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Baltic status

STRUCTURAL SURGERY: Major structural surgery and state-of-the-art engineering skills are transforming a Tyneside landmark.

Convert a derelict 1950s flour mill into a 21st century art 'factory' offering futuristic visual, acoustic and interactive exhibits.

This is the brief now being faced by a team of architects and engineers charged with transforming the vast art deco-style abandoned Baltic Mill on the banks of the River Tyne at Gateshead into a £46M centre for contemporary art, the focal point for a planned 'South Bank' of the North.

And it is appropriate that the initial construction task itself calls for the art of engineering to be stretched to the full. The building's entire interior - some 18,000t of eggbox-shaped reinforced concrete storage silos - must be demolished while retaining intact the 40m high surrounding brick facade.

'We think of this more as a decommissioning task rather than demolition,' explains Paul Ellis, contracts manager at contractor Edmund Nuttall. 'It is a project where everyone involved must function as a single team, co- ordinating the complex demolition sequence, so that we can turn a remarkably difficult task into just a difficult one.'

At the heart of the challenge is a large and innovative £600,000 support frame, currently cocooning the mill's outer walls while the multi-phase internal demolition continues unhindered. This 500t of temporary works - half the weight initially planned - has, claims Ellis, saved client Gateshead Metropolitan Borough Council over £300,000 compared to original pre-tender suggestions.

Foundations for Joseph Rank's mill, 600m downriver from Newcastle's famous cluster of Tyne Bridges, were started in the late 1930s. But the war intervened, and it was 1950 before the building - used as a model for other Rank Hovis mills and described at the time as the 'pride of Tyneside' - was finally completed.

For the next 32 years its honeycomb of 183 silos 40m high stored up to 22,000t of flour, which was ground from wheat brought in by conveyor belt from adjacent ships. The mill now stands abandoned in the midst of a largely derelict riverbank site being redeveloped by the council into Gateshead Quays, an 18ha, £150M cultural and leisure complex.

Just upriver, ground clearance starts this summer for the Sir Norman Foster-designed £60M Gateshead Music Centre with its distinct triple-shell roof. And behind lies land earmarked for hotels, cinemas, shops and offices.

A few metres off the mill's waterfront, a jackup barge is on station, ready to help construct foundations for the Baltic Millennium footbridge with its distinctive eyelid-shaped tilting deck. So what better venue, said Gateshead council, in which to house the largest centre for contemporary art outside London than the mill itself?

Technically, it would have been much easier to demolish the unlisted structure and start again. But, argues the council's consultancy liaison officer John Anderson: 'The mill has become such a prominent and popular local landmark that the public wanted it retained.'

On paper, demolition looks simple: gut the interior, remove most of the end gable walls and replace with vast glazed panels. But the 380mm thick brick and concrete skin of the 53m long mill was never designed to be freestanding, and its four corner towers are in fact false frontages with nothing structural behind them.

While Nuttall was winning the £1.5M design-and-demolish contract, its designer Ove Arup & Partners was examining the two conceptual designs suggested by the client for a 1,000t frame to temporarily support the facade.

However, an internal frame option would have interfered with demolition around it; while an external alternative needed its own new foundations in ground suspected of containing the unmapped remains of previous buildings.

The consultant instead designed a new frame at roughly half the weight and half the cost of the original suggestions. 'We opted for an external frame to allow uninterrupted demolition, but one that could make use of the building's own foundations,' explains Arup associate director Martin Butterworth.

Side walls are supported by 19 vertical trussed frames, each 35m high and bearing onto the mill's existing foundations. The tops of the frames are tied together over the roof by an open truss made up from 508mm diameter steel tubes, and the support box completed across gable ends by diagonal similar size tubular beams.

But the really clever bit is how the total frame acts together to carry loads, from both itself and the facade, down to the building's own foundations.

Nuttall has built 19 concrete blocks, sitting on the mill's existing piled ground slab and running alongside the inside face of the side walls. A horizontal steel beam is cast into each 3m high block and projects out through the adjacent facade to support the base of each of the frame's 19 main vertical trusses. This arrangement allows all vertical loading on the frame to be transferred directly to the piles inside.

Horizontal forces are routed, via the roof trusses, to gable ends where the frame's diagonal bracing carries loads down to each corner of the building.

'We considered the same load transfer arrangement to inside piles,' says Butterworth. 'In the end, however, uncertainty over the mill's ground slab design, and the complexity of connections needed into the existing wall, meant it was easier to form a new single external pile cap.'

More difficulties came from the need for the frame to be erected in some half-dozen meticulously planned stages, integrated totally with demolition contractor O'Brien and Sons' phased removal of the inside.

The dense eggbox of silo cells has a 260mm thick perimeter concrete skin, integral with the mill's single 114mm brick facade. It is thought the whole silo block was slipformed, with brick walls built up during concreting to act as a permanent outer shutter for the slipform. To free this facade from the 2.4m square silos, some 108 sections of connecting 100mm thick cell wall had to be sawcut their full 27m length from top to bottom.

The middle half of each gable end had then to be demolished, to be later replaced entirely with glazing. And it was through one of these removed end walls that O'Brien started nibbling into the concrete silos with pincers mounted on a 25m long articulated excavator arm.

Even with the excavator standing on a 6m high bund, it could not reach the top of the mill. So the uppermost 5m of silo cells, plus a 3m high roof area above, were demolished in advance using two 20t excavators lifted up over the already removed roof to sit on top of the silos.

Phased erection of the frame had not only to accommodate the changing structural state of the facade walls - as silos and gables were cut free - it had also to provide a variety of access gaps for cranes and excavators to enter the building.

Any movement of the facade is recorded automatically every five minutes by a network of 47 electrolevels attached to the brickwork.

'With most of the inside now gone, total displacement of just 14mm is well within our 60mm tolerance,' says Ellis.

Nuttall will have removed all the interior and be off site by next month. But the follow-on £25M contract now being tendered - to install eight main internal floors and fit out the art gallery for opening in autumn 2001 - will not start until September.

This gap allows the mill to itself host an unusual 'open house' summertime exhibition. With a large sheet of red material draped into a sculpture and suspended inside the gutted structure, the public will be invited to walk through the building to experience the large open space.

The Baltic, with its £37M Arts Council lottery grant, is hoping to attract 400,000 visitors a year. Whether Tyneside's down-to-earth Geordies will fulfil this high expectation may be influenced as much by the promise of free admission as by descriptions of the exhibits.

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