Design FiresDesign FiresA Structural Engineering PerspectiveA Structural Engineering Perspective

Dr M GillieDr M Gillie

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

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

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

Compartment FiresCompartment Fires

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

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

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

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

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

……other Evidenceother Evidence

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

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

Proposed Fire ModelProposed Fire Model

200-600C

1200C

Compartment

Structure

CoolHotCool

Need size of fire, gas temperatures and nature of travel!

Proposed Gas TemperaturesProposed Gas Temperatures

1200C

Distance from seat of fire

T

Size offire

Size offire

Good ventilation

Limited ventilation

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

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

Influence on Structural Influence on Structural BehaviourBehaviour

y

x

Travellingfire

Composite frame“Cardiington like”

Influence of TemperaturesInfluence of Temperatures

Influence of TemperaturesInfluence of Temperatures

0

200

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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

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