04b Burgess

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    Theneedtodesignforrobustness

    in

    fire

    IanBurgess

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    Robustnessaworkingdefinition

    2

    Theabilityofastructuretoavoid

    disproportionatecollapsewhen

    subjecttoalocalised failure

    Hence: Onlystructuralresistancefailureisconsidered

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    Whyisitimportantinfire?

    3

    Multiplelocalised structural

    failuresinfire+impact

    damage.

    Collapseofthewhole

    structure,including90%

    unaffectedbyfireorimpact.

    KeyNISTPostWTCRecommendation

    Increasestructuralintegrity

    Developdesigntoolsandmodifycodestopreventprogressivecollapse.

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    Strain

    (%)

    0.5 1.0 1.5 2.0

    Stress(N/mm2)

    0

    300

    250

    200

    150

    100

    50

    20C

    200C300C

    400C

    500C

    600C

    700C

    800C

    Steelbehaviour athightemperatures

    0

    0.5

    1.0

    1.5

    2.0

    2.5

    200 400 600 800

    Temperature(C)

    ExpansionCoeff /C(x105)

    Steel

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    Sourcesofdisproportionatecollapse

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    Multistoreyconstruction:compositebuildings

    6

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    Multistoreyconstruction:compositebuildings

    7

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    Multistoreyconstruction:compositebuildings

    8

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    Wholestoreyfire

    9

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    Wholestoreyfire

    10

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    Beamfailure

    12

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    Columnbuckling

    14

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    Columnbuckling

    16

    Column loadingsredistribute. Mayoverload othercolumns.

    Column buckles. Fire spreads.

    Possibility of pull-in collapse locally.

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    Columnbuckling

    18

    Further columns buckle. Frame collapse.

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    Columnbuckling

    19

    Further columns buckle. Frame collapse.

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    Verticalexternalfirespread

    22

    Flamescausewindowsto

    breakonburningstorey(A)

    andthefloorabove(B).

    B

    E

    D

    C

    A

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    Verticalexternalfirespread

    24

    FIrepropagatesonFloorC,spreadsthroughwindowsto

    FloorD.

    B

    E

    D

    C

    A

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    Pullinenhancedbucklingofcolumns

    25

    B

    E

    D

    C

    A

    Heatedbeamspullingoncolumn

    Greatlyincreasedeffectivelength

    Columnitselfweakenedbyheating

    Inwardbucklingof

    column. Partialor

    overallcollapse.

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    WTC1:Observedcolumnpullin

    26

    WTC1exteriorcolumns

    bowinginwardacross

    mostofthesouthface

    betweenfloors95to98at

    10.23am.

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    WTC1:Numericalmodelling

    27

    Perimeterwallanalysisfound

    an

    inward

    pull

    force

    of

    27kNateachcolumnatfloors

    95to99,starting80minutes

    aftertheaircraftimpact,

    causedamaximuminward

    bowingof790mm.

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    C i f i C fl d l SO83 fi

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    ConnectionforceinWTCfloormodel: ISO834fire

    29

    100

    50

    0

    50

    100

    1500 10 20

    Time(min)

    Horizontalreaction(kN)

    Time(min)

    Deflection(mm)

    1200

    1000

    800

    600

    400

    200

    0

    0 10 20

    C ti f i WTC fl d l ISO834 fi

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    ConnectionforceinWTCfloormodel: ISO834fire

    30

    Differentialthermalexpansion

    Pushout

    Time(min)

    Deflection(mm)

    1200

    1000

    800

    600

    400

    200

    0

    0 10 20

    100

    50

    0

    50

    100

    1500 10 20

    Time(min)

    Horizontalreaction(kN)

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    Connection force in WTC floor model: ISO834 fire

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    ConnectionforceinWTCfloormodel: ISO834fire

    34

    Diagonalstrutsatendsbucklein

    cascade

    Time(min)

    Deflecti

    on(mm)

    1200

    1000

    800

    600

    400

    200

    0

    0 10 20

    Pullin

    100

    50

    0

    50

    100

    1500 10 20

    Time(min)

    Horizon

    talreaction(kN)

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    Tying forces on connections

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    Tyingforcesonconnections

    37

    Capacityat100%top

    chordtemperature

    Capacityat80%

    topchord

    temperature

    110

    90

    70

    50

    30

    1010

    30

    50

    70

    90110

    0 100 200 300 400 500 600 700 800 900Topchordtemperature(C)

    Horizontalre

    action(kN)

    Reaction

    force

    TENSION

    COMPRESSION

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    Connectionfracture

    39

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    Generation of connection failure

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    Generationofconnectionfailure

    Pushoutforce

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    Initialextensionofbeams

    outstripsshorteningdueto

    thermalbowing

    Columnpushedout,compressiveforce

    onconnections.

    CardingtonBeamColumnJointFireTest7

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    g

    43BeamflangebucklingBeamshearbuckling

    Connectiontyingforce

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    y g

    Catenary tensionsinbeams.

    Athighdeflectionconnectionforces

    reversetotension.

    Verticalfirespread

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    Firespreadsacrossupperstorey.

    Effectivelengthofheated

    columnincreases.

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    Connectionfracture

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    Compartmentintegritybreached.

    Debrisloadingontofloor

    maypropagateconnection

    failuredownwards.

    Connectionfracture

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    Compartmentintegritybreached.

    Debrisloadingontofloor

    maypropagateconnection

    failuredownwards.

    Connectionfracture

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    Compartmentintegritybreached.

    Debrisloadingontofloor

    maypropagateconnection

    failuredownwards.

    Consequencesdiffer:

    Columnsarekeyelements failuremaybe

    disastrous.

    Jointfailuremayinitiatefirespreadandprogressivecollapse.

    Axialforceinsteeldownstand ofcomposite

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    beam(Ding&Wang)

    50

    400

    200

    0

    200

    400

    600

    800

    0 200 400 600 800 1000 1200

    Temperature(C)

    AxialForce(kN)

    Axialforcein

    restrainedbeam

    TENSION

    COMPRESSION

    Strength

    Heating

    Cooling

    Jointfailuresincooling

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    Onesidedfailures

    ofpartialdepthend

    plates

    Boltshear

    infinplate

    Nutthreadstrippingin

    endplate

    FractureincoolingatCardington

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    Temperature

    A

    xialForceinrestrainedbeam

    COMPRESSION

    Cooling

    Onesidedfailureofpartialdepthendplates

    duringcoolingphase.

    Reducedstiffnessretainsjointintegrity.

    Partialfracturemayhappenwhencooling

    fromnetcompression

    TENSION

    Heating

    Partialfracture

    52

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    Whataretheconsequencesofconnection

    failureinfire?

    WTC7

    53

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    Sequenceofevents onSeptember11,2001

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    Time Event

    09:59 WTC2Collapse

    WTC1NorthTower

    08:46 WTC1Impact~92nd floor

    Boeing767200,750km/h

    WTC2SouthTower

    09:03 WTC2Impact~78th floor

    Boeing767200,945km/h

    10:28 WTC1Collapse;other

    buildingimpacts

    WTC7

    17:20 WTC7 Collapse

    SimulatedfireprogressonFloor12

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    Firesignitedonupto10floorsatsimilartimes.

    Fireslastedcontinuouslyonlyonfloors7

    9and11 13.

    Observedparticularlyfrom14.30to17.20.

    BurningsimultaneouslyinNEcorneron

    severalfloorsinmidafternoon.

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