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Presentation Outline
 General Fire Concept
 Existing Codes and Standards for
Fire Resistance Design of Steel
Buildings
 Design of Steel Structures for Fire
 Steel Reliability after Fire Incident
 Conclusion and Future Research
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Brief Guide to Fire Resistance of Steel Buildings
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General Concept for Fire Resistance



The fire nature and its development
process in steel members.
Fire is a chemical process of thermal
transition.
Based on the fuel sources, fires could
be divided into two groups of:
 Hydrocarbon fires:
Combustion of oil & Petrochemicals
are the source of Hydrocarbon fires
which occurs at high temperature
 Cellulosic fires:
combustion of materials like wall or
floor coverings , furnishings are
usually the main source for cellulosic
fires.
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

Fire loads :
amount of combustible material and building
contents, could be represented either by the
wood equivalent weight of building contents
in kg/m2 or the potential heat of building
content per unit floor area (MJ/m2 )or the
energy per unit of mass (MJ/kg)
Calculating the amount of released
energy during the fire :
The released energy could be calculated
by multiplying the fire load density into
the potential heat energy of combustible.
For example the fire load density of 30
kg/m2 of an office building with potential
heat energy of 40 MJ/m2 would release
1200 MJ/m2.
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General Concept for Fire Resistance


There is a correlation between the
fire load on a wood equivalent basis
and fire endurance time as per table
below:
Average fire Average fire Equivalent fire
load( psf)
load(kg/m2)
endurance
(hours)
5
24.4
½
7½
36.6
¾
10
48.8
1
15
73.2
1½
20
97.6
2
30
146.5
3
40
195.3
4½
50
244.1
6
60
292.9
7½
Table 1(Inberg,1928)
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what does fire protection mean?
Fire protection means to provide ways to
preserve life safety of the building
occupants & minimize the structural &
environmental damage.
Fire Stages and Fire Heat Transfer
Heat can be transferred by 3 ways:
Conduction, convection, Radiation
 In protected steel members heat is
transferred by conduction between
the protective material and the steel.
Convection and radiation heat transfer
doesn’t occur due to the fire protection
on the members. Conduction is
negligible for unprotected steel where
radiation heat transfer is dominant.
 Radiation is the transfer of thermal
energy through electro-magnetic waves
and objects exposed to the radiation
source that absorb the energy(Buchanan
2001). Radiation is the strongest
mechanism of heat transfer in fires.
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General Concept for Fire Resistance

Fire stages: Incipient, growth, burning &
decay
 Incipient stage: is the stage that
the Fuel source starts heating
but there is no visible smoke or
flame.
 Growth stage : The visible flame
develops in the this stage
 Burning stage : Once the fire
reaches really high temperatures
it becomes fully developed which
is not easy to control. The beginning
of this stage is marked as flashover
point .CISC defines the flashover
the rapid transition to a state of
total surface involvement in a fire of
combustible materials within an
enclosure.(Handbook of steel
constructions)
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
Fire Ratings of Structural Assemblies
what does fire resistance rating mean?
Fire resistance rating is the time that the
element can no longer satisfy the structural
insulation, stability and temperature
transmission criterions in the standard fire
test. The period of time a building element ,
component , or assembly maintains the
ability to contain a fire continues to perform
a given structural function as determined by
test or methods on tests.(Handbook of steel
constructions)
The fire –resistance rating of a building
assembly has traditionally been assessed
by subjecting a replicate of the assembly to
the standard fire test CAN/ULCS101.The
time at which the assembly no longer
meets the failure criteria of the test is the
fire resistance rating.
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General Concept for Fire Resistance

Fire resistance of a steel member is a
function of its mass, its geometry, the load to
which it is subjected, its structural support
conditions, and the fire to which it is
exposed.(AISC)
Fire protection systems :
 Active systems:
increase in steel and provides enough
time for building occupants to exit,
combustibles to burn out and fire fighters
to arrive to extinguish the fire.
A number of alternative passive fire
protection systems are:
- Intumescent or mastic coatings
- Sprayed coatings (SFRM)
- Concrete covers and composite
member protections
-Integrated structural members
- Insulating boards
- Suspended ceilings
- Structural fire protection design
Water sprinkler systems and water filled
structural members, smoke and fire
detectors, alarm systems are Examples
of active fire resistance systems. These
systems function with external activation
(human action or automatic device) to
suppress the fire.
 Passive systems:
Pictures below illustrate application of
A passive fire protection system
above mentioned methods for fire
delays the rate of temperature
protection:
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General Concept for Fire Resistance
Example of intumescent coating for exposed
structural steel in a commercial building
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intumescent coatings-Ontario College of Art
and Design's addition in downtown Toronto
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General Concept for Fire Resistance
Using studs and gypsum boards for wall and
floor assemblies, mineral fibre boards or
covers made of material with light insulating
aggregates like perlite or fibre silicate for
insulating wall boards, suspended ceiling
assemblies on the bottom face of the floor
beams are integrated structural systems
used for fire protection in light steel framed
buildings .
Example of spray fireproofing, using a gypsum
based plaster in a low-rise industrial building in
Vancouver, British Columbia. The plaster
provides a layer of insulation to retard heat flow
into structural steel to prevent collapse.
Stud wall with gypsum board system
Insulating Wall Boards
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General Concept for Fire Resistance
Existing codes for fire resistance
design of steel buildings:
Suspended Ceilings
Other than fire protection function
,suspended ceilings are used to hide
plumbing and mechanical equipments

Fire resistant design of structural
members conforming the codes
regulations and fire resisting
requirements using either
prescriptive or performance-based
approaches is the most effective
method of passive fire protection
systems used for hot rolled
steel framed buildings.
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-
-
Canadian National Building code ,
part3 ,division B ,Fire protection
CAN/ULC-S101 , Fire endurance tests
of building construction and material.
CAN4-S104-M , Fire test of door Assemblies
CAN4-S106-M , Fire test of Window and
Glass block assemblies
CAN/ULC-S112-M,Fire test of Fire
Damper Assemblies
CISC Hand book of steel construction ,
Annexes k
AISC, Fire Resistance Construction
ASTM Specification E119, Standard
Methods of Fire Tests of Building construction
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Existing Codes and Standards for Fire Resistance
Design of Steel Buildings

Goals of Codes and Standards for
Fire Design :
Building codes specify fire-resistance
requirements as a function of building
occupancy, height, area, whether or not
other fire protection systems (e.g.,
sprinklers) are provided.
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 NBCC (Limitation for heights and
area for various occupancies)
NBCC classifies buildings into various
major occupancy categories and their sub
division. Any building or part there of
Shall be classified according to its major
occupancy as belonging to one of the
groups or divisions in table below: (NBCC)
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Existing Codes and Standards for Fire Resistance
Design of Steel Buildings


Articles 3.2.2.20 to 3.2.2.28 prescribes  The maximum allowable area has a
the allowable height and area ,sprinkler correlation with the frontage separation
and sprinklers. For example a building
requirements combustibility and fire
classified in group C having not more
resistance requirements for each
than 3 stories in building height has the
classified occupancy These limitations
maximum building area as per table
are summarized in table 3.2 in the CISC
below: (NBCC)
publications , “fire facts for steel
buildings” .
No. of
Maximum Area ,M2
For instance a building classified as
Stories
group A ,division 1 shall be of nonFacing 1 Facing 2 Facing 3
combustible construction and sprinklered
street
streets
streets
throughout ,floor assemblies shall be fire
separations with a fire resistance rating
1
1800
2250
2700
not less than 2 h ,Mezzanines shall have
a fire resistance rating not less than 1
2
900
1125
1350
hour and load bearing walls ,columns
3
600
750
900
and arches shall have a fire resistance
rating not less than the required for the
supported assembly .
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Design of Steel Structures for Fire
Fire can affect a building’s structural
capacity in two ways:
1. Prolonged exposure of structural
components or subsystems to elevated
temperatures degrades their engineering
properties, thus resulting in the reduction
of overall structural capacity.
2. Exposure to elevated temperature may
induce internal forces (due to restraint of
thermal expansion) or axial deformations
in structural members due to plastic and
creep strains or buckling, which may
adversely affect the global stability of the
building.
To understand how steel structures
are designed for fire we need to
have knowledge of:
 Material Properties
 Steel
 Thermal properties
 Mechanical properties
 Fire Protection Materials
There is lack of understanding
of these materials’ properties in
elevated temperatures.
 Current Fire Protection Strategy
 Design by Qualification Testing
 Design by Engineering Analysis
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Design of Steel Structures for Fire
 Design is conceptually similar to
design of structures under normal
temperatures with the following
differences:
 During fire, service loads are
smaller than normal conditions
 Members’ thermal expansion
can cause internal stresses
during fire
 High temperatures reduce
materials’ strengths
 Cross-sectional areas can be
reduced by charring or spalling
 Lower safety factors can be used
due to low probability of fire
 Deflections can become important
as they affect strength
 Different failure mechanisms
should be considered
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 To address low probability of fire, in
Canada, NBCC 2010 uses the
following load combination for fire
design:
D + Ts + (αL or 0.25S)
where,
D = the specified dead load
Ts = effects due to expansion,
contraction, or deflection caused by
temperature changes due to fire
α = 1.0 for storage areas, equipment
areas, and service rooms, and 0.5 for
other occupancies
L = the specified live load
S = the specified snow load
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Design of Steel Structures for Fire
Steel Thermal Properties:
 Thermal Expansion: As temperature
of steel goes up it expands and
coefficient of thermal expansion is
used to determine steel’s expansion
as a function of temperature.
Δl/l = α(T)ΔT
Coefficient of thermal expansion is
the slope of the thermal elongation
curve and is taken as 1.4x10-5 /°C
for simple calculation purposes.
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where,
Δl = change in length due to rise in
temperature
l = initial steel member length
α(T) = coefficient of thermal expansion
ΔT = temperature change
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Design of Steel Structures for Fire
Steel Thermal Properties:
 Specific Heat: The amount of heat
per unit mass required to raise the
temperature of steel by 1 °C.


Specific heat of steel is taken equal
to 600 J / kg°C for all temperature
ranges for simple calculation
purposes.
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
Specific heat of steel for most
temperature ranges remains
relatively constant with moderate
increase between temperatures 500
°C to 700 °C.
At temperature of 750 °C steel goes
under a phase change and there is
an abrupt increase in specific heat.
At temperatures above 800 °C
specific heat remains constant.
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Design of Steel Structures for Fire
Steel Thermal Properties:
 Thermal Conductivity: Required in
order to determine temperature rise
in members when subject to fire or
heat flux. This property is affected by
the microstructure of steel and is
simply the ability of steel in
conducting heat and is measured in
watts per Celsius-meter or watts per
Kelvin-meter

Thermal conductivity of 12 low-alloy steels
and Eurocode approach as a function of
temperature.
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
With increasing temperature thermal
conductivity of steel reduces.
For simple calculation purposes
thermal conductivity of steel is taken
equal to 45 watts/m°C.
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Design of Steel Structures for Fire
Steel Mechanical Properties:
 Prediction of mechanical properties
of steel in fire requires understanding
of its stress-strain relationship at
elevated temperatures.
 As temperature of steel rises its
strength and stiffness are reduced.
 In structural analysis of steel frames,
the deterioration of strength and
stiffness of structural members
should be taken into account.
 This is done by applying reduction
Strength and elasticity reduction factors for
steel at elevated temperatures.
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factors obtained from tests to
different design parameters such as
Young’s modulus and yield stress.
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Design of Steel Structures for Fire
Design by Qualification Testing:
jmnjk
Inside perimeter of fire protection
(D) for columns
Inside perimeter of fire protection
(D) for beams
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Based on the building’s occupancy and fire
safety objectives, designers select prescriptive
fire-resistant design methods from various
documented test reports and special
directories of testing laboratories and ensure
their designs satisfy building code fire rating
requirements.
 Used primarily in the US; testing should be in
accordance with ASTM standards. In Canada,
tests should be in accordance with CAN/ULCS101 (Standard Methods of Fire Endurance
Tests of Building Construction and Materials).
 W/D Ratios: The most common factor used in
fire resistant design. It’s ratio of the weight per
unit length of the member (W) to its inside
perimeter of fire protection (D). Steel sections
with large W/D ratios experience slower rates
of temperature rise and are more fire resistant
than low W/D members.
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Design of Steel Structures for Fire
Design by Engineering Analysis:
 Requires collaboration between fire
protection engineers and structural
engineers.
 Fire protection engineers perform
heat transfer analysis in order to
determine the spread of heat from the
fire and temperature rise of structural
elements.
 Structural engineers use these results
in order to determine the response of
the structure.
 Two Major Analysis Methods:
 Simple Calculation Methods
 Advanced Calculation Methods
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 Simple Calculation Methods
 Evaluation of performance of individual
members at elevated temperatures by
making simplified assumptions such as
ignoring thermal expansion, temperature
independence, idealized boundary
conditions.
 Procedure: obtain maximum steel
temperature (T), assume T is constant
through the cross-section, use reduction
factors to find steel properties at T,
determine member resistance based on
new properties using same formulas as
normal temperature.
 Advanced Calculation Methods
 finite element models that incorporate
geometrical and material nonlinearities;
used to analyze entire building structure.
 The limiting assumptions associated with
simple methods are not allowed in advanced
models. Mostly used in design of unusual or
innovative structural systems.
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Steel Reliability after Fire Incident
Primary goal of fire protection in
building design is to preserve life
safety.
 Failure Mechanisms:


Ultimate limit states failure, includes
local web/flange buckling of beams,
girders and columns, overall member
buckling of columns, and connection
failures.
Serviceability limit states failure,
includes large end rotation and
vertical deflections from several
inches up to extreme cases of 4 feet.
Fire Damage Assessment: performed
to evaluate whether steel members
can be continued to be used or need
to be repaired or replaced after fire
■ Normally a visual inspection is enough
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Conclusion
Fire protection strategies used for
buildings include a combination of:
•
•
Active systems: (Fire alarms, Sprinklers)
Passive systems: built into structural
system by:
Choice of building materials
Dimensions of building components
Compartmentation
Fire protection materials
• These control fire spread and its effect by
providing sufficient fire resistance to prevent
loss of structural stability within a prescribed
time period, which is based on the building’s
occupancy and fire safety objectives.
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 Current Canadian practice for fire
resistance design of structures is based
on qualification testing, which follows
locally adopted codes which specify
minimum fire endurance ratings for
building elements in accordance with
CAN/ULC-101.
 This prescriptive method does not
provide information about actual
performance of the structure in a real fire
environment (does not look at whole
structure, connections, effects of thermal
expansion). Therefore this method
cannot quantify maximum fire endurance
time of a structure.
 A performance-based approach is
more suitable for important structures or
high-rises with longer evacuation time to
provide the necessary fire resistance
consistent with the required level of
protection.
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