An important factor in establishing the basis for the assessment of the fire risk pertaining to any building is the concept of ‘fire load’ which indicates the quantity of heat liberated per unit area when a building and its contents are completely burnt.
All occupies / buildings, etc. can be graded according to their fire hazard and are to be provided for with suitable fire precautions on the basis of the fire load.
Hence, grading of buildings according to both fire load and fire resistance can be made.
The formula for calculating fire load is as stated.
The calculation of the fire load is the basis for the determining the classification of the occupancies for the fire grading of buildings.
Fire grading of the structures
Structural element of buildings are graded according to the time factor which is nearly equal to but does not exceed the test period which the element fulfills its specified requirements.
Accordingly, all structural elements have been graded under the following five categories depending upon their five resistance viz.
Grade 1…………………… 6 hours
Grade 2…………………… 4 hours
Grade 3…………………… 2 hours
Grade 4…………………… 1 hours
Grade 5…………….…… 0.5 hours
Classification of Occupancies
The fire offices committee (U.K.), in its report of fire grading of buildings in 1946, has recognized 3 main classes of occupancies on the basis of the fire load.
This also conforms to the relevant I.S. specifications and may be stated as:
Occupancies of low fire load:
ordinary buildings for residential purposes, hotels, offices, schools, etc, or occupancies having a fire load not exceeding 2,75,000 kcal/sq.mtr of net floor area of any compartment, nor exceeding an average of 550,000 kcal/sq.mtr on a limited isolated area. (for reference, the maximum for this type in F.P.S. system is 1,00,000 B.Th.U/sq.ft)
The fire resistance required by buildings of this category to withstand the complete burn-out of their contents without collapse is 1 hour as has been found after tests.
Extensive investigations carried out in Switzerland and Germany have shown that the fire load in offices varies from 10 kg to 30 kg/sq.mtr wood equivalent to 43,356 to 130,068 kcal/sq.mtr whereas this type of occupancy has a one hour rating with maximum fire loading up to 270,978 kcal/sq.mtr equivalent to 60 kg/sq.m.
Occupancies of moderate low fire load:
Retail shops, bazaars, stalls, factories, etc. hence the fire load exceeds, 2,75,000 kcal/sq.mtr, and is up to 550,000 kcal/sq.mtr. This is equivalent to the fire load of 2,75,000 kcal/sq.mtr, not exceeding an average of 1,100,000 kcal/sq.mtr on limited isolated area as per relevant I.S. specifications. Occupancies of this type should have a fire resistance of two hours.
Occupancies of high low fire load:
Godowns, warehouses, etc. this category as per I.S. specifications exceed the fire load by 550,000 kcal/sq.mtr, but does not exceed an average of 1,100,000 kcal/sq.mtr of floor area. A fire resistance of 4 hours for these types of occupancies is considered sufficient. (for reference, the maximum for this type in F.P.S. system is 4,00,000 B.Th.U/sq.ft exceeding an average of 2,00,000 B.Th.U/sq.ft).
Example:
The manufacturing process industry uses the following material. Calculate the Fire load by using the following data:
Note: 1 calorie = 4.18 Joule
