More than just a crossing

Why is a new Thames bridge creating tension?

The first new bridge across the Thames in central London for over 100 years is getting above the plimsoll line. Quite high above it, in fact, with a whopping great pair of piers standing 9.5 metres high at a landing below St Paul's and on the South Bank outside the new Tate Modern. These piers hold high tension cables in lofty outstretched arms.The cables are anchored deep in the ground at either landing, where steps and ramps connect the bridge to the bank.

When Lord Foster of Thamesbank designed the bridge, with Chris Wise from Ove Arup and the sculptor Sir Anthony Caro, as a "place to walk through, under, over and around with platforms over the water to view and browse", he described the design as "minimal intervention". But engineering and shipping demands have put on a bit more height and girth than the architect intended for his minimalist design.

Eight cables on reels, each weighing 70 tons to support the bridge over its 350 metre span, are suspended from high lines at 30 metre intervals. A billboard outside the construction site at Bankside describes this bridge-building technique "like pulling a curtain on hooks". Tenterhooks, more likely.

The difficulty of sinking piers into the chalky riverbed in the fast flowing tidal Thames, caused some anxiety and setbacks to the schedule earlier in the year, but the Queen still has it in her diary to open the bridge in June. A river barge will deliver the bridge deck sections within the next few weeks when the steel parts, to be decked in wood, will be lifted straight into place. Each 16-metre section is in a holding yard opposite the Dome where the bridge's remaining component parts - the handrails, walking surfaces and lighting - will be installed. Fibre optic light embedded in the bridge decking will turn it by night into a "blade of light".

In the notes that accompanied his drawing of St Paul's linked to the Tate Modern, Lord Foster described the axis of the light box - the symbol of the new Tate - as a new marker. Bridges symbolise so much more than a mere crossing place, and his expressed wish for the Millennium Bridge to be a link between north and south and art and commerce seems to be supported by the funding.

Two years ago, when the lottery-funded Millennium Commission gave half the estimated costs towards the bridge, it was to cost £14.2m, but costs are spiralling. Now the £15.9m raised is said to be for project costs, without a final figure for the bridge. The Millennium Commission gave £7.1m, the Corporation of London £3.5m, HSBC Holdings £3m and Cross River Partnership, a trust originally set up in the 13th century to build the first London Bridge, £1.8m. But clearly more money is required.

The idea for a public subscription for the bridge - both Waterloo and Westminster bridges were built that way - was launched last year. David Bell, chairman of the Millennium Bridge Trust, expressed his astonishment at public interest in the bridge and offered them the chance "to give that interest a lasting expression". Private individuals can become patrons of the bridge and have their names recorded on the structure for posterity if they give £500.

Membership details are available from Millennium Bridge Trust, Pearson plc, 3 Burlington Gardens, London W1X 1LE.