The construction of Vietnam's Phu My Bridge

Nearing completion, the Phu My Bridge spans the Saigon River between Districts 7 and 2 in Ho Chi Minh city, Vietnam. The bridge will form part of a new ring road currently under construction around the city.  The ring road will be an important transport link from the southern Mekong delta region to the central and northern parts of Vietnam.

The Phu My Bridge contract includes the design and construction of a 705 metre-long cable-stayed main bridge with a clear span of 380 metres, as well as the approach structures on either side.

The bridge is being constructed by the Bilfinger Berger Baulderstone Hornibrook (BBBH) Consortium – joint venture between two companies both within the Bilfinger Berger group – at a cost of US$105 million.

Substructure

Each main bridge pylon is founded on two groups of 14, 2.05 metre-diameter piles, up to 80 metres depth. In addition, the piles have been base grouted in order to ensure the base capacity of the foundations. Each pylon pile cap is composed of two sections of reinforced mass concrete capping each group of 14 piles.  These are locked together by means of a two cell box girder.

The pilecap construction was carried out in phases: A metallic frame was fixed at the top of the pile, supporting thin (200mm) concrete slabs, on which a first pour (approximately one metre) was carried out. The rest of the pile cap was poured when the first lift reached sufficient capacity to support the load.

The lateral formwork of the pile cap is made of precast skirting panels, keeping the pilecap base above low water level and ensuring a dry working area.

Towers

The pylons towers are H shaped, with each leg comprising a box section. The outer dimension varies from 5.5 x 7 metres to 3 x 5 metres. The stay anchorages are located inside the box and the legs are linked by two cross beams.

Tie down piers

The tie down piers are located at the end of each cable stay back span. They are twin rectangular columns with vertical pre-stress, supported on large diameter bored piles. The tie down pier is used to connect the last three stay cables on the back span. The deck slab is solid concrete and is pre-stressed in both the longitudinal and transverse directions.

Superstructure

The total width of the main span deck is 27 metres, this incorporates three lanes of traffic in each direction (two for cars and trucks and one for motorbikes and bicycles) and footpaths.

There are two vertical planes of stay cables. The structure of the deck is composed of two longitudinal concrete girders linked by transverse pre-stressed concrete cross girders at five metre centres. The stay cables are connected to the longitudinal edge beams by precast anchor pods located every 10 metres.

Tower construction

The towers were built using a jump form in four metre lifts. A tower crane was located on the pilecap next to one leg and a hoist was attached to the other. The towers were constructed in the following sequence:

Construct 56 metres of tower leg and then install the lower strut/lifting beam 48 metres above the pilecap. The lower strut was used to control the forces and deflections in the tower leg and also for lifting the lower cross beam and piertable falsework.

Construct the lower cross beam on the pilecap, attach the pier table falsework and the precast anchor pods for the first segments.

Use the lower strut to lift the lower cross beam and pier table falsework. This heavy-lift was carried out by Freyssinet and involved lifting the 1200 tonne assembly 30 metres into the air. Once the cross beam was in its correct position, stitch the lower cross beam to the tower leg;

Once the lower cross beam was connected and stitched, the pier table could be cast. It was important that the deck construction could start as soon as possible and not have to wait until the completion of the towers;

Continue building the tower legs and install the intermediate strut 70 metres above the pilecap. Remove lower strut. The intermediate strut is a needle beam used to control the forces in the tower legs;

Install upper strut. The upper strut acts as both a brace for the tower legs and to support the formwork for the upper cross beam. Construct the upper cross beam, remove the intermediate and upper strut and complete the tower legs construction.

Install and stress the first stays between the pier table and the tower. Release and lower the pier table formwork and then lift the form travellers into position ready to start deck construction. At this stage the geometry of the deck is controlled by the length of the stays more than the force, since the deck was free to pivot on the bearing. The stays were installed in 25 percent increments up to 75 percent of their installation force.

Deck construction

The following overall construction sequence was used for the main bridge.

Build deck out from D7 tower (on the District 7 side of the bridge). The deck is constructed one segment at a time starting on the river side. The sequence between the two sides is partially offset to reduce the out of balance moments and speed up the construction, i.e. the traveller on the river side is launched to the next segment prior to casting the land side.

Install buffeting cables. After seven segments are cast on either side, the buffeting cables are installed. Only the land side is tensioned. The river side is kept slack unless high winds occur. This limits the bending moments in the tower at deck level.

Install temporary tie down cables. After 11 segments, the temporary tie down cables are installed on the land side. The land side buffeting cables are released and the river side ones tensioned. This then limits the bending moments in the whole tower.

D7 sidespan closure. After 15 segments are cast, the 1.5 metre long side span closure is cast. A survey is carried out prior to the closure and the deck is position by releasing the temporary tie down cable.

Move travellers and repeat for D2 deck (on the District 2 side of the bridge). After the sidespan closure is complete the three remaining river side segments are cast and the travellers moved to the D2 side to repeat the process.

Main span closure. The 10 metre main span closure is cast using one of the travellers. A full survey and adjustment of the stay cables is carried out prior to casting. After casting the SDL loads are placed and any further stay adjustments are carried out if necessary.

The deck has been constructed in balanced cantilever starting from each tower. Ten metre long segments were cast using a form traveller. After the initial learning phase this cycle was regularly achieved in five days with the sidespan cycle lagging two days behind the main span cycle.

The project required significant planning and design to agree and optimise the construction methods. This involved a lot of collaboration between the contractor and construction engineer and has resulted in relatively few issues occurring during the construction process.

The bridge should be opened for traffic in October 2009, two months earlier than planned.

• This article is taken from a paper by Cardno principal Colin Edmonds; BBBH Consortium design manager George Moir; LAP project manager Peter Walser; and LAP structural engineer Martin Romberg, which was presented at the recent Austroads Bridge Conference in Auckland. For a copy of the full paper, visit  the website: www.austroads2009.co.nz.