Construction of Cantilever Bridge - List of Various Technologies Used
For cast-in-place and precast segmental bridges a balanced cantilever bridge is mostly constructed. Cast-in-place bridges are constructed for short bridges with longer span, and for longer-bridges with shorter span precast segmental bridges are constructed.
There are two ways to construct the cantilever bridge – one, by self-launching suspension movable scaffolding system which works on free erection sequences, two, linear movement erection process.
To construct the balanced cantilever bridge using precast segments ground cranes, lifting frames and self-launching gantries are used. Self-launching gantries work on the principle of linear movement erection. Whereas, for free erection sequence process ground cranes and lifting frames works.
Usually, box girder sections are used for the making of a balanced cantilever bridge. Precast segments can be used for a span of 50-70m if the depth of the span is constant. In case the depth of span is varying then the demand for flexural capacity of the span would be more. In which case it is difficult to use the precast segment. Lifting and transportation of heavy segments also becomes difficult. Thus, for such scenarios cast-in-segments are used.
Listed below are the most common erection technologies used for precast and cast-in-place segments for balanced cantilever bridges:
• Ground Cranes
For the erection of a balanced cantilever bridge a good access is required throughout the length. The quickest and easiest erection system with least investment is provided by cranes. Different segments can be erected using cranes at once. The reason why a bridge is constructed using ground crane is to provide access to the site and height of the piers as a balanced cantilever bridge is mostly constructed in inaccessible terrains.
• Deck-supported Lifting Frames
For the construction of balanced cantilever bridge which will be of long and curved span, tall piers and spans located over water deck-supported lifting frames are used. In cases like this exceptional lifters can deal with heavier sections and canal boat conveyance can help in minimizing geometry and weight limitations.
For cable-stayed bridges lifting frames are used. It is also considered a standard solution for erection when the site and time factors restrain the use of in-place casting. Even though there are interruptions while moving to the next pier, the lift frames can handle erection conditions which usually are inconsistent with ground cranes and self-launching gantries.
• Fixed Lifting Frame
The connection of the fixed lifting frames is secured to the tip of the cantilever and has a particularly restrictive load-carrying capacity. The turning arms can lift from behind or horizontally and are used to balance cantilever bridges because of the twist that they can transfer to the deck. It is easier to place segments on the pier table and the anchoring of segments to the deck becomes necessary during the operation.
• Self-launching Gantries
It is used to complete a quick erection process and also limit ground interruptions. Using the gantries direct erection of support from one abutment to another is possible and the precast segments can be directly deployed over the finished surface of the bridge.
If two bridges are being constructed nearby then the gantry can be used by moving it from one bridge to another. A faster construction rate can therefore be achieved. To move and lift the precast segment a couple of derricks are used. In case the precast segments are transported to the bridge then the derrick picks them up at the backside end of the gantry.
The old gantries had the same length as of the span to be erected. A small length of span and gantries leads to closer supports. In case of shorter gantries, the front end of the cantilever has to face more of the loading. Thus, the erection of the pier table becomes a complex process.
The new gantries are twice the length of the bridge span and support is provided by the middle span. Therefore, there is little or no need for extra support. New age gantries can be used to construct a longer span truss. The cost of erection comes down as the continuous pre-stressing throughout the length leads towards decrease in the quantity of reinforcement.
Now, self-launching gantries are even more widely used as no ground cranes are required during the process, the requirement for labour also reduces and the launching and placement of pier tables is simple as well.
• Form Travelers
Form travelers are used to erect bridges by in-place casting when the span of the bridge is long but the overall length of the bridge is short for segmental precasting. For the erection of a curved bridge in-place casting is required as it becomes difficult otherwise to produce consistent curved segments. The length of the casting cell for the form traveler is 5m which can be designed for a carrying capacity of up to 500 tons.
To create more working space for the labour working platforms are built. The stressing platform is suspended from the front bulkhead so that the fabrication and tension of the top-slab tendons can be made possible.
A pair of form travelers must be accommodated by the pier table with a length of 8-10m during the initial stage of balancing a cantilever.
It takes 2-4 months for the casting of the complete cells as the form travelers have the restriction of working space, complex geometry during operation and different phases for casting the entire segment.
• Suspension Movable Scaffolding System
It is used to erect cast-in-place balanced cantilever bridge. It is supported on one side of the complete bridge and the other side of the leading pier. Two casting cells are used at the same time between the support and the pier table at the mid-span.
After the casting is done on the mid-span, the girder is launched on the next span. Accordingly, the position of the casting cells is shifted to the next span for the construction of a new segment.
For rectilinear or somewhat curved spans of 100-120 m suspension movable scaffolding system is used. The main girder of the bridge is 1.5 times longer than the bridge span.
The movable scaffolding system helps in improving access from the finished bridge and settles the support during erection, along with it less pre-stressing is required for the bridge so as to reduce the construction loads.
Using this technology, cast-in-place segments can be formed which is up to 20m wide and 12m in length. Moving the casting cells to the next pier requires just hours rather than weeks. Movable scaffolding system helps in minimizing the use of ground cane and therefore the construction process becomes much faster and the labor cost is also reduced substantially.