After five years' gestation, London's Millennium Dome is finally giving birth to its future heir, the O 2. Ruby Kitching reports.
Building a building inside a building is not an easy task, but that is exactly what the team of engineers from consultant Buro Happold and contractor Sir Robert McAlpine are currently trying to do inside London's landmark Millennium Dome.
Walking into the Dome and onto the construction site for the new 22,000 seater O 2 Arena, as the new structure will be called, feels like walking into the structural engineering equivalent of a womb. Vital organs in the form of underground service ducts and water tanks have already been installed, while the slipform core limbs and roof truss rib structure of the Dome baby are just taking shape.
The new O 2 Arena structure will be 150m in diameter and 45m high, allowing for 5m clearance to the Dome fabric and sitting within its ring of yellow masts. Since the original tensile fabric canopy is so mind-bogglingly vast - up to 50m high and covering an area of 80,000m 2 or eight football pitches - it provides a welcome shield from wind and rain during construction.
'But building a building within the Dome is the main constraint that dictated the final solution, ' says Buro Happold associate director Ken Jones. The limited headroom has meant that construction will take place without the use of tower cranes and tall piling rigs (see box).
The main structure of the arena comprises four pairs of reinforced concrete cores to house services, lifts and stairs.
The cores are slipformed so tower cranes are not required.
They will eventually form the support points for the colossal 4,500t steel roof truss.
Jones explains that the original Dome was designed purely as an exhibition space, but the new arena will house concerts, so it requires its own roof in order to create excellent acoustics and blackout conditions, limit noise breakout, provide insulation to the seating bowl and ice pad, as well as flexibility when the arena is used for shows and exhibitions.
Having multiple uses means that the 135m span steeltrussed roof is heavily laden with services. It is also designed to carry heavy point loads including a 20t centre-hung scoreboard and 50t of suspended end-stage concert rigging.
Trusses are up to 11m deep and incorporate upper and lower tubular steel chords 750mm in diameter and up to 55mm thick to be robust enough to prevent excessive deflection.
The cores are the main stability structures for the arena and are designed to take all wind loads, since the building has to be 'future proofed' for the possibility that the Dome's enveloping structure may be removed at some point in the future. Structurally, the former Dome will be little more than an umbrella to the O 2, although it will continue to provide its trademark identity.
Client for the multi-million pound development is Anschutz Entertainment Group, which signed a naming rights deal with mobile phone network provider O 2 in May this year.
Buro Happold was novated to contractor Sir Robert McAlpine a month later.
The arena seating bowl comprises two tiers of precast concrete terraces, supported by steel beams and columns.
The tiers are separated by a double bank of hospitality suites.
Outside the arena building itself, but still within the Dome, there will be entertainment and leisure venues including cinemas, restaurants and bars.
'There are pinch points where we're limited by keeping the top 5m clear for air flow between the fabric canopy and the arena, and we're limited by the height of the top level of seating, ' says Buro Happold project manager Tanya Ross.
She adds that it is fortunate that the Dome height can accommodate the deeper truss sections in the centre of the roof where the design loads are greatest.
'Of course, we couldn't use any masts to keep the structure more slender, so we're just spanning the arena using traditional trusses, ' she says, adding that the primary driver for the roof is functionality rather than aesthetics.
Owing to the high acoustic specification to prevent 'slap back' - the term for the ghastly reverberations which can drown out music in old-style arenas - the O 2 roof will be thick with sound insulation, bringing the final roof weight to 4,500t.
Truss elements are fabricated by Watson Steel and welded together on the arena floor on 2m high temporary platforms.
The ends of the main truss elements, which will eventually rest on top of the concrete cores, are connected to 'quadrapod' supports.
These are steel-framed, rectangular-based pyramid structures that are narrow enough in one dimension to slip between pairs of concrete cores (see diagram).
When the entire roof structure is complete, a strandjack cable dropped down from the top of the concrete cores will be connected to the apex of the four quadrapods. The entire roof will then be lifted off the platforms until the truss reaches the height of the core. The quadrapod is then rotated 90° so that the corners rest on top of the cores.
To study the geometrical, detailing and co-ordination issues associated with the roof cladding layers, a full size mock-up of four roof panels 2), complete with purlins, trusses, cladding, insulation and waterproofing, has been constructed on site.
'This has helped us to work out the best sequencing and iron out any health and safety issues, ' says Sir Robert McAlpine project manager John Davoren. He adds that the mock-up has also helped roofing contractor WWR, which would usually connect roof panels from the underside.
'But here, we're working from the top of the structure, ' he says. Since the roof panels are connected at angles to each other to follow the dome profile, the trial run has also enabled checks to be carried out over whether steel elements will achieve the required warp during erection and in its final state.
With maximum prefabrication and using repeatable operations, Davoren hopes to lift the roof as swiftly as possible: 'The first roof lift will be in March next year, when it will be hoisted up to a halfway point.' At this point we can get on with other superstructure work below it, he adds. 'The final roof lift should take place in May 2006.'
Dome history February 1996 Government's Millennium Commission announces that the Greenwich peninsula site should be used to house a millennium exhibition and spearhead regeneration in the area.
September 1997 English Partnerships leases land to the New Millennium Experience Company until 30 June 2001.
June 1997 Construction of the Dome and associated infrastructure begins.
February 1999 Dome's structure is completed.
January 2000 Dome opens to the public on 1 January.
September 2000 Prospective Dome buyer pulls out.
July 2001 English Partnerships manages the Dome until buyer is found.
May 2005 Anschutz Entertainment Group joins forces with mobile phone company O 2 to create the O 2- the new name for the Millennium Dome. Construction on the site begins in October.
Early 2007 The O 2 to open to public.
Piling box The Greenwich peninsula site was originally packed full of nasties left in the ground from a gas works, a benzol plant and coking works.
Although the site was cleaned up for the original Millennium Dome structure (NCE 24 February 2000), further piling was required to take the increased loads of the new arena. Twenty five percent of the new arena columns are supported by the existing grid of piles from the original Millennium Exhibition, and many further existing piles are re-used in the support of the partially suspended ground slab.
However, around 1,500 new piles have also been required by the scheme.
Since Buro Happold worked on the original Dome structure, its engineers had a good understanding of what challenges lay ahead.
'We knew there was made ground 3m to 5m deep, and that beneath the clean cover system installed for the Millennium Experience the ground was contaminated, ' says Ross.
To ensure minimum ground disturbance, driven precast and driven cast insitu piles have been used where possible, together with mini piles near the Dome perimeter where the headroom is restricted.
CFA piles have been used to carry the heaviest loads, and sleeved piles have been used in the vicinity of the Blackwall Tunnel.
'We've got some of the worst conditions here you could ever imagine - the ground is marshy, there used to be a munitions factory nearby and the site is on a former gasworks, ' says Buro Happold geo-environmental engineer Tom Skailes.
'So we wanted to minimise the amount of spoil produced - it's a good idea from the perspective of reducing the amount of fill that goes to landfill and it costs the client less.' Driving extra piles meant taking up the existing ground slab and gas membrane. The Buro Happold team initially tried to limit the amount of slab disturbed during piling by building on top of the slab where no new piling was required.
But co-ordination with services engineers revealed that new underground services would carve up the slab anyway.
'We also realised that we'd have to reinstate bits of slab and gas membrane in patches, which would require lots of stitching to ensure its integrity, ' says Jones.
Raising the slab level by 150mm across The O 2 footprint saved 2,500m 3 of cart away, and also reduced the volume of arisings from the pile cap excavations.
Other preliminary works involved demolishing six existing buildings (so called 'core' buildings) in the Dome and removing obstructions in the ground from the former gasworks where new piles were to be installed.