Travelling Fires in Building Structural Design (Madrid 2011)

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

11

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

13

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

14

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

15

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