Airport operator BAA has made a name within the construction industry for demanding that its designers and contractors abandon convention and rethink the way they tackle things from fi rst principles.
So it should come as little surprise that Heathrow's new air traffic control tower is not being constructed using the normal airport industry approach.
Control towers are typically constructed in slipformed concrete with components making up the 'cab', from which air and ground traffic controllers operate, being craned into position at the top of the completed and fitted-out mast. Heathrow's control tower, by contrast, is composed of just six prefabricated steel components, already fitted with stairs, lifts and services, which are being connected on site.
Heathrow needs a new tower to meet the demands imposed by the construction of T5 and the arrival of the Airbus doubledecker A380 'super jumbo'. Its construction cost is lumped into T5's £4bn price tag. And those demands mainly concern ground traffic control rather than control of planes in the air, says Arup associate Richard Matthews, who has led design of the tower.
'Ground controllers have to be able to see the tail fin of each plane as it moves around the airport, ' Matthews explains.
Heathrow's existing control tower dates from the 1950s and is a stumpy 45m tall. For a clear view over the roof of T5 and new double height piers for A380s which need to be built at the centre of the Heathrow campus, the new tower will, at 87m tall, be almost twice as high.
Design and construction has been shaped by site constraints, says Mace project manager Peter Czwartos. The tower is sandwiched between live aircraft stands off Terminal 3, Pier 7.
BAA was unwilling to sacrifice revenue by closing the stands, or to hand over sections of taxiway to construction, effectively ruling out the use of tower cranes or construction methods requiring large volumes of concrete.
Aside from large diameter bored, cast insitu pile foundations, installed last year by Laing O'Rourke, no concrete will be used on site until a lightweight three storey, steel-concrete composite building is constructed around the finished tower this summer.
By adopting an all steel design, Arup and Mace could limit movements across airside space to no-fly hours during the night. Weighing in at 750t, 'a lot more than a fully loaded 747', notes Matthews, the cab and top 12m of mast was delivered to site complete with glazing and air conditioning last October. It has been followed by five 12m sections of mast.
The tower is being strand jacked skyward - it is growing from the bottom like a giant bamboo, with its base repeatedly lifted to allow insertion of a new mast section. Lifting subcontractor is Dorman Long, and the penultimate lift took place on Monday.
To lift the structure a special yoke has been designed.
An equilateral triangle of heavy steel girders embraces the mast.
Mounted on the girders are hydraulic 'jaws' that close in on the mast's steel skin. Czwartos emphasises that the structure is not being hoisted aloft by skin friction, but that the jaws bear on flanges connecting sections of the mast - its 12m lengths are each composed of 3m 'cans'. The flanges allow bolted connections to be made.
Strandjacks mounted on three temporary heavy lifting towers linked to the corners of the yoke by cable begin the assembly cycle, lifting assembled tower sections 15m into the air.
A new 12m section of mast is then mounted on a dolly and slid under the mast. The mast is lowered onto the new section so that the adjoining flanges are flush, and bolted connections are made.
Then the mast is lifted again to enable the dolly to be extracted.
Finally, the tower is put back down on the ground and the yoke dropped, ready for the next lift.
It is an apparently simple operation underpinned by meticulous planning and some intricate temporary works.
The tower is kept steady by three pairs of temporary stays.
These have to be payed out and taken back in as the tower is raised and lowered.
To ensure that a perfectly flush marriage is made between the bottom flange of the completed mast and each new section introduced - 'so we don't get moments being introduced when we put the mast down' - there are two systems of reciprocal hydraulic jacks, Mace temporary works co-ordinator Anthony Bateman adds.
'Each set of 12 is arranged in a circle, ' Bateman explains, one is mounted on the yoke to adjust the pitch of the jaws, and the other at ground level to ensure proper orientation of the mast section.
While loads are being transferred to them they are passive.
'Opposite jacks are linked, so that if one is pushed down the other is raised by an equal amount. It ensures forces are evenly distributed across the flanges.' When equilibrium has been achieved the jacks can be activated to tweak vertical alignment.
Such ne tuning is required to achieve the 1mm in 12m vertical tolerance to which the tower is being built.
After the last section of mast has been installed next month, the ground level jacks will be permanently locked off.
The temporary guys will be replaced by 150mm diameter locked coil cable stays. 'They'll only be tensioned to 160t - a tenth of their capacity, ' notes Arup structural engineer Sean McGinn.
But the heftiness of the cables will help combat harmonic vibration.
'The comfort of the controllers is of paramount importance, ' Czwartos adds.
To this end the mast is fitted with small spoilers which prevent vortex shedding - wind turbulence which can result in buffeting.
The tower is also fitted with a pair of Japanese active mass dampers, believed to be a European first.
Construction is due for completion at the end of this year, allowing National Air Traffic Services 12 months to equip and trial the tower before it becomes operational in 2007.