By Morgan J. Hurley, P.E., FSFPE | Fire Protection Engineering
The theme of this issue is "fire risk assessment.” Fire risk
assessment differs from other forms of fire safety analysis or design in
how the probability of an event is considered. With most forms of fire
safety analysis, the only consideration of the probability of an event
occurring is in the context of whether or not the event is likely enough
that it should be addressed. All events that are considered likely
enough to merit protection are then considered equally.
This occurs in both performance-based
designs (at least those that are not conducted on a risk basis) and
within prescriptive codes. For example, many prescriptive codes contain
requirements that are predicated on a single fire occurring at any time.
That is not to say that more than one fire is not possible, only that
the developers of these codes considered more than one fire happening at
once to be sufficiently unlikely that it is not necessary to protect
against this occurring.
Conversely, in fire risk assessment, the consequences
of a fire event are weighted by the probability of the event occurring.
The traditional way to do this is to multiply the consequences of an
event (lives lost, cost of damage, hours of downtime, etc.) by the
event’s frequency (e.g., once every 100 years) and then sum these
products for all scenarios. The result is a measure of the risk
associated with an activity, in units like dollars of fire loss per
Simpler methods of fire risk assessment are available as well. The
viewpoint by Dr. John Hall on page 4 summarizes these approaches nicely.
Unlike other forms of fire protection
analysis or design, rare events can’t be excluded from fire risk
analysis solely on the basis that they are highly unlikely. Even
extremely rare events must be considered, but their (usually extremely
high) consequences are weighted by the low frequency at which these
events would occur.
There are two major challenges associated with fire risk
assessments: the time that they take to perform and the availability of
data. For analyses with a large number of scenarios, the time necessary
to conduct a fire risk assessment can be substantial. It can also be
difficult to find data associated with the frequency of events occurring
or the reliability of fire protection systems.
In some cases, it might be necessary to
apply engineering judgment. When this is done, the uncertainty
associated with the values used should be considered – such as by
selecting conservative values or conducting an uncertainty analysis.
Similarly, system reliability data can be hard to find. An article
summarized sprinkler system reliability studies,1 which
identified system reliabilities ranging from 81.3% to 99.5%; this shows
that even published data can vary. For other types of fire protection
systems, finding good reliability data can be a real challenge.
However, fire risk assessment can be a
very powerful, cost-effective tool. If a few scenarios dominate the fire
risk, then the fire risk can be reduced by focusing on ways to reduce
either the frequency of the scenarios occurring or their consequences.
This allows for resources to be applied in a very efficient manner.
With the exception of a few industries –
like the nuclear industry and the petrochemical industry – fire risk
assessment is rarely used in fire protection engineering design. The
articles in this issue provide an excellent overview of the approaches
that can be used to conduct a fire risk assessment and the references
that are available to assist.
Budnick, E. "Automatic Sprinkler System
Reliability,” Fire Protection Engineering, Winter, 2001, pp. 7-12.
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