Schoolof EngineeringUniversityof Edinburgh

IntroductiontoFireDynamicsforStructuralEngineers

byDrGuillermoRein

TrainingSchoolforYoungResearchersCOSTTU0904,Malta,April2012

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

FireService/Sprinkler

RoomCritical

FloorCritical

BuildingCritical

100%

time

Structural Integrity

P

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C

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Progressive collapse

Untenable conditions

FireSafety:protectLives,PropertyandBusiness

from Physical Parameters Affecting Fire Growth,Torero and Rein, CRCpress

Boundaryat256sH

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Time

DisciplineBoundaries

Fire StructuresHeat Transfer

Fire & Structures

Fire

Fire & Structures

Failure of structures at

550+X CStructures

LameSubstitutionof1st kind

Structures

Fire & Structures

0

300

600

900

1200

0.1 1.1 2.1 3.1Burning Time [hr]

Fire

LameSubstitutionof2nd kind

Fire & Structures

0

300

600

900

1200

0.1 1.1 2.1 3.1Burning Time [hr]

Failure of structures at

550+X C

LameSubstitutionof3rd kind

Before ignition After 5 minutes After 15 minutes

Ignition fuelexposedtoheat Material start to decompose giving off gasses:

pyrolysis

Ignition takes place when a flammable mixture of fuel vapours is formed over the fuel surface

PyrolysisvideoIris Chang and Frances Radford, 2011 MEng project

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]

TimetoignitionTimetoignitionExperimental data for PMMA (polymer) from the literature. Thick samples

2

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oigig q

TTck

4t

Heatflux

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Flammability

VideofromWPI(USA)Effect of heat Release Rate on Flame height

http://www.youtube.com/watch?v=7B9-bZCCUxU&feature=player_embedded

Burningrate(perunitarea)

phq

m

from Quintiere, Principles of Fire Behaviour

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 timeAmmh

Q

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.

HeatofCombustion

from Introduction to fire Dynamics, Drysdale, Wiley

*IGNITION GROWTH MASS BURNING

Burningarea

area of the fire A increasing with time

AA

A

AmhQ c

HBurnoutandtravellingflames

mHt outb

near burn-out,location running out of fuel

Recently ignited by flame

burn-out

a)

b)

FlameSpreadvs.Angle

http://www.youtube.com/watch?v=V8gcFX9jLGc

Rateofflamespreadoverstripsofthinsamplesofbalsawoodatdifferentanglesof15,90,15and0.

TestconductedbyAledBeswickBEng2009

On a uniform layer of fuel ignited, spread is circular

22

22

constantS if

rate spread flame

tSmhAmhQStRA

StR

SdtdR

cc

S

222 ttSmhQ c

R

Flamespread

if flame spread is ~constant, the fire grows as t2

A

~material properties

Tabulated fire-growths of different fire typestsquaregrowthfires

8

6

4

2

00 240 480 720 960

slow

mediumfastultra-fast2tQ

time (s)

H

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R

(

M

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Sofafire

Peak HRR= 3 MW

Average HRR ~1 MW

residual burning+ smouldering

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

growth burn-out

FireTestatBREcommissionedbyArup20094x4x2.4m smallpremiseinshoppingmall

190s

285s

316s

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3500

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4500

5000

0 100 200 300 400 500 600 700

H

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Time (seconds)

FireTestatBREcommissionedbyArup20094x4x2.4m smallpremiseinshoppingmall

Suppressionwith water

Freeburningvs.Confinedburning

Time (s)B

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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

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)

Mechanism for flashover:Fire produces a plume of hot smoke

Hot smoke layer accumulates under the ceiling

Hot smoke and heated surfaces radiate downwards

Flame spread rate and rate of secondary ignition increases

Rate of burning increases

Firepower larger and smoke hotterFeedback

loop

Flashover

Compartmentfires

(a) growth period(b) fully developed fire(c) decay period

(a)

(b)

(c)

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Fire development in a compartment - rate of heat release as a function of time

flashover

foQ

maxQ

DisciplineBoundaries

Fire StructuresHeat Transfer

Fire & Structures

GI GO

Iftheinputisincomplete/flawed,thesubsequentanalysisisflawedandcannotbetrustedfordesign

Fireistheinput(boundarycondition)tosubsequentstructuresanalysis

DesignFires

The Titanic complied with all codes.

Lawyers can make any device legal,

only engineers can make them safe"Prof VM BranniganUniversity of Maryland

TraditionalDesignFires

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EC - Short

EC - Long

Standard

Standard Fire ~1917 Swedish Curves ~1972 Eurocode Parametric Curve ~1995

Traditionalmethodsarebasedonexperimentsconductedinsmallcompartment experiments(~3m3)

1. Traditionalmethodsassumeuniform fires thatleadtouniformfiretemperatures(?)

2. Traditionalmethodshavebeensaidtobeconservative(?)

Stern-Gottfried et al, Fire Risk Management 2009

TraditionalMethods

Limitations

Forexample,limitationsaccordingEurocode:

Nearrectangular enclosures Floorareas