1

Dr Guillermo Rein

University of Edinburgh

Travelling Fires in

Structural Design

6th Int Conf Fire Safety Eng

APICI Madrid, Feb 2011

Contributions from J Stern-Gottfried, A Law,

A Jonsdottir, M Gillie and J Torero

Structural Design for Fire Safety

�Fire is a source of heat that

weakens the structure

�Assessment of structural

response to avoid collapse

� In order to determine structural

detailing and fire protection

requirements

�Enhancement of:� Integral safety

� Robustness

� Safe innovation

� Cost savings

2

GI -> GO

�Cold behaviour ≠ Hot behaviour

�Fire dynamics and resulting environment

are the input and boundary condition to

subsequent Fire & Structures Analysis

�If the input is incomplete, the

subsequent analysis cannot be

trusted

Traditional Methods

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

3

Buildings are Different

Then… …and Now

� Architecture is always seeking out of bound

– higher, larger, new shapes

TraditionsTraditions

�Traditional methods assume uniform fires

that lead to uniform fire temperatures

�Traditional methods are based on

experiments conducted in small

compartment experiments (~3 m3)

�Traditional methods have been said to be

conservative (?)

Stern-Gottfried et al, Fire Risk Management 2009

4

Size MattersSurface Area to Volume Ratio vs Floor Area for a 3m High Square Compartment

0

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000 2500 3000

Floor Area (m²)

Sur

face

Are

a/V

olum

e (1

/m)

Fire Tests

Real Buildings

Stern-Gottfried et al, Fire Risk Management 2009

Limitations in Traditional Methods Limitations in Traditional Methods

For example, limitations according Eurocode:

�Near rectangularrectangular enclosures

� Floor areas < 500 m< 500 m22

�Heights < 4 m< 4 m

�No ceilings openingsopenings

�No low or high thermal-inertia lininglining

5

Sydney Opera HouseSydney Opera House

Near rectangular?Near rectangular?

© KPF Architects

Pompidou CentrePompidou Centre

Proposed WTC Transit HubProposed WTC Transit Hub

< 500 m< 500 m22 floor?floor?

<4 m high?<4 m high?

Excel, LondonExcel, London

6

London Bridge TowerLondon Bridge Tower

Only insulating lining?Only insulating lining?

The Gherkin TowerThe Gherkin Tower

© Renzo Piano

©

Proposed WTC MemorialProposed WTC Memorial

No ceiling opening?No ceiling opening?

Arup CampusArup Campus

© Arup/Peter Cook/VIEW

©

7

Edinburgh Survey: 3,080 compartments� 19-20th Century buildings:

66% of volume within limitations

� 2008 building: 8%

� Suggests modern architecture increasingly produces buildings out of range

Jonsdottir et alFire Risk Management 2009

Travelling Fires Methodology

� Real fires have been 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

Rein et al, Interflam 2007, London

8

Fire environment split

into two:

Near-field ≈ 1000-1200 ºC

Far-field ≈ 200-1200 ºC

(Alper’s correlation)

Te

mp

er

atu

re

Distance

Travelling Fires

Fire environment split

into two:

Near-field ≈ 1000-1200 ºC

Far-field ≈ 200-1200 ºC

(Alper’s correlation)

Te

mp

er

atu

re

Distance

Total burning

duration is a function

of the area of the fire

Travelling Fires

9

Far Field Temperature

�Maximum temperature at ceiling jet. Average

calculated over the correlation with the distance

from the fire (Alpert’s correlation)

( )H

rQ38.5TT

32

max

&

=− ∞

nfff

r

r

4

4

ffrr

drTT

ff

nfmax

−=∫

�Burning at average heat release per unit area

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 & ′′

∆′′=

� 50 MW fire on 200 m2 burns for 30 min� 50 MW fire on 1000 m2 burns for 15 min

Conservation of Mass – burning time

Rein et al, Interflam 2007, London

10

�Each structural element sees a combination

of Near Field and Far Field temperatures

as the fire travels

Travelling Fires

Stern-Gottfried et al, SPFE PBD, 2010, Lund

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

0200

400600800

100012001400

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

Te

mp

era

ture

(ºC

)

Point B, Rebar temperature

Point B, Gas temperature

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Family of possible fires

Stern-Gottfried et al, SPFE PBD, 2010, Lund

Case Study:

Generic Multi-Storey Concrete Structure

Law et al, Engineering Structures 2011

Stern-Gottfried et al, SPFE PBD, 2010, Lund

12

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

Max Rebar Temperatures vs. Fire Size

1h 18 min

Law et al, Engineering Structures 2011

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Max Deflection vs. Fire Size

1h 54 min

Law et al, Engineering Structures 2011

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0% 50% 100%

Fire area

No

rmal

ize

d s

tres

s_

Reb ar temperatureStandard FireParametric - Short hotParametric - Long co ol

00.1

0.20.3

0.40.50.6

0.70.8

0.91

0% 50% 100%

Fire area

No

rma

lized

def

lect

ion_

Deflect ionStand ard FireParametric - Sho rt ho tParametric - Long cool

0

0.01

0.02

0.03

0.04

0.05

0.06

0% 50% 100%

Fire area

No

rmal

ize

d s

trai

n_

Sagg ing strainStandard FireParametric - Short hotParametric - Lo ng coo l

00.02

0.040.06

0.080.1

0.12

0.140.16

0.180.2

0% 50% 100%

Fire area

No

rma

lized

str

ain_

Hog ging strain

Stand ard FireParametric - Sho rt hot

Parametric - Long co ol

Structural Behaviour

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Results for Insulated Steel:

Parametric vs. Travelling firesJonsdottir et al, Interflam 2010, Nottingham

� Compared to parametric fire, 110% higher temperatures

for a protected steel with 39 mm-gypsum

Conclusions

� In large compartments, a post flashover fire

is not likely to occur, but a travelling fire

�Novel framework developed to compliment

traditional methods

�Provides range of possible fire dynamics

�Travelling fires give more onerous conditions

for the structure

�Strengthens collaboration between fire and

structural fire engineers

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ThanksThanksThanksThanks

Collaborators:

J Stern-Gottfried

A Law

A Jonsdottir

M Gillie

J Torero

Sponsors:

ARUP

Jonsdottir et al, Interflam 2010, Nottingham

Law et al, Engineering Structures 2011

Rein et al, Interflam 2007, London

Stern-Gottfried et al, SPFE PBD, 2010, Lund

Stern-Gottfried et al, Fire Risk Management 2009

Jonsdottir et al, Fire Risk Management 2009