design fires a structural engineering perspective dr m gillie
TRANSCRIPT
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Design FiresDesign FiresA Structural Engineering PerspectiveA Structural Engineering Perspective
Dr M GillieDr M Gillie
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ContentContent
Existing fire modelsExisting fire models– Standard FireStandard Fire– Swedish curvesSwedish curves– Parametric firesParametric fires
CFD modelsCFD models Future directionsFuture directions
– Rein’s firesRein’s fires Structural behaviour in travelling firesStructural behaviour in travelling fires
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Standard Fire TestStandard Fire Test Defined using a furnace Test (ASTM-Defined using a furnace Test (ASTM-
E-119, ISO-834, BS-476 Part 8)E-119, ISO-834, BS-476 Part 8)
0
250
500
750
1000
1250
0 30 60 90 120 150 180time [minutes]
Tem
per
atu
re [
oC
]
Fire (BS-476-Part 8)Critical Temperature
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Pros and ConsPros and Cons
Has many limitationsHas many limitations– Not based on real fire dataNot based on real fire data– Test repeatability difficultTest repeatability difficult– No cooling phaseNo cooling phase– Uniform heatingUniform heating– Uses gas temperature “not fair”Uses gas temperature “not fair”
ButBut– Widely usedWidely used– Can be useful for crudely comparing Can be useful for crudely comparing
productsproducts
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Compartment FiresCompartment Fires
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Energy balance for a Energy balance for a compartment – Swedish compartment – Swedish
MethodMethod
openinghrough t
radiatedEnergy
t wallscompartmen
gh lost throuEnergy
gases of exchange
by lost Energy
combustionby
releasedEnergy
R
w
l
c
Q
Q
Q
Q
RQwQL
QcQ
Qc
QW
QW
QL
QR
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Assumptions in Swedish Assumptions in Swedish methodmethod
No heat built-up in pre-flashover No heat built-up in pre-flashover phase of firephase of fire
Temperature uniform in the Temperature uniform in the compartmentcompartment
Uniform heat transfer coefficient in Uniform heat transfer coefficient in compartment boundariescompartment boundaries
All combustion takes place in the All combustion takes place in the compartmentcompartment
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Pros and ConsPros and Cons
LimitationsLimitations– CrudeCrude– Rather severeRather severe– Implicit expressions (Eurocode parametric Implicit expressions (Eurocode parametric
curves solve this) curves solve this) – ***Uniform fire…******Uniform fire…***– ***…hence maximum size of compartment******…hence maximum size of compartment***
ButBut– ““Not bad”Not bad”– Can be used in performance-based designCan be used in performance-based design
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Zone ModelsZone Models
Extension of parametric modelsExtension of parametric models Assume uniform temperatures in Assume uniform temperatures in
each zoneeach zone Normally computer basedNormally computer based Several commercial codes available Several commercial codes available
eg. Ozone, CFasteg. Ozone, CFast Similar drawbacks and benefits to Similar drawbacks and benefits to
parametric curvesparametric curves
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Fires TravelFires Travel
Large compartment test at BRE showed Large compartment test at BRE showed travelling fire behaviourtravelling fire behaviour
Travel times of the order of 45 minutes Travel times of the order of 45 minutes within compartment 5m deepwithin compartment 5m deep
Crib Crib Crib Crib
Ventilation
5.5m
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……other Evidenceother Evidence
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Defining Temperature Defining Temperature LoadingLoading
Need simple model that can capture effects ofNeed simple model that can capture effects of– Travelling fireTravelling fire– Ventilation conditionsVentilation conditions– CoolingCooling
Preliminary work undertaken on simplified Preliminary work undertaken on simplified travelling fire loading by Rein travelling fire loading by Rein et. al.et. al.
NOTE: NOTE: CFD calculations too complex for CFD calculations too complex for design work and…design work and…
……can not predict burning behaviour anywaycan not predict burning behaviour anyway
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Rein’s Fire ModelRein’s Fire Model
Rein proposes a near field and far Rein proposes a near field and far field model of temperature loadingfield model of temperature loading
Fire field moves around Fire field moves around compartmentcompartment
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Proposed Fire ModelProposed Fire Model
200-600C
1200C
Compartment
Structure
CoolHotCool
Need size of fire, gas temperatures and nature of travel!
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Proposed Gas TemperaturesProposed Gas Temperatures
1200C
Distance from seat of fire
T
Size offire
Size offire
Good ventilation
Limited ventilation
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Travelling BehaviourTravelling Behaviour Typical office fire load =570MJ/mTypical office fire load =570MJ/m22
Rate of burning = 500 kW/mRate of burning = 500 kW/m22
Therefore 19 minutes in every locationTherefore 19 minutes in every location Total fire time= 19ATotal fire time= 19Acc/A/Aff
Local fireT=0 Local fire
T>0 Local fireLater on
Fire compartment
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Travelling BehaviourTravelling Behaviour
What “path” does the fire take?What “path” does the fire take? We can’t knowWe can’t know Depends on details of compartment Depends on details of compartment
and chanceand chance
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Influence on Structural Influence on Structural BehaviourBehaviour
y
x
Travellingfire
Composite frame“Cardiington like”
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Influence of TemperaturesInfluence of Temperatures
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Influence of TemperaturesInfluence of Temperatures
0
200
400
600
800
1000
1200
1400
0 1000 2000 3000 4000 5000 6000Time (s)
Tem
pera
ture
(de
gC)
25 NT1 Start25 NT1 Middle25 NT1 End25 NT2 Start25 NT2 Middle25 NT2 End25 NT3 Start25 NT3 Middle25 NT3 End25 NT4 Start25 NT4 Middle25 NT4 End
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ConclusionsConclusions
Current assumptions Current assumptions probably probably conservativeconservative
Travelling fires in large compartments Travelling fires in large compartments may lead to significantly lower design may lead to significantly lower design forcesforces– Current assumptions very severeCurrent assumptions very severe
Possibilities for significant savings in Possibilities for significant savings in fire protectionfire protection
Work to date of conceptual natureWork to date of conceptual nature