Simple mass structures which use their weight to support loads, e.g. a wall.

Shell structures which support their load by using their shape or geometry e.g. an igloo.

Frame structures which support their load using a combination of different members (such

as beams, columns, struts and ties all acting together), e.g. a bed.

Rim

Hub and spindle

A-frame legs

Spoke cables

Back-stay cables

Capsules

Boarding platform

Restraint towers

Pier

Collision protection boom

Compression base

Tension base12

11

10

9

8

7

6

5

4

3

2

1

Structures all have one thing in common they need to safely carry the loads applied to them. The way a structure

performs depends on the form it takes and the materials used.

There are several different structural forms, including:

Examining the structure of The London Eye

Teacher sheet

Structures

1

1

2

3

4

5

6

7 8

9

10

11

12

Recognising structures

Teacher sheet

Structures

2

Frame structuresThe London Eye is an example of a frame structure. The

design has two large tapered legs that stand

20 metres apart at their base and measure over 58 metres in

length to form an A shape. If they were to stand alone the

A-frame legs would fall over but they are held upright by cable

ties. Together this tied A-frame supports a load of about

1200 tonnes (the weight of the wheel, i.e. the rim, capsules

and cables). The spindle itself is a beam member that carries

load by a bending action and cantilevers (supports from one

side only) the wheel over the River Thames.

The A-frame legs, which have been set at an incline of 65,

are stabilised by four large back-stay cables which anchor

them to the ground. The A-frame legs and the back-stay

cables are built on foundations up to 33 metres deep.

The design of the wheel part of the London Eye is similar to a

bicycle wheel, with a spindle, (i.e. axle) and a central hub

connected to the rim by cables, (i.e. spokes). The rim is a

triangular

truss (another frame structure) and supports the weight of

the capsules, each of which are connected to the rim by two

circular rings.

The capsules themselves are shell structures which rely on the structural form of the curved glass to provide structural

strength. The glass that provides passengers with uninterrupted views is strong enough to support the weight of passengers

should a capsule be tilted in any way.

Shell structures

Pupil Exercise 1What type of structures can you identify on the London Eye?

Indicate the different types of structure on the diagram supplied.

Teacher sheet

Structures

3

There are many forces affecting structures when they are subjected to loads. The materials from

which structures are built are chosen to resist these forces in order to make a successful structure.

Forces affecting structures

SQUASHINGIS

APART

ISTWISTING

LOAD

IS

SHEAR IS TRYING TO

Compression

BENDINGTension

SLICE

SQUASHINGIS

APART

ISTWISTING

LOAD

IS

SHEAR IS TRYING TO

Compression

BENDINGTension

SLICE

SQUASHINGIS

APART

ISTWISTING

LOAD

IS

SHEAR IS TRYING TO

Compression

BENDINGTension

SLICE

SQUASHINGIS

APART

ISTWISTING

LOAD

IS

SHEAR IS TRYING TO

Compression

BENDINGTension

SLICE

SQUASHINGIS

APART

ISTWISTING

LOAD

IS

SHEAR IS TRYING TO

Compression

BENDINGTension

SLICE

Compression

Bending

the squashing of a structure. the squashing of a structure

SQUASHINGIS

APART

ISTWISTING

LOAD

IS

SHEAR IS TRYING TO

Compression

BENDINGTension

SLICE

this type of force twists a structure.the stretching of a structure.

this is trying to sliceacross a material.

bending occurs when a force isapplied to a structure when bothends are supported, or one endis fixed. The load has to get tothe end of the structure, thereforethe top section of a beam is incompression when it bends andthe lower section is in tension.

Tension Torsion

Shear

Teacher sheet

Structures

4

Forces at work on the London Eye

Pupil Activity Practical demonstrationSubmit a compression force to a variety of materials, e.g. card, aluminium, balsa wood, plastazote,

MDF, plywood, acrylic. Make a chart to show how each one withstands compression forces.

Included below are suggestions for simple practical exercises to demonstrate the different forces for class

demonstration or for the pupils to try themselves. Pupils should record and analyse the results and draw conclusions.

This presents opportunities for links to ICT. You may wish to use the same range of materials for each test for

comparative purposes.

Note for teachers

The steel A-frame legs are compression members and

support the weight of the wheel (about 1200 tonnes) from

one side. These legs sit on concrete plinths on top of a

reinforced concrete compression base. The compression

base is itself supported on a large number of steel and

concrete piles buried approximately 33 metres below

ground level. This structural system stabilises the

London Eye and keeps it in position.

Compression

Teacher sheet

Structures

5

Pupil Activity Practical demonstration Cut two equal strips from a plastic bag one from the length of the bag and the other across.

Stretch the two pieces to varying lengths. Examine and record the differences in the way they look.

The rim is held in place to the hub of the London Eye by

pre-stressed steel cables. The pre-stressing enables the

cables to carry the weight of the rim without going slack

at the top as the wheel rotates. The cables transmit the

load from the rim to the hub and are tension members.

The inclined A-frame legs are stabilised by a set of steel

back-stay cables which are also tension members.

These prevent the legs from falling over into the river.

These cables (measuring 110mm in diameter) are

anchored to the ground on a reinforced concrete

tension base. This base is also supported on a large

number of special piles (called tension piles) which

anchor the base 33 metres deep into the clay that lies

beneath the site.

The tension base is linked to a compression base by two

reinforced concrete struts which distribute the load evenly

between the three points. This load distribution is an

effective way of stabilising a structure.

Forces at work on the London Eye tension

A-frame legs from cables

base

base

on normal piles

on tension piles

Struts

Tension

Tension

Spindle

LoadLoad

up/down the cable

Hub

CompressionCompression

A-frame leg

Cables

Teacher sheet

Structures

6

The wheel is cantilevered out over the

river on the spindle which is 23m long,

(the height of an eight storey building).

The whole load is carried on the spindle

before being transferred to the A-frame.

The shape of the spindle and the choice

of steel from which the spindle was

fabricated were critical to resist bending

when subjected to this load. The quality

and thickness of steel were carefully

selected to prevent cracking or

excessive deformation.

A circular hollow tube is one of the most

efficient shapes for a beam. This is the

shape used for the spindle on the

London Eye, which has a tube wall up

to 230mm thick.

Forces at work on The London Eye bending

Pupil Activity Practical demonstrationTake a block of foam and draw centre lines along the length of each side. Bend the foam in

several ways. Record which surfaces are in compression and which are in tension.

Pupil Activity Class discussionIs the top of the spindle of the London Eye in compression or in tension?

Teacher sheet

Structures

7

As the rim of the wheel is turned by the rim motor drive,

the hub is subjected to a torsion load. As well as the 64

radial cables attached from the rim to the hub, a further

16 rotation cables are attached at a tangent to the hub

to ensure there is no lag between the turning of the rim

and the turning of the hub. Eight rotation cables are used

for each direction the wheel rotates.

More information on how these cables assist the turning

of the London Eye can be found in the Mechanisms

section.

Forces at work on The London Eye torsion

Pupil Activity Practical demonstrationSelect a variety of materials, e.g. card, aluminium, balsa wood, plastazote, MDF, plywood, acrylic.

Place one end in a vice and place a metal rod through a hole in the other end. Holding either side of the

rod attempt to twist the material once and then two or more times. Record and analyse the results.

Spoke cable

Rotation cableRotation cable

The rotation cables are attached at a tangent to the hub.

Teacher sheet

Structures

8

The spindle (which has been designed to resist bending) also has to

resist shear forces where the load from the weight of the whole wheel

structure could cut through or shear the spindle.

Forces at work on The London Eye shear

SpindleHub

A-frame leg

wheel

Line of shear

Pupil Exercise 22a) Look at the diagram of the London Eye. Where do the different forces apply? Mark the

direction of these forces on the diagram use colour coding to differentiate the forces.

2b) Think of everyday examples to demonstrate each of the forces (compression, tension,

bending, torsion, shear).

Pupil Activity Practical demonstrationTest a variety of materials (as listed above for torsion) to see whether a pair of

scissors can cut through them. Record your findings.

Tiein tension

Strutin compression

Load

Teacher sheet

Structures

9

Materials are selected for their properties to resist forces

and hence perform as structures.

One of the main types of material used to build the

London Eye was steel. Some of the pieces were cast

from molten steel (for example the hollow steel tube of

the spindle). In other cases the steel was joined up by

welding or bolting sections together.

One of the best examples of this is the rim. It is made

from hundreds of steel tubes welded together to form a

triangular truss. The geometry of the tubes help to

strengthen the rim. Many different designs were

considered before agreeing on the final configuration.

For more information on the matereials used to build the

Eye see the Materials section of this resource.

Structural strength frame structures

TriangulationTriangles are one of the most effective ways of forming a

structure from individual members. To create triangles,

individual members called struts and ties are joined

together in a geometry of triangular shapes. The struts

carry compression and the ties carry tension. Triangulated

frames carry loads in a different way to beams.

Pupil Exercise 33a) Build a simple box from straws and push in one corner. Record what happens. Now glue

members from corner to corner of the box and repeat the exercise and record your findings.

3b) Replace the straws with ties made of cotton and repeat the exercise.

Teacher sheet

Structures

10

Pupil Exercise 44a) Triangulation has been used to strengthen the steel rim of the London Eye. Can you think of any

other ways in which triangulation has been used to strengthen the structure of the London Eye?

4b) Look at the design of the triangular truss used on the London Eye. Create a model of a small

section of this rim using art straws.

4c) Plan and design a second truss using a different configuration of triangles.

Devise a test to assess which of the two structures is strongest.

Pupil Exercise 5 Using sheets of A4 paper, investigate different ways of strengthening sheet materials by rolling,

folding and corrugation. Devise the most effective weight to support an eraser. Record your

actions and analyse your findings.

Pupils to build a completely circular truss. Experiment with cotton or string cables to hold the rim in place to a hub.

Extension activity

Shell structures gain their strength by their shape. This can be demonstrated simply by using paper.

Structural strength shell structures

Teacher sheet

Structures

11

Structures are designed and built to support different types of loads.

The loads that a structure might be required to support are classed as either

static or dynamic loads. Loads can be applied either vertically or

laterally (horizontally).

A static load is a stationary, non-moving weight whereas a dynamic or moving

load produces larger forces on the structure. The type of load a structure is

designed to support will be a deciding factor in the way it is designed.

An example of a static load is books on a bookshelf (non moving) and a typical

example of a dynamic load is a car travelling over a bridge (a moving load).

Static and dynamic loads

Even when the London Eye is turning, its moving load is not considered

dynamic as the acceleration is not sufficiently significant. The primary dynamic

load on the London Eye is from gusts of wind; this wind load is applied

horizontally, whereas the loads from all the mass of the capsules, structure etc

act in a vertical direction.

The loads carried by the Eye

Dynamic loads are calculated as mass multiplied by

acceleration as the mass moves. Therefore if a mass is

accelerating at over 10m/second2, the dynamic load

exceeds the static weight, since the acceleration due to

gravity (i.e. 1g) = just under 10m/s2

Teacher sheet

Structures

12

The capsules that carry passengers are fixed to the

outer rim of the London Eye. To ensure maximum

passenger comfort it was decided that, rather than

relying on gravity to keep the capsules level, they

would be fitted with a motorised stability system.

Designing the capsules to carry a moving load

Each capsule is supported on two circular bearings. These bearings have a toothed rail against which the cogs of a motor

drive mechanism can turn. This in-built stability system keeps the capsules level as the rim rotates. Sensors respond to

movement of the load, so even if all the passengers were to move to one side of the capsule, the drive system can keep

the capsule level.

More information on the capsule stability system is provided in the Mechanisms and Systems and Controls sections of this resource.

The wind was the most important environmental

factor that had to be considered when the

London Eye was designed.

The London Eye is designed to withstand winds

of a speed equal to the worst storm anticipated

to occur once in a period of 50 years.

Each capsule is aerodynamically shaped for least

wind resistance and 64 mass-spring dampers

are fixed around the rim to prevent any vibration.

These are positioned above each capsule on

either side. Without this system the capsules

would have moved approximately 1.5 metres in

normal wind conditions. It is certain that

passengers would have then been affected by

motion sickness as the capsule swayed from

side to side during windy conditions.

Weather conditions and structural stability

Mass on low friction wheels

Rim truss

Spring

Mass spring damper

Cables Cables

This shows the workings of a mass-spring damper, 64 of which arefixed around the rim of the London Eye to prevent vibration

Teacher sheet

Structures

13

Pupil Exercise 6Pupils use their knowledge of structures to plan and design a bridge.

6a) In groups of three, create two identical bridges between two desks (gap of 20cm exactly)

using only 10 sheets of A4 paper, scissors and masking tape. Start by brainstorming and

sketching your initial ideas and record all the changes you make.

Using one of the bridges, test for the greatest weight this structure can safely support? Record your

findings and record the ways in which you have improved the strength of your structure.

6b) Once you have found the maximum weight the first bridge can safely carry, drop the same

weight from a height of 20cm onto the second bridge. Record what happens and suggest why?

6c) Repeat this exercise but this time build your bridge structure from drinking straws or other

construction materials available. Record your findings. Explore the use of triangulation to

strengthen your structure.

Pupil Exercise 77a) Plan and build a cardboard structure to support a cardboard turning wheel.

7b) Increase the weight of the wheel by using different materials to form the wheel and evaluate how you

need to adapt your structure to support the increased weight.

Keep a design folio to record how your designs develop and the reasons for any changes you have made.

strutea01strutea02strutea03strutea04strutea05strutea06strutea07strutea08strutea09strutea10strutea11strutea12strutea13