University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Modes of Failure

solids held together by bonds between their atoms

these bonds can be compressed

or extended

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

tension

compression

shear

bending

stress pattens may be complex but consist of only 3 basic states of stress tension - compression - shear

Modes of Failure

buckling

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Tension

state of stress where material tends to be pulled apart

cable with weight becomes longer under pull

lengthening depends on X-section, length & load

larger the diameter - smaller the elongation

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Compression

state of stress in which particles pushed against the others

column under load shortens squashes

shortening of material

a steel column under compression shortens as much as a rod of same steel

lengthens in tension under same stress

4/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Compression (cont.)

can have no tension elements but must have compression elements

materials weak in tension often strong in compression

with modern materials of high compressive strength, e.g. steel can build columns much slimmer

but slenderness introduces new problem

compression elements very commonmust channel loads to ground

5/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Buckling

buckling is a basic design factor for slender elements in compression

buckling occurs even if load perfectly central

as compressive load increases, reach value where slender elements instead of shortening buckle & usually break

buckling load depends on:material, length, shape of X-section, restraints at ends

6/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Shear

state of stress in which particles of material slide relative to each other

• rivets tend to shear

• a hole puncher uses shear to punch out holes in paper

• load on short cantilever tends to shear beam from wall

7/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Bending

consider plank loaded as shown

upper fibres lengthen

plank ends move down

section between stones deflects up

curve is arc of circle

lower fibres shorten

middle fibres remain original length - Neutral Axis

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Bending (cont.)

concrete beam fails in tension due to bending

may fail in diagonal tension due to shear due to bending

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Behaviour of Materials

stress

strain

elasticity - plasticity - brittleness

safety factors

selecting appropriate materials

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Modes Of Failure - Under Stress

tension

compression

buckling

shear

bending

stress patterns complex but consistonly of three basic states of stresstension - compression - shear

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

General Load-DeformationProperties Of Materials

application of load produces dimensional changes in a member

member undergoes change in size or shape or both

deformation may be reversible or irreversible

elastic or plastic

12/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Stress

internal forces developed within a structure due to action of external forces

stress is force intensity - force per unit area

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Fi = Fe

Fe

Fe

Stress (cont1.)

consider member in tension

stress is force intensity - force per unit area

Fe

Fe

X X

Fe

Stress = Force / Area f = F / A

Fe

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University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Stress (cont2.)

stress is force per unit area

1 pascal = 1 newton per square metre

15/22

A load of 1 N on each square metre represents

an average stress of 1 N/m2, or 1 Pa

1m1m

1m

1m1m

1m1m

1N1N

1N1N

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Stress (cont3.)

we use stress in megapascals (MPa) for most materials

1 MPa = 106 N/m2 = 1 N/mm2

we use stress in kilopascals (kPa) for floor loads and foundation pressures (loads distributed over an area)

( remember 1 m2 = 106 mm2)

16/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Stress (cont4.)

internal force not concentrated at single spot

stress developed DOES NOT DEPEND ON

MATERIAL OF MEMBER

distributed over entire cross-section

stress in a member depends only on force applied and cross-section f = F / A

17/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

Strength of Members

strength depends on many factors

in tension, failure will occur by pulling apart at weakest location

weak spot (point of reduced X-section) determines capacity of whole member

f = F/A higher because of smaller A

if material can sustain stress member will carry load

as load increases stress increases eventually material fails (pulls apart)

18/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

ratio ofchange in size or shape of element

to original size or shape

Strain

response to stress

have stress --> get strain

strain to do with change in size or shape

19/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

STRAIN (cont.1)

for member subject to simple tensile force

dimensionless - millimetre / millimetre

strain =increase in length

original length e =LL

20/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

STRAIN (cont2.)

determined by:

except for rubber bands, strains very small usually not visible

more a material strains under load - more the structure deflects

taking member of known length

subjecting it to a known load

measuring elongation

21/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure

Mike Rosenman 2000

STRESS & STRAIN SUMMARY

force

stress

causes

strain

puts material under

deformation

results in

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