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OM Engineering specialises in the design and supply of custom-designed formwork systems, ranging from building, bridge and industrial tank jump forms to major temporary works projects.

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Bridge Design and Engineering Magazine First Quarter Issue 2009

By Bridge Design and Engineering Magazine
October 29, 2009
 
Ho Chi Minh City, Vietnam ---

Construction of a new cable-stayed bridge in Vietnam is set for completion in October this year. Iain Hubert reports on progress so far.

Almost two years on from the start of construction work, the landmark Phu My bridge can now be seen from many parts of Ho Chi Minh City. With one of the two main towers completed it is already possible to see how the new link will dominate the entrance to the Saigon River and become an integral part of the skyline of the city.

The bridge is being constructed by the Bilfinger Berger Baulderstone Hornibrook Consortium, a joint venture of two companies which form part of Bilfinger Berger, based in Germany.

The Phu My Bridge will link district 7 in the south west with district 2 in the north east of Ho Chi Minh City, crossing the Saigon River close to the point where it becomes the Dong Nai River. It will form part of a new ring road currently under construction around Ho Chi Minh City, becoming a vital part of the south eastern section of the ring road and an important transport link connecting the southern Mekong delta region to the central and northern parts of Vietnam. Goods traffic from the south will then be able to bypass the city centre to the east.

The client for this US$105 million project is a private consortium made up of both state and private companies. The Phu My Bridge Corporation holds a 30 year concession for the project; it will build and operate the bridge as part of a toll road scheme before transferring the bridge to public ownership.

Private finance raised by PMC is combined with export credit guarantees from Germany, France and Australia to fund the scheme. As part of the financing arrangements, almost half of the contract expenditure must be in these three countries. Three aspects of the finance arrangements – private sector infrastructure, private sector BOT scheme, and export credit finance - are firsts for Vietnam.

When construction work first began, the team anticipated that it would take some 34 months to complete the project, but optimisation of the construction methods and the use of specialist equipment has enabled some of this time to be saved. The contractor is currently targetting to open to traffic in September this year, a total construction period of just 30 months.

The scope of BBBH’s contract includes the design and construction of the main bridge, a 705m-long cable-stayed crossing with a central span of 380m, as well as the approach viaduct structures on either side of the river. These extend to approximately 760m length on the district 7 side of the river, and 640m on district 2.

Design of the main bridge and approach structures was split into two packages, with the design of the cable-stayed structure being carried out by French consultant Arcadis. The approach viaducts were designed in Australia by Cardno; all the designs were coordinated by BBBH staff in Vietnam. Independent checking was carried out by Tony Gee & Partners.

Navigational passage for river traffic is provided with a minimum 45m vertical clearance, at high tide, over a 250m wide zone at the centre of the bridge.

The total width of the main span deck is 27m, this incorporates three lanes of traffic in each direction, two car and truck lanes, a separated motorcycle lane and footways for pedestrians.

Each of the two main bridge towers is supported on twenty eight bored piles each approximately 2.1m diameter and 80m length; meanwhile the towers themselves are H-shaped and are approximately 140m high. The main bridge deck is designed as an in situ, reinforced concrete slab supported on longitudinal and transverse beams and suspended from the towers by the stay cables. The deck is constructed in balanced cantilever fashion using formwork travellers to carry the deck elements while installation of the stay cables proceeds simultaneously with the casting of the deck.

To maintain control of the critical path, BBBH elected to construct all elements of the main bridge, including pilecaps, towers and deck, using direct labour. Stay cables are being installed by specialist Freyssinet, while a large part of the approach structures is subcontracted to Vietnamese contractor Chau Thoi Concrete Corporation 620. All subcontractors are closely supervised by BBBH staff.

Foundation works for the main bridge started in July 2007, however the team quickly decided that the locally-available equipment and resources which had been planned to be used for construction of the 2.1m diameter bored piles would not be able to meet the requirements of the programme. Instead, equipment and supervisory staff had to be resourced from offshore; however the three month delay that was initially caused by this decision was subsequently recovered by accelerating the subsequent piling works and tower construction.

Both of the main tower pile caps are located in the river so piling equipment worked from barges, with concrete delivery by pump from the adjacent riverbank. The pilecaps themselves are located above low water level so permanent formwork comprising precast concrete panels was used to form the soffit and sides. The soffit panels were supported on a combination of the permanent pile steel casings and some temporary piles in between the two permanent pile groups to allow casting of the bottom 800mm-thick strip of the pile cap. The precast side panels were then mounted on the completed 800mm thick strip and the remainder of the pile cap was cast inside them.

Each leg of the H-shaped bridge towers is a box section whose outer dimensions vary from 5.5m by 7m to 3m by 5m. Stay anchorages are located inside the box. Loop tendons in the tower head, ranging from four, seven-wire 15.7mm-diameter strand to 13 seven-wire 15.7mm-diameter strand, allow the box to resist the stay cable forces and 40mm and 50mm-diameter prestressing bars are used to counteract the bursting forces from the anchorages.

The pylons are being built using self-climbing jump forms which were designed specifically for the project to accommodate the change in section of the shaft (see box). Reinforcement for the towers was prefabricated, with two elements for each 4m lift. On the upper tower these elements were match-fabricated and incorporated the stressing and the stay cable anchorage tubes which were held in position by a temporary steel frame. This use of prefabrication, together with couplers forming the connections on the majority of vertical bars, ensured that a four-day pour cycle was achieved.

The legs of the towers are linked by two cross-beams; the lower of which was built on top of the pile cap. The steel falsework and formwork required for the first three deck segments was then erected on the cross-beam and the combined 1200t unit was raised 40m to its final position using strand jacks located on a temporary strut between the tower legs. When it reached its permanent location the cross beam was stitched and stressed to the tower legs. This method minimised the amount of work to be carried out at height and resulted in a programme saving as the completed pier table was available for deck construction earlier.

Balanced cantilever construction is used for the bridge deck, with 10m-long segments poured in situ using a form traveller. Each segment consists of two, 2m by 1.5m longitudinal beams, two 2m by 400mm transverse cross girders, two 1.5m by 2m by 3.2m anchor pods in which the stay cable tubes are located, and the 250mm-thick deck slab. The deck is prestressed transversely at each of the cross-girders and longitudinally over the central 170m of the span. As with the tower, maximum use is made of prefabrication in its construction. The anchor pods and transverse beams are precast and the reinforcement cage for the longitudinal beams is prefabricated.

The design concept of the form traveller is developed from those used by the consortium on previous bridge projects, and therefore the detailed design, fabrication, assembly, commissioning was managed directly by the project team. The two travellers, weighing 280t each including formwork and platforms, were assembled on a barge and subsequently lifted into position on the end of the pier table using strand jacks. The experience the contracting joint venture gained from previous projects such as Vietnam’s My Thuan Bridge (Bd&e issue no 18) and the Centennial Bridge over the Panama Canal (Bd&e issue no 35), enabled a five-day cycle to be consistently achieved for each 10m segment.

The stay cables for the bridge are supplied and installed by Freyssinet and vary in size from 60m long with 26 strands and a dead load of 280t, to 200m long with 80 strands and a dead load of 785t. In addition to their permanent function the stay cables are also designed to be used in the construction phase to support the front of the form traveller during placement of concrete.

When the deck concrete has reached the required strength, the form traveller is lowered from the previously cast segment. During this process prestressing is installed in the two cross girders.

At each side of the traveller there are C-shaped frames which enable the weight of the fully-lowered traveller to be transferred to longitudinal launching beams which sit on the completed deck spanning from anchor pod to anchor pod. The launching of the traveller is achieved using hydraulic jacks acting against the launching beams. During the launching process the precast pods and cross girders are lifted into position on the traveller.

Following the 10m launch the traveller is raised to the correct level using hydraulic jacks and locked into the position. The level of the front of the traveller is adjusted by inserting shims between the trailing end of the traveller and the previous deck segment.

At the same time as the reinforcement is being installed, the two stay cables are erected between the towers. Before stay installation the two precast pods are each anchored to the traveller by ten, 40mm- diameter stress bars. The stay cables are only partially tensioned at this stage. After a final survey check concrete is placed – delivered by pump from ground level, some 140m3 for each segment. After curing and a further survey check, the stay cables are fully stressed.

Bridge geometry is checked immediately after completion of each deck section, and the results fed into the design model. If required, adjustments are made to the shim values and to the first and second stage stay cable stressing values for the subsequent segment. It is anticipated that some further minor adjustments to stay cable forces may be required once the deck is complete and the concentrated loads from the travellers have been removed.

The contract programme was for a 34 month construction period, with completion due to be achieved by the end of December this year and an agreement to target actual completion two months early. However good progress has been achieved and at the client’s request BBBH have agreed to target completion by 2 September, which is Vietnam’s National Day.

As Bd&e went to press in mid January, the first half of the bridge deck was complete, and the form travellers were in the process of being transferred to the pier table on the east tower. Deck construction was due to restart in mid February.


Iain Hubert is BBH Consortium project director

Leg lifts

Specialist OM Engineering was awarded a contract to design, fabricate and supply the equipment to be used for the construction of the bridge towers on the main bridge. Construction was divided up into 36 concrete lifts of maximum height 4m. From the pile-cap to the cable zone, each leg of the pylon leans in at an angle of 4º, whereas in the cable zone the legs are vertical. From the pile-cap to the deck level, the tower legs also taper transversely from 5.5m to 3m and from the deck level to the cable zone they taper longitudinally from 7m to 5m. In the cable zone, the tower leg geometry is a constant 5m by 3m.

There were also variations in the thickness of the concrete walls – the internal geometry – at the highly-stressed areas at the base of the pylon and at deck level.

BBBH had elected to use a single set of travelling formwork for the deck cantilevers – two form travellers - intending to build them in series, rather than in parallel. Hence the H-shaped towers did not need to be built simultaneously. However, to reduce the lag time between the towers, BBBH asked for proposals to construct the second tower from the pile-cap to the deck level using a crane-picked system. OM Engineering offered a very cheap solution consisting of two light external support frames sharing one set of external formwork. Once construction picked up speed, BBBH asked OM Engineering to modify the crane-picked system so that it could continue beyond the deck level. This modification enabled the system to be used for more than half of the full height of the pylons, up to lift number 21.

The contractor’s intention was to prefabricate as much of the tower rebar as possible. On bridge jump forms, the inner part of the jump form is either connected to the outside part, requiring overhead beams which interfere with the rebar, or it is completely separate. To allow for maximum prefabrication of rebar, OM Engineering designed the jump form such that the overhead beams were reconfigured twice and then removed completely, all during the course of tower construction. This early planning greatly increased the efficiency of the rebar operation.

The jump form system was to be used on both towers, hence it needed to be capable of being rapidly and safely relocated from one side of the Saigon River to the other. A great deal of planning went into the detailed design of the jump form frames, to ensure that they could be assembled in modular units, greatly reducing the time required for installation. The average time required for the first tower was reduced to approximately two days. Furthermore, OM Engineering planned the first concrete lift to ensure that the contractor did not need temporary access scaffolding or other additional equipment before the jump form was installed.

Through all the transition stages of construction of the tower, the jump form provided a safe and enclosed environment, ensuring ample space for all construction activities. An additional advantage, which had been unforeseen at the planning stage, was that the stay cable subcontractor had enough space and weight capacity to complete the installation and stressing of loop tendons and anti bursting stress bar in the tower head cable zone from within the jump form, thereby reducing construction time and saving on additional scaffolding at height.

Advanced three-dimensional software was used by OM Engineering to demonstrate the operation of the jump form from the pile cap, including every cross-section transition, to the top of the pylon. This gave the contractor’s team chance to familiarise itself with the process before the equipment arrived on site, as well as the opportunity to review the construction methodology and request changes. At lift number 27, for example, the legs of the towers transition from a 4º slope to vertical. To ensure that this procedure was fully understood by the site crew and that it was completed safely, OM Engineering prepared a step by step 3D cinematic of every activity required. As a result of fully understanding the process, the site team was able to complete it successfully, exactly as intended.

Bridge Design and Engineering Magazine First Quarter Issue 2009
 
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