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Steely resolve

Structures - Steel

A steel basement and structural frame are turning convention on its head at a residential development in Leeds.

Convention has it that the basement walls of multi-storey residential buildings and the superstructures they support are not built in steel. Fire protection and acoustic dampening make it 'too tricky a problem'.

'Well, all that's gone out the window on this job, ' says Peter Haines, director of structural consultant Terence E Dudley & Associates. The project he refers to is Whitehall Waterfront, a quality high rise residential development being built close to Leeds city centre. 'Our client demands value and early completion but is open minded to innovation.'

And innovation is what the client, York-based developer KW Linfoot, is getting. Whitehall Waterfront appears to be a conventional residential building on the outside but beneath its cladding are remarkable departures from the norm, including a steel basement supporting a structural steel frame.

Leeds is claimed to be Britain's fastest growing city and is now second only to London as a financial centre.

Young people in particular are moving back into the city centre and demand for accommodation is constant. Whitehall Waterfront is providing 192 apartments plus shops and office space in 17 storeys above ground, and 5500m 2of car parking on two levels below.

Innovation starts below ground level. 'The basement was initially seen as being in concrete. But at tender stage, the eventual winner of the shell and core design and build contract proposed using sheet steel piles, ' Haines says, 'from whose pilecap the outer walls of the building above would spring.'

This form of construction is a first for a multi-storey residential building in the UK, on anything like this kind of scale. Barr Construction was the successful contractor (later it negotiated the fit out as well);

and its specialist subsidiary Barr Steel designed and carried out much of the work with the sub and superstructure. Dew Pitchmastic was the piling contractor.

'The advantages of a steel piled basement are relatively profound. They include substantial programme and cost benefits, no need for internal linings to mask rough concrete piles and with much less concrete to cut, less risk of vibration white finger, ' says Linfoot's managing director Michael Luscombe.

'But we had reservations.'

Most fundamental of these was whether the basement walls could support the load of Whitehall Waterfront's external walls. This matter was relatively easily resolved by opting for a robust design following complex structural analysis. Heavy section Larssen LX32 piles were used. These were kept stable during basement excavation by temporary ground anchors until completion of a hefty capping beam and the propping effect of the superstructure kicked in.

Other concerns were water penetration, the steel piles' design life and fire protection.

'Dealing with the first, the clutches of the Larssen piles were cleaned, shot blasted and fully welded, ' says Haines. 'This and a hydrophilic water bar at the joint of the basement's base slab and the piled wall is keeping the basement dry.'

Steel company Corus helped resolve the issue of the basement walls' design life by providing evidence that buried steel piles do not tend to corrode.

'Those used at Whitehall Waterfront are of hefty section and their outside face is unlikely to deteriorate, ' says Jim Wilson, Corus' regional technical manager for piling in the north of England. 'Even if you write off 3-4mm as a sacrificial layer during the structure's 120 year design life, there will still be plenty of steel left.'

As for added fire protection, there is none. The advancing science of fire engineering was used to prove the basement walls' true fire rating and it was found to be in excess of the two hours prescribed in the Building Regulations, without additional protection.

A factor at Whitehall Waterfront was the effect of the soil behind the basement walls which would act like a heat sink in the event of fire. But says Haines; 'In reality, the temperature of the steel could never get so high that the material would start to lose its strength. Fire protection is just not needed' Fire engineering also helped ensure that steel was the medium chosen for the superstructure. Client Linfoot already had a preference for steel as did its consultant. 'We have other, although smaller, residential buildings in steel and appreciate the material's advantages, especially speed of construction, ' says Linfoot's projects manager Chris Hildreth.

He makes the point that compared to concrete, steel shaves 16 weeks off the construction programme. 'This is reducing the period in which we have to finance our development, the earlier payback saving a good six figure sum.'

Protecting steel from fire using empirical measures can add considerably to the cost of a steel framed building.

But, as with the basement, fire engineering's specific assessment of combustibility resulted in a reduced need for additional fire protection with no loss of safety.

The choice of flooring also played a part in economic viability. Terence E Dudley opted for Slimflor, a shallow depth system where precast concrete floor units are supported on the extended bottom flanges of the steel beams, with in situ concrete infilling at columns.

'The bottom plate is the only steel visible at each floor soffit, so the area to be provided with fire protection is minimal, ' says Corus' northern England regional technical manager Walter Swann. The floor system is also part of the answer to resolving acoustical problems which are commonly perceived as militating against use of steel in residential buildings.

Fulfilling the requirements of a Type 2 floor, the main Slimflor element provides the mass and is topped by a resilient layer and floating finish to ensure minimum transfer below of any impact noise. The soffit of each floor (ie the ceiling of the level below) is additionally clad with 12mm of plasterboard.

'This system is quite commonly used on buildings of up to four storeys. We've just taken the concept and gone up with it, ' says Hildreth.

'Acoustics have not really been a worry for us in choosing a steel framed structure: we reckon we've cracked any problems on our previous steel framed developments.'

Airborne sound between apartments and between apartments and corridors is diminished by use of solid blockwork walls made of standard blocks laid on their side to create 215mm thick party walls, rather than the normal 100mm. Huge attention has been paid to avoiding creation of 'penetrations', to reduce the possibility of horizontal impact sound being transferred. For instance, none of the development's 40,000 electricity sockets are positioned back to back in a wall.

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