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Linking Fire Severity and Fire Resistance for the Assessment of Structural Performance
José L. Torero School of Civil Engineering The University of Queensland Australia
Structural Behaviour in Fire
• Fire Resistance • A regulatory terminology defining compliance
with required rules established to implicitly define performance of structural systems in fire
• Fire Severity • A quantitative, physically based, assessment of
structural solicitation (can and should be linked to performance)
Building Code of Australia
• Most critical design load in accordance with AS 1170.0 (clause B.11(b));
• Actions include thermal effects (clause B1.2(e,vii)). • Resistance of steel structures in accordance with AS
4100 (clause B1.4(c,i)
AS 1170.0
• Structural will not be damaged disproportionally to the original cause (e.g. events like fire) (clause 3.2(b));
• Structural design in accordance with clause 2.2 (clause 3.2);
• Design in accordance with combination actions in section 4 (clause 2.2(g));
• Combination factors include thermal actions arising from the fire (clause 4.2.4)
• In terms of FRL adequate loadbearing capacity as determined by AS 1530.4 (definitions)
D.T.S. Language
Base
d on
Fur
nace
Test
Dat
a ad
opte
d in
the
Euro
code
s (20
08)
• D
True Performance Assessment: The Fire
Fire Dynamics
Heat Transfer
𝜌𝜌𝐶𝐶𝑝𝑝𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕
= 𝑘𝑘𝜕𝜕2𝜕𝜕𝜕𝜕𝑥𝑥2
−𝑘𝑘𝜕𝜕𝜕𝜕𝜕𝜕𝑥𝑥�𝑥𝑥=0
= �̇�𝑞"𝑁𝑁𝑁𝑁𝑁𝑁
𝜕𝜕𝜕𝜕𝜕𝜕𝑥𝑥�𝑥𝑥=𝑥𝑥𝑆𝑆
= 0
𝜕𝜕 𝜕𝜕 = 0 = 𝜕𝜕0
Structural Analysis
Challenge 1 How does thermal expansion effect a
structure?
Structural Analysis
Challenge 2 How does thermal curvature effect a
structure?
Challenge 3 Preventing failure
due to strength degradation
Challenge 4 Limiting deflections
due to loss of stiffness
Wood Cribs
Fire: Heat Fluxes evolve in space and time
0 1 2 3 4 5 6 7 8 9 10 11 120
1
2
3
80
8080
80
60 6060
4040
40
20 20 20
100
100100
100
120
120
120
120140
160
160160140160
140160140
140
180
180
160140
140
18016
060
0 1 2 3 4 5 6 7 8 9 10 11 120
1
2
3
2020
60 60 60
40 40 40
80 80 80
2020
20
20
20
60
4060
40
40 80100
120100
20
20
0 1 2 3 4 5 6 7 8 9 10 11 120123456789
101112 120
120
80
120
120
120
120
120
120
160
120
80
160
160
160
200
200160
120
200200
120200
200
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160
200
160
120
160
8012
080
8080
228kW2
Time
Space
Regime II: Fuel Controlled
Thomas and The Compartment Fire
Regime I: Ventilation Controlled
F,rq ′′
W,rq ′′
g,rq ′′S,rq ′′)TT(hq SgC −=′′
Fire Dynamics – The Compartment Fire
Regime I Regime II
Regime I
Regime II
Theory
Theory
A0 H0
A
𝐴𝐴𝐴𝐴0 𝐻𝐻0
Fire Dynamics – Thermal Load
T [oC]
t [min] tBO
Heating
Cooling
R = 0.1 AwH1/2 (kg/s)
Kawagoe (1958) Thomas (1960)
𝜕𝜕𝐵𝐵𝐵𝐵 = 𝑀𝑀𝐹𝐹𝑅𝑅
𝑘𝑘𝑘𝑘𝑘𝑘𝑘𝑘/𝑠𝑠
= 𝑠𝑠
Parametric Fires - Eurocodes MF
Real Fire
Thomas
Parametric
Standard: Fire Resistance
The Origins
• Regulations and Fire Service are born after the great fires of New York (1835), London (1861), Chicago (1871), Boston (1872) and San Francisco (1906), etc.
• Rules stem from the experience of fire fighters, architects and engineers
• Very poor understanding of human behaviour, fire dynamics and structural behaviour
1850-1940 • Building Separation Distances • Compartmentalization • Size and separation of windows • Maximum Egress distances • Sprinklers • Passive fire protection – fire proofing • 1st Professional Fire Brigade – James
Braidwood (1824)
Fire Resistance
• Developed in the 19th Century as a result of the introduction of steel in construction
• Based on numerous test conducted prior to 1930 • Semi-formalized in the 1920’s • Formalized in 1928 (Ingberg S.H., “Fire loads:
Guide to the application of fire safety engineering principles,” Quarterly Journal of the National Fire Protection Association, 1, 1928.)
Standard Fire + Rating • Worst Case Scenario • Curve is defined by an envelope to all fires • Rating is defined by total fuel consumption
0
250
500
750
1000
1250
0 30 60 90 120 150 180time [minutes]
Tem
pera
ture
[oC
]
Fire (BS-476-Part 8)
Increasing Fuel Load
Restraint + Compartmentation • Allows to approximate global structural
behaviour to single element – Restraint enables effective load transfer
Restraint
Furnace Testing
• Single elements can be tested within a controlled environment
• The environment has to create “worst case scenario” condition
Standard Fire
0
250
500
750
1000
1250
0 30 60 90 120 150 180time [minutes]
Tem
pera
ture
[oC
]
Fire (BS-476-Part 8)
Critical Temperature
Rating
Structural Element
Critical Temperature • Based on material properties – steel
– Steel structural element (steel design) – Reinforcement - prestressing (concrete design)
𝑁𝑁𝐹𝐹𝑁𝑁𝑈𝑈
= 0.7
𝑁𝑁𝐹𝐹𝑁𝑁𝑈𝑈
= 0.5
𝑁𝑁𝐹𝐹𝑁𝑁𝑈𝑈
= 0.2
Importance • The building will be designed in a
manner that allows the test to be a scenario worse than any possible fire
• Fire protection will guarantee that the failure conditions described in the test will never be attained
• Architects will prescribe fire protection
• Separated structural analysis from fire safety through the development of a standardized testing procedure
• Enabled structural engineers to design tall buildings
What is Equivalent?
• Energy input – Equivalence Methods? • If we provide the same amount of energy – will
the outcome be the same?
The Objective is not an – Equivalent level of energy but an Equivalent structural
behaviour?
Broadgate Phase 8 Furnace Recreation
Linking Severity to Resistance
Time (s)
0 10 20 30 40 50 60 70 80 90 100 110 120
Ver
tical
Def
lect
ion
(mm
)
-1000
-750
-500
-250
0
Central deflect ion - Parametric Heat ingCentral deflect ion - Heat Flux Model
δ
Simple
Detailed
Time (s)
0 10 20 30 40 50 60 70 80 90 100 110 120
Ver
tical
Def
lect
ion
(mm
)
-1000
-750
-500
-250
0
Central deflect ion - Parametric Heat ingCentral deflect ion - Heat Flux Model
Linking Severity to Resistance
δ
Simple
Detailed
Summary • Fire Resistance is a regulatory definition – Many
issues with this implicit form of performance • Fire Severity is an explicit physical solicitation of
the structure due to fire – Can be defined at many levels of precision
• Equivalency means equal desired outcomes – Desired outcome is adequate Structural Behaviour
• Outcome: Equal Explicit Structural Performance
