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Bridging millennium; the gap for the

This week deputy prime minister John Prescott launches construction of London's Millennium Bridge. Andrew Mylius gets a preview of the link between London's richest district and one of its poorest.

Minimalist elegance underpinned by sinewy toughness characterises Millennium Bridge.

In the genealogy of bridges, it sits somewhere between suspension and ribbon structures. Arup director Chris Wise describes the 330m structure: 'It's about cables stretched over the river. Then you walk on them.'

Eight 120mm diameter cables will be spun across the Thames - four on either side of a pedestrian deck. They will be supported by Y-shaped arms atop two midstream piers, and the deck will be carried by 'legs' which follow the cables' vertical profile and pull them into a distinctive lateral curve.

Wise heads the engineering part of the design team, working alongside Arup colleagues, architect Foster & Partners and sculptor Sir Antony Caro.

The bridge's toughness stems in part from its vulnerable location. 'Ships quite often hit bridges,' says Wise. There will be massive foundations despite the bridge's slender form.

The £12.5M project, which will provide central London with its first Thames crossing in 100 years, is first and foremost about regeneration, believes Wise. Think of it as an urban umbilical cord between the City, London's richest district, and one of its poorest, Southwark, he says.

The bridge will stretch over the Thames between St Paul's Cathedral on the north bank and the new Tate gallery on the south. Southwark Borough Council is the official client, but the £12.5M project is 50% funded by the Millennium Commission. The Corporation of London, banking group HSBC and private donors have also contributed.

Construction officially starts this week, and one of the first tasks will be preparation for the pier foundations.

Two pairs of 6.3m diameter, 20m deep shafts are to be excavated, allowing installation of segmental concrete linings and construction of grade 40 reinforced concrete caisson piles. Each elliptical reinforced concrete pier will be built on a 3m thick concrete pile cap. It is calculated the piers would yield only 160mm to a direct hit from even the largest vessels on the river.

In June, two cofferdams will be placed mid stream to enable construction of the piers. At high tide there will be 8m of muddy water forcing against the cofferdams, sucked at by 7 knot currents at the peak of ebb and flood. Pumps will race round the clock to keep the hole dry.

When you are working down a 20m hole in the bed of the Thames the last thing you want is a runaway dredger ploughing into your defences. Wise says the cofferdams have been designed with as small a plan as possible to minimise the vulnerable area. They will be elliptical and heavily reinforced internally.

Wise has reassured Sir Robert McAlpine, contractor for construction of the piers, that the cofferdams are collision-proof. Traffic on the Thames is hardly prolific, but some ships weigh 2,000t. Moving with the current, they would deliver an impact of about 35MN.

Caro had been keen on a bridge of steel swirls and arcs - a huge acrobatic ribbon across the Thames. Great for skate-boarders, but a challenging promenade for the average Londoner, Wise says. The lean, clean aesthetic design of the bridge being built came out of engineering.

At the same time, the design team was concerned not to detract from St Paul's, the Tate or from the river itself. 'It's a way of getting you to a point where you can look at London: it's not about looking at the bridge. It's the inverse of ego,' he says.

Standing 12m tall from the river-bed, the tops of the sculpted concrete piers will be 4m clear of high water spring tides, crowned by stainless- steel Ys supporting the suspension cables. The 10m long inclined arms of the Ys hold the cables 7m out from the bridge's centre-line at the piers.

Bent beam 'legs' beneath the deck, attached to the cables with lozenge- shaped clamps, will carry the deck as a continuous horizontal surface. The angles of the legs will vary along the length of the bridge to accommodate the cables' natural parabolic sag. These legs are also being used to give the cables a lateral parabolic profile.

When they are spun across the river between December and January, the cables will be jacked tight over the stainless steel Y arms. This means that, as they stretch from each bank, the cables will diverge 7m either side of the bridge's centre line. They will then be gathered by the legs to within 3m of the centre at the mid point of the main span.

As a suspension bridge, the design team knew Millennium Bridge would behave well in the vertical plane, but lateral and torsional stability was required as well. By placing the cables in a curve outside the deck Wise says they stabilise wind-induced rocking in the same way as outriggers on a trimaran do.

A combination of dynamic and finite element analysis was needed together with traditional hanging chain modelling. 'The engineering was right at the cutting edge,' says Wise.

Two thirds of the bridge's 600t is accounted for by cables, with Bridon as frontrunner supplier. These are required to have a tolerance of +/-2mm and will be tensioned via jacks on the north bank to 2,000t, making Millennium Bridge 10 times flatter than San Francisco's Golden Gate Bridge. Caro describes it as 'strung like a guitar'.

Winds blow up the Thames valley and Millennium Bridge is designed to withstand a 1 in 100,000 year wind - some 45m/second, or twice as violent as the storm that shredded the UK in 1987. With the aid of six wind-tunnel tests Arup has also checked to make sure the bridge will hold up to sustained pressure from lesser winds.

'We were using the tunnel like a design tool... like a sculpting device,' observes Wise. He parallels the development of Millennium Bridge's aerodynamic performance with automotive or aeronautical design.

Wise and the team were concerned that, on completion, the structure would be so light 'people might think they'd be blown into the river' should a good wind get up. While working in the wind tunnel on minimising vibration, the design group also looked at reducing the buffeting experienced by pedestrians.

The profile of deck and balustrade act both to help wind slip smoothly over and under the bridge and as a faring for those on it. Decking will come from superstructure contractor Monberg-Thorsen as prefabricated, extruded aluminium elements.

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