RISK OF cable vibrations caused by wind induced sway have forced engineers to redesign the huge 293m high towers for Hong Kong's 1.01km central span cable stayed Stonecutters Bridge.
Composite construction will now be used for the upper 120m sections instead of an all steel solution to increase stiffness.
News of the change has emerged as five groups of international contractors prepare to submit bids to build the bridge incorporating what will be the world's longest cable stay span.
Halcrow/Flint & Neill won the original competition for the bridge, with a design which featured distinctive all steel upper sections.
But this was a preliminary design and had not been wind tunnel tested before detail design work was handed over to Arup's Hong Kong office.
After a programme of wind tunnel tests, Arup has finally opted for a stiffer stainless steel/concrete composite shell for the upper tower sections.
Arup's solution was developed in conjunction with its principal sub-consultant COWI of Denmark.
A stainless steel outer shell will be doweled into an inner insitu concrete skin, typically 800mm thick. High performance duplex stainless steel 20mm thick was specified for the outer skin.
The Arup design team has a brief to retain much as possible of the visual aspects of the original competition-winning concept.
This meant switching to an all-concrete pylon was undesirable.
'Duplex stainless steels combine durability and strength, ' said stainless steel fabricator Ancon Building Products special projects manager Paul Fisher.
'But they are difficult to weld consistently, because steel temperatures have to be very closely controlled or durability is compromised.'
Recent full scale trials in Ancon's Sheffield factory proved the feasibility of the Arup concept.