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Q&A: What Does Fire Load Mean? and How do you calculate it?

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Q: What Does Fire Load Mean? and How do you calculate it?

A: “Fire load” is a term used to describe the potential severity of a fire within a specified space. It is therefore a form of hazard assessment and is used to determine the level of fire risk that exists within a defined area.

Fire load inspections require the assessment of all materials within the space in order to determine that space’s overall flammability. This includes both the material being used in the structure itself and any flammable objects and substances that may be stored in it

A building’s fire load is a calculated value. The specific calculation used varies by region and context. In the United States, the standard for determining fire load during the building design process is NFPA 557. The generic U.S. calculation for determining fire load within a given space uses the metric system and describes fire load as being equal to the mass in kilograms of all material in that space, multiplied by the value of these materials in calories (kilojoules per kilogram), divided by the area of the space in square meters.

This yields the equation: Fire load = (Mass * Calories) / Area

Various fire safety standards used by occupational safety authorities in the United States and elsewhere are designed to reduce the fire load that exists in areas that are regularly occupied by workers. These include both explicit fire-protection standards, such as those that require certain flammable materials to be kept in storage, as well as standards that are designed to reduce the presence of hazards that may increase fire risks indirectly. OSHA’s housekeeping standards, for example, require workplace safety procedures that simultaneously reduce the risk of workers tripping on items in their environment and limit the amount of potentially flammable material distributed throughout the workplace.

There is no fixed value for when a fire load is “low enough.” A workplace’s fire load is always somewhat dependant on the material characteristics that are inherent to that type of workplace—a paint factory would be expected to have a larger fire load than a swimming pool, for instance. Whether an employer is keeping a sufficiently low fire load is therefore determined by whether or not their fire load deviates significantly from the load that would be expected within their particular industry.

Fire load:

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 occupancies/buildings, etc. can be graded according to their fire hazard and are to be provided 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.
Fire load = (combustibles in kg) x calorific value in kcal/kg/Floor area in square meters

The calculation of the fire load is the basis for 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 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 hour
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 of550,000 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 a 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 upto 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 exceeds 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:

Material Quantity in kg. Area in sq.mtr Calorific value (kJ/kg)
Paper 1000 10 15600
Wood 20,000 20 17500
Coal 100,000 50 20000
Rubber 5000 25 40000
Paetroleum product 50,000 30 43000

Note: 1 calorie – 4.18 Joule

Fire load = (combustibles in kg) x calorific value in kcal/kg/ Floor area in square meters

Fire load (paper) = 1000 x 3732.05 //10 = 373205.74 kcal/sq.mt

Fire load (wood) = 20000 x 4186.602/20 = 418602 kcal/sq.mt

Fire load (coal) = 100,000 x 4784.688  = 9569377.99 kcal/sq.mt

Fire load (rubber) = 5000 x 9569.37 /50  = 1913874 kcal/sq.mt/25

Fire load (Petroleum products) = 50,000 x 10287.081/30   = 17145135.57  kcal/sq.mt

Total fire load = 29420195.3 kcal/sq.mt

The result indicates that the occupancy has a High fire load. So fire resistance should be of 4 hours.

Installation of fire extinguishers:

Example:

  1. Determine the number of fire extinguishers required to give adequate protection for a given property.

Risk: Light engineering workshop (Light hazard) Area: 315m x 112 m. i.e. 35,280 sq. mtr.

Type of hazard:

Class ‘A’ fire due to normal combustibles.

As per IS 2190 this is Light Hazard so one 9 ltr water expelling extinguisher for every 600 sq.mtr of floor area should be installed. Extinguishers should be available within a 25 mtr radius.

Here Total area is 35,280 sq.mt.

So no. of extinguisher = 35,280/600 sq.mt . = 58.8

so no. of extinguisher required is 59.

  1. Determine the number of fire extinguishers required to give adequate protection for a given property.

Risk: Petroleum processing units (High hazard) Area: 300m x 150 m. i.e. 45000 sq. mtr.

Type of hazard:

Class ‘B’ fire due to petroleum products.

As per IS 2190 this is a High Hazard so two 9 ltr foam chemical/mechanical types; or 5 kg capacity dry powder extinguisher for every 600 sq.mtr with a minimum of four extinguishers per compartment shall be installed.

Extinguishers should be available within 15 meter radius. Here Total area is 45000 sq. mt.

So no. of extinguisher = 45000 /600 sq. mt. = 75

so no. of extinguisher required is 75.

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