Fire Severity Design Notes (NRC)
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Transcript of Fire Severity Design Notes (NRC)
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1Winter 2003 Fire Severity 1
FIRE SEVERITY
Winter 2003 Fire Severity 3-2
OVERVIEW
This section will: provide an overview of basic methods for designing
structures for fire safety describe methods for quantifying the severity of post-
flashover fires for comparison with fire resistance describe the standard fire used for fire resistance testing describe the concept of equivalent fire severity which is
used to compare real fires with standard fires
Winter 2003 Fire Severity 3-3
FIRE SEVERITY AND FIRE RESISTANCE
Verification Fire Exposure Models Design Combinations
Winter 2003 Fire Severity 3-4
Verification
The fundamental step in designing structures for fire safety is to verify that the fire resistance of the structure is greater than the severity of the fire to which the structure is exposed, i.e.,
Fire resistance Fire severity
Fire resistance is the ability of the structure to resist collapse or fire spread during exposure to a fire of specified severity
Winter 2003 Fire Severity 3-5
Verification
Fire severity is a measure of: the destructive impact of a fire, or the forces or temperatures that may cause collapse or
fire spread as a result of a fire
Below is a table showing methods for comparing fire severity with fire resistance
Winter 2003 Fire Severity 3-6
Verification
Methods for comparing fire severity with fire resistance
Domain Units Fire Resistance Fire SeverityTime minutes or hours Time to failure Fire duration as calculated or
specified by codesTemperature C Temperature to cause
failure Maximum temperature reachedduring the fire
Strength kN or kN-m Load capacity at elevatedtemperature
Applied load during the fire
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2Winter 2003 Fire Severity 3-7
Verification
The verification may be in the: time domain temperature domain strength domain
Winter 2003 Fire Severity 3-8
Verification - Time domain
The most common verification is comparing fire severity and fire resistance in time domain, i.e.:
tfail ts tfail is the time to failure of a building element,
usually a fire-resistance rating ts is the fire duration or fire severity, usually a
time of standard fire exposure or an equivalent time of standard fire exposure calculated for a real fire in a building
Winter 2003 Fire Severity 3-9
Verification - Temperature domain
Sometimes, verification of design is in the temp. domain, i.e.:
Tfail Tmax Tfail is the temp. which would cause failure of
building elements (thermal or structural failure) Tmax is the maximum temp. reached in building
elements during a fire or the temp. at a certain time specified by codes
Winter 2003 Fire Severity 3-10
Verification - Temperature domain
Temperatures in elements can be calculated by thermal analyses of assemblies exposed to fire
For barriers, failure temp. is the unexposed side temp. causing fire spread to other areas
For structural elements, temp. causing collapse can be calculated based on loads on elements and elevated temp. effect on material properties
Temperature domain is more suitable for barriers than structural elements
Winter 2003 Fire Severity 3-11
Verification - Strength domain
Verifying strength domain is comparing applied loads at the time of fire with the load capacity of structural members throughout the fire, i.e.:
Rf Uf Rf is the minimum load capacity reached during a
fire or the load capacity at a certain time specified by codes
Uf is the applied load at the time of a fire
Winter 2003 Fire Severity 3-12
Verification - Strength domain
The load values may be expressed in units of: force and resistance for the whole building internal member actions such as axial force or bending
moment in individual structural members Load capacity in a fire can be calculated using
thermal and structural analyses at high temp., (limited structural tests available for full burnouts)
Loads at the time of a fire can be calculated using load combinations from national building codes
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3Winter 2003 Fire Severity 3-13
Verification - Example
Behaviour of a steel beam in fire(a) temperature increase (b) loss of strength
Winter 2003 Fire Severity 3-14
Verification - Example
Figure (a) shows the time-temperature of a steel beam during fire exposure
Calculations indicate the beam failing at time tfailwhen the steel temperature reaches Tfail
The code requires a fire resistance or required fire severity of tcode for the beam
Winter 2003 Fire Severity 3-15
Verification - Example
Verifying time domain (check 1 - Figure (a)):time to failure tfail fire severity tcode
Verifying temp. domain (check 2 - Figure (a)):steel temperature causing failure Tfail Tcodetemperature reached in the beam at time tcode
Checks 1 & 2 give the same results since they are based on the same process
Winter 2003 Fire Severity 3-16
Verification - Example
Figure (b) shows the load capacity of the same steel beam during the fire
Applied load at the time of the fire is Uf Load capacity before the fire is Rcold, which
decreases during the fire Load capacity of the beam reduces to Rcode at
tcode Verifying strength domain (check 3 - Figure (b)):
Rcode Uf at time tcode
Winter 2003 Fire Severity 3-17
Fire Exposure ModelsFire models and structural response models
Winter 2003 Fire Severity 3-18
Fire Exposure Models
The Figure shows a range of design situations The first column shows three fire exposure
models representing three different design fires Fire exposure H1 (most common) represents a
standard test fire exposure for a specified period of time, tcode given by a prescriptive code
Prescriptive codes specify required fire resistance (30 min to 4 hrs), no reference to the fire severity
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4Winter 2003 Fire Severity 3-19
Fire Exposure Models
Fire exposure H2 represents a modified duration of exposure to the standard test fire
The equivalent time, te is the exposure time to the standard test fire considered to be equivalent to a complete burnout of a real fire in the same room (equivalent fire severity)
Performance-based codes allow the use of time equivalent formulae to improve on simple prescriptive fire-resistance requirements
Winter 2003 Fire Severity 3-20
Fire Exposure Models
Fire exposure H3 represents a realistic fire that occurs in a room with complete burnout and no fire suppression - for example Swedish curves
The figure also shows that the fire resistance may be assessed considering a single element, a sub-assembly or a whole structure
For each category, the method of assessment is indicated - testing or calculation
Winter 2003 Fire Severity 3-21
Design Combinations
Many design combinations are possible and therefore it is essential for designers to specify clearly the combination that will be used
The Table below illustrates the most common combinations
In very general terms, both the accuracy of the prediction and the amount of calculation effort increase downwards in the table
Winter 2003 Fire Severity 3-22
Design Combinations
Design combinations for verifying fire resistance
Combination Fire exposure model Assessment of fireresistance
Verificationdomain
1 Prescriptive code(H1)
Listed rating orcalculation
Time
2 Time-equivalent formula(H2)
Listed rating orcalculation
Time
3 Predicted real fire(H3)
Calculation Temperature orstrength
Winter 2003 Fire Severity 3-23
FIRE SEVERITY
Fire severity is a measure of the destructive potential of a fire
Fire severity is usually defined as the period of exposure to the standard test fire, but this is not appropriate for real fires which are different
In prescriptive codes, the design of fire severity is usually prescribed
In performance-based codes, the design fire severity is usually a complete burnout fire or the equivalent time of a complete burnout fire
Winter 2003 Fire Severity 3-24
FIRE SEVERITY
The equivalent time of a complete burnout is the time of exposure to a standard test fire that results in an equivalent impact on an element
Damage to a structure is mainly dependent on the heat absorbed by the structural elements
The severity of a fire is mainly dependent on the level and duration of the high temperatures
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5Winter 2003 Fire Severity 3-25
STANDARD FIRE
Fire performance of building elements is assessed using full-size fire resistance tests
Time-temperature curves used in fire resistance tests is called the 'standard fire'
The most widely used test specifications are: ASTM E 119 and CAN/ULC-S101-M89 (North America) ISO 834 (International) British Standard BS 476 Parts 20-23 Australian Standard AS 1530 Part 4
Winter 2003 Fire Severity 3-26
Time-temperature CurvesStandard time-temperature curves
Winter 2003 Fire Severity 3-27
Time-temperature Curves
The figure shows standard time-temperature curves for ASTM E 119 (ULC-S101), ISO 834, Eurocode (EC1)
ASTM E 119 and ISO 834 curves are similar (this is true for most international standards curves)
ISO 834 specifies the temperature T (C) as:T = 345 log10 (8t+1) + To
t is the time (min), To is the ambient temp. (C)
Winter 2003 Fire Severity 3-28
Time-temperature Curves
ASTM E 119 curve is defined by a number of discrete points
The following is an approximate equation of the ASTM E119 curve for temperature T (C) as:
T = 750[1 - e-3.79553 t] + 170.41 t + To t is the time (hours), To is the ambient temp. (C)
Winter 2003 Fire Severity 3-29
Time-temperature Curves
The following table shows temperature values for ASTM E119 and ISO 834
Time(minutes)
ASTM EI19Temperature (C)
ISO 834Temperature (C)
0 20 205 538 57610 704 67830 843 84260 927 945120 1010 1049240 1093 1153480 1260 1257
Winter 2003 Fire Severity 3-30
Time-temperature Curves
The Eurocode hydrocarbon fire curve is intended for use where a structural member is engulfed in flames from a large pool fire
The temperature T (C) in the hydrocarbon fire curve is given by:
T = 1080(1 - 0.325e-0.167t - 0.675e-2.5t)+ To t is the time (min), To is the ambient temp. (C)
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6Winter 2003 Fire Severity 3-31
Time-temperature Curves
The other Eurocode curve (lower temperatures) is intended for designing external structural members located outside a burning compartment
The temperature T (C) for this fire is given by: T = 660(1 - 0.687e-0.32t - 0.313e-3.8t) + To
t is the time (min), To is the ambient temp. (C)
Winter 2003 Fire Severity 3-32
Furnace Parameters
Fire severity depends on the testing furnace characteristics
Two similarly-operated furnaces may not impact test specimens with the same fire severity
Temperatures are not always uniform throughout the furnace (may severely impact test specimens)
Even with similar curves, tests can be considered to give only roughly equivalent thermal exposure
Winter 2003 Fire Severity 3-33
Furnace Parameters
Thermocouple measurements may be different from one furnace to another
Temp. differences are most significant during the first 5 minutes of the tests
Significant differences may exist between heating conditions in various furnaces, depending on the furnace size, fuel type and furnace lining material
These differences affect the heat transfer to the furnace walls and to the test specimens
Winter 2003 Fire Severity 3-34
Furnace Parameters
Most common wall lining materials are fire bricks or ceramic fibre blankets, which have different thermal properties, hence different rates of heat transfer to the test specimens
Temperatures increase less rapidly in furnaces lined with bricks
Winter 2003 Fire Severity 3-35
EQUIVALENT FIRE SEVERITY
Real Fire Exposure Equal Area Concept Maximum Temperature Concept Minimum Load Capacity Concept Time-equivalent formulae
Winter 2003 Fire Severity 3-36
Real Fire Exposure
The equivalent fire severity is a concept used to relate the severity of an expected real fire to the standard test fire
This relation is important for designers who want to use published fire-resistance ratings from standard tests with estimates of real fire exposure
Below is a description of the methods comparing real fires to the standard test fire
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7Winter 2003 Fire Severity 3-37
Equal Area Concept
Ingberg (1928) introduced the equivalency concept by stating that two fires have equivalent severity if areas under each time-temp. curve are equal
The figure below shows the concept The concept is not theoretically sound Although inadequate, the concept was used as a
crude method of comparing fires
Winter 2003 Fire Severity 3-38
Equal Area Concept
Equivalent fire severity on equal area basis
Winter 2003 Fire Severity 3-39
Equal Area Concept The equal area concept is used to correct results
of standard fire-resistance tests if the standard curve is not exactly followed within tolerances
A problem with the equal area concept is that it can give a poor comparison of heat transfer for fires with different shaped time-temp. curves
Babrauskas and Williamson (1978) commented that there could be a big difference between a short hot fire and a longer cool fire
Winter 2003 Fire Severity 3-40
Maximum Temperature Concept
This is a more realistic concept, developed by Law (1971), Pettersson et al. (1976) and others
The concept defines the equivalent fire severity as the time of exposure to the standard fire that would result in the same maximum temperature in a protected steel member as would occur in a complete burnout of the fire compartment
The following figure shows the concept
Winter 2003 Fire Severity 3-41
Maximum Temperature Concept
Equivalent fire severity on temperature basis
Winter 2003 Fire Severity 3-42
Maximum Temperature Concept
The figure compares temp. in a protected steel beam exposed to a standard fire with those when the same beam is exposed to a particular real fire
This concept is applicable to insulating elements when the temp. on the unexposed
face is used instead of the steel temp. materials which have a limiting temp. such as the 300C
temperature of charring onset of wood
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8Winter 2003 Fire Severity 3-43
Maximum Temperature Concept
The maximum temp. concept is commonly used The concept may be misleading when maximum
temp. used in the derivation of a time-equivalent formula are: greater than those causing failure in a building lower than those causing failure in a building
Winter 2003 Fire Severity 3-44
Minimum Load Capacity Concept
The minimum load capacity concept is similar to the maximum temperature concept
In this concept, the equivalent fire severity is the time of exposure to a standard fire resulting in the same load bearing capacity as the minimum that would occur in a complete burnout of a compartment
The following figure shows the concept
Winter 2003 Fire Severity 3-45
Minimum Load Capacity Concept
Equivalent fire severity on load capacity basis
Winter 2003 Fire Severity 3-46
Minimum Load Capacity Concept
The Figure shows the load bearing capacity of a structural member exposed to a standard fire decreases continuously
The strength of the same member exposed to a real fire increases after the fire enters the decay period and the steel temperatures decrease
Winter 2003 Fire Severity 3-47
Minimum Load Capacity Concept
The concept is the most realistic time equivalent concept for the design of load bearing members
However, the concept is difficult to apply for a material which does not show a well defined minimum load capacity
For example, wood members where charring can continue after fire temperatures start to decrease
Winter 2003 Fire Severity 3-48
Time-equivalent formulae
Based on the maximum temp. concept, many empirical time-equivalent formulae have been developed
These formulae are based on maximum temp. of protected steel members exposed to real fires and include: CIB formula Law formula Eurocode formula
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9Winter 2003 Fire Severity 3-49
Time-equivalent formulae -CIB formula
Widely used time equivalent formula (published by CIB W14 group (CIB, 1986)
Formula derived by Pettersson (1973) based on the ventilation parameters of the compartment and the fuel load
Winter 2003 Fire Severity 3-50
Time-equivalent formulae -CIB formula
The equivalent time of exposure to an ISO 834 fire test te (min) is given by:
te = kc w ef ef is the fuel load (MJ/m2 of floor area) kc is a parameter to account for different linings
of the compartment
Winter 2003 Fire Severity 3-51
Time-equivalent formulae -CIB formula
w is the ventilation factor (m-0.25) given by: w = Af / Av At Hv
Af is the floor area of the compartment (m2) Av is the total area of openings in the walls (m2) At is the total area of the internal bounding
surfaces of the compartment (m2) Hv is the height of the windows (m)
Winter 2003 Fire Severity 3-52
Time-equivalent formulae -Law formula
Similar to CIB formula, Law developed a formula based on tests in small-scale and large-scale compartments
The formula is given by:
te = Af ef / Hc Av (At Av) Hc is the calorific value of the fuel (MJ/kg)
Winter 2003 Fire Severity 3-53
Time-equivalent formulae -CIB/Law formulae
CIB and Law formulae are only valid for compartments with vertical openings in the walls
CIB and Law formulae cannot be applied to rooms with openings in the roof
CIB and Law formulae give, in general, similar results (Law formula predicts slightly larger values)
Winter 2003 Fire Severity 3-54
Time-equivalent formulae -Eurocode formula
Eurocode (1994) formula is a modification of CIB and Law formulae
The formula give te (minutes) as: te = kb w ef
kb replaces kc in CIB formula and the ventilation factor w is altered to allow for horizontal roof openings
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Winter 2003 Fire Severity 3-55
Time-equivalent formulae -Eurocode formula
The ventilation factor is given by:w = (6.0/Hr)0.3[0.62+90(0.4-v)4/(1+bv h)] > 0.5
v = Av / Af (0.05 v 0.25)h = Ah / Af (h 0.20)bv = 12.5 (1 + 10v - v2)
Winter 2003 Fire Severity 3-56
Time-equivalent formulae -Eurocode formula
Hr is the compartment ceiling height (m) Af is the floor area of the compartment (m2) Av is the area of vertical openings in walls (m2) Ah is the area of horizontal openings in roofs (m2)
Winter 2003 Fire Severity 3-57
Time-equivalent formulae -CIB/Eurocode formulae
The Eurocode formula comes from an empirical analysis of calculated steel temperatures in a large number of fires
An important difference from the CIB formula is that the Eurocode equivalent time is independent of opening height, but depends on the ceiling height of the compartment
Winter 2003 Fire Severity 3-58
Time-equivalent formulae -CIB/Eurocode formulae
The two formulae can give different results for the same room geometry
The two formulae give similar results for small compartments with tall windows
The Eurocode formula gives much lower fire severity values for large compartments with tall ceilings and low window heights
Winter 2003 Fire Severity 3-59
Time-equivalent formulae -CIB/Eurocode formulae
Values of kc and kb are given in the table below Values of kc and kb depend on materials of the
compartment The general case is used for compartments with
unknown materials kc and kb have slightly different values and units
because of the different ventilation factors in the respective formulae
Winter 2003 Fire Severity 3-60
Time-equivalent formulae
The table below gives values of kc or kb in the time equivalent formulae
k is thermal conductivity (W/m-K), is density (kg/m3), cp is specific heat (J/kg-K)
b = ( k cp)High Medium Low
Formula Term Units > 2500 720-2500 < 720 GeneralCIB WI4 kc min m2.25/MJ 0.05 0.07 0.09 0.10Eurocode kb min m2/MJ 0.04 0.055 0.07 0.07Large compartments kb min m2/MJ 0.05 0.07 0.09 0.09
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Winter 2003 Fire Severity 3-61
WORKED EXAMPLE 1
Calculate the equivalent fire severity using the Eurocode formula for a room 4.0 m x 6.0 m in area, 3.0 m high, with one window 3.0 m wide and 2.0 m high. The fire load is 800 MJ/m2 floor area. The room is constructed from concrete.
Length of room: l1 = 6.0 m Width of room: l2 = 4.0 m Floor area: Af = l1 x l2 = 6.0 x 4.0 = 24.0 m2
Winter 2003 Fire Severity 3-62
WORKED EXAMPLE 1
Height of room: Hr = 3.0 m Fuel load energy density: ef = 800 MJ/m2
For concrete Thermal conductivity: k = 1.6 W/m-K Density: = 2200 kg/m3 Specific heat: cp = 880 J/kg-K Thermal inertia: kcp = 1760 Ws0.5/m2K (medium)
Winter 2003 Fire Severity 3-63
WORKED EXAMPLE 1 Conversion factor: kb = 0.055 Window height: Hv = 2.0 m Window width: B = 3.0 m Window area: Av = Hv B = 2.0 x 3.0 = 6.0 m2
Horizontal vent area:Ah = 0 (no ceiling opening) v = Av / Af = 6.0/24.0 = 0.25 h = Ah / Af = 0 bv = 12.5(1 + 10v - v2) = 43.0
Winter 2003 Fire Severity 3-64
WORKED EXAMPLE 1
Ventilation factor:w = (6.0/3.0)0.3 [0.62+90(0.4-0.25)4/(1+43.0x0)]
= 0.820 m-0.3
Equivalent fire severity:te = ef kb w = 800 x 0.055 x 0.820 = 36.1 min
Winter 2003 Fire Severity 3-65
WORKED EXAMPLE 2 Repeat Worked Example 1 with an additional
ceiling opening of 3.0 m2.
Ceiling opening area:Ah = 3.0 m2; h = Ah / Af = 3.0/24.0 = 0.125
Ventilation factor:w = (6.0/3.0)0.3[0.62+90(0.4-0.25)4/(1+43x0.125)]
= 0.772 m-0.3 Equivalent fire severity:
te = ef kb w = 800 x 0.055 x 0.772 = 34.0 minWinter 2003 Fire Severity 3-66
WORKED EXAMPLE 3 Repeat Worked Example 1 using the CIB
formula and the Law formula.
CIB formula Length of room: l1 = 6.0 m Width of room: l2 = 4.0 m Floor area: Af = l1 x l2 = 6.0 x 4.0 = 24.0 m2
Height of room: Hr = 3.0 m
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12
Winter 2003 Fire Severity 3-67
WORKED EXAMPLE 3
Fuel load energy density: ef = 800 MJ/m2
Total area of the internal surface:At = 2(l1l2 + l1Hr + l2Hr) = (6x4+6x3+4x3) = 108 m2
For concrete Thermal conductivity: k = 1.0 W/m-K Density: = 2200 kg/m3 Specific heat: cp = 880 J/kg-K
Winter 2003 Fire Severity 3-68
WORKED EXAMPLE 3
Thermal inertia: kcp = 1391 Ws0.5/m2K (medium) Conversion factor: kc = 0.07 min-m2.25/MJ Window height: Hv = 2.0 m Window width: B = 3.0 m Window area: Av = Hv B = 2.0 x 3.0 = 6.0 m2
Winter 2003 Fire Severity 3-69
WORKED EXAMPLE 3 Ventilation factor
w = Af /(Av At Hv0.5)0.5 = 24/(6x108x20.5)0.5
= 0.793 m-0.25
Equivalent fire severity:te = ef kc w = 800 x 0.07 x 0.793 = 44.4 min
Winter 2003 Fire Severity 3-70
WORKED EXAMPLE 3
Law formula Net calorific value of wood: Hc = 16 MJ/kg Equivalent fire severity:
te = ef Af / [Hc (Av (At - Av))0.5]= 800x24/[16x(6(108-6))0.5] = 48.6 min
Winter 2003 Fire Severity 3-71
Time-equivalent formulae -Validity
Time-equivalent formulae are empirical and have been derived by calculation using: a particular set of design fires small rooms maximum temp. concept for protected steel members
with various thickness of insulation Formulae are crude, may not be applicable to:
other shapes of time-temperature curve larger rooms other types of protection other structural materials
Winter 2003 Fire Severity 3-72
Time-equivalent formulae -Validity
Time equivalent formulae are applicable to protected steel and reinforced concrete members
Time equivalent formulae are not intended for unprotected steel or for timber construction
It is more accurate to carry out designs using first principles to estimate post-flashover fire temp.