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

,

,

,

,

,

,

,

,

,

,

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

FireTestatBREcommissionedbyArup20094x4x2.4m– smallpremiseinshoppingmall

0

1000

2000

3000

4000

5000

150 170 190 210 230 250 270 290 310

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

0

300

600

900

1200

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.

0

300

600

900

1200

0.1 1.1 2.1 3.1Burning Time [hr]

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

PyrolysisvideoPyrolysisofclearPMMA slab25mmthick

htttp://www.youtube.com/watch?v=UusEwufhWaw

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

Flammability~materialproperty

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

*

IGNITION GROWTH MASS BURNING

area of the fire A increasing with time

AA

A

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

ExamplesofHRR

workstation mattress wood crib

from NIST http://fire.nist.gov/fire/fires

190s

285s 316s

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

Ibelieveinhumanrights,

therefore:

Breakof5min

DisciplineBoundaries

Fire Structures

Fire & Structures

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

0

200

400

600

800

1000

1200

1400

0 30 60 90 120 150 180 210 240

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

0

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000 2500 3000

Floor Area (m²)

Surf

ace

Are

a/Vo

lum

e (1

/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


therefore:

Breakof5min

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

AverageCompartmentTemperature

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

Proposed WTC Transit Hub

<500m2 floor?<4mhigh?

Excel, London

Rectangular?

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

“Problems cannot be solved by the level of awareness that created them"

Attributed to A Einstein

*

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:


therefore:

Breakof5min

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


CaseStudy:GenericMulti‐StoreyConcreteStructure

Law et al, Engineering Structures 2011

StructuralResults– RebarTemperature

0

100

200

300

400

500

0.1 1 10 100

Reb

ar T

empe

ratu

re (°

C)

Time (hours)

2.5%

5%

10%

25%

50%

100%


Familyoffires– notjustonefirecastinstone

rangeofsizes=rangeofspreadrates

Effectoffiresizeandrebardepth


ComparisonwithTraditionalMethods


0

100

200

300

400

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


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


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


Rein et al, Interflam 2007, London

Jonsdottir et al, Fire Risk Management 2009


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

Effectoffuelload


Effectofnearfieldtemperature


Results

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

by dr guillermo rein - cvut.cz

Documents