Climatic conditions at the site of the new high-level cable-stayed road bridge over the Cua Luc estuary were a critical issue for the design of the structure.
The crossing was built as part of a new highway link from Ha Noi international airport to the Chinese border, intended to increase trade between the two countries and stimulate industry and tourism around Ha Long Bay.
Bai Chay Bridge is a 903-metre long, 25-metre wide structure with 4 traffic lanes as well as 2 passing lanes for pedestrians. Concrete towers rise to a total height of 137,5m above foundations.
The bridge owner placed great emphasis on the importance of taking all possible steps to minimise the wind loading – and resulting cable vibration – that the 903 m-long structure would be subjected to.
This included specifying the use of compact cables and internal dampers, as well as reducing the profile of the prestressed concrete deck, which with a main span of 435 m was the longest of its type in the world.
A compact cable system had already been developed by Freyssinet when bidding for another project that was similarly afflicted by high winds. Although the previous bid had not been successful, this system was perfectly suited for use on the Vietnamese project.
Adaptation measures included reducing the thickness of the HDPE coating on every strand by less than a millimetre, to minimise the final cross-section of the cable while still meeting the stringent corrosion-protection specification.
Special tools were needed for the assembly and erection of the strands within the slender cable sheath. Every millimetre was crucial and not only did the butt-welded bead have to be pared off the inside of the sheath to free up extra space, but a very slender shuttle was developed for use in coaxing the final strand through the remaining space.
With the structural elements slimmed down, controlling vibrations was also a critical issue and Freyssinet supplied and installed a range of dampers on the cables to address this.
The 112 cables vary in size from 37 to 75 strands, and in length from 50 m to 230 m; internal radial dampers were used on the 20 longest cables, with internal hydraulic dampers applied to the mid-length elements and elastomeric dampers on the shortest.
Three different shades of yellow were selected by the client, which Freyssinet incorporated in the individual HDPE sheaths to give the cable array the appearance of a sunset.
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