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The links effect

To complement the huge development at Singapore's Marina Bay, the city is building an extraordinary double-helix bridge that will look like a strand of DNA. Report and photographs by Adrian Greeman.

Since the end of the Second World War, Singapore has successfully transformed itself from a colonial outpost to an independent world trade and financial centre. Now, the heart of the high-rise city is being further developed at Marina Bay with major cultural and leisure facilities.

The bay, an estuary to the cleaned-up Singapore River, is being transformed into a freshwater reservoir with a sea barrier. It will become a water store and a focal point for the city's leisure and cultural development.

Developments on surrounding reclaimed land include the Marina Bay Sands complex currently under construction (NCE 12 June), the recently opened Singapore Flyer observation wheel, the completed Esplanade theatres, and high-rise luxury apartments. New infrastructure is also being added, including a metro link and a road.

The highlight of the city's contribution is a new pedestrian walkway across the Marina Bay, which is being deliberately designed as an iconic structure – not just a connection, but also as a viewing platform.

To find a suitably dramatic design, the city tendered the project out to an international design competition. The winner is the "DNA bridge", a double helix structure that should be able to hold its own between two other unusual structures – the still fairly new Esplanade theatres on the far side of the bay, with their extraordinary twin spiky domes (resembling the local durian fruit), and the new lotus-inspired theatre, which is the centrepiece of the Marina Bay Sands complex.

The bridge is an integral part of the new transport link across the bay, alongside the more straightforward dual three-lane road crossing. It will have a curving arc form on plan, approaching the road bridge at a tangent in the centre, as well as a series of five observation platforms. The bridge floor will be transparent at points allowing a view downwards to yachts and water-skiers in the water below.

The design of the 280m-long bridge was produced by Australian architect Cox Group, local Singapore firm Architects 61, and engineering consultant Arup, working from its Australian and Hong Kong offices. "You should not underestimate the drive to have a statement here," says Matthew Clarke, Arup structural engineer in Sydney, who worked on the project. "The city's Urban Redevelopment Authority wanted something very different and eye-catching."

That "something" turned out to be a double-helix – two interlocking tubular steel helices that rotate in opposite directions, reflecting on a huge scale the famous double helix structure of DNA. There are even cross members linking the two spirals in the way that the four DNA bases form a spiralling "ladder" along the genetic material.

Those connections are important in the bridge as they tie the structure together. "The spiral members have an active tendency to want to straighten out," says Clarke. "The crossovers are linked, and when you join them the opposing forces balance each other out."

The resulting spiral truss is actually a very good structure to handle the curved arc of the walkway across the water, adds Clarke. "Curves induce a lot of torsion and other kinds of structures would not be able to deal with that so well. Net and tension structures, for example, which are increasingly commonplace, would be unable to do it."

Deep spiral
A relatively deep spiral for the two complementary main elements allows them to wrap around each other, while leaving sufficient space in the centre through the circular envelope left in the middle for the pedestrian walkway. Glass and mesh canopies will provide shade above the bridge deck and its inset glass panels.

The viewing platforms are also spaced along the length, and it will be possible to walk over on to a pedestrian deck on the road bridge at the centre point. "The client wanted a physical connection," says Clarke.

Analysing the bridge has required some complex software packages, including some of those developed for the Beijing Water Cube Olympic swimming pool and its 'bubble' walls, as well as Arup's GSA software.

Design work was made more complex by the decision to use stainless steel for the bridge.

According to Clarke, the main reason for using such an expensive material is to reduce future maintenance loads. "Singapore has a strict inspection and repair regime with a full survey every five years," he says. "A full repainting [of conventional steel] would also have been needed at least every 20 years, and both things are difficult to do over water."

He argues that this material is less expensive than might be expected because it is possible to use very high-strength duplex stainless steel, similar to that used on the Stonecutters Bridge in Hong Kong. "That is quite a jump up in strength properties over normal 316 steel," says Clarke. "That means you can shave a lot of weight from the bridge and use less steel overall."

As the structure is quite stiff, it therefore benefits from the use of high-strength materials – around 500t of steel will be needed.

Optimisation software was run on the design, which repeatedly tests the bridge elements one by one with material shaved out until it produces cost-effective minimum use of steel against strength. "We interfaced that with another external software package from Strand Seven, which is one of the few doing British codes on stainless steel," says Clarke.

However, the stainless steel will not have a dazzling polished finish. Contrasting matt and grit-blasted finishes are being used for the two different spirals to emphasise their 'oppositeness' and the overall form. Night lighting along the spirals and cross links will perform the same function after dark.

Oddly enough, the high-strength stainless steel is a little easier to weld than other steels. Much of that will be done on site, although the architect and engineering design team have already broken it down into segments, says Clarke. By taking some trouble to refine the design, it could allow the spirals and crossovers to essentially repeat, making a convenient 2.7m-long section.

These could have been made as prefabricated modules, each one rotated relative to the next for assembly and site erection.

"But the contractor, Japanese company Sato Kogyo, has elected to go for conventional erection methods on site," explains Clarke. Piling work started in the spring in the bay is not wholly for the bridge, but also for supporting a temporary work platform.

Foundations will be needed, of course, and there are a series of 1m diameter bored piles needed, 30m or so deep through the soft marine clay of the bay. "They are not so much needed for vertical loads as transverse resistance against boat impact," says Clarke.

One factor that has helped is the transformation of Marina Bay, from a river to a freshwater enclosure. With the link to the sea removed for boats there are likely to be fewer heavier vessels using it. However, the water is bound to be busy with recreational boats, making speedboats and jet-skiers the likeliest source of impact rather than dredgers and freighters.

Singapore has been developing its tourist and leisure industries intensively for the past 15 years, and the new development aims to fit in with these plans. These include the extraordinary 190m-high hotels of the "integrated leisure complex" in the giant Marina Bay Sands project, the shaded transparent domes of the Esplanade, and the ever-growing high rise skyline of the city behind.

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