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The rebirth of a local landmark

Battersea Power Station - Plans to turn the hulk of London's Battersea Power Station into a riverside pleasuredome hinge on some delicate structural engineering, discovers Andrew Mylius.

Cynics reckon that the cover of rock group Pink Floyd's album Animals, on which a pig is depicted flying low over Battersea Power Station, showed uncanny foresight.

Since its turbines stopped spinning in 1982 the station's soot stained brick carcass, topped by four incongrously white chimneys, has mouldered away beside the river Thames.

Finding a viable way of bringing the iconic structure back to life with some postindustrial use has seemed about as likely as, well, flying pigs, especially since developer John Broome ran out of cash during site preparation work in 1996.

But with Hong Kong developer Parkview International staking £1bn or more to a new blueprint drawn up by Arup with architect Nicholas Grimshaw, it is time for unbelievers to rethink their position, says Parkview development manager Steve Kennard. Next year is already set as the start date for construction Parkview has had consultant Buro Happold hard at work on structural analysis of the power station and design of new 'superstructure' to be placed within it. Arup is carrying out structural design on a mini-city of office, apartment, exhibition, conference and hotel buildings that will spring up around the station over the planned four year construction period.

A construction management contractor, Bovis Lendlease, was appointed last year to tackle issues of buildability as designs rapidly evolve, and Parkview plans to invite tenders for construction late this year or early next.

Battersea Power Station is to be reborn as a kind of riverside pleasuredome, packed with chichi boutiques, glitzy restaurants, a couple of cinemas, and a multi-functional 'event arena' for staging fashion shows, product launches, concerts, theatre, dance. Parkview's emphasis is on the spectacular.

'We're building a very large shopping mall - though we hate to use those words, ' sums up Parkview's PR man Ian Rumgay.

'We are trying to create a new commercial, leisure, retail and urban focus for the capital in a building that's a London icon and a local landmark, ' 'It's going to be a secular St Paul's, ' chips in Kennard, 'except it [the cathedral] wouldn't touch Battersea's sides'.

Getting from present day reality to the vision of a sophisticated catwalk cum film studio environment, populated by fashionistas and pulsating to an upmarket urban beat, is no mean challenge, however.

Battersea, constructed in two halves in 1933 and 1953, was built with a heavy duty steel frame clad in brick. It has worn the years badly, with chronic corrosion and spalling of the structure's steel beams popping the brickwork apart (see box).

'Our headline issue is the condition of the power station, ' Kennard states. 'It wasn't built to last hundreds of years; it wasn't even built to be beautiful. We know that when the CEGB [Central Electricity Generating Board] owned the building there was corrosion.'

That corrosion has only worsened since John Broome removed the power station's roof during the early 1990s.

In its favour, the power station was designed according to steel bridge design codes of the day, as there were no guidelines for large steel framed buildings to work to.

Buro Happold partner Steve Brown fleshes out the background: 'The building was designed for heavy industrial use - the kit housed within it was huge. It was big, heavy and dynamic. Its engineers erred on the side of caution and as a result it had enormous residual capacity.'

Even though some steel beams have been eaten to a filigree, there is still enough steel to ensure that Battersea remains structurally sound, reassures Brown. And for Parkview's purposes, over the developer's planned 60 year time frame, it has all the strength that is needed. 'We're replacing heavy plant only with air and people, ' he notes.

Brown's focus is twofold.

Buro Happold must find a way to shore up the Grade II listed structure without damaging or removing too much of the original brickwork.

On the outside, trial brickwork repairs have already been carried out to find the best match between new and old bricks and mortar. Where damage is most acute inside, Brown believes that lining the brick walls with concrete will be the best way of giving them long term integrity. Steel bracing will be used to stabilise walls subject to loading from new internal structures, while corrosion on the old steel frame will be arrested through the use of cathodic protection.

His other concern is to create the extra floor area required by Parkview within the power station without putting too much load on to the old structure. Before grinding to a halt a decade ago, Broome had installed a matrix of piled and pad foundations, which Brown plans to reuse.

'We've modelled these in three dimensions. We know what's there, ' he says. However, he adds: 'The power station poses an interesting geological problem - it's built over two of the biggest sinkholes in London.

'They're scour holes in the London clay and are about 26m deep. What's happened is that the holes filled with gravel.'

Broome piled into the clay around the sinkholes and put in pads over areas of deep gravel.

'The issue isn't about the strength of the foundations that are there but how much they'll move - clay and gravel have different compressive characteristics. It's about differential settlement between pads and piles, and between the individual pads.'

There is no space between the existing foundations to install additional piles or pads, so plans are to construct heavy ground beams, yoking the pads and piles together in groups. In preparation, the second phase of a site investigation is under way to find the underlying clay and gravel's spring co-efficiency, and to test the foundations for differential movement.

Brown is working on the understanding that ground bearing capacity will be variable across the power station's footprint, and that the existing foundations may not necessarily be in the optimum positions to carry new internal superstructure needed to realise Battersea's transformation.

Accordingly, Brown's team has evolved a family of lattice tube towers to channel forces imposed by new mezzanine floors into the ground.

Instead of providing a regular grid of vertical steel columns and horizontal beams, these lattice tube towers will have raked columns and diagonal beams. Their geometry is worked out by applying CAD stress and strain models, so that structural members follow load paths directly to the ground.

'We are moving around the structure to see what moves and by how much, and adjusting loads from the superstructure accordingly. The lattice tubes enable us to spread load into the foundations. The superstructure is likely to be steel to keep weight low, ' adds Brown. 'Start making things heavy and you are compounding the problem.'

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