for the laborers. However, the stressing platform is suspended from the front bulkhead so that fabrication and tensioning of the top-slab tendons can be achieved.
During the initial stages of the construction of a balanced cantilever, a pair of form travelers must be accommodated by the pier table with a length of 8-10 m. Casting cells are deployed on the pier table, supported by the props from the foundation or by pier brackets.
Mostly, the form travelers have the restriction of working space, complex geometry during operation, and different phases for casting the entire segment. Therefore, at least 2-4 months are required for casting the complete cells.
6. Suspension Movable Scaffolding System
A suspension movable scaffolding system is used to erect the cast-in-place balanced cantilever bridge. The system is supported on one side of the completed bridge and the other side of the leading pier. Two casting cells are simultaneously used between the support and the pier table at the mid-span.
Once the casting is done on the mid-span, the girder is launched onto the next span. Subsequently, the position of casting cells is shifted to the next span for the construction of a new segment. Short pier tables are precast toward the end of the pier erection to shorten the process duration of the movable scaffolding system and to reduce the critical path of the activities.
The suspension movable scaffolding system is utilized for directional erection of rectilinear or somewhat curved spans of 100-120 m. The main girder of the bridge is 1.5 times longer than the span of the bridge.
The movable scaffolding system improves the access from the finished bridge and settles the support during erection, and less pre-stressing force is required for the bridge to reduce the construction loads.
With this technology, cast-in-place segments can be formed up to 20 m wide and 12 m in length. Also, moving the casting cells to the next pier requires hours rather than weeks. Movable scaffolding system minimizes the use of ground cranes and thus, the construction process becomes faster and the labor cost reduces substantially.
FAQs
Ground cranes, lifting frames, and self-launching gantries are used to construct the balanced cantilever bridge using precast segments. Self-launching gantries work on the principle of linear movement erection from abutment to abutment. In contrast, ground cranes and lifting frames work on the free erection sequences process.
Ground cranes normally give the easiest and quickest erection systems with least investment and different segments can be erected at once.
There are two technologies used to construct the cast-in-place balanced cantilever bridge, named form travelers and self-launching suspension movable scaffolding system. Form travelers and self-launching suspension movable scaffolding system work on the free erection sequences and linear movement erection process, respectively.
By using self-launching gantries, the construction of the two bridges located nearby can be done simultaneously by moving the gantry from bridge to bridge. Therefore, a faster construction rate can be achieved, which may arrive at 2+2 segments per support per day.
Suspension movable scaffolding system is used to erect the cast-in-place balanced cantilever bridge. Suspension movable scaffolding system is supported on one side of the completed bridge and the other side of the leading pier. Two casting cells are simultaneously used between the support and the pier table at the mid-span.
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