by Dr Guillermo Rein - cvut.cz
Transcript of by Dr Guillermo Rein - cvut.cz
DepartmentofMechanicalEngineering
IntroductiontoFireDynamicsforStructuralEngineers
byDrGuillermoRein
TrainingSchoolforYoungResearchersCOSTTU0904,Naples,June2013
EM-DAT International Disaster Database, Université catholique de Louvain, Belgium. www.emdat.be
ExplosionsandFire
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NOTE: Immediate fatalities as a proxy to overall damage. Disaster defined as >10 fatalities, >100 people affected, state of emergency or call for international assistance.
Jocelyn Hofman, Fire Safety Engineering in Coal Mines MSc Dissertation, University of Edinburgh, 2010
TechnologicalDisasters1900‐2000
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HR
R (k
W)
Time (s)
FireTestin4x4x2.4menclosure~smallpremiseinshoppingmall
Forced suppressionStarts here
Compartmentfires
(a) growth period(b) fully developed fire(c) decay period
(a)
(b)
(c)Hea
t rel
ease
rat
e (k
W)
Time
Fire development in a compartment - rate of heat release as a function of time
flashover
foQ
maxQ
DisciplineBoundaries
Fire Structures
Fire & Structures
Boundarybetweenfireandstructuresistheintersectionofthetwosets
Fire
Fire & Structures
Failure of structures at
550+X ºC
LameSubstitutionofthe1st kind
Whenstructuralengineersareentirelyreplacedbypseudo‐science.Itstillsurvivesinmanystandards
Structures
Fire & Structures
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0.1 1.1 2.1 3.1Burning Time [hr]
LameSubstitutionofthe2nd kind
When fire engineers are entirely replaced by pseudo-science.It is mainstream in structural engineering.
Fire & Structures
Failure of structures at
550+X ºC
LameSubstitutionofthe3rd kind
When both fire and structural engineers are simultaneously replaced by pseudo-science.
Any similarities with reality is a mere coincidence.
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Objectiveofthistalk
Provide an introduction to fire dynamics to the audience, a majority of structural engineers
working on fire and structures
This introduction will make emphasis on the mechanism governing fire growth in
compartments
Then, two most fundamental flaws of current design fire methodologies will be reviewed
Textbooks
Introduction to fire Dynamicsby Dougal Drysdale, 3rd Edition,
Wiley 2011
Principles of Fire Behaviorby James G. Quintiere
~£65
~£170
The SFPE Handbook of Fire protection Engineering, 4th Edition, 2009
~£46
Ignition– fuelexposedtoheat Upon receiving sufficient heat, a solid/liquid fuel
starts to decompose giving off gasses: pyrolysis Ignition takes place when a flammable mixture of
fuel vapours is formed over the fuel surface
Flammable mixture
T(t3)T(t2) T(t ignition)T(t1)Tambient (t0)
Heat flux
Pilot
Temperature
Dep
th
time
0 50 100 150 2000
50
100
150
200
250
300 Classical theory (best fit)
Apparatus AFM Cone calorimeter FPA FIST LIFT Others apparatus with tungsten lamps heat source Others apparatus with flame heat source
Experimental conditions
No black carbon coating or no information Black carbon coating Vertical sample Controled atmsophere (18% < O2 < 30%) Miscellaneous
Dashed area = experimental error Time = 2s Heat flux = +12 / -2 % [35]
TimetoignitionTimetoignition– ThickSamplesExperimental data for PMMA (polymer) from the literature. Thick samples
2
e
oigig q
TTck
4t
Heatflux
Timetoignition
from Bal and Rein, Combustion and Flame 2011
Flamespreadisinversely
proportionaltothetimeto
ignition
FlameSpreadRate
s
ig
s
tS
2
4
e
oigig q
TTckt
Downward Upward
e
igig q
TTct
0
Thick fuel
Thin fuel
FlameSpreadvs.Angle
Upward spread is 20 times faster than downward spread
upward spread
downward vertical spread
TestconductedbyAledBeswickBEng2009
FlameSpreadvs.Angle
http://www.youtube.com/watch?v=V8gcFX9jLGc
Rateofflamespreadoverstripsofthinsamplesofbalsawoodatdifferentanglesof15,90,‐15and0˚.
TestconductedbyAledBeswickBEng2009
FlameandFirePowerEffect of heat Release Rate on Flame height (video WPI)
http://www.youtube.com/watch?v=7B9-bZCCUxU&feature=player_embedded
FirePower– HeatReleaseRate Heat release rate (HRR) is the power of the fire (energy
release per unit time)
AmhmhQ cc
Heat Release Rate (kW) - evolves with time
Heat of combustion (kJ/kg-fuel) ~ constant
Burning rate (kg/s) - evolves with time
Burning rate per unit area (m2) ~ constant
Burning area (m2) - evolves with timeAmm
hQ
c
Note: the heat of reaction is negative for exothermic reaction, but in combustion this is always the case, so we will drop the sign from the heat of combustion for the sake of simplicity
1.
2.
3.
Burningrate(perunitarea)
phq
m
from Quintiere, Principles of Fire Behaviour
In general, it is a material and scenario dependant. In open fires it can be
approximated as a material property only.
HeatofCombustion
from Introduction to fire Dynamics, Drysdale, Wiley
It is a material property only if the combustion
efficiently is also taken into account. Efficiency is scenario dependant.
On a uniform layer of fuel, isotropic spread gives a circular pattern
22
22
tSmhAmhQ
StRA
StR
SdtdR
cc
constantS if
rate spread flameS
222 ttSmhQ c
R
Flamespread
when flame spread is ~constant, the fire grows as t2
A
~ material property in well ventilated fires
Tabulated fire-growths of different fire types
t‐squaregrowthfires
8
6
4
2
00 240 480 720 960
slow
mediumfastultra-fast2tQ
time (s)
HR
R (M
W)
H
Burn‐out
mHtout
near burn-out,location running out of fuel
Recently ignited by flame
burn-out
At some point:
Later on:
Sofafire
Peak HRR= 3 MW
Average HRR ~1 MW
residual burning+ smouldering
from NIST http://fire.nist.gov/fire/fires
growth burn-out
Freeburningvs.Confinedburning
Time (s)B
urni
ng ra
te (g
/m2 .s
)
Experimental data from slab of PMMA (0.76m x 0.76m) at unconfined and
confined conditions
confined free burning
Smoke and walls radiate downwards to fuel items in the compartments
phq
m
from Introduction to fire Dynamics, Drysdale, Wiley
What is flashover?
Sudden period of very rapid growth caused by generalized ignition of fuel items in the room
Some indicators:
• Average smoke temperature of ~500-600 ˚C
• Heat flux ~20 kW/m2 at floor level
• Flames out of openings (ventilation controlled)
NOTE: These three are not definitions but indicators only
Suddenandgeneralizedignition(flashover)
NOTE: Average temperate of 600˚C implies that the room space is occupied mostly by interment flames
GI GO
Whenproblemsariseattheinterfacebetweenfireandstructures,mostconsequencestraveldownstream,ie.towardsthestructuralengineer
Iftheinputisincompleteorwrong,thesubsequentanalysisisflawedandcannotbetrusted
Fireistheinput(boundarycondition)tosubsequentstructuresanalysis.
ViewsoffireArtist
Forester
Mechanical engineer
Structural engineer Rest of the world
Geo
scie
ntis
t
WTC2‐ Eastface
White=nofireRed=firevisibleinside,Orange=externalflaming,Yellow=spotfire
Blue=observationnotpossiblefigures/datafromNIST
9:05am 9:15am
9:30am
10:00am
9:45am
AncientDesignFires
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Time (minutes)
Tem
pera
ture
(°C
)
EC - Short
EC - Long
Standard
Standard Fire ~1880 (on paper in ~1912) Swedish Curves ~1972 Eurocode Parametric Curve ~1995
TraditionalDesignFires
Blindextrapolationfromlimitedexperience
FireinSmallcompartment
FireinNormalcompartment
FireinLargecompartment
FireinMulti‐storeycompartment
blindextrapolation
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Floor Area (m²)
Surf
ace
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a/Vo
lum
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/m)
Scalingeffects
HerebeFireTests
HerebeRealBuildings
Wallareapervolum
eofenclosure[m
2 /m
3 ]
Floorarea[m2]
blindextrapolation
DesignFires
What follows is a review of the current state of the art on design fires in fire and structures.
“TheTitaniccompliedwithallcodes.Lawyerscanmakeanydevicelegal,onlyengineerscanmakethemsafe"
ProfVMBranniganUniversityofMaryland
Traditionalmethodsarebasedonexperimentsconductedinsmallcompartment (~3m3)
1. Traditionalmethodsassumeuniform fires thatleadtouniformfiretemperatures(?)
2. Traditionalmethodshavebeensaidtobeconservative(?)
Stern-Gottfried, PhD thesis, University of Edinburgh 2011.
TraditionalMethods
FuelLoad
Mixed livingroom/office spaceFuel load is ~ 32 kg/m2
Set-up Design for robustness and high repeatability
CompartmentTemperature
Stern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007
Cardington Results
Stern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007
Conclusions on homogeneity Decentlyinstrumentedfiretestsshowconsiderablenon‐uniformityinthetemperaturefield
Whenexposedto80%percentiletemperaturesinsteadofaverage,thetimetofailuredecreasesto15%inProtectedSteelandto22%forConcrete.
Onesingletemperatureforawholecompartmentisnotcorrectnorsafeassumption
Heterogeneityhassignificantimpactonstructuralfireresponse
Firetestswithcrudespatialresolutionhaveledtoerroneousconclusions
FuturetestsshouldbeinstrumentedasdenselyaspossibleStern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007
Limitations
Forexample,limitationsaccordingEurocode:
Nearrectangular enclosures Floorareas<500m2
Heights<4mNoceilingsopeningsOnlymediumthermal‐inertialining
Noceilingopening?
© Arup/Peter Cook/VIEW
©
© Renzo Piano
Insulatinglining?
Arup CampusLondon Bridge Tower
3000compartments
Jonsdottir et al, Out of range, Fire Risk Management 2009
• Buildingsfrom1850‐1990:~66%ofvolumewithinlimitations
• Buildingsfrom2000:~8%ofvolumewithinlimitations(figure)
Modernarchitectureincreasinglyproducesbuildingsoutofrange
WesurveyedmostoftheenclosuresintheKingsBuildingscampusoftheUniversityofEdinburgh.
Traditionalmethodsarebasedonexperimentsconductedinsmallcompartment (~3m3)
1. Traditionalmethodsassumeuniform fires thatleadtouniformfiretemperatures(?)
2. Traditionalmethodshavebeensaidtobeconservative(?)
Stern-Gottfried, PhD thesis, University of Edinburgh 2011.
TraditionalMethods
*
IGNITION SPREAD MAX SIZE
Firespreadinsmallvs.largeroom– ExtrapolationofMaximumSize
area of the fire increasing with time
A Amax Amax
Because all knowledge on fire behaviour came from tests in small rooms, the implicit assumption was to
extrapolate the maximum size
*
IGNITION SPREAD TRAVEL
Thefiretravelsinlargefloors
area of the fire increasing with time
AAmax Amax
NOTE: The name Travelling Fires was incidentally given by Barbara Lane in an email in 2007. Chances are high she does not know this.
H
mHtout
SLtspread
SS S
L
Condition for travellingbehaviour:
TravelingFires
Far field (Tff)Near field (Tnf)Far field (Tff)
Near field (Tnf)
Far field (Tff)
Tempe
rature
Distance
Tempe
rature
Distance
Each structural element sees a combination of Near Field and Far Field temperatures as the fire travels
TravellingFires
Stern-Gottfried and Rein, Fire Safety Journal, 2012
short&hot~1200 ˚Cfor20min
long&cold~100‐600 ˚Cforhours
from Alpert, Ceiling jet flows, SFPE handbook
flame spreadsburnt out fuel
FarField=CeilingJet– butnowittravels!
Time during which the near field burns at any given fuel location:
where tb is the burning time, m” is the fuel load density (kg/m2), Hc is the effective heat of combustion and Q’’ is the heat release rate per unit area (MW/m2)
Qhmt c
b
Fortypicalofficebuildings,burningtimeis~20min
ConservationofMass– burningtimefornearfield
Stern-Gottfried and Rein, Fire Safety Journal, 2012
StructuralResults– RebarTemperature
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Reb
ar T
empe
ratu
re (°
C)
Time (hours)
2.5%
5%
10%
25%
50%
100%
Law et al, Engineering Structures 2011
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500
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Reb
ar T
empe
ratu
re (°
C)
Burning Area
Travelling Fires
Standard Fire - 1h 18min
EC Short
EC Long
MaxRebarTemperaturesvs.FireSize
1h 18 min
Law et al, Engineering Structures 2011
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0% 20% 40% 60% 80% 100%
Def
lect
ion
(m
)
Burning Area
Travelling Fires
Standard Fire - 1h 54min
EC Short
EC Long
MaxDeflectionvs.FireSize
1h 54 min
Law et al, Engineering Structures 2011
Conclusions In large compartments, a post flashover fire
is not likely to occur, but a travelling fireProvides range of possible fire dynamicsNovel framework complementing
traditional methodsTravelling fires give more onerous conditions
for the structureStrengthens collaboration between fire and
structural fire engineers
Thanks
Collaborators:
J Stern-GottfriedA LawA JonsdottirM GillieJ Torero
Sponsors:
ARUP
Law et al, Engineering Structures 2011
Rein et al, Interflam 2007, London
Jonsdottir et al, Fire Risk Management 2009
Stern-Gottfried and Rein, Fire Safety Journal, 2012
Stern-Gottfried, PhD Thesis, 2011
FireTechnologyPeer reviewed journal of the NFPA by Springer.
Interdisciplinary journal spanning the whole range of fire safety science and engineering.
~1 or 2 orders of magnitude larger audience than any other fire journal. Specially read by industry.
Current impact factor is low (IF=0.43) but the sooner you publish with us and cite it the sooner we will reach IF~1.
Editor in Chief: Guillermo Rein
Associate Editors:Luke Bisby
Samuel ManzelloErica Kuligowski
John WattsKathleen AlmandNicholas Dembsey
Craig BeylerJohn Hall
Get the table of contentshttp://www.springer.com/10694
TravellingFires
Real fires are observed to travelWTC Towers 2001Torre Windsor 2005Delft Faculty 2008 etc…
Experimental data (and common sense) indicate fires travel in large compartments
In larger compartments, the fire does not burn uniformly but burns locally and spreads
RoomCritical
FloorCritical
BuildingCritical
100%
time
Structural Integrity
ProcessCom
pletion Progressive
collapse
Untenable conditions
ObjectiveofFireSafetyEngineering:protectLives,Property,BusinessandEnvironment
from Torero and Rein, Physical Parameters Affecting Fire Growth, Chapter 3 in: Fire Retardancy of Polymeric Materials,CRC Press 2009
FireService/Sprinkler
RoomCritical
FloorCritical
BuildingCritical
100%
time
Structural Integrity
ProcessCom
pletion
Untenable conditions
ObjectiveofFireSafetyEngineering:protectLives,Property,BusinessandEnvironment
from Torero and Rein, Physical Parameters Affecting Fire Growth, Chapter 3 in: Fire Retardancy of Polymeric Materials,CRC Press 2009
Rescue operations