The fire resistance of a structural member may be defined as its ability to withstand exposure to fire without loss of load bearing function or ability to act as a barrier to spread of fire. This provides time to permit people to evacuate a building before it collapses and is essential in confining fire to the compartment in which it starts.
Prediction of the fire behavior of structural members involves the calculation of fire temperatures and the temperature, deformation and strength of the members .
FIRE RESISTANT CONSTRUCTION.
In a fire resistant construction, the maximum use of non-combustible material should be encouraged.the structural members like beams ,columns,floors,roofs etc. should be constructed in such a way that they should continue to function as structural members at least for the period which may be sufficient for the occupants to escape.
STRENGTH OF MEMBERS
When temperature rises in fire-exposed members, their strength is reduced. If the fire load is sufficient and the duration of the fire long enough, a stage will be reached at which the strength of the member will no longer be adequate to support the structural load. The fire load that is just sufficient to reduce strength to this critical point is defined as the critical fire load.
COLUMNS AND BEAMS
Reinforced concrete structures should be preferred to steel structures as it is liable to twist ,sag under heavy fire.
If steel is used it should be protected by using concrete,hollow clay tiles, bricks ,and plaster .the treatment given to the columns is given below.
The cover of the protective material like tiles or bricks etc. should be all around the members at least 10 cm. Thick.
The cover of concrete for reinforced concrete members should be sufficient to enable the members function under fire for max. time.
For beams and columns the higher ratio of the surface exposed to fire greater is the fire resistance.
Beams and columns with membrane enclosure protection will have less surface area exposed to fire.
FIRE RESISTANCE OF STRUCTURAL STEELWORK
Although steel is non combustible material it does not behave very well under fire conditions.
Achieved fire resistance for steel members encased in concrete depends upon the thickness of the protective concrete cover,concrete mixture and structural restraint .
For protection of structural elements following methods are adopted :
1)cladding
2)Cooling the element with water
3)Intumescent coatings
4)design of a composite section
LOAD BEARING CONSTRUCTION
In case wooden partition has to be made fire resistant it should be covered with metal lath and plaster.
The load bearing walls should be plastered with fire resistive mortar.
Cavity wall provides a good option for fire resistant construction.
The fire resistance of load bearing walls should not be less than two hours
The partition walls should be of fire resistant material;in general R.C.C., bricks, hollow concrete blocks, tiles or timber.the material should be treated in such a way that they should have fire resistance of not less than one hour.
The load bearing walls should be plastered with fire resistive mortar.
Cavity wall provides a good option for fire resistant construction.
The fire resistance of load bearing walls should not be less than two hours
The partition walls should be of fire resistant material;in general R.C.C., bricks, hollow concrete blocks, tiles or timber.the material should be treated in such a way that they should have fire resistance of not less than one hour.
The load bearing walls should be thicker in section so that they may successfully act as fire barrier for a considerable time.
There are also special fire-retardant paints and coatings that can substantially reduce the flame-spread rating of an interior surface.
Fire resistance of construction with cavity air space will be greater than identical weight construction without airspace
STAIRCASE
For stairs a higher standard of fire resistance is required.
R.C.C. stairs are considered best .
The enclosing walls of the main staircase should have a fire resistance of at least 1 hour.
Access to the staircase should be through doors having fire resistance of at least ½ hour.
All the fire escape stairs should be directly connected to ground floor and their entrance should be away from the internal staircase of a building.
ROOFS
Flat roof should be encouraged as far as possible.
Where sloping roof is necessary either due to site conditions or architectural reasons the ceilings of the sloped roof should be made up of fire resistant material by fixing asbestos cement boards , fiberboard to its framework.
In the installation of roofing, wood cant strips, roof curbs, nailing strips are permitted in noncombustible construction. Roof sheathing and sheathing supports of wood are also permitted provided:
They are installed above a concrete deck
The concealed space does not extend more than 1m above the deck
Openings through the concrete deck are located in noncombustible shafts
Parapets are provided at the deck perimeter extending at least 150mm above the sheathing.
The noncombustible parapets and shafts are required to prevent roof materials igniting from flames projecting from openings in the building face or roof deck.
In buildings that must be of noncombustible construction the roof coverings must have a fire classification of Class A, B or C. In such cases the use of fire-retardant treated wood shakes and shingles on sloped roofs is allowed.
FLOORING
Fire resisting material should be used in construction of flooring
In case of wooden floors thicker joists spaced at greater distance apart should be used.
Fire stops should be provided in wooden floors at suitable intervals
Flooring made from materials like concrete, brick, ceramic tile etc. is considered to be most suitable for fire resistant construction.
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