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 &
StructuresStructures
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 &
StructuresStructures
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 &
StructuresStructures
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
from Introduction to fire Dynamics, Drysdale, Wiley
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
190s
285s
316s
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
∆′′
=′′&
&
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
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 &
StructuresStructures
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.
� Traditional methods are based on experiments
conducted in small compartment (~3 m3)
1. Traditional methods assume uniform fires that
lead to uniform fire temperatures (?)
2.2.2. 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
Stern-Gottfried, PhD thesis, University of Edinburgh 2011.
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
Stern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007
� 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
Stern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007
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
Stern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007
Stern-Gottfried et al., Fire Safety Journal 45, pp. 249–261, 2010. doi:10.1016/j.firesaf.2010.03.007
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
� Temperature rise increase from 4% to 18%
� Time to failure decrease of 5% to 15%
� Concrete
� Temperature rise increase from 5% to 25%
� Time to failure decrease of 6% to 22%
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
Traditional ProblemsTraditional Problems
� Traditional methods are based on experiments
conducted in small compartment (~3 m3)
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 (?)
Traditional ProblemsTraditional Problems
Stern-Gottfried, PhD thesis, University of Edinburgh 2011.
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
Stern-Gottfried and Rein, Fire Safety Journal, 2012
�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
Stern-Gottfried and Rein, Fire Safety Journal, 2012
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
Law et al, Engineering Structures 2011
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
Law et al, Engineering Structures 2011
Effect of fire size and rebar depth
Stern-Gottfried and Rein, Fire Safety Journal, 2012
Effect of fuel load
Stern-Gottfried and Rein, Fire Safety Journal, 2012
Max Rebar Temperatures vs. Fire Size
1h 18 min
Law et al, Engineering Structures 2011
Max Deflection vs. Fire Size
1h 54 min
Law et al, Engineering Structures 2011
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
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
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
Stern-Gottfried and Rein, Fire Safety Journal, 2012