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PASSIVE FIRE PROTECTION IN BUILDINGS

almost 4 years ago

Authored by Dr. D. K Singh and Mr. Vivek Sharma, Assistant Professor, NFSC, Nagpur

Passive Fire-protection,Fire safety,National Fire Service College

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PASSIVE FIRE PROTECTION IN BUILDINGS

Dr. D. K. Singh and Mr. Vivek Sharma

Assistant Professor, National Fire Service College, Nagpur, India



1.   Introduction

Fire affects the structural members of building by degrading its strength and stiffness. Also, fire induces temperature strain in these members. The consequences of strength & stiffness degradation and strain due increase in temperature results in reduction of load carrying capacity in structural members as well as large displacement in these members.

In general active fire protection system and passive fire protection system are the two types of fire protection system in a building. Passive fire protection systems are the system which is always present and effectively working whenever there is fire. Passive fire protection works on the principle of containment of fire which results in controlled and slow spread of fire to adjoining physical spaces and limits the size of the fire by restricting the supply of fresh air. The fire protection technique is based on the behaviour of building materials and other combustible material present in building during fire. Therefore, passive fire protection system controls the fire and its effects by the restricting it to limited space in the building. Typical example of this includes partition walls, slabs and fire resistant doors, fire stops, fireproofing cladding, cable coating etc. These walls are capable of arresting the spread of flames, hot gases and smokes due to their better stability, integrity and insulation. Passive fire protection is often described in terms of fire resistance.


2.   Controlling Fire Spread

One of the main objective of passive fire protection is to control the fire spread. These fire spread can be within a room, room to room, floor to floor and one building to another.


1. 2. 2.1. Fire Spread within Room of Origin

In this, the fire is spread within the room from the origin of fire. Such type of fire spread depends on heat release rate and burning rate of the material on fire. The heat is transferred from origin of fire to other areas of room through convection and radiation. The smoke produced due to fire also act as a thermal reservoir which supply heat to other combustible material in the room. As a result the other combustible material in the room also starts burning and eventually the fire is spread completely in the room. In order to prevent the spread of fire in the room the selection of construction material and other items should be done carefully to minimise the risk of fire. The important properties of material to be considered while selection are ignitability, minimum ignition energy, burning rate, heat release rate, flame spread rate and smoke production rate. Mostly these are inherent properties of material and it can be enhanced used certain chemical treatments.


2.2. Fire Spread to Adjacent Rooms

           Here, the fire spreads from one room to another room through doors, concealed spaces and partition walls etc. The heat is transferred through one room to another through radiation. The movement of fire and smoke depends on building layout. Fire spread in this condition can be stopped by using automatic closing doors or fire curtains having good fire resistance, use of intumescent material which can swell when exposed to fire or high temperature resulting and better sealing of gaps, use of fire stoppers in cavities like seismic and construction joints etc. and using fire dampers in ducts which closes automatically during fire.  


2.3. Fire Spread to Other Storeys

           Here, the fire can be spread from one floor to another floor from inside or outside the building. Inside the building the fire spread through ceiling/floor, vertical concealed spaces, shafts, stairways and service ducts, etc. All the gaps should be sealed with fire stopping material and staircase must be separated from the other occupied spaces. Outside the building the fire spreads through combustible cladding and window. This fire spread can be avoided by well separated windows and fire resistance cladding.


2.4. Fire Spread to Other Buildings

         Here, fire can be spread from one building to another building by radiation, flame contact and flaming brands. This can be prevented by providing sufficient distances between the buildings and using fire barriers. The exterior walls and roofs etc. must be treated with fire resistance material.


3.   Fire Resistance

Fire resistance is the ability of the material to withstand fire usually referred in duration of time. Stability, integrity and insulation are the three domains in which fire resistance is specified. Stability criteria refers to the ability of structural member carrying load to withstand service loads during fire. Whereas, integrity and insulation is important in members used for partitioning. The fire resistance test is carried in accordance with IS: 3809. The material is exposed to high temperature and the duration (in minutes) is measured until the failure occurs in terms of stability or integrity or insulation (see Table 1) due to such exposure.


 

Fire resistance in terms of stability can be determined if specimen can no longer support the test load and collapse. In stability test the specimen should be subjected to loading which will generate the stresses close to actual stresses generated in element. After 30 minutes the heating should be started and time for collapse of specimen should be noted.

Fire resistance in terms of integrity is the time at which the specimen develops crack or fissure that allow hot gases to pass through it. In integrity test a cotton pad of size 100 mm × 100 mm and having thickness 20 mm shall be placed on the crack developed on unexposed face of member and should not catch fire for at least 10 seconds.

Fire resistance test in terms of insulation can be determined by measuring temperature on the unexposed side with the help of thermocouple. In insulation test at least five thermocouples to be placed in the specimen. The average temperature of unexposed surface shall not increase by 140°C and the maximum temperature at any point shall not increase by 180°C.


4.   Fire Propagation:  

The fire propagation is broadly categorized into two zones viz. pre flash-over and post flash over zone. The pre flash-over has two distinct phases namely incipient phase and growth phase, while post flash-over zone has fully developed phase and decay phase. The graphical representation of these phases in temperature verses time is shown in Figure 1. Once the fire is ignited and incipient phase is over the active as well as passive measures of fire safety helps in delaying the period of flash-over and hence the total duration of growth phase is increased. As a result of this more time is available for the evacuation of the people from the building. During the growth phase the active measure of fire safety dominates in containing the fire but once the




Figure 1: Different phases of fire



flash-over occurs and fire is transformed from growth phase to fully developed phase then the passive measures of fire safety plays an important role in containing the spread of fire from compartment to compartment, floor to floor and ultimately from building to building. The fully developed fire is considered severe in terms of property loss as heat release rate and temperature in the enclosure is very high.  The Passive measure of fire safety also helps in maintaining the structural integrity of the building during fully developed fire and thus reduced the risk of property loss. It also helps in providing the window for the continuity of fire fighting operation.

 

5.   Equivalent Fire Severity

Large uncertainty is present in real fire in terms of maximum temperature, duration of burning, fire growth duration, sustained burning and decay phases etc. To overcome this standard fire is usually used. The standard fire is different from real fire in terms of no decay period and period from ignition to flash-over is not considered. Standard fire curve gives uniformity in the fire exposure of building elements. However, the fire rating derived from standard fire curve must be equivalent to real fire. The following are the four concept developed for converting standard fire to real fire.

 

4. 5. 5.1. Equal Area Concept

Here, the area under time-temperature curve of standard fire is made equal to area under time-temperature curve of real fire. This one of the old and most used technique, however, this has no physical significance and lacks in theoretical basis because unit of area is not meaningful (Chandrasekaran and Srivastava 2019).


5.2. Maximum Temperature Concept

Here, the temperature of structural member when exposed to real fire is compared with the temperature of structural member when exposed to standard fire. This is different than the equal area concept because here temperature of member is taken to into account rather temperature of gas in previous case.

Eq. (01) give the curve for standard fire. Where, T is furnace temperature at time t, To is initial temperature and t is time of burning.


5.3. Minimum Load Capacity Concept

Here, the minimum load capacity of structural member when exposed to real fire is compared with the minimum load capacity of structural member when exposed to standard fire.


5.4. Time Equivalent Formulae

These formulae are developed by fitting the empirical curves to results of many calculation of the concept shown in sec 3.2. The most commonly used formula is CIB formula. Other formulae generally used are Law formula and Euro-code formula.


6.   Conclusion

This article provide information on various important points that to be considered in design of passive fire along with the important test to be performed. The passive fire protection plays a very important role in containment of fire at one location within a building and prevents and slows the spread of fire to other building spaces. Thus, it provides more time for occupant for emergency evacuation and saves loss of lives and property. Moreover, this system is the inherent property of the material in a building and hence it is always present. The reliability of these systems can be ensure with proper designing, maintenance and workmanship.

 

7.   Reference

1.    IS 3809-1979, Fire resistance test of structures. Bureau of Indian Standards, New Delhi, India

2.    Buchanan, A. H., & Abu, A. K. (2017). Structural design for fire safety. John Wiley & Sons.

3.    Chandrasekaran, S., & Srivastava, G. (2019). Design aids of offshore structures under special environmental loads including fire resistance. Springer.


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