Safety in Buildings

CHAPTER 1
MODULE I - SYLLABUS
Effect of Temperature on the properties of materials: Concrete, Steel, Masonry and wood.
Behaviour of non structural materials on fire- plastics, glass, textile fibers and other
house hold materials.
Determination of combustibility by fire tube method; brief description on non-
combustibility test and classification of flame spread rate of materials as per relevant
standards(BIS).
Compartment fire-factors controlling fire severity, ventilation controlled and fuel
controlled fire.
Spread of fire in rooms, within building and between buildings.
REFERENCES
1. Smith E.E. and Harmathy T.Z., “Design of Buildings for Fire Safety” ASTM
Special Publication 685.
2. Roytman. M. Ya. “ Principles of Fire Safety Standards for Building
Construction”, Amerind Publishing Co. Pvt. Ltd., New Delhi.
3. Jain, V. K., “Fire Safety in Buildings”, New Age International (P) Ltd., New
Delhi.
4. Butcher E.G. and Parnel, A.C., “ Designing for Fire Safety”, John Wiley and Sons
5. Eric W Marchant, “Acomplete guide to Fire and buildings”, Medical and
Technical Publishing Co. Ltd., England.
1.1 COMBUSTION
Combustion is a reaction between a substance and oxygen ( oxidation ) in which heat is
given out such that the temperature of the reactants is maintained or increased by the
generation of heat. This means that once the material starts burning, further heat source is
not required to keep it burning.
Oxidation can be in different ways like- rusting of steel; removal of hydrogen from a
substance;, etc. However, these reactions between a substance and oxygen cannot be
considered as combustion, as the oxidation may not produce sufficient heat to continue
the reaction, or some times may not produce heat at all.
Ignition of a material ( fuel ) is said to have started when the heat generated due to the
reaction between fuel and oxygen sustains the reaction without any further need for the
applied heat.
Flame is the zone of oxidation of gas, usually characterized by the liberation of heat and
the emission of light.
Combustion takes place when a combustible substance and oxygen are brought together
and heated until the two react together and give off heat. The combustible substance can
be termed as fuel. The rate of combustion is dependent on the ability of the molecules of
the fuel and oxygen to mix together in the appropriate proportion. The most rapid rate
gives an explosion (combustion of explosives) and the slow rate gives a Smoky,
smoldering fire (combustion of husk, chaff, etc.)
The three parts of a combustion process namely Fuel ( Matter) ,Oxygen, and Heat and
the stages involved in starting and stopping combustion can be explained well with the
help of combustion triangle.
1.2 CLASSIFICATION OF FIRE
Fire has been classified as Class A, Class B, Class C, and Class D by BIS in IS 2190.
However, the most widely accepted classification of fire is as follows:
Class A Fire
Fire in easily combustible solid materials of organic nature such as wood, grass, paper,
cloth and products thereof. In such cases, fire is extinguished by reducing the temperature
of burning materials by water spraying the residue is always carbonous materials.
Class B Fire
Fire in volatile and inflammable liquids which provide a homogenous media of fire
expansion in materials like all petroleum products ( petrol, kerosene, naphtha, etc.),
Chemicals, Paints, Oils, Grease, Lubricants, fats, etc.. Absorbing oxygen by creating
smothering atmosphere round the flames is the method used to extinguish the fire under
this class.
Class C Fire
Fire involving flammable gases under pressure including liquefied gases, where it is
necessary to inhibit the gas at fast rate with an inert gas powder or vaporizing liquid
for extinguishments.
Class D Fire
Fire involving evolution of heat by chemical reaction in combustible materials such as
magnesium, aluminium, zinc, sodium, potassium when the burning metals are reactive to
water containing agents; and in certain cases, carbon dioxide, halogenated hydrocarbons
and ordinary dry powders. Special media and techniques are required for controlling
the fire.
Class E Fire
Fires involved in electrical equipment due to short circuit or over loading. Fire fighting
medium has to be non-conductive and non-magnetic. ( BIS does not include Class E Fire,
but makes a mention of this type of fire)
1.3 EFFECT OF TEMPERATURE ON BUILDING MATERIALS
The structural behaviour of a building subjected to fire depends primarily on the
variations developed in the properties of individual materials which are exposed to fire.
Hence, for the purpose of planning a building with regard to fire safety, it is very
essential to understand the various properties of building materials under elevated
temperature.
1.3.1 Material Properties
The material properties which are of importance when a structure is exposed to fire, are
listed below.
1. Thermal Expansion
2. Thermal Conductivity, Thermal Capacity and Thermal Diffusivity
3. Modulus of Elasticity
4. Poisson’s Ratio
5. Stress-Strain Relationship
6. Creep Deformation and
7. Strength
The significance of these properties are discussed in the following section.
1.3.1.1 Thermal expansion
Thermal expansion of materials is one of the important properties as far as fire safety is
concerned. In structures, members restrained from expansion by the surrounding
elements or the development of differential expansion of different materials leads to the
failure of members in a structure, which may ultimately lead to even its collapse.
In general, the coefficient of linear expansion of building materials increases with
temperature. It is represented by . The normal range of in case of different building
materials is presented in Table 1.1
From the table 1.1, it can be noted that the coefficient of thermal expansion of concrete
and steel is almost the same. This is the success of concrete as a surrounding medium for
steel in fire protection works. Further, due to the same reason, in case of reinforced
concrete members, the bond between steel and concrete will be effective even at elevated
temperatures and there by the integral action of concrete and steel will not be lost for a
considerable time in the event of a fire.
Table 1.1 Coefficient of Thermal Expansion of Materials
Sl.
No.
Material
Coeff. of Linear
Expansion,
x 10
-6
/ C
1
Brick
3 – 9
2
Granite
6 – 9
3
Masonry
4 – 7
4
Cement and Concrete
10 – 14
5
Mild Steel
10 – 13
6
Wood – Along the Grain
2.5 – 9.5
7
Wood – Across the Grain
32 – 61
1.3.1.2. Thermal conductivity , Thermal capacity and Thermal diffusivity
The behaviour of building as well as the severity of fire are affected by the following
properties of the materials.
A) Thermal Conductivity
Thermal conductivity is numerically equal to the quantity of heat that flows in one
second through a cube of unit size and when the opposite faces of the cubes is
maintained at a temperature difference of 1 C. This is generally represented as k
and its unit is J/ m / K s.
B) Thermal Capacity and Specific Heat
Thermal capacity of a substance is defined as the heat required to raise the
temperature of whole mass of a substance through 1C. On the other hand,
specific heat is defined as the heat required to raise the temperature of a unit mass
of a substance through 1C. Specific heat is generally represented by the symbol
c and its unit is calorie (cal)
C) Thermal Diffusivity
Since there are practical difficulty in correctly measuring thermal conductivity
and specific heat of a material, a term called thermal diffusivity has been derived
and this can be obtained by dividing the thermal conductivity by the volumetric
thermal capacity.
Thermal diffusivity ,
= k / (
c)
The unit of thermal diffusivity is m
2
/ s
The measurement of diffusivity consists essentially of determining the relation
between time and the temperature differential between the interior and the surface
of a specimen initially at a constant temperature when a change in temperature is
introduced at the surface. This basic approach is used in determining the Integrity
of structural elements in a fire resistance test.
It is to be noted that, if materials have high thermal conductivity, then the temperature of
the structural element will rise faster. Similarly, if the thermal capacity of the materials is
low, then the materials will get hotter more quickly for a given heating condition.
Further, in case of common building materials ( concrete, steel , masonry, wood, etc.), it
is observed that the thermal conductivity is proportional to the density.
That is, k
This means that, for a given material, if the density is less then the thermal conductivity is
also less. This may be due to the fact that there will be voids within the materials and that
the air in the voids is a bad conductor.
The thermal diffusivity is an important factor which gives an indication of how fast the
structure will heat up in the event of a fire. That means, it represents the rate at which
temperature changes can take place within a mass.
The approximate values of density, thermal conductivity, specific heat, and thermal
diffusivity in case of commonly used building materials are given in table 1.2
Table 1.2 Thermal Conductivity, Specific Heat, Density and Thermal Diffusivity of
Commonly Used Building Materials
Sl.
No.
Material
Density
()
Thermal
Conductivity
( k )
Thermal
Diffusivity
=k / ( c)
kg/ m
3
W/ m / K
(x 10
-7
)
m
2
/s
1
Brick Work
1000
2600
0.24
0.86
2.8
3.9
2
Concrete
800
2400
0.23
1.83
3.0
7.9
3
Mild Steel
7800
41
105
4
Wood –Along
the grain
Soft wood
Hard wood
610
770
0.215
0.290
2.8
3.0
5
Wood–Across
the grain
Soft wood
Hard wood
610
770
0.125
0.160
1.6
1.7
It can be observed that the commonly used building materials fall under two extreme
conditions as far as thermal diffusivity is concerned. On one side, some of the materials
are good Insulators such as Brick, Concrete, Wood, etc. On the other side, some other
materials are good Conductors such as steel, (metals).
If a building material is a good insulator, then heat from the fire will not penetrate deep
into the structure. On the other hand, if it is a good conductor, a uniform temperature will
be attained quickly throughout the body of the material and as a result, temperature of the
material will be less than the temperature of the fire.
1.3.1.3 Modulus of elasticity
The variation in the modulus of elasticity of a material with temperature will influence
the deformation characteristics of the structure. This is particularly important in flexural
members exposed to fire.
1.3.1.4 Poisson’s ratio
Poisson, ratio is the ratio of lateral strain to the longitudinal strain. This ratio has
influence in the volumetric change of a material when subjected to stress.
1.3.1.5 Stress-strain relationship
The deformation of a structural members is influenced by the stress-strain relation ship
of its material.
1.3.1.6 Creep deformation
Creep deformation is a time dependent deformation under sustained load effect. For a
member exposed to fire, short term creep deformation is the one which influences its
behaviour.
1.3.1.7 Strength
Strength of materials at elevated temperature is one of the important parameter in the
design of structures against fire safety.
1.4 EFFECT OF TEMPERATURE ON CONCRETE
Concrete is the most widely used material in construction. It is a mixture of different
materials such as cement, coarse aggregate, fine aggregates and water. Each material
itself can be of different kinds as explained below:
Cement - OPC, PPC, etc.
Coarse Aggregate - Siliceous (Sandstone, Flint, Gravel) ;
Carbonates (Lime stone, igneous rocks, crushed
bricks, blast furnace slag , dolomite) , etc.
Fine Aggregate - Natural river Sand, Crushed Aggregate, etc.
In general, the use of concrete in construction can be grouped in to two, viz.
1. Used as structural concrete- Used to construct structural members such as
Reinforced Cement Concrete ( RCC) and Pre-Stressed Concrete (PSC)
members and
2. Used as a protection material to structural steel against fire – Primarily Plain
Cement Concrete ( PCC)
As concrete is made of different materials, its behaviour with temperature depends on
several factors and as such a general remark can only be made with respect to the various
properties.
1.4.1 Thermal Expansion of Concrete
Thermal expansion of concrete is influenced by different parameters such as
- the type of cement
- type of aggregate
- water content at the time of temperature change and
- age of concrete
In general, concrete has a positive coefficient of thermal expansion. That means, the
thermal expansion increases with increase in temperature. However, the rate of increase
is not in a linear fashion.
The thermal expansion reduces with an increase in stress level of concrete. That is, if a
concrete, which is subjected to some stress due to loads, is exposed to fire, its thermal
expansion is less when compared to a concrete, which is not subjected to any stress. As
the stress level increases, the thermal expansion also decreases.
1.4.2 Thermal conductivity, Thermal diffusivity and Specific Heat of Concrete
Thermal conductivity of concrete decreases with increase in the temperature. However,
during subsequent cooling, the change is not reversible.
Light weight concrete, particularly with crushed brick and blast furnace slag as coarse
aggregate has low thermal conductivity. Such concrete stands up better to fire than
ordinary concrete. So, for concrete as a protective material, this type of concrete is better
than ordinary concrete.
Use of Dolomite gravel leads to a very good fire resistant concrete due to the reason that
the calcinations of dolomite is endothermic and as a result, heat is absorbed and further
temperature rise is delayed in concrete.
The thermal capacity or Specific heat increases slowly with increase in temperature.
The type of aggregate has only a small influence on this factor.
The thermal diffusivity decreases with increase in temperature.
1.4.3 Compressive Strength of Concrete
Compressive strength of concrete depends on the relative proportion of individual
materials present in the concrete. Hence the influence of temperature on the strength of
concrete depends on the strength behaviour of various materials present in concrete as
well. However, in general, concrete loses strength with increase in temperature.
It is interesting to note that up to about 200 C, the reduction in the strength of concrete is
nominal. On the other hand, when temperature reaches at 500 C and above, concrete
starts deteriorating its strength rapidly. Table 1.3 shows the variation of compressive
strength with temperature for concrete made of different types of aggregate.
With the development of temperature in concrete, the ferric oxide present in aggregate
becomes dehydrated or get reacted with lime present in cement paste. This results a color
change in the concrete and the process is irreversible. The colour change occurs at a fairly
well defined temperature as indicated below.
up to about 300 C - There is no change in the colour of the concrete
( normal colour is gray )
at 300 C - Colour changes to Pink, Red or Reddish Brown
at 650 C - Pink or Red fades, reverts to almost normal gray; some
coarse aggregate may remain red; concrete becomes
porous and friable.
at 1000 C - colour changes to dull yellow ( Buff Colour)
Hence, it is possible to assess the temperature reached by various parts of a concrete
structure by observing the color change.
Table 1.3 : Compressive Strength of Concrete at High Temperature
Tests have shown that even with a 4 hour fire duration ( Severe fire), the portion of
concrete affected by the fire is only for a depth of about 150 mm from the heated surface.
In practice, even in the case of a severe fire, temperature at which the concrete loses its
strength is rarely attained by concrete throughout its whole mass. As a result, concrete
often maintains its integrity and strength even after being attacked by severe fire.
So, in order to protect the steel form fire, BIS specify minimum thickness of concrete
cover to the steel reinforcement ( IS 456 : 2000 and NBC-Part IV- Fire Protection )
1.4.4 Modulus of Elasticity
Modulus of elasticity of concrete reduces rapidly with increase in temperature. Typical
curve showing the influence of temperature on the Young’s modulus is shown in fig. 1.3.
From this figure, it can be seen that,
At 200 C - Young’s modulus is about 70 – 80 % of that at room temperature
At 400C - Young’s modulus is about 40 – 50 % of that at room temperature
If all other parameters are equal, the deflection of a flexural member is inversely
proportional to the Young’s modulus. Hence it can be stated that, the deflection of a
concrete member will be doubled if its temperature reaches about 400 C .
1.4.5 Poisson’s Ratio
With the available data, a generalized behaviour of Poisson’s ratio with temperature is
not possible in case of concrete. In some cases, it was observed that Poisson’s ratio
decreases with increase in temperature.
Temperature
(°C)
Compressive strength of concrete as % of initial strength with different
types of aggregate
Siliceous Aggregate
Carbonate Aggregate
Light Weight
Aggregate
Stressed
Un-
stressed
Stressed
Un-
stressed
Stressed
Un-
stressed
200
99
92
100
97
98
94
400
95
85
87
81
96
91
600
85
77
56
34
86
74
800
62
40
20
48
42
1.4.6 Stress Strain Relationship
Figure 1.4 shows the stress-strain relationship of concrete with temperature. It can be
observed that as temperature increases, the ultimate strength of concrete decreases and
the ultimate strain increases.
1.4.7 Creep
As the duration of fire is short, the short term creep characteristics are important with
regard to materials subjected to fire. In case of concrete, experiments have proved that
creep plays a very limited role in the overall behaviour of concrete except when the
temperature is above 400 C . For temperature above 400 C, creep deformation will be
more and has influence on the structural behaviour of the member.
1.4.8 Spalling of Concrete
The most common form of damage to concrete when subjected to fire is spalling of
concrete. That is, the breaking off of pieces or layer of concrete. It has been suggested
that spalling of concrete due to temperature rise can occur in a member due to one or
more of the following reasons.
1. Due to excessive compression or restraint of the materials
2. Due to the formation of high pressure steam in the material
3. Splitting of the aggregate used in the concrete mix.
Case 1 : Due to differential thermal expansion, excessive compression or restraints
occurs in concrete. If steel is present, it will restrain concrete from expanding and as
concrete is weak in tension, a concrete layer will spall off at the level of steel.
This kind of spalling can occur to the concrete cover provided to structural members such
as slabs, beams, columns, etc.
Case 2 : Concrete by nature is porous and depending on the various parameters like
water-cement ratio, aggregate gradation, compaction, etc., there can be about 2 – 5 %
voids by volume in concrete. These voids are called capillary pores. The capillary pores
are distributed in random throughout the mass of concrete and in general, they are
interconnected . Depending on the environmental conditions such as temperature,
humidity, etc. to which the member is exposed, these voids may be fully or partially filled
with water and such water is called capillary water ( or free water in concrete)
When a concrete surface is exposed to fire, the water present in the capillaries near the
surface will get converted into steam and thereby develops pressure in the capillary pores.
This pressure will cause the pore water adjacent to the heated surface to migrate inwards,
until a stage is reached when part of the concrete has so much water in it that it can no
longer move quickly enough through the pores to relieve the steam pressure developed.
As a result, the steam pressure will build up until the forces are great enough to cause
lateral fracture of concrete and thereby spalling occurs.
The quantity of pore water present in concrete at the time of fire is the most important
factor with respect to the spalling due to steam pressure.
Case 3 : The thermal expansion and other thermal properties of aggregate used in
concrete is different from those of the cement paste. Hence, depending on the type of
aggregate used, thermal instability may occur and thereby splitting of aggregate occur.
It has been observed that Carbonate aggregates are less prone to splitting. Igneous rocks,
lime stone, crushed bricks, blast furnace slag, dolomite are some of the carbonate
aggregates used in concrete.
Silitious aggregates are prone to aggregate splitting. Flint, Gravel, and Sand stone are the
type of aggregates under this classification.
Light weight aggregates ( Foamed slag, Crushed bricks, etc. ) have the least chance to
undergo aggregate splitting.
As the strength of cement increases, likelihood of aggregate splitting also increases.
Large sized aggregate may also lead to aggregate splitting when compared with small
sized aggregates
The steel in concrete, whether it is reinforcing, prestressing or structural steel, may get
exposed due to spalling of concrete. Once such steel is exposed to fire, the fire resisting
property of the whole structure will be suddenly at risk. Hence spalling of concrete is to
be prevented or controlled.
The spalling of concrete can be prevented by
proper selection of concrete mixes and its constituents – Need for a design
concrete mix.
specifying proper cover to the steel - Depending on the fire resistance rating
required, minimum cover to the reinforcement is specified by Bureau of
Indian standards
providing a secondary layer of reinforcement with wire mesh between the
steel and surface of concrete, so that the even if concrete get cracked, it will
not spall off and thereby direct exposure of steel to the fire is prevented ( IS
456 : 2000 and NBC-Part IV- Fire Protection )
1.5 EFFECT OF TEMPERATURE ON STEEL
Steel can be used in two different ways in building construction
1. As structural steel
- steel beams, columns, etc.
- They are normally made of Mild Steel and rarely with Alloy Steel
2. As reinforcement in concrete
- In reinforced cement concrete (RCC) construction. They are normally
made of Mild Steel and rarely with Alloy Steel
- In prestressed concrete (PSC) construction. They are made of High
tensile steel. Depending on the type of process, these steels are further
classified as i) Heat treated; ii) Cold drawn and iii) As drawn ( Alloy
Steel which is self hardening).
The changes in the properties and behaviour of these steel when exposed to fire are
different. Hence general comments can only be made with respect to these steel.
1.5.1 Thermal Expansion
The coefficient of thermal expansion for steel increases as temperature increases.
However, the variation is not linear.
1.5.2 Thermal Conductivity and Thermal Capacity
The thermal conductivity of steel, in general, decreases with increase in temperature up to
about 800 C and then after, it slightly increases. Steel has low thermal capacity. This
means that
A uniform temperature will be achieved very quickly
Even in a small fire, a temperature of 800 C to 1000 C will be reached
quickly.
The rate at which temperature of structural steel rises when exposed to a fire depends on
the mass of steel and on the surface area exposed to heating. If the ratio of surface area to
the mass of a structural steel member is more, the temperature rise will also be at a faster
rate.
1.5.3 Young’s Modulus
Figure 1.5 shows the variation of Young’s modulus of steel with the rise in temperature.
In general, it can be stated that, the modulus of elasticity decreases with increase in
temperature. This variation is linear up to about 500 C for mild steel as well as hot rolled
alloy steel.
The rate of decrease of young’s modulus is more in case of cold drawn steel when
compared with hot rolled bars. The variation is as much as 20 %.
Irreversible deformations of elements subjected to bending and loss of stability of axially
loaded structural members may occur due to a decrease in the modulus of elasticity.
1.5.4 Creep
Creep due to a rise in temperature – called thermal creep – may be critical in case of
steel. Tests have shown that although high temperatures are reached only over a short
period during a fire, they still cause an increase in plastic deformation, which is
irreversible. When steel is subjected to heating, the deformation due to thermal creep is
considerably high when compared with that due to the change in the modulus of
elasticity.
In case of prestressing steel ( high tensile steel) the thermal creep deformation is so
critical that the prestressing is completely lost at a temperature of about 250 – 300 C. So,
in the event of a fire, a prestressed concrete structural member may become an ordinary
reinforced concrete member with irreversible deformations, which cannot be used further.
1.5.5 Stress- Strain Relationship
The influence of temperature on the stress-strain relationship of steel is presented in fig.
1.6 and 1.7. Figure 1.6 shows the behaviour of reinforcing steel ( mild steel),where as the
behaviour of prestressing steel ( high tensile steel) is shown in fig 1.7 . In general, it can
be stated that, with an increase in temperature, the material becomes non-linear at an
early stress level. The yield point of reinforcing steel gets reduced with an increase in
temperature.
1.5.6 Strength
When exposed to fire, cold drawn and heat treated steel lose their strength more rapidly
when compared to mild steel or alloy steel.
On cooling,
Cold drawn and heat treated steel lose part of their strength permanently if they
were heated to above 300 – 400 C prior to cooling.
Mild steel and as drawn wires regain almost all their original strength.
The variation of the ultimate strength of different types of steel at higher temperature is
presented in table 1.4 as well as in figure 1.8
Table 1.4 Variation of Ultimate strength of steel with temperature.
Temperature
( C )
Ultimate strength in terms of % age strength
at room temp. (20C)
Mild
Steel
High
Strength
Alloy Steel
Cold Drawn /
Heat treated
Prestressing
Steel
20
100
200
300
400
500
600
700
100
102
115
112
82
55
30
20
100
98
102
97
82
60
38
20
100
97
94
80
55
34
16
8
From table 1.4, the following conclusions can be made.
Up to about 300 C There is no loss in strength for mild steel as well as high
strength alloy steel. In fact, the strength increases slightly
in this range.
Cold drawn and heat treated wires lose their strength with
temperature rise and at 300 C, its strength is reduced by
20 %
The temperature at which the failure occurs ( Ultimate strength) at 50 % of the ultimate
strength corresponding to room temperature ( 20 C ) is generally considered as the
critical temperature. Now, for steel, in general, failure is likely if the temperature crosses
the critical temperature. From the table 1.4, it can be observed that the critical
temperature is different for different types of steel. The commonly accepted value of
critical temperature for different types of steel is presented below:
Type of Steel Critical Temperature ( C )
Mild Steel and Alloy Steel 550
Cold Drawn and Heat treated Steel 400
1.5.7 Concluding Comments
The temperature in steel will cross the critical temperature even in a small fire, if they are
directly exposed to fire. Hence structural steel must be protected from the effects of heat.
There are different protection methods such as
i) Cladding Most common method
Encase the steel member by thermal insulating materials viz.
cement concrete, plaster, paint, mineral fiber, etc.
ii) Placing screens between the element and the fire
False ceilings, partitions, etc.
iii) Cooling the element with water - Less common
iv) Intumescent Coatings - Less common
Materials which can be painted on the surface of steel and which
gets converted into high heat resisting foams on application of high
heat intensities are called intumesent paints.
1.6 EFFECT OF TEMPERATURE ON BURNT CLAY BRICKS
Masonry is used for the construction of walls, columns, piers, etc. in buildings. In
general, a masonry structure can be either Load bearing or Non-load bearing structure.
Behaviour of masonry structures with respect to temperature depends on the type of
material used for making the individual masonry units as well as the type of bonding
material used for construction.
The individual masonry units can be of different types such as
Burnt Clay Bricks ( Solid)
Burnt Clay Hollow Blocks
Stones ( different types)
Sand-Lime bricks
Lime based Blocks
Concrete Blocks ( Solid and Hollow)
Gypsum Partition Blocks
Autoclaved Cellular Concrete Blocks
In general, masonry construction behaves well in fire conditions possibly because of the
fact that the numerous joints present in the masonry structure prevent thermal stresses
from building up over a large area.
A masonry wall, when exposed to fire on one side will deflect towards the fire due to the
greater expansion of the surface layers on the side of the fire. The failure of such walls
will be due to the excessive distortion caused by fire ( Stability failure ) rather than the
material failure.
Similarly, when hollow block clay or concrete blocks are used in floor construction, the
floor will deflect towards the source of heat and the ultimate failure of such floors will be
primarily due to this deflection.
Clay is the commonly used material in the manufacture of bricks. In general, compressive
strength of burnt clay bricks and blocks at elevated temperature follows a similar pattern
to that of concrete.
Variation in strength
Up to 400 C There is only slight deterioration in the compressive strength
At 500 C The strength starts to decrease
At and above 600 C The rate of decrease of strength becomes rapid.
Behaviour of solid bricks are better than the perforated or cavity (hollow ) bricks with an
equivalent thickness ( That is with the same over all ) thickness. The improvement
expected in thermal insulation due to air cavities in perforated or cavity bricks is
insufficient to compensate for the loss of solid materials
In case of masonry with burnt clay bricks exposed to fire, the development of residual
stresses may not occur. This is due to the fact that during the manufacturing process
itself, bricks are subjected to high temperature and later they are cooled down at a slow
rate. As a result, residual stresses developed, if any, would have been released during the
cooling process and development of residual stresses may not occur when they are
subjected a temperature rise again.
1.7 EFFECT OF TEMPERATURE ON WOOD
Timber is one of the oldest structural materials used in buildings. Timber finds its
application in building construction as
Structural elements
Interior finishing material
External Claddings
Construction of different compartments of buildings such as doors,
windows, etc.
In general, timber can be classified in to two viz.
Hard Wood - Density 400 kg / m
3
Soft Wood - Density 400 kg / m
3
Wood has many advantages as a building material such as
- high strength to weight ratio
- Low Thermal Expansion
- Low Thermal Conductivity
- Reasonable level of Fire Resistance
The behaviour of wood when exposed to fire can be summarized as follows:
Over 100 C - Wood will become over dry, discoloured, distorted and will loose
weight
Above 150 C - Wood can be easily ignited
The ignition temperature of wood depends on the type of species, its density, intensity of
heat radiation, etc. and the normal ignition temperature ranges from 275 C to 300 C.
Wood burns at a fairly constant rate from its ignition temperature. However, the rate of
combustion will be less for a wood with higher density. Further, the combustion rate
depends on the heat radiation intensity and the type of species. The variation of rate of
burning with density of wood is as presented below:
Density ( kg/ m
3
) Rate of Burning ( mm / Minutes)
400 – 500 0.60 – 1.10
600 – 800 0.30 – 0.60
When wood burns, it forms a layer of charcoal on the burnt surface, which helps to
insulate and protect the un-burnt wood below the charred zone. Due to the low thermal
expansion, the char layer stays in place even with continued heating. Further, due to the
low thermal conductivity the undamaged timber below the char retains its strength.
Flame retarding treatment of wood has no effect on the rate of combustion. Flame
retarding treatment only delays the appearance of flames at the wood’s surface and it
reduces the speed at which the flame propagates.
The normal practice in designing structural members with wood is to provide extra cross
section for wood when compared with the structural requirement. The additional quantity
required is arrived at, based on the rate of combustion and the required duration of fire
resistance, so that, the portion of wood as per structural requirement will be available
even at the end of the required duration of fire resistance.
Example 1
A wooden beam requires a cross section of 200 mm x 300 mm to resist the dead load and
live load coming over it. Determine the cross section required, if it has to resist a fire of 4
h duration. Assume the rate of combustion as 0.3 mm / min
Thickness required to resist 4 h duration of fire = 0.30 x 4 x 60
= 72 mm
So, required size of beam
- Depth = 300 + 2x 72 = 444 mm
- Width = 200 + 2x 72 = 344 mm
So, the minimum size of beam required is 344 mm x 444 mm.
1.8 BEHAVIOUR OF PLASTICS ON FIRE
Plastic covers a wide range of materials. The use of plastic is increasing for various
structural and non-structural purposes in building construction such as in walls,
partitions, ceilings, floorings, roofing etc. Plastics consists of three basic groups of
materials
Resins – They act as binders and they can be of synthetic, artificial or natural
Fillers – Can be of organic or mineral origin and may be in solid, liquid or
gaseous state.
Plasticizers
A wide range of plastics can be made out by the appropriate combination of the above.
Behaviour of these plastics at high temperature depends on their physical, mechanical
and chemical properties.
According to the thermal stability of resins, plastics can be grouped in to two, namely
Thermoplastic and Thermosetting plastics.
Thermoplastic materials will lose their mechanical strength and their physical shape very
quickly when heat is applied. When cooled, they will regain their mechanical strength.
Most of the plastics under this group get softened at temperature even below 100 C and
in a fire, they will burn as a molten material.
Thermosetting plastics, upon heating will get converted to an insoluble and infusible
state, which is irreversible. They retain their mechanical strength and their physical shape
in a fire until decomposition. Most of the plastics under this group have the maximum
temperature of destruction in the range of 250-400
o
C.
All plastics are to some degree combustible. The fire retardant treatment does not make
the particular plastic non-combustible. Fire retardant plastics will only make the plastic
less easily ignited and when exposed to a fully developed fire they will burn readily. The
physical form of plastics is important when its fire potential is considered. Plastics with
larger exposed surface area such as foam plastic will burn more rapidly than the same
material in a solid or sheet form. Plastics have greater thermal expansion than steel.
Laminated plastics behaves like light metals. Thermoplastics without filler have very
high coefficient of linear expansion
In general, polymer materials possess comparatively low thermal stability. Maximum
temperature of softening and decomposition is about 300-400
o
C only. Fire resistance of
such materials is only for a few minutes. Almost all plastic materials break down under
fire to form undesirable and toxic gases. Smoke impairs visibility and thereby affects the
escape of the occupants of a building.
The common hazard due to plastic in buildings are due to the use of plastic
- as the internal lining of the wall or ceiling or
- as a thermal insulating material in the form of plastic foam
in both cases, there is a danger of rapid fire spread.
The classification of plastics and their possible hazard is described in table 1.5
Table 1.5 : Classification of Plastics and the Possible Hazard
Group
Uses
Plastic
Possible
Hazard
Thermoplastics
High
density
Sheet,
mouldings,
extrusions
Acrylic, PVC,
Nylon
Flaming
droplet Hazard
Low
density
Flexible
foam, rigid
foam
Polyurethane
Polythene
Burns very
rapidly with
flaming droplet
hazard
Thermosetting
Plastics
High
Density
Sheet,
mouldings,
extrusions
Bakelite
Formica
Polyesters
Burns as a solid
Low
Density
Flexible
foam, rigid
foam
Area
formaldehyde
Polyster
Burns rapidly
as a Solid
1.9 BEHAVIOUR OF GLASS ON FIRE EXPOSURE
Depending on the purpose, various forms and types of glass are being used in
construction. There are several types of glass treatment and some of the common types
and their manufacturing process are as follows
Float Glass – All modern manufactured glass is float glass. Molten glass “floats”
on top of a pool of molten metal, usually tin, and the liquid tin and the liquid glass
form perfectly flat surfaces. The glass is drawn off one end of the molten tin, and
gradually cools and hardens into a glass sheet.
Annealed Glass – This type is most common. Annealing is part of the normal
glass manufacturing process. Glass is cooled gradually under controlled
conditions to remove undesirable stresses and to spread the minor residual stress
evenly throughout the cooled glass. This minimizes the tendency for spontaneous
breakage.
Heat-Treated Glass – Clear and tinted annealed glass can be heat treated to
increase strength and resistance to thermal stress. Not all coatings applied to glass
are suitable for subsequent heat treatment. Here, glass is heated almost to its
softening temperature, and cooled quickly to lock in compressive stresses. The
heated glass in the centre shrinks as it cools, putting the outer surfaces of the glass
into compressive stress. Depending on the rate of cooling, glasses are classifies as
tempered or heat strengthened glasses. If the outer surfaces of glass are cooled
very quickly to retain very high compressive stress, the result is tempered glass;
otherwise, the result is heat-strengthened glass. Tempered glass (and to a lesser
extent heat-strengthened glass) can resist higher impact loads, wind loads, and
temperature changes than ordinary annealed glass. Fully tempered glass is
approximately four times as strong as annealed glass of equal thickness. Tempered
glass tends to break into small cubical pieces.
Laminated Glass – Laminated glass can be fabricated by bonding two or more
glass panes with a transparent, flexible interlayment material. When broken,
laminated glass tends to remain in place with glass particles adhered to
interlayment.
Wired Glass - Wired glass is formed by rolling and has an embedded wire mesh
to prevent shattering and to withstand fire exposure. It is accepted as safety glazing
in fire-rated doors and windows. Woven stainless steel wire is used in diamond or
square mesh pattern.
As far as fire safety is concerned, window/door glazing systems are important, as
breaking of these glasses may influence the fire severity in ventilation controlled fires.
The breaking of glass when exposed to fire depends on the properties such as the glass
thickness, thermal conductivity, thermal diffusivity, Young’s modulus, fracture stress,
shading depth, thermal expansion coefficient and the distance to the flame.
When a window pane of ordinary float glass is first heated, it tends to crack when the
glass reaches a temperature of about 150 - 200ºC.
The temperature differences between the exposed glass surface and the glass shielded by
the edge mounting play the dominant role in controlling cracking. A temperature
difference of about 80°C between the heated glass temperature and the edge temperature
is needed to initiate cracking. This is dependent on the thermal and mechanical properties
for glass and may vary.
The first crack initiates from one of the edges. At that point, there is a crack running
through the pane of glass, but there is no effect on the ventilation available to the fire. For
the air flows to be affected, the glass must not only crack, but a large piece or pieces must
fall out.
Tempered glass shatters upon initial cracking, but the initial cracking does not occur until
the glass reaches rather high temperatures. An exposed-surface temperature of 290-380ºC
has been found to be needed, with the unexposed surface temperatures being about 100ºC
lower. Plain" glass was found to "break" when the exposed side reached 150-175ºC, with
the unexposed side being at 75-150ºC.
Heat-strengthened and tempered glass (unspecified thickness) was found not to break at
an irradiance of 43 kW m
-2
. The latter heat flux corresponded to 350ºC on the exposed
face and 300ºC on the unexposed face.
The oldest category of the latter is wire glass. Nowadays, several types of patented fire-
resistive glasses also exist which are not wired glass. These are usually multi-layered
structures, generally involving some polymeric inner layers.
Fire-resistive glasses will normally be accompanied by a laboratory report of the
endurance period. Such glasses can be assumed to have no ventilation flow until after
their failure time.
It is, very difficult to predict when glass will actually break enough to fall out in a real
fire. 300°C appears to be a reasonable lower bound.
3 mm window glass will break around 340°C. For thicker, 4-6 mm glass, the mean
temperature of breakage would appear to be around 450°C. Double-glazed windows
using 6 mm glass can be expected to break out at about 600ºC. Tempered-glass in not
likely to break out until after room flashover has been reached.
Factors such as window size, frame type, glass thickness, glass defects, and vertical
temperature gradient may all be expected to have an effect on glass fall-out. Over-
pressure due to gas explosions is an obvious glass failure mechanism. Yet, normal fires
do show pressure variations and these could potentially affect the failure of glass panes.
Conventional glazing materials used to be the weak spots in a building, but nowadays
fire-resisting glass can help to limit the spread of fire. Wired glass is held in position by
the wire net for a limited time. Special borosilicate glass without wire net withstands this
temperature difference during the heating-up phase due to its low thermal expansion
coefficient, 1/3 of normal glass, and an additional heat-strengthening process.
Borosilicate glass also shows higher viscosity in comparison to float glass and wired
glass, respectively, and, therefore, provides a long fire-rating time at temperatures above
1000°C. Using special steel frames which can keep the melting glass within the frame,
borosilicate glass is able to provide an integrity and stability up to 2h. In case heat
insulation is required, appropriate glass systems must be chosen. Two different systems
are available: designs consisting of multiple layers of glass and intumescent interlayers
and such using an aqueous gel in between two tempered glass panes. Some of the
international certificates are shown as examples, followed by typical designs of
construction elements. Typical applications of the non-wired glass types are shown in
slides taken from fire doors, -screen and -windows providing fire ratings from 30 min in
timber frames up to 2h in special steel frame
1.10 BEHAVIOUR OF TEXTILE AND OTHER HOUSE HOLD MATERIAL
ON FIRE EXPOSURE
Behaviour of textile fibers when exposed to fire depends on the type of fibers used for
making fabric. Fibers can be divided in to three groups
- fibers which have a resistance to fire
- fibers which melt away due to fire
- fibers which burn readily
Many furnishing materials are made by blending more than one type of fibers and their
behaviour when ignited will be different from the individual fiber type. Materials with
light weight structure and loose woven structure will tend to be more flammable than
heavy structures.
Wool is normally considered as a fire resistant fabric. However, light weight wool fabric
will burn more rapidly.
Fabrics with PVC blend, when exposed to fire, behaves differently based on the quantity
of fire resistant materials added while manufacturing it. PVC, Polyester, Polyamide, etc.
will melt and causes flaming droplet hazard.
Cellulose based materials such as cotton, linen, viscose, etc. are readily ignitable and
burns rapidly. By proper chemical treatment, fabric made out of the above cam be
considered as fire resistant.
1.11. DETERMINATION OF COMBUSTIBILTY OF BUILDING MATERIALS
The combustibility of materials can be determined using various testing methods such as
Fire tube method
Calorimetric method
Non-Combustibility Test as per BIS ( IS : 3808)
Non-Combustibility Test as per BS ( BS : 476 )
Based on the combustibility of materials, the Bureau of Indian standards ( BIS ) classifies
materials in to two groups viz.
1) Combustible materials and
2) Non-Combustible materials
In this case, a material is said to be combustible, if it burns or adds heat to a fire when
tested for non-combustibility.
1.11.1 Fire Tube Method
Fire tube method is a simple method of determining the combustibility of building
material..
Experimental Setup
Specimen dimension - 10 mm x 10 mm x 150 mm
Fire tube - Metal cylinder of length 165 mm and diameter 50 mm
Burner flame - Flame length of 40 mm and temperature of 1000–1100C
Procedure
1. Weigh the specimen before the test ( A)
2. Place the specimen in the fire tube
3. Ignite the specimen for a period of one minute
4. Observe the independent burning or glowing of the specimen, if any, and its
duration.
5. Weigh the specimen after the ignition ( B)
6. Determine the percentage weight loss , C = (A-B) 100 / A
Now, the specimen is considered to be low combustible (Low combustible materials are
called fire resistant materials – they are actually combustible materials ), if its weight
loss is less than 20 % of its initial weight and the time of independent burning or glowing
does not exceed thirty seconds.
It is to be noted that, the BIS classifies materials into either as Combustible materials or
as Non-Combustible materials.
1.11.2 Non-Combustibility Test ( BIS Method)
The various aspects of Non-combustibility test has been described in IS 3808 – 1996
“Method of Test for Non-combustibility of Building Materials” and some of the salient
points are discussed here.
The non-combustibility test apparatus consists essentially of a furnace with a stand and
an air flow shields; conical air flow stabilizer; draft shield, etc.
Procedure
1. Prepare a representative specimen of cylindrical shape. The cylinder is to be
of 50 mm long with a diameter of 45 mm ( volume is to be 80 5 cm
3
)
2. The specimen shall be conditioned by keeping it in a ventilated oven at 60
5 C for at least 20 h and shall be cooled to ambient temperature.
3. Heat the furnace and maintain the temperature of the furnace at 800 to 850 C
in a standard manner.
4. Insert the specimen with the help of specimen holder and keep the specimen
in the furnace for 20 minutes. The time for heating is counted from the
moment the specimen is inserted in to the furnace.
5. Obtain the temperature rise in the specimen in a standard manner( from the
thermocouple readings) and the maximum duration of any sustained flaming
(for flames of duration more than 5 seconds ).
6. Estimate the mass loss at the specimen and if the average mass loss is not
more than 50 % of the average initial mass, then the material is not a
combustible material
1.12 FLAME SPREAD RATE OF SURFACE MATERIALS
Many factors influence fire spread within buildings, and one of the most important is the
speed with which interior finish materials spreads flame. Fires often develop slowly, but
when flammable interior linings are involved they tend to speed up the development so
that the flashover point is reached more rapidly. When fire breaks out in a building, the
nature of interior surfaces may determine the time available for the escape of occupants.
The surfaces of escape routes, corridors, and stairways require special attention, as they
are part of evacuation route.
Surface finishes shall be such that
- the flame spread rating is not more than the specified values and
- they do not generate toxic smoke / fumes.
IS 1277 (1989): Fire safety- Flame spread of products – Method for classification
classifies materials based on flame spread rating. Based on the rate of spread of fire,
surfacing materials are classified in to 4 classes and their details are given in table 1.6
Table 1.6 : Classification Surface Finishes based on Flame Spread rating
Class
Description
Effective Spread of Fire
Typical use
Class 1
Surfaces of
Very Low
Flame Spread
≤ 19 cm
May be used in any
situation
Class 2
Surfaces of
Low Flame
Spread
≤ 30 cm in first 1 ½ minutes and
≤ 60 cm
May be used in any
situation except on walls,
façade of buildings,
staircases and corridors.
Class 3
Surfaces of
Medium
Flame Spread
≤ 30 cm in first 1 ½ minutes and
≤ 85 cm during first 10 minutes
May be used only in living
rooms and bed rooms (not
in rooms on the roof) and
only as lining to solid
walls and partitions.
Class 4
Surfaces of
Rapid Flame
Spread
> 30 cm in first 1 ½ minutes or
> 85 cm during first 10 minutes
Shall not be used in
kitchens
1.13 FACTORS INFLUENCING FIRE SEVERITY
When a fire breaks out in a building, the behaviour of various structural elements and
their effects on the building depends on the fire severity. The fire severity can be defined
as the condition of a fire (compartment fire), which is related to the maximum
temperature reached and to the duration of burning. There are several factors that
influence the fire behaviour and thus control fire severity. The various factors that
influences the fire severity can be brought under two groups, namely,
The factors concerning to the combustible contents in the building
The Nature, Amount and Arrangement of fuel are the important factors
brought under the above head
The factors concerning to the type of building
The Size and Shape of Room, Area and Shape of Windows and the
Thermal Insulation of Walls and Ceilings are the important factors
under this heading.
It is to be noted that almost all these factors are inter related and makes the assessment of
fire severity a difficult one. However, the influence of each factor on the fire severity is
discussed in the following sections.
1.13.1 Nature of the fuel
The combustible materials found in buildings will vary greatly in nature and composition.
The important feature for the present discussion is to see how much heat will be released
on combustion and how quickly this release takes place. The total amount of heat
released depends on the nature of material and is referred to as the calorific value of the
material. However, different materials having the same weight and same calorific value
may behave differently in their properties like ease of ignition, speed of burning,
liberation of heat, etc. and create different hazardous condition. Based on the above
factors, materials are classified as No-Hazardous (NH), Hazardous (H) and Extra
Hazardous (EH).
Some process employ heat applied to materials under manufacture that may be
combustible or there may be considerable heat generated in the process of manufacture of
materials, which are in themselves liable to combustion. These materials are classified as
abnormal occupancy group. These materials are grouped into 9 groups as per IS: 1641 –
1960. So, H and EH materials are further grouped based on the abnormal occupancy
group and is listed in table 1.7 wherein, typical examples in each group are also present.
1.13.2 Amount of Fuel or Fire Load
The amount of combustibles present in a building is called the fire load. Fire load is one
of the important factors in deciding how severe the fire will be if one does occur in a
building. Now, the fire load density gives an idea about the total amount of heat that can
be released from the compartment. It is a common practice to express the fire load
density in terms of wood equivalent. The calorific value of different materials in terms of
wood equivalent is presented in Part IV of N.B.C 1983 ( SP: 7). Table 1.8 shows the
calorific value of some of the materials in terms of wood equivalent.
Table 1.7 : Classification of Materials Based on Abnormal Occupancy Group
Group
Design –
ation No
Description of Materials
Example
1
Explosives
Blasting powder(EH), Cement (H),
Celluloid (H), Safety fuses (H), Coal
Dust(EH)
2
Compressed permanent liquefied
and dissolved gases
Butane(H), Chlorine (H), Ethane(H),
Hydrogen(H)
3
Substances which become
dangerous by interaction with
water or air
Aluminium Dust(EH), Calcium(EH),
Lime (un-slaked) (H), Potassium (EH),
Sodium (EH), Zinc Powder (EH)
4
All substances with flash point
below 65 C
Benzene (EH), French Polish(EH),
Gasoline(EH), Petrol(EH), Tar (H)
Corrosive substances
Aqua Regia (EH), Caustic Soda(H),
Hydrochloric Acid (H), Sulphuric Acid
5
(EH, H)
6
Poisonous Substances
Ammonium cyanide(EH), Mercuric
Chloride(H), Phosgene(H) Poisons(H)
7
Miscellaneous Substances
Fel t(Asphalted, Bituminous, Roofing,
Tarred) (H)
7A
Oxidizing Agents
Ammonium Nitrate(EH), Calcium
Chloride(EH),
7B
Substances Liable to Spontaneous
Combustion
China Grass(H), Copra(H), Fodder,
Dried(H),
7C
Readily Combustible Solids
Areca Nuts( complete with shell)(H),
Bamboo mats(H), Camphor(H),
Carbon(H)
8
Substances likely to spresd fire by
flowing from one point to another
Beeswax(H), Candles(H), Creosote(H),
Rubber(H), Wax of all kinds (H)
9
Substances in such a form as to
be readily ignitable
Cellophane Paper(EH), Coconu Fibre(H),
Ganja(H), Wood shavings( H), cotton(H),
coal dust(EH), etc.
Table 1.8 : Calorific Value of Materials in Terms of Wood Equivalent
Materials
Calorific
Value
(x10
3
J/kg)
Wood
Equivalent
(kg/kg)
Remarks
1. Wood (hard or soft)
17.60
1.00
1Cal = 4.2 J
(Calorific value of
wood
= 445 kcal / kg.)
2. Benzene
39.60
2.25
3. Charcoal
28.40
1.61
4. Methane
52.80
3.00
5. Straw
13.20
0.75
Strictly speaking the fire load density in a building / compartment varies from time to
time. For example, the furniture and goods will move in and out of a building during the
normal working hours. Hence, it is difficult to assess the exact fire load in a building. A
reasonably accurate fire load density based on the type of occupation has been suggested
in National Building Code ( Table 26 of NBC, Part IV, fire protection) and the same is
reproduced in Table 1.9
Table 1. 9 Fire Load Density for Different Occupancies
Sl.
No.
Building Type
Fire Load Density in
Wood Equivalent
( kg /m
2
)
1
2
3
4
5
6
7
Residential ( A)
Institutional & Educational ( B & C)
Assembly (D)
Business (E)
Mercantile (F)
Industrial (G)
Storage & Hazardous ( H & J)
25
25
25 – 50
25 – 50
upto 250
upto 150
upto 500
1.13.3 Arrangement of Fuel
The arrangement of various combustible materials contained in the building influence the
spread of fire. If materials are not arranged properly in a compartment, a small fire,
starting at one particular spot, may spread over the other combustible materials quickly
and may become a large fire within no time.
If materials are tall, so that flames reach the ceiling quickly, fire spread to other materials
will be accelerated due to the rapid side way movement of flames under the ceiling. The
same behaviour can be expected if combustible materials are used to line the walls or if
they are hung on the walls. Vertical or inclined materials or structural members ( for
example a column) will burn more intensely than horizontal ones ( Example – beam).
This is because, in the former case, they are oriented in the direction of heat flux and
thereby their heat receiving surfaces are increased.
The rate of burning of combustible materials depends also on the surface area exposed to
the air and to the ignition heat. This means that, if sufficient oxygen supply is ensured, a
material with high ratio of surface area to mass will burn rapidly compared with the same
material and fire load, but with a low value of the said ratio. This is the reason why wood
shavings burn much more quickly than the same weight of wood in block or plank form.
Similarly, paper, when exposed to fire in single sheets burns rapidly but stacks of paper,
books on a shelf, or files in a cabinet will burn much more slowly. Another common
example is the plastic foams. Plastic foams have a very high surface area due to their
foam structure. Hence they, when exposed to fire, will burn in a very rapid manner.
So, if various combustible materials are spread throughout the building with good
separation between each item or groups of items, then it may take more time for the fire
to spread. Accordingly, large floor area is required for the building. However, if building
is to be designed for its optimum utilization, then it may not be possible to arrange the
combustible contents so as to achieve a minimum fire development.
1.13.4 Size and Shape of Windows
The size and shape of windows controls the air supply and loss of heat in a compartment
and thereby influences the fire severity. In general, all openings into a compartment,
other than window openings are required to be closed by fire resisting doors or shutters.
Hence, it is generally assumed that the air can reach a fire only through window
openings. It is further assumed that any glazing present in the window will fall at an early
stage of the fire.
Two activities can occur through a window opening when a compartment is on fire. The
cold air enters through the lower level of the window and smoke and hot gases escape
from the window through the upper level. A schematic diagram showing the flow of air
through a window is presented in figure 1.11
Depending on the amount of air supply available to a fire, two kinds of fire behavior are
identified, namely ventilation controlled fire and fuel controlled fire.
Ventilation Controlled Fire
When the window opening is small, the amount of air reaching a fire will be limited. If
the window opening is increased, the rate of burning of the fire will also increase due to
the greater air supply. This type of fire is called ventilation controlled fire. Based on the
experimental studies, it has been found that the shape of the window is important in
ventilation controlled fires.
The factor which considers the shape of window openings with regard to the fire severity
of a ventilation controlled fire in a compartment is A
H where , A is the area of window
openings and H is the height of window openings. This means that, for the same area of
window openings, the rate of burning of combustibles will be less for a wide and short
opening compared with a narrow and tall opening
Fuel Controlled Fire
In this case, the window openings are in such a way that there is sufficient air supply for
the combustibles. Any further increase in the area of window openings will not increase
the rate of burning. Under this condition, the rate of burning is determined by the nature
and arrangement of the combustible material. Any increase in air supply will cool the
fire. Air will be entrained into the smoke and flame and thereby reduce the fire
temperature. The transition from ventilation controlled fire to fuel controlled fire takes
place approximately at a fire load per unit window opening of 160 kg/m
2
for a fire with
wood crib as combustible material. If fire load per unit window opening is less than 160
kg/m
2
of wood crib
, then it will be fuel controlled fire.
1.13.5 Size and Shape of Room
If area of the room is larger, then the total quantity of combustible materials stored will
be greater( Fire load will be higher). In a room with larger height, the cooling air flowing
in and out of the window openings will not be able to influence all the burning material
and as a result, the temperature of the fire will be more compared with a compartment of
low height.
So, in the fire protection point of view, for a minimum fire severity, the room /
compartment must be as small and as shallow as possible. However, it must be important
to note that the above criteria cannot be adopted due to the other functional requirements.
1.13.5 Thermal Insulation of Walls and Ceiling
A room or compartment bounded by walls and ceiling with materials having low thermal
conductivity will conserve the heat from the fire and hence causes a rapid increase of the
fire temperature. If the surface of the ceiling above the fire reaches a high temperature
quickly, heat will be re-radiated from this surface back in to the fire or nearby
combustible materials and the fire development will be rapid when compared to a case
where there is no such rise in ceiling temperature. If inside surfaces are lined with a
material with low thermal conductivity, ceiling will get heated up faster. On the other
hand, if inside surfaces and ceiling can absorb and conduct heat, ceiling will not get
heated up quickly. This means that thermal insulation to walls and ceilings are to be
carried out carefully. The better positioning of a thermal insulator will be within the wall
/ ceiling panel and not on the inside surface of a compartment / room. Figure 1.12
compares such an arrangement.
1.13.6 Mechanical Loads
During a fire, structural elements may be subjected to additional loading such as
Pressure of product at combustion,
Falling debris
Water used for extinguishments
Explosion
Effect of these factors is to reduce the fire resistance rating of structural elements. The
pressure due to product of combustion during a fire is usually insignificant unless the fire
occurs in enclosed premises such as theaters, window-less buildings, etc. In general,
practical measures are taken to relieve pressure due to explosion through vents. Figure
1.13 summarizes the various factors that influences the fire severity and the measures to
be taken to minimize the fire severity in each case.
1.14 SPREAD OF FIRE WITHIN A COMPARTMENT
The development of fire within a compartment exhibits three distinct phases and they can
be called as
- Initial growth phase
- Rapid fire development phase and
- Fire decay phase
When fire starts, one item or part of one item is ignited. The flame on this one item
gradually grows and fire spreads to whole of this material, then to the adjacent items and
so to all the items in the compartment. During this initial growth phase, people within the
compartment will be exposed to smoke, fumes and heat. The initial growth period of fire
in a compartment vary from less than a minute to several hours and this depends on
various factors. People must be evacuated from the compartment during this phase of fire
development.
During the course of this development of fire, materials will give off volatile combustible
constituents. When concentration of these volatile constituents reaches its flammability
limit and under favourable conditions, fire spreads rapidly over the entire surface of
combustible materials- called flash over. Once flash over occurs, escape from the
compartment is practically not possible.
When all the surfaces of combustible materials in a compartment is under fire, the fire is
considered to be a fully developed fire. This stage occurs when temperature of fire
reaches 500°C or more. During the rapid fire development stage, the building under fire
causes greatest exposure hazard to adjacent buildings. In this stage, the fire resistance of
the containing building elements is critical to the safety of building and this depends on
the fire severity. Maximum temperature attained by the fire will be in the range of 900°C
to 1200°C. During this stage, the rate and development of burning in a compartment fire
is given by the following equations.
R = 6A√H
t α F/(6A√H)
where,
A = Area of window opening ( m
2
)
H = Height of window ( m )
F = Fire load ( kg )
R = Rate of burning kg/m
t = duration of burning ( min.)
If 2/3
rd
of fuel is assumed to be burning during the time t, then fire duration can be
written as
t = F/(9A√H)
The third phase, fire decay, begins when most of the combustible materials have been
consumed and their carbonaceous residue burns to ash.
1.15 SPREAD OF FIRE WITHIN THE BUILDING
Spread of fire within the building –from one compartment to another-happens primarily
through openings and voids. The geometry and construction of the compartment have
significant effect on the time taken to spread fire from one compartment to another.
Compartments in a building require openings in order to fulfill its functions for which
buildings are constructed. Different openings are required for different purposes namely,
Doors - Openings through walls
Required for movement of people and materials
from one compartment to another.
Stairs, Lifts - Openings through floors
Required for vertical movement
Openings for services - Through walls and floors
Required for water, power, drainage, etc.
Unless these openings are fire tight -with fire resisting assemblies fitted with-,fire and
product of combustion may spread to the near by compartment, corridors, evacuation
passages, etc.
Another mode of spread of fire within a building is through void spaces available in a
building. Void spaces are formed within a building
- While forming ventilation ducts
- While constructing decorative cladding on external walls
- While constructing false ceiling below floor
- While taking service lines, voids are formed around them.
These voids contain many a time combustible materials used for various purposes namely
for insulation, packing, etc. and they cause a rapid spread of fire within the building.
Provision of fire stoppers and use of non-combustible materials in voids will reduce the
danged of fire spread through openings.
Fire can spread to other parts of the buildings through flames emerging from windows.
When fire emerges from a window, it will try to hug the face of building and in some
cases, it get sucked into the open window. Shape, width and height of windows are
important parameters influencing the fire spread. When there are combustible materials
near to the windows of floors above the fire affected floor, they get ignited and fire
spreads to these floors. Ideally, for fire safety, windows must be as small as possible.
Increasing either height or width increases the fire spread through windows.
1.16 FIRE TRANSMISSION BETWEEN BUILDING
The transmission of fire from a building to another building can be due to the
transmission of heat by
Conduction
Convection and
Radiation
Fire Transmission by Conduction of Heat
Conduction of heat refers to the heat transfer through a solid material. Contribution to the
transmission of heat by conduction is less, particularly at the initial stags of fire
transmission.
Fire Transmission by Convection of Heat
Transfer of heat through a fluid ( liquid or gas) medium by its movement refers to
convective heat transfer. In general, transfer of heat by convection by way of the hot
gases and flames emerging from a burning building is relatively limited in range- say if
they are separated by one or two meters.
However, under certain climatic conditions, the convection currents set up by hot gases
emitted from the burning building may be strong enough to lift and carry pieces of solid
burning material (flying brands). The flying bands can create secondary fires if the
surfaces of the receiving buildings have inferior fire characteristics with respect to
ignitiability, flame prevention and flame spread. Even though flying brands can contain
only small amount of heat, which is rarely sufficient to ignite other materials, they can
become a pilot source of ignition when there is a considerable amount of heat transfer by
radiation and thereby may cause combustion of the exposed material earlier than if
radiation only was present.
The convective gas temperatures in normal building fires are too low to cause ignition.
However, depending on the direction of wind, this may boost the effect of radiation.
Fire Transmission by Radiation
The amount of heat radiated from the surface of a building depends on a number of
factors such as
The degree of compartmentation within the building
The fire load in each compartment
The area of openings on external wall
The calorific value of the combustible contents
Wind condition
Installed fire fighting equipment to protect the openings
It has been observed that for a single compartment fire, the fire becomes hotter and of
longer duration as the fire load per unit window opening is increased up to about 150 kg/
m
2
.
If fuel load is in excess of 150 kg/m
2
, then the fire will become ventilation controlled
fire. Under this condition, the duration of fire will increase. However, the intensity of
radiation emitted from the openings will not increase.
The radiation of heat emitted from the openings in an external wall is time dependent.
The maximum radiation occurs when the fire becomes a fully developed fire. It is to be
noted that the period of development of a fire in to a fully developed fire depends on so
many factors. The area of openings in a external wall defines the amount of radiated heat
that could be emitted.
Measures to Prevent Fire Transmission between Buildings
Radiation is the major way of heat transfer, which leads to the fire transmission between
buildings. So the preventive measures includes the methods to reduce the amount of
energy transmitted to combustible components or contents of the receiving building.
There are two methods by which radiation energy can be reduced namely
By providing an imperforate, non-combustible barrier between the fire and the
combustible materials.
By providing sufficient space between the fire and adjacent combustible materials
to ensure that the intensity of radiation received by them will be below that which
could cause ignition. The limiting amount of radiation is 0.3 cal cm
2
/ s.
IS 1643 : 1988 – “code of practice for fire safety of buildings (General) Exposure
Hazard” specifies the guidelines with respect to the openings in external walls, the
spacing, location and nature of protection of window openings, distance between
buildings, etc. to prevent the fire transmission between buildings. Depending on the
occupational classification, the fire transmission between buildings is controlled by
specifying
the maximum Floor Area Ratio (FAR)
the minimum open space around the building and
limiting the maximum Height of building.
Floor Area Ratio (FAR) is the ratio of the total covered area (plinth area) on all the floors
to the plot area. The FAR of a building has been fixed based on the width of street, the
type of construction and the combustible content of different occupancy. Table 19 of
SP7-1983 ( or Table 1 of IS1643-1988 ) specifies the maximum limit of FAR for
different occupancies.
The maximum height of a building is restricted based on the occupancy classification, the
type of construction and based on the width of street.
Similarly, the minimum open space around the building is dependent on the occupancy
classification, the width of street, the height of building and the type of buildings
(Detached, semi-detached, non-type, toner-like, high rise etc).
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