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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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41
Generationofconnectionfailure
Pushoutforce
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42
Initialextensionofbeams
outstripsshorteningdueto
thermalbowing
Columnpushedout,compressiveforce
onconnections.
CardingtonBeamColumnJointFireTest7
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g
43BeamflangebucklingBeamshearbuckling
Connectiontyingforce
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44
y g
Catenary tensionsinbeams.
Athighdeflectionconnectionforces
reversetotension.
Verticalfirespread
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45
Firespreadsacrossupperstorey.
Effectivelengthofheated
columnincreases.
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Connectionfracture
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47
Compartmentintegritybreached.
Debrisloadingontofloor
maypropagateconnection
failuredownwards.
Connectionfracture
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48
Compartmentintegritybreached.
Debrisloadingontofloor
maypropagateconnection
failuredownwards.
Connectionfracture
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49
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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51
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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55
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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FloorstructurearoundColumn79
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At~500Ca15mcomposite
(secondary)beamhasafree
thermalexpansionof~90mm.
79
44
40
38
42
FloorstructurearoundColumn79
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At~500Ca15mcomposite
(secondary)beamhasafree
thermalexpansionof~90mm.
Ifthisisrestrainedbythenoncompositegirder,itcreateslarge
transverseforcesonthegirder.
79
44
40
38
42
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GirdertocolumnconnectionatColumn79
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GirdertocolumnconnectionatColumn79
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Resultantrestraintforce
fromsecondarybeams Shearforcesonbolts
FloorstructurearoundColumn79
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At~500Ca15mcomposite
(secondary)beamhasafree
thermalexpansionof~90mm.
Ifthisisrestrainedbythenoncompositegirder,itcreateslarge
transverseforcesonthegirder.
Boltsonseatingplateandlocating
cleatfractureinshear(probably
onFloor13),andthegirder44
79collapses.
Thisisrepeatedinsequenceon
lowerfloorsduetoimpactsand
similarrestraintforcesfrom
simultaneousfires.
79
44
ThefallofWTC7
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67
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Canwepredictconnection
behaviour infire??
Principalcomponentzonesofendplate
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69
BeamwebColumn
web
Beam
flange,web
Column
web
Compression
Beamweb
Columnweb Column
flangeEnd
plate
Tensionbolts
Tension
Hogging
Moment
Slip
TheComponent methodwithaxialforce
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70
K1
K2
Kc
Componentmodeldealswithloadcombinations
automatically,thoughMfcurveschangeduetothrust.
TheComponent methodwithaxialforce
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71
FcKc
F2
F1
K1
K2
Ft
M
Componentmodeldealswithloadcombinations
automatically,thoughMfcurveschangeduetothrust.
TheComponent methodwithaxialforce
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72
Componentmodeldealswithloadcombinations
automatically,thoughMfcurveschangeduetothrust.
FcKc
F2
F1
K1
K2
FtF
ComponentBasedConnectionElement(Block)
Compression
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73
Beamendandcentrelineofcolumnassumedtoremainplane
Tensionandcompressionforceshavedifferentlinesofaction
Onlydependsonthegeometryandthematerialoftheconnection
Compression
springs(column
web)
Onesetoftensionspringsperboltrow
(Tstubs,bolts)
Shearspring(bolts)Zerolength
i j
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Componentbasedconnectionelement:beam
shearpanel
7/31/2019 04b Burgess
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75
1
u
w
f
F
S
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Componentbasedconnectionelement:
includingbothshearpanels
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w
u
f
F
S
Componentbasedconnectionelement: centre
linemodelincludingshearpanelsColumnelement
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F
Sw
u
f
element
ColumnShear
Panelelement
Connection
element
BeamShear
Panelelement
Beam
element
7/31/2019 04b Burgess
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Beamsectionfractureincooling
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81
Beamtemperature
Normalised axial
forceinrestrained
beam
TENSION
COMPRESSION
Heating
AxialForce/steelstrength
Reducedrestraint,
higherductility
Tensilefailureofbeamsection
Stiffrestraint
Cooling
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Howcanwefullymodel
progressivecollapse?
Dynamicanalysistoidentifyrestabilization
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85
Stableregion
Unstable
region
Load
Deflection
Stable
regionre-st
crit
Pcrit
Dynamic
Wherenext?
7006005004003002001000
0
Temperature(C)
800
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J1
C1
Forcesinboltrows
againstrotationatJ1
0 20 40 60 80 100 120ForcesinComponent(KN)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Rotation(rad
)
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Rotation
(rad) RotationatJ1against
temperature
Wherenext?
70060050040030020010000
Temperature(C)
800
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200
400
200
100 200 300 400 500 600 700 800
600
800
1000
0
Displacement(mm) Temperature( C)
J1
C1
DisplacementoftopofcolumnC1
againstTemperature
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Rotation
(rad) RotationatJ1against
temperature
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Willthecomponent
basedmethodwork?
Characterizingcomponentsforconnectionelement
Displacement (mm)
7/31/2019 04b Burgess
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90
25 20 15 10 5 00
20
40
60
80
100
120
Displacement(mm)
Force
Testsonflushendplateconnections
10
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320
300
tp400
90
133.4
70
50
51.7
UC25489
UB30540Connector
(LoadingAngle)
ViewonAA
90
200
10
Grade8.8M20
bolts
91
DeformedShapeandFailureMode
At 20oC
7/31/2019 04b Burgess
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At20oC
92
Comparisonwithwebcleattests:Loadingat35
7/31/2019 04b Burgess
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0
50
100
150
200
250
0 3 9 12 18 21
Rotation(Degrees)
F
orce(kN)
156
ComponentModel
Test
20C
450C
550C
650C
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