The arch is the most natural bridge shape in nature. Originally constructed of stone, today such bridges are built of reinforced or precast concrete. They are often the most economical choice where a bridge is required to cross over inaccessible landscapes.
The development of modern arch construction methods has made the use of arch construction more economical by removing the need of expensive centring formwork. Though abutments still must be well founded on rock or soild ground.
Two construction techniques are most commonly used today.
(i) Cast-in-situ free cantilever method
This method involves the partially built arch tied back to rock anchors in the valley side slopes.
(ii) Slip formed sections
This method involves half arch sections being held vertically over each abutment and then rotating each arch section into position.
Tee-beams are generally used for arch bridge decks for their functionality and self weight.
The technique of cable-stayed construction has been used and continually developed over the last 50years. It is the most common construction choice today when a bridge is required to span more than 300. Cable-stayed bridges can be either concrete or steel though a combination of both materials is often chosen.
Fig: Stay cable anchorages on a concrete deck
For concrete cable stayed bridges free cantilever construction is considered economical. With this method the deck segments can be either precast or cast-in-situ by travelling shutter arrangement.
In ca cable stayed bridge, depending on its design, the cables carry the bridge deck from one or both sides of the supporting tower. The stay cables carry the deck and transfer all bridge loads to the foundations. This is done by transmitting the cable stay forces, through its extremeties, at it anchorage points. Stay cables are firmly attached to the anchorages which are designed to resist the buckling forces of the loads.
Detailing of all anchorages should allow for their safe construction and accessibility for inspection and maintenance on completion. In concrete stay-cabled anchorages are placed under the deck.
What is balanced Cantilever method of Bridge Construction?
The balanced cantilever method of bridge construction used for bridges with few spans ranging from 50 to 250m. The bridge can be either cast-in-place or precast.
Moreover, the basic concept of balanced cantilever construction method is to attach the segments in an alternate manner at opposite ends of cantilevers supported by piers.
Furthermore, this method is easily adaptable to irregular and long span lengths, congested project sites, rough and water terrain, rail crossings, and environmentally sensitive areas.
Additionally, it is highly suitable for building cable-stayed bridges. This is because once segments are placed, they will be supported by new cable-stays in each erection stage. Therefore, no auxiliary supports are required, and hence it is both economical and practical method for long cable-stayed bridges.
Finally, this article presents balanced cantilever method of bridge construction.
Fig.1: details of cantilever method of bridge construction (Cast in situ segment)Fig.2: details of cantilever method of bridge construction (precast segments)
Procedure for balanced cantilever method of cast-in-situ bridge Construction
1- After the construction of lower infrastructure of the bridge is completed, fig.3. Bridge construction begins at each pier. Special formwork is positioned and cast-in-situ pier segment is begun, fig.4. The complete pier segment is then used as an erection platform to support a form traveler for cast-in-place segments.
Fig.3: construction of lower infrastructure of bridgeFig.4: Positioning of special formwork
3- Thereafter, soffit shuttering, shuttering for web & deck shuttering is fixed on both sides of pier as shown in fig.5 and fig.6.
Fig.5: Soffit, web, and Deck shutteringFig.6: shuttering soffit, web, and decks
5- Then concreting is done on both sides of the pier as shown in fig.7 and fig.8. The segment production rate for form travelers is usually one segment every 5 days per traveler.
Cast-in-situ segments range between 3m to 5m in length with formwork moving in tandem with each segment. Segment construction is continued until a joining midpoint is reached where a balanced pair is closed as demonstrated in fig.9. The construction of closer section of a bridge is shown in fig.10.
Fig.7: concrete placementFig.8: Concrete placementFig.9: bridge construction progressionFig.10: Construction of closer section of the bridge
Sequence of balanced cantilever method of precast bridge construction
1- Foundations and pier shafts of all permanent piers in the main unit are constructed.
2- Once the piers are built, they are used as an erection platform for precast segments.
Fig.11: Pier construction
3- Assemble and install lifting frame
Fig.12: lifting frame
4- Place first pair of precast segments.
5- Construct wet joints
6- Cure the wet joints
7- Then, install next pair of precast segments. After that, construct and cure joints. This procedure continues till the end of construction process.
8- Finally, remove the lifting frame.
Fig.13: End span segment o false work
Casting of precast segments
There are two methods for precast segment casting which include:
Short line method: In this rate of segment production is slow. Three or four segments cast at a time.
Long line method: In this rate of segment production is fast. Segments equal to one span cast at a time.
Fig.14: Short line segment castingFig.15: Long line segment casting
Cast-in-Place Segments Vs precast segments
Cast-in-place construction proves to be very beneficial when large, considerably heavy segments are required to be constructed. So, instead of handling the segments, only materials have to be transported thus influencing the type and size of required equipment.
Alignment variations and corrections are more easily accommodated in cast-in-place construction; but more corrections will probably be necessary.The increase in alignment corrections for cast-in-place construction compared to precast construction relates directly to the age of the concrete when loaded. By and large, the concrete is much younger when loaded in cast-in-place construction.
INCREMENTAL LAUNCHING METHOD OF BRIDGE CONSTRUCTION
Incremental Launching Method of Bridge Construction:
For bridge decks greater than 250m in length, the method of incremental launching can be considered. With this method of construction the bridge deck is built in sections by pushing the structure outwards from an abutments towards the pier. It is most suited to the rapid construction of bridges with a constant radius of curvature such as constant depth of box girder segments.
The construction sequence begins behind one abutment where a highly mechanized prefabrication deck mould is set up removing the need for temporary supports with this method. A rigid framework is then attached enabling the production of cast-in-situ segments. In-situ deck segments range in length from 5m to 30m. when each segment is complete it is placed on sliding bearings and pushed through into the span.
A steel nose is also positioned at the front of the first span formwork. This allows for the necessary deck cantilever length as the span approaches the first pier.
Bridge decks of 1km can be built by this method with a rate of construction of 30m per week.
SPAN BY SPAN CASTING METHOD OF BRIDGE CONSTRUCTION
Span by Span Casting method of Bridge Construction:
Span by span is a relatively new construction technique historically associated with cantilever construction but the advancement in external prestressing has enabled its own potential use to grow. Today it is considered to be the most economic and rapid method of construction available for long bridges and viaducts with individual spans upto 60m.
Decks are begun at one abutment and constructed continuously by placing segments to the other end of the bridge. Segments can be positioned by either a temporary staying mast system through more commonly using an assembly truss.
Before segments are placed the truss with sliding pads is braced over two piers. Depending on the bridge location the segments are then transported by lorry or barge to the span under construction. Each segment is then placed on the sliding pads and slid into its position. Once all segments are in position the pier segment is then placed.
The final stage is then begun by running longitudinal prestressing tendons through segments ducts and prestressing entire span. Deck joints are then cast and closed and ducts grouted. When the span is complete the assembly truss is lowered and moved to the next span where construction cycle begins until the bridge is complete.
