The cable-stayed bridge is being built as part of Project 18, for Vietnam's Ministry of Transport, and forms part of a new link between Ha Noi international airport and Bac Luan, on the Chinese border. It carries the road over the Cua Luc estuary, just north of the World Heritage site of Ha Long Bay, a crossing which is currently made by ferry. The new road is expected to improve trade links between Vietnam and China, and stimulate industry and tourism in Ha Long Bay and the nearby city of Hai Phong.

In fact the new bridge will not replace the ferries, which are planning to continue running once the link is open to traffic. Much of the traffic using the ferries at the moment is local traffic, and crossing the estuary by boat will continue to be the most convenient route for these travellers. The shipping clearance required results in the deck being some 50m above the water level, hence the nearest junctions giving access to the bridge's approach roads are some kilometres back from the abutments. The continued use of the ferries will not only be more convenient for local travellers, but it will reduce the traffic on the bridge.

Financing for the project has been provided by the Japan Bank for International Cooperation, an arrangement which required a certain percentage of the loan to be spent on Japanese goods and services. Design work is being carried out by a number of companies, including Japan Bridge & Structure Institute; Pacific Consultants International; Transport Engineering Design Incorporation, and Hyder Consulting. The main contract was awarded in May 2003 to a Japanese joint venture of Shimizu-Sumitomo Matsui, and work started on site in August of the same year.

As well as being the showcase for new cable technology, the bridge will be the longest of its type in the world - its 435m-long central span will be the longest for a bridge in prestressed concrete with a single, central plane of cables. This new structure will overtake the other giants of its type - the Elorn Bridge in France, the USA's Sunshine Skyway, and the Coatzacoalcos Bridge in Mexico.

The feasibility study for the new bridge began in 1999, but a lot of investigation had to be carried out into the environmental impact of the new bridge, considering the proximity of Unesco-protected Ha Long Bay Ha Long Bay, in the Gulf of Tonkin, was listed as a World Heritage Site in 1994 - it includes some 1,600 islands and islets, forming a spectacular seascape of limestone pillars. Because of their precipitous nature, most of the islands are uninhabited and unaffected by a human presence, and the site's outstanding scenic beauty is complemented by its great biological interest.

Although the bridge is not actually inside the protected area, it can be seen from there, hence a lot of thought went into the aesthetics of the structure. In the initial feasibility stage, a tunnel was also considered for the crossing, but was found to be too expensive since it was relatively short. In the detailed design stage, five bridge alternatives were studied, four of which were cable-stayed bridges and which could span the strait without needing piers in the water. Three different shades of yellow were chosen for the HDPE ducts on the bridge, and they will be used from centre of the towers outwards, with the intention of evoking impressions of a sunset.

Foundations for the new bridge were built in concrete caissons measuring up to 18m by 22m, which were sunk under compressed air to a depth of up to 26m. Underlying rock is sandstone, with thin mudstone or medium, partly fine sandstone above. Completion of the foundation construction took about a year, after which the tower erection began. The bridge is a total of 903m long, with a main span of 435m, back spans of 129.5m on each side, and additional approach spans of 41m and 81m on one end, and 87m on the other. The concrete towers rise to a total height of 137.5m above the top of the foundations, and the shipping envelope is 200m wide by 50m high.

The 25.3m-wide deck is supported by a total of 112 stay cables, and will carry two lanes in each direction, and two walkways. The cables vary in size from 37 strands to a maximum of 75 strands, and from 50m length to 230m at the longest. Contractor Shimizu Sumitomo Matsui Joint Venture is currently working from both towers at the same time, but casting only at one location at a time - workers prepare and cast at the end of one cantilever, then at the other end from the same tower, then move to the other tower to cast elements at each end of the cantilever. Closure of the span is expected to take place in April next year.

One of the main criteria for the final bridge design is to reduce the effect of wind loading on the structure, by making the deck as slender, and the cables as compact as possible. The bridge was originally designed to resist a design wind speed of 45m/s, but this was later upped to 50m/s after review by the client's advisory committee - in addition, the factor of safety was revised to 1.3. The purpose of these revisions was to enhance the safety and durability of the structure, and required changes to be made during the design review and checking phase.

In order to meet these revised requirements, additional vertical tendons were added to the tower design, and the compact stay cable system was developed in order to keep cables as slender as possible and hence reduce wind loading on the structure.

The strand is being produced by a Japanese supplier under a licence granted by Freyssinet, in order to meet the requirement for sourcing 50% of the materials from Japan. But installation is being carried out by Freyssinet on site, and construction manager Craig Robertson explains that many of the site staff carrying out the cable erection are veterans of Vietnam's My Thuan Bridge, which was finished in 2000 (Bd&e issue no 18).

To absorb vibrations, the bridge is fitted with three types of internal dampers, according to the length of the stays. On each tower, 20 internal radial dampers are installed on the longest cable, 20 internal hydraulic dampers on the intermediate lengths, and 16 elastomer internal dampers on the shortest cables.

The monolithic concrete box girder bridge deck is being cast in situ using the balanced cantilever erection method. A specially-designed formwork carriage at the end of each cantilever enables 6.5m-long segments to be cast one at a time, followed by cable stay erection. As Robertson explains, the strand by strand erection process that is being used here is subtly different to the usual method, in order to make the cables more compact than normal.

The first stage of strand by strand erection involves the cable duct being lifted into position, after which a 'shuttle' is pulled up inside the duct, with a length of strand attached. Operatives at the top unclip the strand from the shuttle and attach it to the anchorage, then the shuttle is pulled back to the deck level ready for the next length of strand to be attached.

As more and more strands are erected, the space in the duct is reduced; when the shuttle is pulled back down for the last time, and removed, there is still a certain amount of space left in the duct - the space taken up by the shuttle.

According to Freyssinet Vietnam director Roger Raymond, the decision to attempt to make the cables as slender as possible was not taken until after the contract had been signed. And at the same time as the designers decided to introduce a compact duct, they also asked Freyssinet to produce the ducts in three different shades of yellow.

At first the company's technical department was not confident that they could achieve the developments in the time required. One of the first steps was to shave half a millimetre off the coating of each monostrand - by using a coating of 18.5mm thickness rather than 19mm. Although such a modest reduction would not make any difference in terms of corrosion protection, in a cable of 75 strands it would make quite a contribution to space saving.

A new shuttle was developed that took up a lot less room that the standard one, and was specially shaped to avoid damaging the strand coating as it was pulled through the duct in the final stages of erection, when space was down to a minimum. In its laboratory, Freyssinet tested a 30m-long section of the cable duct and found that the new design worked, albeit more slowly than the standard shuttle.

On site at the Bai Chay Bridge, the first of the compact ducts was under erection when Bd&e visited. Robertson explains that the new shuttle is only used for erecting the last ten or so stays - the traditional shuttle is used for the rest of the cable. The disadvantage of saving 20% on the cable size is that it takes approximately 20% longer to erect each cable.

At the end of September, the cable crews were working on an eight-day cycle of cable erection, but Freyssinet says it is on track to complete its work as scheduled by May 2006. The opening of the new bridge is due to take place on 9 August 2006.

Client: Socialist Republic of Vietnam

Consultants: Japan Bridge & Structure Institute; Pacific Consultants International; Transport Engineering Design Incorporation; Hyder Consulting

Main contractor: Shimizu-Sumitomo Mitsui Joint Venture

Subcontractors: Vinaconex (girder construction); Chi Thang Construction, Engineering & Trading Company (tower construction); Freyssinet Vietnam (cable installation); Civil Engineering Construction Corporation No 1/Vinaconex (substructure construction); Shiraishi Corporation (pneumatic caisson construction)

Printer friendly version

Email this article to a friend