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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
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
Scope of Presentation
• Summary of current U.S. practice• Structural fire engineering – What
are needed?• Unique concrete issues
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
Spalling Explained
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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.
Workshop on Structures in Fire: Research Needs Workshop on Structures in Fire: Research Needs –– Michigan State UniversityMichigan State University
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.