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The Great Bridge of China
It might not last as long as the Great Wall, but Hong Kong's new single span cable-stayed
bridge has been designed to last 120 years
By Carroll McCormick
Nickel Magazine, July 2005 -- Its deck will soar 75 metres above the
entrance to Hong Kong’s Kwai Chung container port, and its two pole towers will rise 290 metres into the sky.
When it is completed, in 2008, the 1,600-metre-long Stonecutters Bridge will be a key component
in China’s global trade activity.
The difficulty of providing access to, and performing maintenance on, the upper part of the towers,
prompted the designers at Arup Hong Kong to insist on a 120-year maintenance-free service life. To satisfy
the rigorous structural and surface finish requirements, Arup Materials Consulting in London, England, chose
S32205 duplex hot-rolled plate (containing 4.5 to 6.5% nickel) to form the top 120 metres of
the towers. About 2,000 tonnes of S32205 will be used.
Arup also specified
S30400 stainless steel reinforcing bar (containing 8.0 to 10.5% nickel) in the concrete piers and main
tower splash zones. Italian-based Cogne will supply 2,882 tonnes of rebar in diameters up to 50 millimetres
(mm).
Arup Hong Kong is supervising the four-year construction, while assuming responsibility for all
engineering (civil, structural, mechanical, electricl and geotechnical), as well as traffic-, wind- and
maine-related work. COWI Consulting Engineers and BMT Asia Pacific Ltd. are providing support to Arup.
The international consortium Maeda-Hitachi-Yokogawa-Hsin Chong Joint Venture won the US$350-million
construction contract. Outokumpu in Sweden is supplying the S32205 and the welding consumables.
Curved S32205 plates, most of which are 20 mm thick, will be welded and bolted together to form the skin.
Plates will be welded together to form a section size appropriate for lifting. The top and bottom of each
section will have a 25-mm-thick, 130-mm-wide structural flange, so adjacent sections can be bolted
together.
Assemblies of bolted-up sections will then be lifted into place to form a structural "can" into which
concrete will be poured. About forty-thousand 300-mm-long S32205 studs, 16 mm in diameter, will be welded to
the back of the plates on 300-mm centres, providing the necessary bond to the concrete. Loads will be
transferred from the concrete to the skin and the flanges.
In seeking material for the skin, designers decided that although carbon steel had the necessary
structural strength of 450 Newtons per mm squared, it did not offer the required zero maintenance. "The
strength that was required could not be met by an austenitic stainless steel, which has a design strength of
about 300 N/mm²," explains Graham Gedge, Arup’s specialist in project materials. "It had to be thicker and
thus heavier and more expensive: with S32205, we knew we could achieve a strength of 450 N/mm² with hot
rolled plate."
There was another reason for discounting standard austenitic stainless steel: long-term performance
in this polluted marine environment would have required a carefully controlled surface preparation. The
durability assessment of the environment in which these materials are expected to perform is C5M, the worst
atmospheric exposure possible under the ISO environmental classification.
S32205 is ideal for the finish the designers specified. "S32205 is less susceptible to pitting and
staining than other candidate alloys, and allows us more flexibility in choice of final surface finishes,"
Gedge explains. "The control of final surface roughness becomes less critical, even if it will trap some
dirt and salt."
A matte, non-directional finish was chosen, partly for aesthetic reasons but mainly to simplify
fabrication. "We didn’t want two great huge mirrors in the Hong Kong harbour," says Gedge, "and so
much welding is required, it was crucial that we chose a finish that's easy to restore after
fabrication."
Outokumpu will polish the plate to a standard finish. Once the sections have been fabricated, they will be
blasted with an aluminum oxide abrasive. Then the surface will be finished with glass beads to blunt the
aluminum oxide’s sharp finish.
To ensure success at the design stage, Arup contracted Ancon Building Products, in Sheffield, England, to
fabricate a full-size prototype of a skin section. Barnshaw Bending, in Glasgow, Scotland roll-formed the
plate and Metal Improvement Company, in Derby, England, applied the surface finishing.
The prototype was made in two halves, each about 1.75 metres high and seven metres long, curved on plan.
Each half was built up from butt-welded welded-up plate, and the two halves connected by a bolted flange
identical to what will be used in the actual structure.
Plates were roll-formed to a conical shape (the tower radius tapers from 30 metres where the S32205 skin
begins, to 27 metres at the top) and details such as welds, flanges, studs and cable conduit holes were
carefully worked out.
"The prototype forced us to re-think the flange connections, because the weld was distorting too much,"
says Gedge. "We did a second, smaller fabrication with the altered weld detail, which reduced the distortion
significantly. Our biggest challenge was how to achieve the least distortion and the highest structural
strength.
"The protoype also enabled us to produce the required finish and ensure that the welds are
not obtrusively visible. And we determine that the finish can be achieved on heavy, hot-rolled plate."
The combination of duplex towers and stainless steel reinforcing bar should result in a bridge that will
endure.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: Arup Materials Consulting
Graham Gedge
Associate Director
Arup Materials Consulting
Arup Campus
Blythe Gate
Blythe Valley Park
Solihull
West Midlands B90 8AE
United Kingdom
Tel: 44 (0)121 213 3431
Fax: 44 (0)121 213 3001
Mobile: 44(0)7711 696099
E-mail: graham.gedge@arup.com
Web site: www.arup.com
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