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Design for life

The Stonecutters design dates back to 1999, when the Hong Kong Special Administrative Region (HKSAR) Highways Department launched a competition to find a landmark bridge. Halcrow/Flint & Neill/ Dissing + Weitling/Shanghai Municipal Engineering Design Group JV won the competition and developed the reference scheme, but the detailed design contract went to Arup in 2001 (NCE 1 March 2001). While the aesthetic aspects of the reference scheme had to be adhered to, Arup made some design changes.

In the original design, the top third of the concrete towers was to be made of steel to create a 'modern look' and to speed up construction.

'But at the review stage, we found the frequency of the cable stays would be the same as the steel [in the tower] and could cause resonance problems. So we decided to add weight to the top of tower by making it composite, and to give it a frequency outside the cable frequency, ' says Arup director Klaus FalbeHansen.

The top 118m of the 300m tall concrete towers will now be sheathed in stainless steel to cope with the harsh marine climate. An outer layer of stainless steel reinforcement will also be incorporated in the concrete to increase its durability and achieve the 120 year design life. The towers will be constructed using a jump form climbing formwork system, rather than slip forming, to create a 'smoother, marble-like finish'.

Typhoon winds are the main design challenge for the bridge, which has been designed for wind speeds of up to 100m/s.

'Wind doesn't blow consistently and the effect of big gusts on the flexible structure is of great concern, ' says Falbe-Hansen, adding that large wind loads could introduce resonance problems.

'We've introduced a kink into the [steel] deck cross section to deal with some of the aerodynamic issues, ' he says.

He explains that the original steel deck cross section had 'smooth belly' profile where the wind could flow along the underside and then 'slip'. But these wind loads could then ricochet off other parts of the structure, causing damage.

'By introducing the kink, we can predict where the vortexes might set up and guide the wind away from the structure, ' he says (see diagram).

Arup associate director Fergal Whyte adds that the other advantage of introducing a defined angle is that it helps to locate the deck section with the cross girders. Arup also removed the monolithic connection between the deck and towers to reduce stresses at this junction during construction when the steel deck sections cantilever out from the main tower.

Ship impact will be accommodated by a hydraulic 'shock absorbing system' between the deck and tower which will also help with effects of slow moving traffic and possible earthquakes.

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