In our July 1996 column, we began our look at fire risk analysis. We have looked at target hazards and the need for effective record-keeping systems. Let us now build further upon what we have read. There are many ways that a risk can be controlled or minimized. By knowing exactly what type of...
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In our July 1996 column, we began our look at fire risk analysis. We have looked at target hazards and the need for effective record-keeping systems. Let us now build further upon what we have read.
There are many ways that a risk can be controlled or minimized. By knowing exactly what type of risk protection is needed, the fire administrator can make informed decisions about which resources to acquire and what programs to use.
Fire flow analysis forms the basis of determining risk levels in your community. Should you decide to perform a risk analysis in your community, there are two fire flows which must be calculated for the identified target hazards.
- Initial attack
- Sustained attack
These flows are crucial, in that they coincide with a fire department’s ability to (1) provide an aggressive offensive interior attack and halt fire spread within a structure (initial attack flow) or (2) provide a defensive operation capable of limiting the spread of a major fire from building to building (sustained attack flow).
The initial attack flow is calculated by using the “ideal rate of flow” formula developed by Keith Royer and Floyd Nelson at Iowa State University back in the 1960s.
The volumetric measurements of the largest single open area of a building must be calculated and divided by a constant (100) to achieve an approximate number of gallons per minute (gpm) of fire attack water. The formula reads as follows:
- Length of Building x Width x Height/100 = Gallons-Per-Minute
- (L x W x H/100 = GPM)
Let us use the example of a warehouse building which is 100 feet long, 100 feet wide and has a ceiling height of 10 feet.
- 100’ x 100’ x 10’/100 = 1,000 GPM
A building whose largest open area was of these dimensions would require an initial attack flow of 1,000 gpm to control a fire using the offensive interior attack mode. The importance of this figure for planning fire department operational resource allocation will soon become apparent.
According to the National Fire Academy (NFA) course Fire Risk Analysis, a fire department must generate its required initial attack flow within 10 minutes. It must accomplish this to receive maximum credit for meeting risk analysis standards. In rural areas, allowances may have to be made for increased response times. A review of past response times can provide guidance in making this determination.
The calculations required for developing a sustained attacked flow figure are more complex. The formula used by the NFA in its Fire Risk Analysis course is derived from the Guide for Determining Needed Fire Flow developed by the Insurance Service Office (ISO) for use in its fire risk analysis program.
In its simplest form the formula involves using a square footage figure to develop a basic gpm fire flow. The flow will vary based upon size, construction type and occupancy of the building being studied. This flow is an excellent planning tool for ascertaining the array of forces needed to control large-scale fires
A fire department’s time frame for developing sustained attack flow is also expanded to 30 minutes. This was done to allow for the increased response time for mutual aid fire department personnel and equipment.
Copies of the Sustained Attack Fire Flow form may be found in the NFA’s Fire Risk Analysis course booklet or in my textbook, Managing Fire Service Finances. We strongly recommend that you secure a copy from either source.
The sustained attack flow figure serves a useful purpose as the basis for planning to meet current and fire department needs. This information would also be valuable for use as part of any pre-fire planning program.
This formula is useful for determining the commitment of resources which might have to be made during a major fire in one of your target hazard occupancies. Once the necessary gpm has been calculated, comparisons can be made between what is required by the target hazard and what is actually available in the community.