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

Transcription

1 International Workshop on Infrastructure Applications of FRP Composites Performance-based Approach for Fire Resistance Design of FRP-Strengthened RC Beams Dr Jian-Guo Dai Associate Professor Department of Civil and Environmental Engineering The Hong Kong Polytechnic University, China

2 Presentation Outline Background Existing guidelines for fire resistance design of FRP-strengthened RC 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 RC beams: FE analysis and simple design method Case study Conclusions

3 Background FRPs are widely used for strengthening applications. Tunnel Beam Column Fire safety is a very important concern for indoor applications.

4 Background Poor performance of FRPs at elevated temperatures Glass transition temperature (Tg) : 45~82 (fib 21; ACI 28) pt / f p Normalized tensile strength f GFRP sheets (Chowdhury et al. 211) CFRP sheets (Chowdhury et al. 28) CFRP sheets (Cao et al. 211) FRP sheets (Cao et al. 29) CFRP plates (Wang et al. 211) FRP bars (Wang et al. 27) GFRP bars (zhou 25) Bisby's (23) model for CFRP Bisby's (23) model for GFRP Temperature ( o C) Bond stress ratio f,t / f, Poor bond performance of FRP-to-concrete interfaces at elevated temperatures (Dai et al. 213) Interfacial slip (mm)

5 Existing fire resistance design guidelines Very limited guidance on the fire resistance design of FRPstrengthened RC members is availabe [e.g., fib Bulletin 14 (fib 21); ACI 44.2R-8 (ACI 28)]. When a fire insulation layer is adopted, ACI 44.2R-8 recommends that the contribution of the FRP strengthening system be taken into consideration if the FRP temperature remains below its critical temperature (e.g., Tg). With no fire insulation layer, it is suggested that the mechanical resistance of the EB FRP system be ignored in cases of fire. That is, the original RC member is expected to be efficient to sustain the new (i.e., possibly increased) service load throughout the required fire resistance period.

6 Proposed frame for three levels of fire resistance design Three-level fire resistance design Temperature field analysis of insulated beams Fire resistance analysis of RC beams

7 Level-III fire resistance design No need for mechanical response analysis. Thermal analysis only. (Gao et al. 215) 12 1 t6 min Temperature ( o C) Insulation 122 C B A t12 min 2 ASTM E119 fire curve Furnace temperature Fire exposure time (min) 5 t18 min Finite Element Analysis Gao, W.Y., Dai, J.G. and Teng, J.G. (215), Finite Element Modeling of Insulated FRP-strengthened Reinforced Concrete Beams Exposed to Fire, ASCE, Journal of Composites for Construction, Fire exposure time (min) Temperature ( o C) Longitudinal positions 1,2 &3 Williams et al.'s prediction Present prediction Present prediction (no FRP)

8 Level-III fire resistance design Equivalency between insulated and enlarged concrete members Gao. W.Y, Dai, J.G. and Teng, J.G. (215), 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,

9 Level-III fire resistance design,, exp exp 2 ln 2 One-dimensional heat transfer ln Two-dimensional heat transfer Gao, W.Y., Dai, J.G., and Teng, J.G. (214). A simplified approach for determining the temperature fields of concrete beams exposed to fire. Advances in Structural Engineering. Vol. 17, No. 4, pp

10 Level-III fire resistance design Beam section Column section

11 Level-I fire resistance design: FE analysis Validation of the FE model (for RC beams) t=6min t=12min t=18min t=24min Gao, W.Y., Dai, J.G., Teng, J.G. and Chen, G.M. (213), Finite Element Modeling of Reinforced Concrete Beams Exposed to Fire, Engineering Structures, 52, July 213,

12 Level-I fire resistance design: FE analysis Validation of the FE model (for RC beams) -5-5 Mid-span deflection (mm) Test data of Beam I -25 Test data of Beam II Perfect bond -3 Upper bound Lower bound Fire-exposure time (min) Mid-span deflection (mm) Test data of Beam III -3 Perfect bond Upper bound Lower bound Fire-exposure time (mm) Predicted and measured mid-span deflections of Beams I and II Predicted and measured mid-span deflections of Beam III

13 Level-I fire resistance design: FE analysis Validation of the FE model (for RC beams) t=min t=3min t=6min t=9min t=16min Stress distributions over the mid-span cross-section Concrete spalling zones

14 Level-I fire resistance design Total 512 specimens Aggregate type Placement of tension steel rebars Beam width

15 Level-I fire resistance design: FE analysis BS code predictions (min) ACI code predictions (min) Unsafe Safe Safe FE results (min) BS ACI 216 FE results 1 (min) FE results (min) FIP/CEB 1 FE results 2 (min) FE results (min) code <.5.55 <.5.5 <.5 < FIP/CEB report predictions (min) Eurocode predictions (min) 3 Unsafe Unsafe 2 1 Kodur and Dwaikat's predictions (min) <.5 Unsafe Unsafe Safe Safe Safe code EurocodeKodur and code Dwaikat (211)

16 Level-I fire resistance design: design equations l A sc l A R,, c,,,, b c,, b ag s s ag d A d A st st 5 a a a a c, s c 1 l 2, 1 2 d 1.4 ag A A sc sc ξ ξ ξ 1 2 A A st st Total 512 specimens sc Formulae predictions (min) % -1% Predictions FE results Mean = 1. COV = 4.355% Calcareous aggregate concrete Siliceous aggregate concrete FE results (min)

17 Level-I fire resistance design: design equations l A sc l A R,, c,,,, b c,, b ag s s ag d A d A st st 25 a a a a c, s c 1 l 2, 1 2 d 1.4 ag A A sc sc ξ ξ ξ 1 2 A A st st sc Formulae predictions (min) Unsafe Total 512 specimens Safe Existing fire test data (min) Wu et al., 1993 Lin et al., 1981 Dotreppe and Franssen, 1985 Hertz, 1985 Blontrock, 21 Dwaikat and Kodur, 29 Choi and Shin, 211

18 Level-II fire resistance design: FE analysis Dai, J.G., Gao, W.Y., and Teng, J.G. (214). Finite element modeling of insulated FRP-strengthened reinforced concrete beams exposed to fire. Journal of Composites for Construction, 1.161/(ASCE)CC ,

19 Level-II fire resistance design: FE analysis Tension softening behavior of concrete at elevated temperatures Normalized fracture energy At elevated temperatures (Bazant and Part, 1986) At elevated temperatures (Zhang and Bicanic, 26) After cooled down (Zhang and Bicanic, 26) After cooled down (Zhang et al., 2) After cooled down (Baker, 1996) After cooled down (Nielsen and Bicanic, 23) After cooled down (Tang and Lo, 29) f t,t / f t Temperature ( ) Crack opening displacement Fracture energy of concrete at elevated temperatures Tensile stress-crack displacement relationship of concrete

20 Level-II fire resistance design: FE analysis Tension stiffening behavior of steel rebars at elevated temperatures Normalized bond strength 1.4 At elevated temperatures (Diederichs and Schneider, 1981) 1.2 At elevated temperatures (Hu, 1989) 1 At elevated temperatures (Morley and Royles, 198).8 After cooled down (Milovanov and Salmanov, 1954) After cooled down.6 (Reichel, 1978) After cooled down.4 (Hu, 1989) After cooled down (Haddad et al., 26).2 Proposed upper bound Proposed lower bound Temperature ( ) Bond stress ratio ( s,t / max, ) CEB-FIP model Interfacial slip (mm) Normalized bond strength and proposed bound lines Proposed local bond stressinterface slip relationships

21 Level-II fire resistance design: FE analysis Debonding behavior of FRP plates at elevated temperatures B ( x) 2 B ( x) ( x) 2Gf B e e Gf ( T) 1 T 1 tanh b2 b3 Gf 2 T g, a 2 BT ( ) B 1c T 1c tanh c c T ga, 2 Bond stress ratio f,t / f, Interfacial slip (mm) Proposed local bond stressinterface slip relationships

22 Level-II fire resistance design: FE analysis Constitutive laws of FRP laminates at elevated temperatures Normalized tensile strength f pt / f p GFRP sheets (Chowdhury et al. 211) CFRP sheets (Chowdhury et al. 28) CFRP sheets (Cao et al. 211) FRP sheets (Cao et al. 29) CFRP plates (Wang et al. 211) FRP bars (Wang et al. 27) GFRP bars (zhou 25) Bisby's (23) model for CFRP Bisby's (23) model for GFRP pt / f p Normalized tensile strength f GFRP sheets (Chowdhury et al. 211) CFRP sheets (Chowdhury et al. 28) CFRP sheets (Cao et al. 211) FRP sheets (Cao et al. 29) Proposed equation f pt 1b1 T 1b1 tanh b2 b3 fp 2 T g, p Temperature ( o C) Normalized temperature T / T g,p Tensile strength of FRP composites at elevated temperatures Normalized tensile strength of FRP sheets at elevated temperatures

23 Level-II fire resistance design: FE analysis Constitutive laws of FRP laminates at elevated temperatures Normalized elastic modulus E pt / E p GFRP sheets (Chowdhury et al., 211).3 CFRP sheets (Chowdhury et al., 28) GFRP bars (Zhou, 25) FRP bars (Wang et al., 27) CFRP model (Bisby, 23) GFRP model (Bisby, 23) Temperature ( ) pt / E p Normalized elastic modulus E GFRP sheets (Chowdhury et al. 211) CFRP sheets (Chowdhury et al. 28) Proposed equation EpT 1a1 T 1a1 tanh a2 a3 Ep 2 T g, p Normalized temperature T / T g,p Elastic modulus of FRP composites at elevated temperatures Normalized elastic modulus of FRP sheets at elevated temperatures

24 Level-II fire resistance design: FE analysis Validation of the FE model (for FRP-strengthened RC beams) t= 6 min t= 12 min t= 18 min t= 24 min Temperature distributions of cross-section at various fire-exposure times [Beam II was tested by William et al. (28) with a 38mm VG (cementitious plaster)]

25 Level-II fire resistance design: FE analysis Validation of the FE model (for FRP-strengthened RC beams) Temperature ( ) Insulation material CFRP Thermocouples FRP-toconcrete bond ASTM E119 Furnace temp. FRP/concrete (TC1) FRP/concrete (TC16) FRP/concrete (TC43) Model prediction (Williams) FE model prediction Temperature ( ) Insulation material CFRP Thermocouples Unexposed surface (TC1) Unexposed surface (TC2) Unexposed surface (TC4) Unexposed surface (TC7) Unexposed surface (TC9) Model prediction (Williams) FE model prediction Unexposed surface Fire exposure time (min) Fire exposure time (min) Temperature distributions of cross-section at various fire-exposure times [Beam II was tested by William et al. (28) with a 38mm VG (cementitious plaster)]

26 Level-II fire resistance design: FE analysis Validation of the FE model (for FRP-strengthened RC beams) Temperature ( o C) FRP-to-concrete interface and rebar temperatures 3 ISO 834 fire curve Test rebar temp. Predicted rebar temp. Test interface temp. Predicted interface temp. 2 Insulation 2 4 Mid-span deflection (mm) Mid-span deflection Test prediction Fire exposure time (min) Fire exposure time (min) Results of insulated CFRP-strengthened RC beams tested by Blontrock et al. (2)[Beam 6 was protected with a 4/2mm Promatect H (calcium silicate boards)]

27 Level-II fire resistance design: FE analysis Validation of the FE model (for FRP-strengthened RC beams) FRP-to-concrete interface and rebar temperatures Mid-span deflection Temperature ( o C) ISO 834 fire curve Test rebar temp. Predicted rebar temp. Test interface temp. Predicted interface temp. 2 Insulation Mid-span deflection (mm) Test Prediction Fire exposure time (min) Fire exposure time (min) Results of insulated CFRP-strengthened RC beams tested by Blontrock et al. (2)[Beam 7 was protected with a 25/12mm Promatect H (calcium silicate boards)]

28 Level-II fire resistance design: FE analysis Validation of the FE model (for FRP-strengthened RC beams) Effect of bond degradation on the mid-span deflection -2-2 Mid-span deflection (mm) Test data Insulated FRP-RC beam (bond-slip) -1 Insulated RC beam Insulated FRP-RC beam (no slip) RC beam Fire exposure time (min) Mid-span deflection (mm) Test data Insulated FRP-RC beam (bond-slip) -1 Insulated RC beam Insulated FRP-RC beam (no slip) RC beam Fire exposure time (min) Referred to Beam 6 Referred to Beam 7 Conclusion: The contribution of the FRP strengthening system to the fire resistance evaluation can be ignored.

29 Level-II fire resistance design: Category I Temperature field analysis of insulated RC beams 5 o C isotherm method

30 Level-II fire resistance design: Category I Temperature field analysis of insulated RC beams 5 o C isotherm method

31 Level-II fire resistance design: Category I Moment capacity M R (kn.m) Moment capacity M R (kn.m) 4 Moment capacity (t in =1mm) 35 Fire load action 3 =.7 s =.8%, t CFRP =.3mm 25 2 =.5 s =.8% 2 15 t in =5mm t 1 in =1mm 15 t in =15mm = t in =2mm t in =3mm Fire exposure time (min) Fire exposure time (min) Time-dependent moment capacity Determination of fire resistance period

32 Level-II fire resistance design: Category I Predicted fire resistance periods (min) Predictions FE results Mean =.95 COV = 6.8% +1% -1% FE results (min)

33 Level II fire resistance design: Category II Fire insulation 4.2 m CFRP laminates 2 mm Anchorage zone: Thick insulation Central part: Thin insulation

34 Case study = 7.5 kn/m, = 9. kn/m 4 m 4 mm 2ϕ 14 = 3 MPa = 375 MPa = 1.2% = 2/3 4 mm 3 mm ϕ 8 mm stirrups 2 mm (a) Elevation and cross-section of the reference RC beam Case 1: = 7.5 kn/m, = 1.5 kn/m Case 2: = 1.5 kn/m, = 18. kn/m Case 3: = 15. kn/m, = 18. kn/m Fire insulation (if required) 3.8 m Length of the end anchorage, =.4 m CFRP laminates 16 mm (b) Elevation and cross-section of the CFRP-strengthened RC beam

35 Case study 18 Fire resistance period (min) Case 1 I II Case 2 Case 3 III Load ratio (M/M u,rc ) Step I: Conceptual design

36 Case study 3 Fire resistance period (min) FE results Design-oriented method Fire insulation thickness (mm) Step II: Fire insulation design (Category I, Level II)

37 Case study 6 (Level-II) Temperature (C) 4 2 Tensile rupture Debonding failure t in =1mm t in =2mm t in =3mm t in =4mm t in =5mm t in =6mm t in =7mm (Level III) Fire exposure time (min) Step III: Threshold temperature design

38 Case study Results of fire resistance design

39 Conclusions The fire resistance design of un-protected FRP-strengthened RC beams (i.e., Level-I design) can be approximated by that of bare RC beams. Explicit design equations previously proposed by the authors are applied for the fire resistance evaluation 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 standard fire, a design-oriented method has been established based on the simple 5 o C isotherm method to enable the prediction of their time-dependent moment capacity. The fire resistance results obtained from the design-oriented method are in good agreement with the FE predictions, making it more attractive for use in practical design due to its simplicity yet good accuracy.

40 Conclusions The Level-III design of FRP-strengthened RC beams can be realized through simple threshold temperature design. For the Level-II design of RC members, the fire insulation thickness can also be determined based on two principles: (a) a thick fire insulation layer to prevent the debonding failure at two anchorage zones during fire exposure; and (b) a relatively thin insulation along the central part of the beam to avoid a significant reduction of the tensile strength of the FRP laminate at elevated temperatures. However, this partial fire protection approach needs further research.

41 Acknowledgements Thanks are due to the National Basic Research Program of China (i.e. the 973 Program); National Natural Science Foundation of China (NSFC) and PolyU Postdoctoral fellowship for supporting this research project. Thanks are also due to Dr Wan-Yang GAO, who completed this research project as his PhD dissertation and Prof Jin-Guang Teng, who was the co-supervisor of Dr Gao s PhD dissertation.