Just how is the millennium wheel going to be built? Lisa Russell explains.
F inished components for the British Airways London Eye are sitting in factories across Europe. Sections of the enormous wheel's 135m diameter rim, its massive castings, bearings and lengths of cable are ready to make the journey to London.
Ahead lies assembly at the site on the Thames, followed by one of the most spectacular engineering events ever seen in this country - raising the wheel into position from the horizontal in a single 24-hour operation.
The Eye will be the fourth highest structure in London and the tallest observation wheel in the world.
Passengers will be carried in capsules for a half-hour gentle rotation giving them staggering views of the capital from the site alongside the old County Hall (NCE 10/31 December 1998). The ride is due to start turning on December 31, and open to the public in early spring 2000.
The Eye will work like a bicycle wheel, says Allott & Lomax director and IStructE president-elect Dr John Roberts. The wheel and hub will turn on bearings around a fixed spindle, driven by a motor with wheels that will run against the rim.
The spindle will be cantilevered out over the Thames, supported by a single A frame founded on the shore and anchored back to a tension foundation. The frame will essentially work like one of the forks on a bicycle.
In theory, it would have been possible to build the structure upright in situ, says Roberts. But huge temporary works would have been needed to give lateral support.
Furthermore, the cables could not be stressed until the circle was complete; and stressing in the vertical position is much more complex, he adds.
Laying the Eye flat and then raising it into position was a far better option.
To accommodate this plan, piled platforms are being built at seven spots in the Thames where most of the rim of the Eye will rest, with one river platform for the wheel spindle. There is just one rim platform on land.
The first rim sections are leaving Hollandia's fabrication yard in Rotterdam now, and will be delivered to the site by barge (NCE last week).
The wheel laid flat will occupy half the width of the river. Fortunately, says Roberts, the shipping channel is on the other side. Barges will be anchored precisely so that each section can be jacked into position on the platform.
The rim will be built up in sections (see diagrams), starting with the quadrant on land. Welding enclosures will be set up on the platforms to join the sections.
Before the circle can close, the passenger boarding platform will be brought in and the hub, spindle and A frame legs connected. The A frame will be fitted flat against the wheel, with its legs pinned at plinths on the massive 11m high concrete foundation platforms.
Once the circle is complete, a temporary radial bridge will be built between the rim and centre, and this will be turned, allowing the 70mm diameter spoke cables to be installed.
Initially they will be simply 'hand tight' - not quite the right phrase says Roberts, as no hand could turn the nuts.
Cables have been previously prestretched and cut to length by the fabricator in Italy to ensure correct tensioning. A second set of cables will make the hub and rim turn together instantaneously whichever way the wheel turns.
Dramatic as the assembly over the river will be, the real spectacle will come when the wheel is pulled upright.
First, a temporary steel 90m high A frame truss has to be built on the ground, fixed to the permanent frame's foundations.
A crane can only lift the truss upright part of the way, so cables fixed to the wheel's centre will be used to raise the temporary truss up to the vertical. The same cables will then be jacked again to pull the wheel and permanent A frame legs up. As they reach their final angle, they will be locked off and the wheel rotated to the vertical.
This operation might take only a few dozen words to describe, and 24 hours to perform, but the day it happens in late summer will be a memorable one for the construction team and one suspects, an awful lot of spectators.