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Because if you don’t.............Because if you don’t.............

ont De Millau

Strength: Tension

Compression

straw wood stone steel

What material should we use....What material should we use.... Bridge?Bridge?Ship?Ship?Opera house?Opera house?

Elasticity

durability

Bending

Does it return to initial state?

How do we determine the properties of the materials?How do we determine the properties of the materials?

Vertical tension

Vertical compression

shear torque

Apply tension

What happens at the atomic scale???

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Solids are composed of

coupled atoms

An elastic solid

Forcearea

re)  

l/l

(pressur

Mild steel

Cast iron

lead

Aluminium

Stress-strain measurement

Variable tension

Sample

Brittle fracture, cold metals, ceramics

Moderately ductilefracture

Very ductile, soft metals (e.g. Pb, Au)

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Hyatt Regency Hotelin Kansas City

(Where everything’s up to date)

Hyatt Regency Hotel walkway collapse

WHY?WHY?

The steel supports for the walkways were made of a steel that was brittle, and broke when the load exceeded itsand broke when the load exceeded its designed strength

If we build a structure that is going to be loaded…

The material must be elastic within the range of distortion

The atoms in the upper The atoms in the upper section are pushed closersection are pushed closer

The upper section is in The upper section is in COMPRESSIONCOMPRESSION

The atoms in the lower The atoms in the lower section are pulled apartsection are pulled apart..

COMPRESSIONCOMPRESSION

The lower section is The lower section is under TENSIONunder TENSION The spacing of atoms  in this The spacing of atoms  in this 

“neutral layer” is unchanged. “neutral layer” is unchanged. 

If this structure was the roadway of a bridge…

….the compression and tension would be continually increasing and

decreasing as traffic passed.

October 1970October 1970

What do you think might be the long-term consequence?

Failure of the materialFailure of the material

The lower section is The lower section is under TENSIONunder TENSION

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The Tacoma Narrows Bridge

Looks great…doesn’t it???

Now there are two of them!

Let’s look at the type of load-bearing beams that are used in a typical bridge.

Note coupling between the girders.

Note the height of the girders to give strength

Did you notice………the cross section shape of the girders?

Why aren’t they solid rectangular in cross section?

forceforce

The top region is The top region is under under

compressioncompression

The lower region The lower region is under tensionis under tension

So, because the middle So, because the middle suffers no significant suffers no significant net force, there is no net force, there is no need to have unwanted need to have unwanted mass there. mass there. (and expense)(and expense)

The de Havilland Comet

THE WORLD’S FIRST COMMERCIAL JETLINERTHE WORLD’S FIRST COMMERCIAL JETLINER(1952)(1952)

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1952Comment on pressurization , 1st commercial jetliner3  Broke up in flight in the 1st year of commercial flight.  videoWhat was the cause?.....mention structure of materials

The failure was a result of metal fatigue caused by the repeated pressurisation and de‐pressurisation of the aircraft cabin.

Also, the supports around the windows were punch‐rivetedwhich may have caused the start of fatigue cracks around the rivetwhich may have caused the start of fatigue cracks around the rivet.Unlike drill riveting, the imperfect nature of the hole created by punch riveting caused manufacturing defect cracks which may have caused the start of fatigue cracks around the rivet.  Square corners provided centres for stress.  Ask  what shape are Plane windows 

Again we have a situation where continual stress and relaxation leads to metal fatigue.

Note the square windows

Note the windows

All solids under stress have successively regions of . elasticity, . plasticity . rupture.

The characteristics of solids can be determined experimentally.

SummarySummary

Materials must be chosen to be suitable for the project.

Continual cycling of a solid can produce metal fatigue