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Nuts and Bolts: London Eye Construction

When the architects at Marks Barfield sat down to consider what structure would best commemorate the turning of the century, they noted that London didn't have any observation points for people view the skyline and surrounding landscape. A tall, rotating wheel would not only allow a unique vantage point of the city, but would allow large numbers of people to see that view at the same time.

The London Eye is a modern take on a traditional Ferris wheel with a few distinct differences. For one, the passengers sit in fully enclosed capsules rather than dangling gondolas. Two, the entire structure of the London Eye is supported on one side only, allowing the wheel to hang over the River Thames.

The London Eye is an excellent example of a frame structure. Its steel design forms an "A" shape, with two large tapered legs at the base -- 65 feet (20 meters) apart and each over 190 feet (58 meters) in length. The legs lean toward the river at a 65-degree angle. Cable backstays keep the frame from tilting into the river -- they're anchored to the top of the frame and then buried in a concrete foundation 108 feet (33 meters) deep.

The wheel part of the London Eye resembles a bicycle wheel -- with a spindle and hub connected to the rim by 64 cables, or spokes. Sixteen additional rotation cables are attached to the hub at an opposing angle to ensure there's no lag between the turning of the rim and the turning of the hub. The spindle itself is supported by the frame on one side only (cantilevered), and the frame holds the wheel over the river. The London Eye can withstand winds of a 50-year storm, the worst storm anticipated to occur once in a period of 50 years, and if it's ever struck by lightning, the strike would be conducted to the ground with no harm to passengers.

The London Eye rotates around the hub much like a bicycle wheel, but motorized. Hydraulic motors, driven by electric pumps, provide energy to turn the wheel. The drive systems are located in two towers, one at each end of the wheel's boarding platform. Here's how the wheel turns: Standard truck tires along the rim of the wheel act as friction rollers. Hydraulic motors turn the tires, and the rotation of the tires turns the wheel. A computer controls the hydraulic motor speed for each tire.

The main components of the London Eye were built offsite. Once they were completed, barges transported them piece by piece up the River Thames to the construction site on the South Bank. Workers assembled the London Eye horizontally on a temporary support platform over the river, which made construction faster, easier and safer than if it had been built vertically. Once it was assembled, hydraulic lifts and cables slowly raised the 1,322 ton (1,200 tonnes) structure over the course of one day, until it reached its 65-degree angle. Once it was in final position, the 32 capsules were attached to the rim, which took eight days.

Instead of being suspended and swinging, the passenger capsules turn within circular mounting rings fixed to the outside of the main rim. As the wheel rotates, the capsules also rotate within their mounting rings to remain horizontal. If the capsules didn't rotate, by the time your capsule went around the wheel, you and your friends would be upside down. Each capsule has its own heating and cooling system, bench seating and is fitted with special glass that can handle weather fluctuations. Capsules also have a built-in stability system, meaning the capsule will stay level even if all the passengers suddenly move to one side. There are 32 capsules, one to represent each borough of London.

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