Performance-based Approach for Fire Resistance …web/@eis/@research/...Existing fire resistance design guidelines • Very limited guidance on the fire resistance design of FRP- strengthened

Research Group in Sustainable Materials and Structures (SMAS) December 7, 2015

Performance-based Approach for Fire Resistance Design of FRP-Strengthened

RC Beams

Dr Jian-Guo DaiAssociate ProfessorDepartment of Civil and Environmental EngineeringThe Hong Kong Polytechnic University, China

International Workshop on Infrastructure Applications of FRP Composites

Presentation Outline

Background Existing guidelines for fire resistance design of FRP-strengthenedRC members Proposed three-level performance-based fire resistance design Fire resistance of fully protected FRP-strengthened RC beams Fire resistance of unprotected FRP-strengthened RC beams Fire resistance of partially protected FRP-strengthened RCbeams: FE analysis and simple design method

Case study Conclusions


Background

• FRPs are widely used for strengtheningapplications.

ColumnBeam

• Fire safety is a very important concern for indoor applications.

Tunnel


Background


• Poor performance of FRPsat elevated temperatures

Glass transition temperature(Tg) : 45~82 (fib 2001; ACI2008)

0 100 200 300 400 500 6000

0.2

0.4

0.6

0.8

1

1.2

Temperature (oC)

Nor

mal

ized

tens

ile s

treng

th f

pT /

f p0

GFRP sheets (Chowdhury et al. 2011)CFRP sheets (Chowdhury et al. 2008)CFRP sheets (Cao et al. 2011)FRP sheets (Cao et al. 2009)CFRP plates (Wang et al. 2011)FRP bars (Wang et al. 2007)GFRP bars (zhou 2005)Bisby's (2003) model for CFRPBisby's (2003) model for GFRP

• Poor bond performance ofFRP-to-concrete interfacesat elevated temperatures

(Dai et al. 2013)

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Interfacial slip (mm)

Bon

d st

ress

ratio

f,T

/ f,0

20�40�50�60�70�80�90�100�120�200�

Existing fire resistance design guidelines

• Very limited guidance on the fire resistance design of FRP-strengthened RC members is availabe [e.g., fib Bulletin 14 (fib2001); ACI 440.2R-08 (ACI 2008)] .

• When a fire insulation layer is adopted, ACI 440.2R-08recommends that the contribution of the FRP strengtheningsystem be taken into consideration if the FRP temperatureremains below its critical temperature (e.g., Tg).

• With no fire insulation layer, it is suggested that themechanical resistance of the EB FRP system be ignored incases of fire. That is, the original RC member is expected tobe efficient to sustain the new (i.e., possibly increased)service load throughout the required fire resistance period.


Proposed frame for three levels of fire resistance design

Fire resistance analysis of RC beams

Temperature field analysis of insulated beams

Three-level fire resistance design


Level-III fire resistance design


No need for mechanical response analysis. Thermal analysis only. (Gao et al. 2015)

t 60min

t 120min

t 180min

Finite Element Analysis

0 60 120 180 2400

200

400

600

800

1000

1200

Fire exposure time (min)

Tem

pera

ture

(o C)

ASTM E119 fire curveFurnace temperature

1220

38Insulation

150

250

300

BA

C

0 60 120 180 2400

100

200

300

400

500


Tem

pera

ture

(o C)

Longitudinal positions 1,2 &3Williams et al.'s predictionPresent predictionPresent prediction (no FRP)

Gao, W.Y., Dai, J.G. and Teng, J.G. (2015), Finite Element Modeling of Insulated FRP-strengthened Reinforced Concrete Beams Exposed to Fire, ASCE, Journal of Composites for Construction, http://dx.doi.org/10.1061/(ASCE)CC.1943-5614.0000509



Gao. W.Y, Dai, J.G. and Teng, J.G. (2015), Simple Method for Predicting Temperatures in Insulated Fiber-Reinforced Polymer (FRP)-Strengthened Reinforced Concrete Members Exposed to a Standard Fire, ASCE, Journal of Composites for Construction, 04015013-1-16.

Equivalency between insulated and enlarged concrete members



Gao, W.Y., Dai, J.G., and Teng, J.G. (2014). “A simplified approach for determining the temperature fields of concrete beamsexposed to fire.” Advances in Structural Engineering. Vol. 17, No. 4, pp. 573-590.

∆ ,

, ∙ exp

ex p 200⁄ ln 200⁄

∆ ln 1 1 ∙ ∙ ∙

1.26 1.32 0.881

One-dimensional heat transfer Two-dimensional heat transfer

0.759 4.37 10 1.71 10



Beam section

Column section

Level-I fire resistance design: FE analysis

Validation of the FE model (for RC beams)

t=60min t=120min t=180min t=240min


Gao, W.Y., Dai, J.G., Teng, J.G. and Chen, G.M. (2013), Finite Element Modeling of Reinforced Concrete Beams Exposed to Fire, Engineering Structures, 52, July 2013, 488-501.


0 20 40 60 80 100 120 140-350

-300

-250

-200

-150

-100

-50

0

Fire-exposure time (min)

Mid

-spa

n de

flect

ion

(mm

)

Test data of Beam ITest data of Beam IIPerfect bondUpper boundLower bound

0 20 40 60 80 100-350

-300

-250

-200

-150

-100

-50

0

Fire-exposure time (mm)

Mid

-spa

n de

flect

ion

(mm

)

Test data of Beam IIIPerfect bondUpper boundLower bound

Predicted and measured mid-span deflections of Beams I and II

Predicted and measured mid-span deflections of Beam III




Stress distributions over the mid-span cross-section

t=0min t=30min t=60min

t=90min t=106min

Concrete spalling zones



Total 512 specimens

Aggregate type

Placement of tension steel rebars

Beam width

Level-I fire resistance design


0 100 200 300 400 5000

100

200

300

400

500

FE results (min)

BS

cod

e pr

edic

tions

(min

)

≥0.5<0.5

Safe

Unsafe

BS 8110 code

0 100 200 300 400 5000

100

200

300

400

500

FE results (min)

AC

I cod

e pr

edic

tions

(min

)

≥0.5<0.5

Safe

Unsafe

ACI 216 code

0 100 200 300 400 5000

100

200

300

400

500

FE results (min)

FIP

/CE

B re

port

pred

ictio

ns (m

in)

≥0.5<0.5

Safe

Unsafe

FIP/CEB code

0 100 200 300 400 5000

100

200

300

400

500

FE results (min)

Eur

ocod

e pr

edic

tions

(min

)

≥0.5<0.5

Safe

Unsafe

Eurocode0 100 200 300 400 500

0

100

200

300

400

500

FE results (min)

Kod

ur a

nd D

wai

kat's

pre

dict

ions

(min

)

≥0.5<0.5

Safe

Unsafe

Kodur and Dwaikat (2011)



2 31 2 3 4

0 1

20 1 2

1 2

, , , , , , , ,

,

,

1.04

ξ ξ ξ

sc scag s s ag

st st

s

ag

sc sc

st st

A Al lR c b c b

d A d A

a a a a

c c

ld

A AA A

0 100 200 300 400 5000

100

200

300

400

500

FE results (min)

Form

ulae

pre

dict

ions

(min

)

Calcareous aggregate concreteSiliceous aggregate concrete

-10%

+10%

PredictionsFE resultsMean =1.000COV =4.355%

Total 512 specimens

Level-I fire resistance design: design equations


2 31 2 3 4

0 1

20 1 2

1 2

, , , , , , , ,

,

,

1.04

ξ ξ ξ

sc scag s s ag

st st

s

ag

sc sc

st st

A Al lR c b c b

d A d A

a a a a

c c

ld

A AA A

Total 512 specimens 0 50 100 150 200 2500

50

100

150

200

250

Existing fire test data (min)

Form

ulae

pre

dict

ions

(min

)

Wu et al., 1993Lin et al., 1981Dotreppe and Franssen, 1985Hertz, 1985Blontrock, 2001Dwaikat and Kodur, 2009Choi and Shin, 2011

Unsafe

Safe

Level-I fire resistance design: design equations


Level-II fire resistance design: FE analysis


Dai, J.G., Gao, W.Y., and Teng, J.G. (2014). “Finite element modeling of insulated FRP-strengthenedreinforced concrete beams exposed to fire.” Journal of Composites for Construction,10.1061/(ASCE)CC.1943-5614.0000509, 04014046.

Fracture energy of concrete at elevated temperatures

Tensile stress-crack displacement relationship of concrete

0 100 200 300 400 500 600 7000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Temperature (� )

Nor

mal

ized

frac

ture

ene

rgy

At elevated temperatures(Bazant and Part, 1986)At elevated temperatures(Zhang and Bicanic, 2006)After cooled down(Zhang and Bicanic, 2006)After cooled down(Zhang et al., 2000)After cooled down(Baker, 1996)After cooled down(Nielsen and Bicanic, 2003)After cooled down(Tang and Lo, 2009)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Crack opening displacementf t,T

/ f t0

20-100�200�300�400�500�600�700�800�

Tension softening behavior of concrete at elevated temperatures



0 0.2 0.4 0.6 0.8 1 1.2 1.40

0.2

0.4

0.6

0.8

1

1.2

Interfacial slip (mm)

Bon

d st

ress

ratio

( s,

T/ m

ax,0

)

20�100�200�300�400�500�600�700�800�

Tension stiffening behavior of steel rebars at elevated temperatures

Normalized bond strength and proposed bound lines

Proposed local bond stress-interface slip relationships

CEB-FIP model

0 200 400 600 800 1000 12000

0.2

0.4

0.6

0.8

1

1.2

1.4

Temperature (� )

Nor

mal

ized

bon

d st

reng

th

At elevated temperatures(Diederichs and Schneider, 1981)At elevated temperatures(Hu, 1989)At elevated temperatures(Morley and Royles, 1980)After cooled down(Milovanov and Salmanov, 1954)After cooled down(Reichel, 1978)After cooled down(Hu, 1989)After cooled down(Haddad et al., 2006)Proposed upper boundProposed lower bound



Debonding behavior of FRP plates at elevated temperatures

( ) 2 ( )( ) 2 B x B xfx G B e e

2 30 ,

( ) 1 1tanh2 2

f

f g a

G T Tb bG T

1 12 3

0 ,

1 1( ) tanh2 2g a

c cB T Tc cB T

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Interfacial slip (mm)B

ond

stre

ss ra

tio

f,T / f,0

20�40�50�60�70�80�90�100�120�200�

Proposed local bond stress-interface slip relationships



Constitutive laws of FRP laminates at elevated temperatures

Tensile strength of FRP composites at elevated temperatures

Normalized tensile strength of FRP sheets at elevated temperatures

0 0.5 1 1.5 2 2.5 3 3.5 40

0.5

1

1.5

2

Normalized temperature T / Tg,p

Nor

mal

ized

tens

ile s

treng

th f

pT /

f p0

GFRP sheets (Chowdhury et al. 2011)CFRP sheets (Chowdhury et al. 2008)CFRP sheets (Cao et al. 2011)FRP sheets (Cao et al. 2009)Proposed equation

0 100 200 300 400 500 6000

0.2

0.4

0.6

0.8

1

1.2

Temperature (oC)

Nor

mal

ized

tens

ile s

treng

th f

pT /

f p0

1 12 3

0 ,

1 1tanh2 2

pT

p g p

f b bTb bf T

GFRP sheets (Chowdhury et al. 2011)CFRP sheets (Chowdhury et al. 2008)CFRP sheets (Cao et al. 2011)FRP sheets (Cao et al. 2009)CFRP plates (Wang et al. 2011)FRP bars (Wang et al. 2007)GFRP bars (zhou 2005)Bisby's (2003) model for CFRPBisby's (2003) model for GFRP



Constitutive laws of FRP laminates at elevated temperatures

Elastic modulus of FRP composites at elevated temperatures

Normalized elastic modulus of FRP sheets at elevated temperatures

0 100 200 300 400 5000

0.3

0.6

0.9

1.2

Temperature (� )

Nor

mal

ized

ela

stic

mod

ulus

E pT/ E

p0

GFRP sheets (Chowdhury et al., 2011)CFRP sheets (Chowdhury et al., 2008)GFRP bars (Zhou, 2005)FRP bars (Wang et al., 2007)CFRP model (Bisby, 2003)GFRP model (Bisby, 2003)

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

Normalized temperature T / Tg,p N

orm

aliz

ed e

last

ic m

odul

us E

pT /

Ep0

GFRP sheets (Chowdhury et al. 2011)CFRP sheets (Chowdhury et al. 2008)Proposed equation

1 12 3

0 ,

1 1tanh2 2

pT

p g p

E a aTa aE T



Validation of the FE model (for FRP-strengthened RC beams)

t= 60 min t= 120 min

t= 180 min t= 240 min

Temperature distributions of cross-section at various fire-exposure times[Beam II was tested by William et al. (2008) with a 38mm VG (cementitious plaster)]




Temperature distributions of cross-section at various fire-exposure times[Beam II was tested by William et al. (2008) with a 38mm VG (cementitious plaster)]

0 50 100 150 200 2500

200

400

600

800

1000

1200


Tem

pera

ture

(�) ASTM E119

Furnace temp.FRP/concrete (TC10)FRP/concrete (TC16)FRP/concrete (TC43)Model prediction (Williams)FE model prediction

InsulationmaterialCFRP

Thermocouples

0 50 100 150 200 2500

20

40

60

80

100


Tem

pera

ture

(�)

Unexposed surface (TC1)Unexposed surface (TC2)Unexposed surface (TC4)Unexposed surface (TC7)Unexposed surface (TC9)Model prediction (Williams)FE model prediction

Unexposed surface

FRP-to-concrete bond

InsulationmaterialCFRP

Thermocouples




Results of insulated CFRP-strengthened RC beams tested by Blontrock et al. (2000)[Beam 6 was protected with a 40/20mm Promatect H (calcium silicate boards)]

Mid-span deflectionFRP-to-concrete interface and rebar temperatures

0 20 40 60 80 100 1200

200

400

600

800

1000

1200


Tem

pera

ture

(o C)

ISO 834 fire curveTest rebar temp.Predicted rebar temp.Test interface temp.Predicted interface temp.

200

300

40Insulation

20

0 20 40 60 80 100 120-25

-20

-15

-10

-5

0

Fire exposure time (min)M

id-s

pan

defle

ctio

n (m

m)

Testprediction




Results of insulated CFRP-strengthened RC beams tested by Blontrock et al. (2000)[Beam 7 was protected with a 25/12mm Promatect H (calcium silicate boards)]

Mid-span deflectionFRP-to-concrete interface and rebar temperatures

0 20 40 60 80 100 1200

200

400

600

800

1000

1200


Tem

pera

ture

(o C)

ISO 834 fire curveTest rebar temp.Predicted rebar temp.Test interface temp.Predicted interface temp.

200

300

25

Insulation

12

0 20 40 60 80 100 120-50

-40

-30

-20

-10

0


id-s

pan

defle

ctio

n (m

m)

TestPrediction




Effect of bond degradation on the mid-span deflection

0 20 40 60 80 100 120-120

-100

-80

-60

-40

-20

0


Mid

-spa

n de

flect

ion

(mm

)

Test dataInsulated FRP-RC beam (bond-slip)Insulated RC beamInsulated FRP-RC beam (no slip)RC beam

0 20 40 60 80 100 120-120

-100

-80

-60

-40

-20

0


id-s

pan

defle

ctio

n (m

m)

Test dataInsulated FRP-RC beam (bond-slip)Insulated RC beamInsulated FRP-RC beam (no slip)RC beam

Referred to Beam 7 Referred to Beam 6Conclusion: The contribution of the FRP strengthening system to the fireresistance evaluation can be ignored.



Temperature field analysis of insulated RC beams

“500 oC” isotherm method


Level-II fire resistance design: Category I

Temperature field analysis of insulated RC beams

“500 oC” isotherm method



0 60 120 180 240 300 3600

5

10

15

20

25

30

35

40


Mom

ent c

apac

ity M

R (k

N.m

)

tin=5mmtin=10mm

tin=15mmtin=20mmtin=30mm

s=0.8%

Time-dependent moment capacity0 60 120 180 240 300 3600

5

10

15

20

25

30

35

40


Mom

ent c

apac

ity M

R (k

N.m

)

Moment capacity (tin=10mm)

Fire load action

s=0.8%, tCFRP=0.3mm=0.7

=0.5

=0.3

Determination of fire resistance period




0 100 200 300 4000

100

200

300

400

FE results (min)

Pre

dict

ed fi

re re

sist

ance

per

iods

(min

)+10%

-10%

PredictionsFE resultsMean = 0.95COV = 6.08%


Level II fire resistance design: Category II


4.2 m

Fire insulation

CFRP laminates

200 mm

Anchorage zone:Thick insulation

Central part:Thin insulation

Case study

200 mm

40 mm = 30 MPa = 375 MPa = 1.2% ⁄ = 2/3

300 mm

ϕ 8 mm stirrups

40 mm

= 7.5 kN/m, = 9.0 kN/m

4 m

2ϕ 14

(a) Elevation and cross-section of the reference RC beam

3.8 m

Case 1: = 7.5 kN/m, = 10.5 kN/mCase 2: = 10.5 kN/m, = 18.0 kN/m Case 3: = 15.0 kN/m, = 18.0 kN/m

Fire insulation(if required)

CFRP laminates

Length of the end anchorage, = 0.4 m 160 mm

(b) Elevation and cross-section of the CFRP-strengthened RC beam


Step I: Conceptual design

0 0.25 0.5 0.75 1 1.25 1.50

30

60

90

120

150

180

Load ratio (M/Mu,RC)

Fire

resi

stan

ce p

erio

d (m

in) � �

I

II

III

Case 2

Case 1

Case 3


Case study

Step II: Fire insulation design (Category I, Level II)

0 10 20 30 400

60

120

180

240

300

Fire insulation thickness (mm)

Fire

resi

stan

ce p

erio

d (m

in)

FE resultsDesign-oriented method


Case study

Step III: Threshold temperature design

0 60 120 180 2400

200

400

600


Tem

pera

ture

( C

)

Debondingfailure

Tensilerupture

tin=10mm

tin=20mm

tin=30mm

tin=40mm

tin=50mm

tin=60mm

tin=70mm

(Level-II)

(Level III)


Case study

Results of fire resistance design


Case study

Conclusions


The fire resistance design of un-protected FRP-strengthenedRC beams (i.e., Level-I design) can be approximated by thatof bare RC beams. Explicit design equations previouslyproposed by the authors are applied for the fire resistanceevaluation of these un-protected beams.

For the Level-II design of FRP-strengthened RC beams (i.e.,equivalent to insulated RC beams) exposed to a standardfire, a design-oriented method has been established basedon the simple “500 oC isotherm method” to enable theprediction of their time-dependent moment capacity. The fireresistance results obtained from the design-oriented methodare in good agreement with the FE predictions, making itmore attractive for use in practical design due to its simplicityyet good accuracy.

Conclusions


The Level-III design of FRP-strengthened RC beams can berealized through simple threshold temperature design.

For the Level-II design of RC members, the fire insulationthickness can also be determined based on two principles:(a) a thick fire insulation layer to prevent the debondingfailure at two anchorage zones during fire exposure; and (b)a relatively thin insulation along the central part of the beamto avoid a significant reduction of the tensile strength of theFRP laminate at elevated temperatures. However, this partialfire protection approach needs further research.

Acknowledgements


Thanks are due to the National Basic ResearchProgram of China (i.e. the 973 Program); NationalNatural Science Foundation of China (NSFC) andPolyU Postdoctoral fellowship for supporting thisresearch project.

Thanks are also due to Dr Wan-Yang GAO, whocompleted this research project as his PhDdissertation and Prof Jin-Guang Teng, who wasthe co-supervisor of Dr Gao’s PhD dissertation.

Documents

Performance-based Approach for Fire Resistance …web/@eis/@research/...Existing fire resistance design guidelines • Very limited guidance on the fire resistance design of FRP- strengthened