Singapore bridge demolition by heavy lifting equipment

Singapore bridge demolition by heavy lifting equipment

Haixue, Liao

During the past few years, traffic loadings have increased steadily and the structural specification for bridges has changed. Many bridges built in the 1970s and 1980s now need strengthening or replacing. Assuming their foundations are adequate, simply supported and continuous structures can be strengthened by external prestressing(1). However, some structures may not be suitable for strengthening, because of inadequate foundations or unsuitable superstructures, such as concrete frames. In the following example, the existing flyover had to be demolished and a sophisticated system was developed to do this without using scaffolding or diverting roads.

Built in 1974, the Adam Road/Pan Island Expressway (PIE) Singapore six-lane flyover needed replacing. Work began with the construction of a new north-bound carriageway immediately alongside. Demolition of the flyover – a frame with a 36.6m span, five webs and five abutment ribs – was carried out in April 2001 with T. Y. Lin (the original designers) as consultant. The south-bound carriageway, built on the site of the old flyover, has two carriageways, each 25m wide with spans of 22.6, 41.2 and 22.6m.

Original proposal

The initial proposal was to:

Stage 1: Demolish approach structures, remove backfill and construct temporary diversion roads

Stage 2: Install scaffolding and temporary supports, cut central part of beam and slab, remove segments by crane

Stage 3: Demolish remainder of the slab structure

Stage 4: Demolish abutments, ribs and footings.

Alternative proposal

The alternative proposal presented by main contractor, Wee Poh Construction Pre Ltd, and prestressing specialists, OVM, offered a more cost-effective approach. The scheme involved cutting the connection between the abutment and deck structure, installing a bespoke sliding system, and pulling the whole deck structure – some 15,000kN – during a night closure (12:30am to 5:30am). Final demolition of the deck structure, abutment and footing could then be undertaken at ground level, and there was no need to construct diversion roads.

Heavy lifting system

Following major improvements in strand-type heavy lifting systems, lifting speeds of 40 m/h are now possible, compared with 2m/h ten years ago(2). Indeed, systems can now lift, rotate and lower loads or structures of virtually any size and configuration(3). With the tight time constraints, it was decided to use such a system for this project, the required lifting speed to be above 30m/h. This was the first time in the world that a strand-type heavy lifting system was used to demolish a major flyover.

Operation sequence

Stage 1: Preparatory work

1. Build the north-bound carriage– way of the new flyover and divert all traffic onto it

2. Demolish the approach slabs

3. Excavate backfill behind abutments to the required level to produce temporary supports for the heavy lifting system and track laying

4. Remove kerbs and central reserve, excavate and make concrete foundations, then level the running surface on the PIE for the rolling system

5. Install prefabricated temporary supports 1 and 2

6. Grout interface between temporary supports and the deck

7. Cast four deadmen and four anchor blocks

8. Install remaining device

9. Cast five concrete track beams

10. Apply initial force of 2200kN at support 2, then cut abutment 2

11. Apply initial force of 2200kN at support 1, then cut abutment 1

12. Cast abutment 1 to the level of track 1

13. Install pulling system and cable

14. Cut prestressed cables on abutments 1 and 2 and prepare for pulling operation.

Step 2: Pulling operation

1. Close the PIE at 12:30am

2. Install m thick steel plate of track 2 across the PIE

3. Release the rolling system block

4. Start pulling operation

5. Adjust roller alignment during pull

6. Remove support 1

7. Replace broken roller

8. Remove 4mm thick steel plates

9. Open the PIE to traffic when sup port 2 reaches the other abutment

10. Adjust level of support 2 to that of track 1

11. Remove deck structure from abutment 1

12. Remove support 2.

Stage 3: Structural demolition

The deck structure, abutments and footings on the ground were demolished using hydraulic breakers and crushers mounted on the excavator

Detailed considerations

During demolition, the structure originally a frame – was transformed into a simple structure on temporary supports, after which the prestressed cables in the abutment were cut. Although there are no traffic loads on the bridge when cutting the abutments, the span between temporary supports close to the abutment was too great for the collapsing structure and raked supports were installed to reduce the span. These temporary supports were placed close to the abutment as no traffic is affected by the demolition work.

Timing the cutting of abutment cables was crucial. First, the flyover was closed and jacks on the temporary supports applied 2200kN to each web (11000kN each side) so that the structure would switch smoothly from a frame to a simple structure on temporary supports. The rollers were blocked by stoppers and the structure-retaining system installed. The cables were then cut.

As the demolition of the flyover was critical to Singapore’s traffic system and the pulling operation was to be completed in only five hours, simulation tests were required by the client, the Land Transport Authority. The tests – covering installation of track 2, pulling speed, roller system efficiency, connection joints and actual friction force were carried out under factory conditions using a 150-tonne steel structure and one set of heavy lifting equipment. The results showed the structure could be pulled at 36m/h, the friction coefficient being initially 0.074 and 0.056 during movement. It was necessary to close the PIE before track 2 was laid. This was a time-consuming procedure and a critical factor to project success. Installation of track 2 on the PIE involving laying five 32-metre tracks, which took two hours. Simulation tests had established that the actual operation should take three and half hours.

As the original structure was cast in-situ, its soffits were uneven. Interfaces were grouted so that the temporary supports would make full contact. On each side, there were five temporary supports, parts of which were prefabricated in the factory, corresponding to the five webs of the deck. Each support foot had four rollers running on track 2: the latter consisted of four steel plates, 1.8m wide, 8m long and 40mm thick. As there was no guide rail, small differences in friction force, loading on temporary supports, track levels and foundation settlements caused some roller misalignment and attempts were made to remedy this. Initially, one set of heavy lifting equipment was tried, and then chain blocks were used. Neither method worked and the tracks had to be relaid to match the roller misalignment. In future, this could be remedied by installing guide rails. Upper rollers to the level of track were levelled by the iterative operation of vertical jacks and nuts.

Conclusion

Demolishing a bridge using heavy– lifting equipment saves time and money compared with existing methods. It also reduces traffic disruption.

References

1 LIAO, H. Strengthening Singapore’s Aljunied Flyover with external prestressing, CONCRETE, Vol.35, No. 3, March 2001, pp.39-41.

2 MOSCHLER, E. and PHILIPS, M. Specialised bridge erection technique. Proceedings: Bridges into the 21st Century. Hong Kong Institution of Engineers, 1995.

JACOBAN, K. Prefabrication of structures with erection using strand type heavy lifting equipment. Proceedings: the thirteenth FIP Congress on challenges for concrete in the next millennium, Balkema, Rotterdam, 1998. pp.201-204.

Liao Haixue, Zhong Ruihui and Huang Weijun,

OVM Prestress Co. Pte Ltd, Singapore

Copyright The Concrete Society Nov/Dec 2001

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