introduction to fire dynamics for structural engineers (school for young researchers, 2012)

School of Engineering

University of Edinburgh

Introduction to Fire Dynamics

for Structural Engineers

by Dr Guillermo Rein

Training School for Young Researchers

COST TU0904, Malta, April 2012

Textbooks

Introduction to fire Dynamics

by Dougal Drysdale, 3rd Edition,

Wiley 2011

Principles of Fire Behavior

by James G. Quintiere

~£65

~£170

The SFPE Handbook of Fire

protection Engineering, 4th

Edition, 2009

~£46

EM-DAT International Disaster Database, Université catholique de Louvain, Belgium. www.emdat.be

Explosions and Fire

,

,

,

,

,

,

,

,

,

,

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

Technological Disasters 1900-2000

,

,

,

,

EM-DAT International Disaster Database, Université catholique de Louvain, Belgium. www.emdat.be

Jocelyn Hofman, Fire Safety Engineering in Coal Mines MSc Dissertation, University of Edinburgh, 2010

Technological Disasters 1900-2000

Fire and Explosions

Room

Critical

Floor

Critical

Building

Critical

100%

time

Floor E

vacuat

ion

Structural Integrity

Pro

cess

Co

mp

leti

on Progressive

collapse

Untenable conditions

Objective of Fire Safety Engineering:

protect Lives, Property, Business and Environment

from Torero and Rein, Physical Parameters Affecting Fire Growth, Chapter 3 in: Fire Retardancy of Polymeric Materials,CRC Press 2009

Ro

om

ev

acu

atio

n

Fire Service/Sprinkler

Room

Critical

Floor

Critical

Building

Critical

100%

time

Floor E

vacuat

ion

Structural Integrity

Pro

cess

Co

mp

leti

on

Untenable conditions

Objective of Fire Safety Engineering:

protect Lives, Property, Business and Environment

from Torero and Rein, Physical Parameters Affecting Fire Growth, Chapter 3 in: Fire Retardancy of Polymeric Materials,CRC Press 2009

Ro

om

ev

acu

atio

n

Rescue operations

Boundary at 256s

Heat release

rate (kW)

Time

The boundary between fire

engineers and structural

engineers is at the onset of

flashover.

Discipline Boundaries

Fire Structures

Heat Transfer

Fire & Fire &

StructuresStructures

The boundary between fire and structures is the intersection of these two sets of expertise. In the 21st C. we are very lucky this intersection is recognized now by all to take place under the realm of heat transfer

– that the fire insult to the structural element is a heat flux.

Fire

Fire & Fire &


Failure of structures at

550+X ºC

Lame Substitution of the 1st kind

When structural engineers are entirely replaced by pseudo-science. It can still be observed in several papers and standards

Structures

Fire & Fire &


0

300

600

900

1200

0.1 1.1 2.1 3.1Burning Time [hr]

Lame Substitution of the 2nd kind

When fire engineers are entirely replaced by pseudo-science. It is mainstream all over science and technology of structural

engineering.

Fire & Fire &


Failure of structures at

550+X ºC

Lame Substitution of the 3rd 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]

Objective of this talk

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

The two most fundamental flaws of current

design fire methodologies will be reviewed

Before ignition After 5 min After 15 min

Ignition – fuel exposed to heat

� 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

radiant heat radiant heat

Ignition – fuel exposed to heat

Flammable mixture

T(t3)T(t2) T(t ignition)T(t1)Tambient (t0)

Heat flux

Pilot

Temperature

Dep

th

time

by Nicolas Bal

Pyrolysis of solid

Pyrolysis

When a solid material heats up, it eventually reaches a temperature threshold where it

begins to chemically break down. This process is called pyrolysis and is similar to

gasification but with one key difference – pyrolysis is the simultaneous change of

chemical composition (eg, long hydrocarbon chains to shorter chains) and

physical phase (ie, solid or liquid to vapour) and is irreversible. When a solid is

burning with a flame, it is actually the pyrolysis vapours (aka pyrolyzate) directly above

it that is burning, not the solid itself.

Pyrolysis of liquid

from Introduction to fire Dynamics, Drysdale, Wiley

Pyrolysis videohtttp://www.youtube.com/watch?v=UusEwufhWaw

Iris Chang and Frances Radford, 2011/2012 MEng projectPyrolysis of clear PMMA slab 25mm high

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% < O

2 < 30%)

Miscellaneous

Dashed area = experimental error Time = 2s Heat flux = +12 / -2 % [35]

Time to ignitionTime to ignition – Thick SamplesExperimental data for PMMA (polymer) from the literature. Thick samples

2

e

oig

igq

TTck

4t

′′−

ρπ=&

Heat flux

Tim

e t

o i

gn

itio

n

from Bal and Rein, Combustion and Flame 2011

Flammability

� Thermally thin (hτ/k=Bi<0.1) means that one single temperature represents the whole slab

� τ is thickness, ρ is density, c is specific heat� No temperature gradients – lumped system

� For thin fuels exposed to high radiant heat fluxes, tigcan be plotted against q”e to give a straight line of

gradient τρc.

( )e

0ig

igq

TTct

′′−

τρ=&

Time to ignition – Thin Samples

Candle burning on Earth (1g) and

in microgravity inside the ISS (~0g)

Buoyancy – controls flame shape

Flame and FirepowerEffect of heat Release Rate on Flame height (video WPI)

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

Firepower – Heat Release Rate� 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 timeA

m

m

h

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.

Burning rate (per unit area)

ph

qm

∆′′

=′′&

&

from Quintiere, Principles of Fire Behaviour

Heat of Combustion


Temperature of the plumefrom I

ntr

od

ucti

on

to

fir

e D

yn

am

ics, Drysdale, Wiley

*

IGNITION GROWTH MASS BURNING

Fire spread

area of the fire A increasing with time

A

A

A

AmhQ c′′∆= &&

H

Burn-out and travelling flames

m

Ht outb

& ′′=−

ρ

near burn-out,location running out of fuel

Recently ignited by flame

burn-out

a)

b)

Flame spread is inversely

proportional to the time to

ignition

Flame Spread Rate

sδ

ig

s

tS

δ∝

2

4

′′−

=e

oig

igq

TTckt

&ρπ

Downward Upward

( )e

0ig

igq

TTct

′′−

τρ=&

Thick fuel

Thin fuel

Flame Spread vs. Angle

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

Rate of flame spread over strips of thin samples of balsa wood at different angles of 15, 90, -15 and 0˚.

Test conducted by Aled Beswick BEng 2009

Flame Spread vs. Angle

Upward spread up to 20 times faster than downward spread

upward vertical spread

downward vertical spread

Test conducted by Aled Beswick BEng 2009

On a uniform layer of fuel, isotropic spread gives a circular pattern

( )22

22

constantS if

rate spread flame

tSmhAmhQ

StRA

StR

Sdt

dR

cc ′′∆=′′∆=

==

=⇒=

==

&&& πππ

S

222 ttSmhQ c απ =′′∆= &&

R

Flame spread

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

A

~ material properties

Tabulated fire-growths of different fire types

t-square growth fires

8

6

4

2

00 240 480 720 960

slow

mediumfastultra-fast2tQ α=&

time (s)

HRR (MW)

Sofa fire

Peak HRR= 3 MW

Average HRR ~1 MW

residual burning+ smouldering

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

growth burn-out

Examples of HRR

workstation mattress wood crib

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

Fire Test at BRE commissioned by Arup 2009

4x4x2.4m – small premise in shopping mall

Fire Test at BRE commissioned by Arup 2009

4x4x2.4m – small premise in shopping mall

Suppressionwith water hoses

Free burning vs. Confined burning

Time (s)B

urn

ing

rat

e (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

ph

qm

∆′′

=′′&

&


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

Sudden and generalized ignition

(flashover)

NOTE: Average temperate of 600˚C implies that the room space is

occupied mostly by interment flames

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 hotter

Feedback

loop

Flashover

Compartment fires

(a) growth period

(b) fully developed fire

(c) decay period

(a)

(b)

(c)

Heat release rate (kW)

Time

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

flashover

foQ&

maxQ&

Discipline Boundaries

Fire Structures

Heat Transfer

Fire & Fire &


GI ⇒⇒⇒⇒ GO

�When problems arise at the interface between fire and structures, most consequences travel downstream, ie. towards the structural engineer

� If the input is incomplete or wrong, the subsequent analysis is flawed and cannot be trusted

� Fire is the input (boundary condition) to subsequent structures analysis.

Design Fires

“The Titanic complied with all codes.

Lawyers can make any device legal,

only engineers can make them safe"

Prof VM Brannigan

University of Maryland

What follows is a review of the current state of

the art on design fires in fire and structures.

Traditional Design FiresTraditional Design Fires

0

200

400

600

800

1000

1200

1400

0 30 60 90 120 150 180 210 240

Time (minutes)

Temperature (°C)

EC - Short

EC - Long

Standard

� Standard Fire ~1917

� Swedish Curves ~1972

� Eurocode Parametric Curve ~1995

� Traditional methods are based on experiments

conducted in small compartment (~3 m3)

1. Traditional methods assume uniform fires that lead

to uniform fire temperatures (?)

2. Traditional methods have been said to be

conservative (?)

Stern-Gottfried, PhD thesis, University of Edinburgh 2011.

Traditional MethodsTraditional Methods

LimitationsLimitations

For example, limitations according Eurocode:

� Near rectangularrectangular enclosures

� Floor areas < 500 m< 500 m22

� Heights < 4 m< 4 m

� No ceilings openingsopenings

� Only medium thermal-inertia lininglining

Proposed WTC Transit HubProposed WTC Transit Hub

< 500 m< 500 m22 floor?floor?

<4 m high?<4 m high?

Excel, LondonExcel, London

Rectangular?Rectangular?

No ceiling opening?No ceiling opening?

Arup CampusArup Campus

© Arup/Peter Cook/VIEW

©

ShardShard

© Renzo Piano

Insulating lining?Insulating lining?

Edinburgh Survey of 3,080 compartments

Jonsdottir et al, Out of range, Fire Risk Management 2009

We surveyed most of the enclosures in the Kings Buildings campus of the

University of Edinburgh. Results:

•Buildings from 1850-1990 have on average 66% of its volume within limitations

•Newest building from 2008 has 8% of its volume within limitations (see figure)

Conclusion: Modern architecture increasingly produces buildings out of range

Size EffectsDuring the last few years, we have been working on two

problems that stem from the assumptions made by current design fires used in the field

The main problem is that in a large enclosure, the concept of flashover is not possible. Average

temperature of 600˚C implies the whole enclosure is an massive intermitting flame, which could not be possibly fed by enough ventilation. This scenario would resemble

an explosion and be short-live instead.

We currently do not know what the upper enclosure size for flashover is, but Eurocode suggests is in the order of

500 m2 floorplate.



1. Traditional methods assume uniform fires that

lead to uniform fire temperatures (?)

2.2.2. Traditional methods have been said to be Traditional methods have been said to be Traditional methods have been said to be

conservative conservative conservative (?)(?)(?)

Traditional ProblemsTraditional Problems


Fuel Load

�Mixed livingroom/office space

�Fuel load is ~ 32 kg/m2

�Set-up Design for robustness and high repeatability

Average Compartment Temperature

Compartment Temperature

Stern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007

Cardington Results


� Peak local temperatures range from 23% to 75% above

compartment average, with a mean of 38%

� Local minimum temperatures range from 29% to 99%

below compartment average, with a mean of 49%

Temperature Distributions


Three different beams used

�Unprotected steel I-beam

� Protected steel I-beam to 60 min (12mm

high density perlite)

�Concrete beam with 60 min rating

Example: Cardington

2

Structural Results at 80th

Percentile

� Criteria taken as steel or rebar temperature of 550°C

� Unprotected steel

� Temperature rise increase from 4% to 15%

� Time to failure decrease of 5% to 26%

� Protected steel



� Concrete



Conclusions on homogeneity

� Fire tests show considerable non-uniformity in the

temperature field of post-flashover fires

� One single temperature for a whole compartment is not a

correct assumption

� Heterogeneity has significant impact on structural fire

response

� Fire tests with crude spatial resolution have led to erroneous

conclusions

� Future tests should be instrumented as densely as possible

“Problems cannot be solved by the

level of awareness that created

them"Attributed to A Einstein




1.1.1. Traditional methods assume uniform fires that lead to Traditional methods assume uniform fires that lead to Traditional methods assume uniform fires that lead to

uniform fire temperatures (?)uniform fire temperatures (?)uniform fire temperatures (?)

2. Traditional methods have been said to be

conservative (?)



Travelling Fires

� Real fires are observed to travel

�WTC Towers 2001

�Torre Windsor 2005

�Delft Faculty 2008

� Experimental data indicate fires travel

in large compartments

� In larger compartments, the fire does

not burn uniformly but burns locally

and spreads

� Flashover in large compartment has

never been observed

Travelling Fires

Reject homogenous

temperature assumption.

Fire environment split into

two regions:

Near-field, short & hot ≈ 1000-1200 ºC

Far-field, long & cold ≈ 200-1200 ºC

Stern-Gottfried and Rein, Fire Safety Journal, 2012

Te

mp

era

ture

Distance

Travelling Fires

spread

Te

mp

era

ture

Distance

spread

Ceiling Jet

from Alpert, Ceiling jet flows, SFPE handbook

�Each structural element sees a combination

of Near Field and Far Field temperatures

as the fire travels

Travelling Fires


�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)

Q

hmt cb & ′′

∆′′=

� For typical office buildings, burning time is ~20 min

Conservation of Mass

– burning time for near field


Case Study:

Generic Multi-Storey Concrete Structure

Law et al, Engineering Structures 2011

Example – 25% Floor Area fire in a 1000 m2

�Near field temperature 1200ºC for 19 min

� Far field temperature ~ 800ºC for 76 min

Structural

Element

Core

0200400600800

100012001400

0 50 100 150 200 250 300 350 400Time (min)

Te

mp

era

ture

(ºC

)

Point B, Rebar temperature

Point B, Gas temperature

Structural Results – Rebar Temperature


50% burn area

400ºC

0ºC600 minutes 1200 minutes

Tem

pera

ture

Time

2.5% burn area5% burn area10% burn area

25% burn area

100% burn area

Rebar Temperature


Effect of fire size and rebar depth


Effect of fuel load


Max Rebar Temperatures vs. Fire Size

1h 18 min


Max Deflection vs. Fire Size

1h 54 min


Steel Structure

Stern-Gottfried and Rein,Fire Safety Journal, 2012

Conclusions

� In large compartments, a post flashover fire

is not likely to occur, but a travelling fire

�Provides range of possible fire dynamics

�Novel framework complementing

traditional methods

�Travelling fires give more onerous conditions

for the structure

�Strengthens collaboration between fire and

structural fire engineers

Thanks

Collaborators:

J Stern-Gottfried

A Law

A Jonsdottir

M Gillie

J Torero

Sponsors:

ARUP


Rein et al, Interflam 2007, London

Jonsdottir et al, Fire Risk Management 2009


Stern-Gottfried, PhD Thesis, 2011

Encouraging initial reactions to this work?

� Abstract submitted to 2008 Structures in Fire (SiF) conferenceTitle: “ON THE STRUCTURAL DESIGN FIRES FOR VERY LARGE

ENCLOSURES”

Outcome: Rejected� Reviewer #1: This abstract does not it fit with [conference] theme.� Reviewer #2: This paper is outside the scope of the conference

� Reviewer #3: The authors are encouraged to submit their paper somewhere else

� Abstract submitted 2012 Structures in Fire (SiF) conference Title: “TRAVELLING FIRES IN LARGE COMPARTMENTS: MOST

SEVERE POSSIBLE SCENARIOS FOR STRUCTURAL DESIGN”Outcome: Rejected

� Reviewer 1: Several works has been done and published � Reviewer 2: No significant input

� Reviewer 3: Authors must provide examples for typical case studies

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

Attributed to A Einstein

Effect of near field temperature


introduction to fire dynamics for structural engineers (school for young researchers, 2012)

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