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.
From these formulas will come the answers to a few critical questions regarding fire department resource allocation:
- How are these flows to be achieved?
- How many people will be needed to apply them?
- How many pumpers will be needed to supply them?
Fire Chief William E. Clark, in his text, Firefighting Principles And Practices, states that “... a study of 250 large fires disclosed that ... there was (on average) one pumper present for each 500 gallons-per-minute pumped.“ He further states that “... this water requirement method (the ideal rate of flow formula mentioned above) could be used to determine how many firefighters and how many pumpers are needed.”
A figure of 50 gallons-per-firefighter emerged from Clark’s research as the basic yardstick for manpower at firefighting operations. For planning purposes, Clark recommends the following as a minimum operational level:
- One pumper/500 gpm required.
- One firefighter/50 gpm required.
- One aerial truck company or unit to perform truck-related duties for every two pumpers assigned.
The ideal rate of flow formula was not developed for risk analysis use. However, the widespread acceptance of it and its ability to meet the needs of this risk analysis format has led to its use for initial attack flow calculations.
Bear in mind at all times that the results of this planning mechanism MUST be considered as MINIMUM operational levels. It may well be that greater resource levels are needed in your community. A number of factors can influence your decision to provide a higher level of protection.
Some of these considerations are:
- High population density.
- Densely packed housing stock.
- Hazardous industrial operations in close proximity to residential and commercial areas.
There is a distinct difference, however, in the manner in which a fire department’s ability to generate these required fire flows is calculated. In the case of the initial attack flow formula, a department must be observed operating on the drill ground. The speed of the initial attack must be clocked and recorded. Remember that the initial attack flow must be attained within the 10-minute time frame recommended by the NFA. The sustained attack flow will be more difficult to develop owing to the large commitment of resources which must be made and the fact that, in many cases, outside assistance will be required to achieve the necessary flows.
You can compare a representative target hazard’s sustained flow requirement with the fire department’s available resources and see if it was within their ability to supply the flow. A large-scale mutual aid drill might be scheduled which could serve as the focus for evaluating a department’s ability to deliver required sustained attack flow.
The object in both of these cases is to determine capability to supply the flows needed to combat any fire which might occur in the identified target hazard occupancies.
In this column, we have concentrated on protecting things. In our next Command Post, we will look at the concept of life hazard.
Harry R. Carter, Ph. D., a Firehouse® contributing editor, is a battalion chief with the Newark, NJ, Fire Department and past chief of the Adelphia, NJ, Fire Company.
