Balanced Cantilever Method of Bridge Construction

Madeh Izat Hamakareem

10 years ago

Balanced cantilever methods of bridge construction

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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 bridge

Fig.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 shuttering

Fig.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 placement

Fig.8: Concrete placement

Fig.9: bridge construction progression

Fig.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 casting

Fig.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.