Steel trees will start sprouting from T5's other major building in early April. Dave Parker reports on T5B - future home of the super-Jumbos.
At any other time, on any other airport, T5 Concourse B would be a landmark £350M project. At Heathrow itself, it dwarfs Terminal 4, and would be the biggest project undertaken by BAA in the last 15 years - apart from T5 Concourse A, of course. Some might be tempted to view T5B just as an unusually large satellite to T5A, but they would be wrong, says BAA T5B project leader Sandy McIver.
'T5B is an integral part of the T5 project. T5A can't function without it.' he says. 'The main transfer baggage hub for the whole of British Airways' operation will be in T5B, and when the A380 Airbus arrives, T5B will be the only T5 facility able to handle it.'
Its footprint may 'only' be 442m by 52m compared to T5A's 400m by 180m, but T5B faces very similar design and construction challenges. The 42m radar ceiling also applies, ground conditions are identical, and, if anything, the logistical restrictions are even more challenging.
Aircraft stands at the south east corner of the building have to be handed over to Heathrow Airport operations as soon as possible.
Target date is 1 October, less than 11 months after work on the superstructure began.
'Getting these stands operational on time is the biggest single driver on T5B at the moment, ' says McIver.
'Unfortunately, this means that for most of the construction period, we will have very restricted access, as this side of the site will then be airside operation.'
Roof abutment erection will begin just weeks after the first lift on the landmark waveform T5A roof. A similar if smaller roof profile will take shape on T5B, although a slightly different structural philosophy has been adopted for the main superstructure.
McIver explains: 'Like T5A, our roof will have box section steel rafters spanning right across the building, supported on a steel tube abutment structure. But the abutment 'trees' carrying the rafters only start at Departures level - below that loads from the roof and the top level of curtain walling are taken by precast concrete perimeter columns which are part of the main superstructure.'
Although the trees may only stretch up the equivalent of two storeys, they are still major fabrications. Four, 400mm diameter steel tubes spring from a complex cast steel node on the concrete floor. One pair of 'branches' picks up the end of one main rafter, the second pair supports the end of the adjacent rafter 18m away.
Working out how to get these trees into position at second floor level caused some preliminary headaches. 'Weight isn't really the problem, ' explains Laing O'Rourke T5B production leader Bob Kwist.
'A complete tree only weighs 30t.
'But it's the size of a fourbedroomed house - much too big to prefabricate off site.'
Prefabrication on site, however, is a realistic option. Two factory areas are being set up close to the building and substantial jigs installed. Once Watson Steel has finished welding each tree together, tree and jig will be transported to the building and positioned adjacent to one of the two tracked tower cranes servicing the project, located at apron level.
'Then we lift the tree out of the assembly jig and drop it straight into a second jig at Departures level, ' Kwist explains.
'This holds the tree in place until the rafters are assembled and craned into position and the straps and ties which provide lateral stability are in place.'
As on T5A, the rafters arrive in sections, which are bolted together at slab level before being lifted into position - individual rafter weight is lower, however, so the tower cranes can handle the lift.
Design solutions more common on City office developments have found their place inside T5B. Ultra slim, flat, post tensioned insitu concrete was selected for the main Arrivals and Departures floor slabs, a total of more than 29,000m 2. Slimmer floors give significantly more room for services where floor to ceiling heights are restricted by planning constraints. In a heavily serviced building like T5B, whose height above original ground level was restricted to 19.86m by the public inquiry, post tensioned insitu concrete floors sitting on a 9m by 9m grid of 500mm by 500mm precast concrete columns is the most effective solution.
Stability comes from four steel framed service cores running from apron level to the mezzanine CIP (commercially important passenger) level nestling up under the roof.
Despite its island location airside, T5B will be fully integrated into the new T5 transport networks. Its three basement levels sit directly above the new extensions to the Piccadilly Line and the Heathrow Express, and the lower two of the three basement levels are largely given over to the Tracked Transit System (TTS).
Initially, the TTS will whisk passengers a mere 260m between T5A and T5B: later there could be an extension of the TTS linking into the projected T5C satellite to the east.