London eye physics[1] (1)

By Sarega Gurudas

The London Eye is an excellent example of a frame structure.

The components of the London Eye needed to be strong enough to support an immense

load. For this reason the designers needed to use a material on the frames which would be tough enough to not only support the weight of the 32 capsules (each weighing 11 tonnes),

but the load of the passengers within each capsule each containing up to 25 passengers

weighing another 1.9 tonnes.

HOW WAS IT BUILT?

It was agreed that steel was the best choice; as it met the strength criteria necessary for the project. Steel is made from iron and

carbon and is very adaptable in that it can be produced with specific properties by varying the proportions of carbon and other alloyed

metals (e.g. chrome) that are added to the basic iron.Steel is an example of a ferrous metal and as such is prone to

rusting when left outside for any period of time. Rusting is caused by exposure of the metal to oxygen and water and was therefore a concern for the engineers working on the London Eye, as it would be positioned on the bank of the Thames, subjected to water from

the river and rainy weather conditions.Paint was then applied to the sections to protect them from rust. There were two main options available for the capsules; glass or

perspex. Although perspex is as strong as glass and easy to form it can be scratched over time. The most sensible alternative was to use

glass as this would provide the desired optical quality.

CHOICE OF MATERIAL

Tests had to be completed on various parts of the London Eye before it was able to go into operation. Various parts of the

London Eye were tested for their strength by subjecting them to loads far in excess of what they would be required to

tolerate in everyday use.

TESTS

Specimens for testing were taken from all the main components. These confirmed the material’s strength and provided information

about its toughness and flaws.As further protection, it was decided to ‘proof test’ the whole

spindle. Proof testing puts a structure under a greater load than is anticipated in service. This makes the component tougher as well as

testing it. Under overload at normal temperature, any existing cracks that have not been discovered will have yielded at their tips under tension. When the load is released, the material around the

crack tip will then contract to leave the crack tip in a state of residual compression. The spindle then has additional protection

against fracture because cracks only propagate when the material at their ends is in tension.

The engineering team must also ensure the structure does not move in response to oscillatory forces; for example it must not sway in the

wind. The structure itself must also withstand occasional storm conditions. One way to limit the amplitude of oscillations is by

damping, ultimately it will reduce the velocity of the oscillations and the structure loses energy on each cycle.

The engineers had to make sure that the parts of the wheel would not vibrate in a breeze, so they put dumb-bell shaped masses, called Stockbridge dampers at the positions on the cables which maximise the energy that they absorb.

LIFTED INTO POSITION

It was first raised at 2 degrees per hour until it reached 65 degrees, then left in that position for a

week while engineers prepared for the second phase of the lift.

A temporary mast held one end of the supporting cable above the ground. A chain provided a counterforce to

keep the mast vertical. The wheel was lifted in sections and joined together while resting on temporary islands

built on the river bed. The cable was attached to the centre of the wheel and then tensioned so that the leg is able to pivot. Hydraulic jacks at the top of the mast

pulled the cable, and brought the wheel up to an upright position.