Introduction

to

EurocodeStructural

FireEngineeringStructural

Steelwork

Eurocodes10.51.0

1.5Strain

(%)2.0Stress

(N/mm2)03002502001501005020°C200°C300°C400°C500°C600°C700°C800°CSteel

softens

progressivelyfrom100-200°C

up.Only

23%

of

ambient-temperature

strengthremains

at

700°C.At

800°C

strength

reducedto

11%

and

at

900°C

to

6%.Melts

at

about

1500°C.Steel

stress-strain

curves

athigh

temperatures21.00.90.80.70.60.50.40.30.20.101420°C200°C400°C600°C800°C1000°C2

3Strain

(%)Normalised

stressConcrete

also

losesstrength

and

stiffnessfrom

100°C

upwards.Does

not

regainstrengthoncooling.High

temperatureproperties

depend

mainlyon

aggregate

type

used.Concrete

stress-straincurves

at

hightemperatures3The

fire

triangleHeatFuel

+

Oxidant

=

Combustion

productsCH4+

O2

=

CO2+2H20Reactionoccurs

whenOxygen/fuelmixture

hotenoughOxygenFuel4Stages

of

a

natural

fire

-

andthe

standard

fire

test

curveCooling

….Ignition

-SmoulderingPre-FlashoverHeatingPost-Flashover1000-1200°CNatural

fire

curveISO834

standard

firecurveTimeTemperatureFlashover5The

EC1

(ISO834)

standardfire

curve10009008007006005004003002001000060036001200

1800

2400

3000Time

(sec)Gas

Temperature

(°C)9458427817396755766200400600800100012000120036002400Time

(sec)Gas

Temperature

(°C)Typical

EC1Parametric

firecurveExternal

FireStandard

FireHydrocarbon

FireFire

resistance

timesbased

on

standardfurnace

tests

-

NOT

onsurvival

in

real

fires.EC1

Parametric

Firetemperature-time

curves.Based

on

fire

load

andcompartment

properties(<500m2). Only

allowedwith

calculation

models.Different

EC1time-temperature

curves7CompartmentTemperatureLoad-bearingresistanceTimeTimeFire

severity

timeequivalentUsed

to

rate

fire

severity

orelement

performance

relativeto

furnace

test.Matches

times

to

giventemperature

in

a

natural

fireand

in

Standard

Fire.Fire

resistancetime

equivalentStandard

fireNatural

fireElementTime-equivalence8Furnace

tests

on

structuralelementsFire

TestingLoad

kept

constant,

firetemperature

increased

usingStandard

Fire

curve.Maximum

deflection

criterionfor

fire

resistance

of

beams.Load

capacity

criterion

forfire

resistance

of

columns.ProblemsLimited

range

of

spansfeasible,

simply

supportedbeams

only.Effects

of

continuity

ignored.Beams

fail

by

“run-away”.Restraint

to

thermal

expansionby

surrounding

structureignored.9Standard

fire

resistance

furnacetest100200012002400

3600Time

(sec)Deflection

(mm)30010Standard

fire

resistancefurnace

test100200012002400

3600Time

(sec)Deflection

(mm)300Standard

FireSpan2/400dIf

rate

<span2/9000dSpan/3011Structural

fire

protectionPassive

ProtectionInsulating

BoardGypsum,

Mineral

fibre,

Vermiculite.Easy

to

apply,

aesthetically

acceptable.Difficulties

with

complex

details.Cementitious

SpraysMineral

fibre

or

vermiculite

in

cementbinder.Cheap

to

apply,

but

messy;

clean-up

may

beexpensive.Poor

aesthetics;

normally

used

behind

suspended

ceilings.Intumescent

PaintsDecorative

finish

under

normal

conditions.Expands

on

heating

to

produce

insulating

layer.Can

now

be

done

off-site.12“Slim-floor”

Systems

Downstand

Beam

Shelf-angle

BeamInherent

fire

protection

tosteel

beams13Structural

fire

protectionComposite

sectionsDownstand

BeamPassive

Protection

–

Composite

sectionsTraditional

downstand

beamtop

flange

upper

face

totally

shielded

by

the

slab14Structural

fire

protectionComposite

sectionsEncasedBeamPassive

Protection

–

Composite

sectionsBeams

with

concrete

encasementHave

high

fire

resistance

(up

to

180

minutes).Involve

complicated

construction

of

joints.Require

formwork.15Structural

fire

protectionComposite

sectionsPartially

Encased

BeamPassive

Protection

–

Composite

sectionsSteel

beams

with

partial

concreteencasementConcrete

between

flanges

reducestherate

of

heating

of

the

profile's

web

andupper

flange.Concrete

between

flanges

contributes

tothe

load-bearingresistance.The

beam

can

be

fabricated

in

theworkshop

without

the

use

offormwork.Simple

construction

of

joints.16Load

reduction

factor

infireRelative

toambient-temperaturedesign

resistanceEither…..Relative

toambient-temperaturedesign

load

(more

conservative)Or

moreusefully…..17Establishing

FireResistance:

StrategiesEurocodes

allow

fireresistance

to

be

establishedin

any

of

3“domains”:Time:Load

resistance:Temperature:tfi.d

>

tfi.requRfi.d.t

>

Efi.d.tcr.d

>

dUsually

only

directly

feasible

using

advancedcalculation

models.Feasible

by

handcalculation.

Findreduced

resistance

atdesign

temperature.Most

usual

simple

EC3method. Find

criticaltemperature

for

loading,compare

withdesigntemperature.18Material

propertiesSteelMechanical(effective

yield

strength,elastic

modulus,

...

)Thermal(thermal

expansion,thermal

conductivity,specific

heat)ConcreteMechanical(compressive

strength,secant

modulus,

...

)Thermal(thermal

expansion,thermal

conductivity,specific

heat)19Strength/stiffness

reductionfactors

for

elastic

modulusand

yield

strength

(2%strain).0.51.0

1.5Strain

(%)2.0Stress

(N/mm2)03002502001501005020°C200°C300°C400°C500°C600°C700°C800°CElastic

modulus

at

600°Creduced

by

about

70%.Yield

strength

at

600°Creduced

by

over

50%.Steel

stress-strain

curves

athigh

temperatures20RftDegradation

of

steelstrength

and

stiffness0300120010080604020%

of

normal

value600

900Temperature

(°C)Effective

yield

strength(at

2%

strain)SSElastic

modulusSSRftStrength

and

stiffnessreductions

very

similarfor

S235,

S275,S355structural

steels

and

hot-rolled

reinforcing

bars.(SS)Cold-worked

reinforcingbars

S500

deterioratemore

rapidly.(Rft)21100500200

400

600

800

1000

1200Temperature

(°C)654321Strain

(%)Strength

(%

ofnormal)Strain

atmaximumstrengthDegradation

of

concretestrength

and

stiffnessNormal-weightConcreteConservative

for

normaldensity

concretewithcalcareous

aggregates,.Lightweight

ConcreteConservative

for

light-weight

concretes.

Alltypes

treated

the

same.Strength

reduction

factorsAccurate

for

normaldensity

concretewithsiliceous

aggregates.22Concrete

strength

inheating

and

coolingStress-strain

relationshipin

cooling

from

700°C

(at400

C)Stress-strain

relationshipin

heating

phase

(700

C)250,03Stress-strainrelationship

atambient

temperature15Stress-strainrelationship

in

heatingphase

(400

C)50,01

0,02Stress-strain

relationshipafter

cooling

from700°C(at

20

C)23Thermal

expansion

of

steeland

concrete04,54,03,53,02,52,01,51,00,5100

200

300

400

500

600

700

800

900Temperature

(°C)ExpansionCoeff

/°C

(x

10-6)SteelSteel

thermal

expansionstops

during

crystalstructrure

change

in

the700-800°C

range.Normal-weightconcreteConcrete

unlikely

to

reach700°C

in

time

of

abuildingfire.Lightweight

concreteLight-weight

concretetreated

as

having

uniformthermal

expansioncoefficient.24

a=45W/m°K

(EC3

simplecalculation

model)Thermal

conductivity(W/m°K)2010403050600 200

400

600 800

1000

1200Temperature

(°C)Steelca=600J/kg°K(EC3

simplecalculationmodel)Other

steel

thermalpropertiesSpecific

Heat(J/kg°K)50000200

400

600 800

1000

1200Temperature

(°C)4000300020001000Steel25Other

concretethermalpropertiesNCNCMay

assume

constantvalue

for

NC:1,60

W/m.KMay

assume

constantvalue

for

NC:1000

J/kg.Kcc*40080010001200LC200

600 1000

°CSpecific

heat

cc

(J/kg.K)Thermal

conductivity

c

(W/m.K)LC200

600 1000

°C12326Thermal

analysisThermal

analysis:both

EC3

Part

1.2

and

EC4

Part

1.2unprotected

and

protected

steel

beamsLower

and

upperflangesConsiderablydifferenttemperaturesoftemperatures

!proper

calc