fire safety design of concrete structures - what are needed? wksp pres/sess… · ·...

Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University

Fire SafetyDesign of Concrete Structures

- What are needed?

Long T. Phan, Ph.D., P.E.Building and Fire Research Laboratory

National Institute of Standards and Technology


Scope of Presentation

• Summary of current U.S. practice• Structural fire engineering – What

are needed?• Unique concrete issues


Current PracticePrescriptive – Fire Resistance Rating (FRR)

Int’l. Building Code (IBC) 2006Int’l. Code Council (ICC)

Bldg. Cons. & Safety CodeNFPA 5000

Nat’l. Fire Protection Assn.

• Qualification testing (NFPA 251)

• Calculation methods (ASCE 29)

• Other methods based on NFPA 251’s exposure

• Qualification testing (ASTM E119)

• Calculation methods (ACI 216)

• FR design by approved sources

• Tabulated data

ACI/TMS 216.1-07: Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies

ASCE/SFPE 29-05: Standard Calculation Methods for Structural Fire Protection

ASTM E 119: Standard Test Methods for Fire Tests of Bldg Const & Matls

NFPA 251: Standard Methods of Tests of Fire Endurance of Bldg Const & Matls

Differ only in sampling rate for furnace temperature

Acceptance Criteria

Heat Transmission: Min thickness to limit temperature rise on unexposed surface

Load Carrying Ability: Min cover so that reinforcement yield strength is ≥ 50% of value at ambient


Current Practice

• FRR measures relative performance under standardfire, not actual performance in real fire

• Depending on fuel loads and duration, real fires could be more severe

• Component-oriented, structural interaction between components ignored

• FRR of building is assumed to equal that of component with least FRR, but connections are rarely tested

• Unique concrete issues: spalling, strength degradation, global structural stability not addressed


Structural Fire Engineering

General framework for structural fire calculation

Input:Descriptionof structure, contents

Design FireScenarios

T-t, fluxeshistory

ThermalResponse

Thermalproperties

StructuralResponse

Constitutivematerial models

(σ-ε(T), E(T). f’c(T))

Meet FailureCriteria?

YesEnd

No


Structural Fire EngineeringThermal response calculation• Material properties

• Thermal Conductivity• Specific Heat• Density

HSC & NSC: Similar properties

• Calculation methods• Closed-form (Lumped Heat Capacity, Semi-Infinite Slab):

Uniform temperature increase• FEM, FDM (Fires-T3, Safir, Firetrans, Ceficoss, Ansys):

Complex geometryUncoupled with structural response calculationMoisture transport depends on material models used

Temperature (°C)

Ther

mal

Con

duct

ivity

, (λ)

(W

/m.K

)

Temperature (°C)

Volu

met

ric S

peci

fic H

eat,

( ρC

p) (M

J m

-3/K

)

2000

2100

2200

2300

2400

0 200 400 600 800 1000 120 Temperature T [°C]

Den

sity

ρ [k

g/m

3 ]

0


Structural Fire Engineering

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛

εε

+ε

ε=σ

θθ

θ3

,1c

c,1c

,ccc

2

f3

Structural response calculation• Sophisticated FE Programs:

• Complex geometry• Uncoupled with thermal calculation

• Material models• Stress-strain • Strength-Temperature• Spalling

− Corner Spalling− Explosive Spalling

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000

Phan (HSC mix I)Phan (HSC mix II)Phan (HSC mix III)Phan (HSC mix IV)

Rel

ativ

e St

reng

th

Temperature (oC)

NIST HSC Tests

Abrams (Carbonate)Abrams (Siliceous)

Other NSC Tests

Castillo Khoury

Other HSC Tests

(a)

CEN (1993)CalcareousSiliceous

(Phan, 2003)

CEN Class 2CEN Class 3


Concrete SpallingCorner Spalling Explosive Spalling

Channel Tunnel (11/18/1996)(Paul Acker - Laboratoire Central des

Ponts et Chaussees)

Fire test of 6-story building (Lennon et al., - BRE)

Spalling in small specimen (NIST)Laboratory fire tests at U. of Liege

(by J-M. Franssen)

Concrete columns in real fire


Spalling Explained


Model for Spalling• Facts:

Contributing/Mitigating Factors- w/cm ratios- Initial Moisture Content (IMC)- PP fibers- Heating Rate- Silica Fume

Occurrences:- In HSC (w/cm ≤ 0.37; IMC ≥ 5%)- Concrete temperature: 220 °C – 260 °C- Spalling depth: 25 mm – 75 mm, or at interface with reinforcement

• Spalling Prediction:Input data: Transport properties typically not available to practicing engineersSimplified Approach: Use concrete type (HSC.NSC) and concrete

temperature (from thermal analysis) as spalling indicator. Remove concrete up to spalling depth if conditions met.


Summary

• Existing FRR methods do not adequately consider many unique concrete issues.

• Structural fire engineering method requires accurate characterization of material properties.

• More material properties available and codified.

• Spalling can be mitigated through material design.

• Spalling prediction requires properties typically not available to practicing engineers. Simplified method based on experimental observations can be developed to handle potential for spalling.

• Mechanical properties of HSC and NSC vary differently at elevated temperatures. HSC sustains higher strength loss in the intermediate temperature range (100 °C to 400 °C) than NSC.


Thank you!

fire safety design of concrete structures - what are needed? wksp pres/sess… · ·...

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