rational design for frp strengthened concrete structures in fire · · 2017-08-05rational design...

Rational design for FRP strengthened reinforced concrete structures in fireRational design for FRP strengthened reinforced concrete structures in fire

Mark F. Green and Luke A. BisbyQueen’s University at Kingston University of Edinburgh

Rational Approaches for Fire Resistance Design of Concrete Structures12 April 2015, ACI Spring Convention, Kansas City

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Outline

• FRP and High Temperature• Description of current ACI 440F recommendations• Description of proposed changes to ACI 440F• Loading during fire event• Thermal modelling• Structural modelling• Examples

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3

Glass fibreroving

Carbon fibre roving

Unidirectional glass FRP bar

Carbon FRP prestressing

tendon

Aramid FRP grid

Examples of FRP

High strength fibres in a polymer matrix

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FRPs & Fire: Primary Concerns

• Fire is recognized as a critical research need for FRP:– a primary factor preventing widespread application

of FRPs in buildings

• Potential concerns during fire:1. Loss of strength and stiffness2. Loss of interaction (bond) w/ concrete3. Smoke generation and flame spread

Current 440F Repair Guidelines ‐ Fire

• “The strength of externally bonded FRP systems is assumed to be lost completely… unless it can be demonstrated that the FRP temperature remains below its critical temperature (for example, FRP with a fire-protection system)”

• “The critical temperature of an FRP strengthening system should be taken as the lowest glass-transition temperature Tg of the components of the repair system,”

• “The structural member without the FRP system should possess sufficient strength to resist all applicable service loads during a fire” (1.0DL + 1.0LL)

• “The fire endurance of FRP-strengthened concrete members may be improved through the use of certain resins, coatings, insulation systems, or other methods of fire protection”

• Reference is made to ACI 216R guidelines 5

Proposed 440F Repair Guidelines ‐ Fire

• “The strength of externally bonded FRP systems is assumed to be lost completely in a fire, unless it can be demonstrated that the FRP will remain effective for the required duration of the fire (for example, FRP with a fire-protection system).”

• “In most cases, the structural member without the FRP system should possess sufficient strength to resist applicable service loads during a fire” (1.2DL + 0.5LL + 0.2SL + 1.0Ak)

• “The resistance should be computed for the time period required by the member’s fire-resistance rating—for example, a 2-hour fire rating—and should not account for the contribution of the FRP system unless the continued effectiveness of the FRP can be proven through testing.”

• “Until better information on the properties of FRP at high temperature isavailable, the critical temperature can be taken as the lowest Tg of the components of the system.”

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Rationale for new load factors

• Old load factors for fire (1.0DL + 1.0LL) are based on ASTM E119 provisions for fire testing (no statistical basis)

• New ASCE 7 load factors are based on probabilistic studies and are applicable for new structures (Ellingwood)1

–New loads have less than 5% probability of being exceeded in a given year

–Average live load is 0.6 kPa or 12 psf (approx. 0.25 LL)

71 Ellingwood, private communication with Tarek Alkhrdaji (August 2014)

Comparison of Loading Combinations

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Load

in Fire

Con

ditio

n

Live Load to Dead Load Ratio (LL/DL)

216/E119 440F/ASCE

200 mm (8 in.) slab with 2.4 kPa (50 psf) loading

200 mm (8 in.) slab with 4.8 kPa (100 psf) loading

Procedure for finding fire endurance

Necessary analysis steps:

1. Find temperatures in member at different times during the fire exposure

– Guides (ACI, Eurocodes), test data, analytical models, numerical tools

2. Evaluate reductions in strength and modulus of the materials (steel, concrete, FRP) depending on temperature – in most cases the FRP will not contribute

– ACI 216, structural Eurocodes, research literature, testing

3. Calculate member strength– Classical plane sections analysis– Simplified methods (e.g. Eurocode 500oC Isotherm Method)– Finite element methods

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Philosophy for Fire‐Safety

Temp.(ºC)Load

Time

Initial Service Life Retrofitted Life

Strength

Service Load

Fire Event (hrs)0 1 2 3 4

1000

100

Retrofit

Fire Event Onset

Construction

Conceptual design approach

Fire resistance

Flame spreadprotectionAmou

nt of Stren

gthe

ning

Protect FRP Itself

Design example (after ACI 440.2R)

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(610 mm)

(305 mm)

(34.5 MPa)

(414 MPa)

(361 kN.m)

As = 3.0 in2 (1935 mm2)

Analysis Approach and Assumptions

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1. Find temperatures at key locations– FRP – determined from furnace gas temperature– Steel reinforcement – determined from ACI 216 – Concrete compressive stress block – determined from ACI 216

900 F (480 C)

ACI 216 ‐ 2 hours


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2. Evaluate reductions in material strengths– FRP – determined from Green et al. (2014) Bisby et al. (2005)– Steel Reinforcement – determined from ACI 216– Concrete compressive stress block – determined from Eurocodes

0.0

0.5

1.0

0 100 200 300 400 500 600 700Redu

ction factor

Temperature (°C)

FRP Strength FRP Modulus FRP Bond Steel Strength


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2. Evaluate reductions in material strengths– FRP – determined from Green et al. (2014) Bisby et al. (2005)– Steel Reinforcement – determined from ACI 216– Concrete compressive stress block – determined from Eurocodes

3. Calculate member strength– Classical plane sections analysis assuming full strength in ‘residual’ section

– Eurocode ‘500oC Isotherm Method’ used to treat compressive stress block width

Unstrengthened beam in fire

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* Fire Resistance > 3.0 hours

LL/DL = 1.2

050100150200250300350400450500

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Mom

ent (kN

.m)

Time (hours)

Mr 216/E119 440/ASCE

FRP Strengthened beam in fire

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LL/DL = 1.8

050

100150200250300350400450500

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Mom

ent (kN

.m)

Time (hours)

Mr 216/E119 440/ASCE

0

100

200

300

400

500

600

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Mom

ent (kN

.m)

Time (hours)

Mr 216/E119 440/ASCE

Beam FRP strengthened by 50% in fire

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* Fire Resistance ≈ 1.5 to 3 hours

LL/DL = 2.7

Insulation ‐ Fire Test: Beam C (15 mm)

Beam strengthened by 50% in fire – with insulation

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0

100

200

300

400

500

600

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Mom

ent (kN

.m)

Time (hours)

Mr 216/E119 440/ASCE

LL/DL = 2.7


Conclusions

• ACI 440.2R is proposing to modify the load demand equation for the fire limit state to harmonize with ACI 562 and ASCE 7

1.0DL + 1.0LL

1.2DL + 0.5LL + 0.2SL + 1.0Ak

• Practical guidance has been given on how to determine the fire resistance of FRP strengthened beams, with or without supplemental fire insulation

• In most cases it is possible for FRP strengthened concrete structural elements to achieve satisfactory fire resistances

Acknowledgements

• Natural Sciences and Engineering Research Council of Canada (NSERC)

• Leverhulme Trust

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Questions?

Mark Green ‐ [email protected] Bisby ‐ [email protected]

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rational design for frp strengthened concrete structures in fire · · 2017-08-05rational design...

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