1 Framed Static Structures

8/3/2019 1 Framed Static Structures

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Framed structures



Objective

Stress / strain

Tensile testing

Understand different types of frames;

Redundancy

Stability Calculate forces in frames.



Definitions

Tensile force

Compressive force Shear force

Stress

Strain

Shear stress Shear strain

Elasticity

Elasticity modulus



Material Testing.

Materials are tested

to determine their:

± Physical & Mechanical

properties.

± Physical condition.

± & therefore their

suitability for a

particular application.



Categories of material test

± Non Destructive

Testing.

± Destructive Testing.



Destructive Testing.

The material is tested

until it is:

Deformed

Damaged

Fractured




± Readings recorded.

± Readings comparedwith standards.

± Material properties

identified.




Destructive testing

undertaken to

determine a

material¶s:

± Tensile strength.

± Toughness.

± Hardness.

± Ductilit .



Tensile Testing.

± Material in

TENSION when

stretched.

± Stretching called

TENSILE LOAD.



Tensile Testing.

± Tensile test is carried

out on a

± TENSOMETER.



Tensile Testing.

± Throughout test

readings of:

± applied load and

± Extension

± recorded.



± Test results converted

into a graph.

± Graph shape

compared with other tensile test graphs.

Tensile Testing.



Tensile Testing.

± Throughout test

readings of:

± applied load and

± extension

± recorded.



Tensile Testing.

1. Place sample in machine jaws.

2. Turn the hand wheelclockwise until the pointer onthe DTI moves.

3. Note the position of thegroove on the hand wheel.

4. Zero the DTI.

5. Turn hand wheel through onerevolution.

6. Record DTI reading.7. Continue this sequence.

8. For EACH revolution of handwheel record DTI reading.



Undertake the Tensile Test.



Converting the results.

± Specimen elongation given by subtracting DTI reading from number of turns.

eg. 3 turns giving a DTI reading of 2.83(3 ± 2.83 = 0.17mm extension).

± Force is given by DTI reading x 100.

eg. DTI reading = 2.83 x 100 =

283 Newtons force.



Typical Force / Extension Curve.



Tensile test practice

Do handout questions.

Perform tensile test in lab.



Truss ± Framed structure



Uses:

Bridges -A

Roof- B

Joints are assumed to

be frictionless pin-joints



Parts

Each part is called a ³member´.

They are either in tension or compression,depending on the structure and loading.

In compression they are called struts

In tension they are called ties



Plane trusses

In groups of 2, build a truss:

± 3 members, 3 joints, length=200mm, 150mm,150mm

± 4 members, 4 joints, 200mm, 200mm, 250mm,

100mm.

± 5 members, 4 joints, 200mm, 200mm, 250mm,

100mm, 5 member to fit across opposite corners.



Stability?

Which of the trusses you made are stable,

and which are unstable? Can you think of a formulae that would tell us

if it going to be stable?

What if we add an additional member to

example 3 without additional joints?



Redundancy

If we have a stable structure and we add one

more member, that member will beredundant .



Statically determinate:

Structure j m

Unstable:

m+3<2j

Stable with no

redundancy:

m+3=2j

Stable with

redundancy:

m+3>2j



Analysis of frameworks:

Example 1



Analysis of frameworks:





Factor of Safety / Safety factor

This is a value engineers use in DESIGN in order to

ensure they are well below the UTS of the component.

This is based on:

± the criticality of the component

± economics of weight (e.g. aircraft)

± Accuracy of design method and analysis.

. .

Allowable

UTS F O S

W

!



Factor of Safety / Safety factor

But often we do not want the component to deform

permanently so,

. .

yield

Allowable

F O S

W

W

!



Exercise sheet: Framed structures

Documents

1 Framed Static Structures