The Occupancy Factor
Knowing a building’s intended use is key to developing strategy and tactics.
NFPA Journal®, May/June 2009
Having a pre-fire plan and knowing a building’s occupancy is essential when confronted with a structure fire in a large, complex building, such as a hospital or convention center. Central to pre-fire planning, then, is formulating an incident action plan based on a risk-versus-benefit analysis. The level of risk to firefighters must be commensurate with the probable life safety and property conservation benefits, and determining the life safety benefit necessitates evaluating the occupied status of the building. Knowing a building’s intended use helps the incident commander (IC) determine whether it is occupied and estimate the total number of people likely to be in the building at the time of a fire.
NFPA 101®, Life Safety Code®, establishes the occupant density, or maximum occupant load per square foot, by occupancy type. For example, the occupant density of an office building might be one person per 100 square feet (9 square meters). Places of public assembly may be posted with the maximum number of occupants. In hotels and apartment buildings, the number of rooms or apartments is an indicator of maximum occupant load. Time factors such as hours of operation, combined with maximum occupant load and evacuation status, help determine the number of people endangered and the possible need for assistance.
Assembly occupancies tend to be occupied by large numbers of people during hours of operation, but are unoccupied, or minimally occupied, at other times. Many large-loss-of-life fires have occurred at assembly occupancies at times of peak use when large numbers of people are gathered in relatively small areas, often exceeding the maximum capacity specified in NFPA 101. Conversely, fires tend to gain considerable headway before detection when nonprotected assembly buildings are unoccupied, often resulting in defensive operations.
Occupancy type not only helps the IC determine how many people are threatened by the fire, but also indicates the effort required to evacuate the building. Evaluating the ability of occupants to self-evacuate, as well as the difficulty in performing rescue operations, is essential when determining resource needs. In most cases, it is impractical, if not impossible, to fully evacuate a large, multistory hospital. Due to protective measures required by fire and building codes, however, a complete evacuation is seldom necessary. Structural Fire Fighting estimates occupant rescue difficulty using a numerical matrix to assess occupant mobility, age, awareness, familiarity, density, and leadership.
These are general ratings, however, and may change due to individual circumstances. For example, most hospital patients are less mobile than the general population, and some are totally unable to escape on their own. They may be asleep or under the influence of medications, and they are usually unfamiliar with the building’s layout. Hospital patients rely on the assistance of others when threatened by smoke or fire, and some must remain tethered to life-support systems.
As a result, most hospitals practice a defend-in-place strategy, moving patients in imminent danger to a place of safe refuge on the same level. Some areas within a hospital, such as administrative offices, outpatient services, and other treatment areas, are only occupied at specific times. Some hospitals do not have intensive care units or trauma centers where patients are totally immobile, which could change their rating.
On the positive side, hospitals are required to provide leadership and assistance for patients, and the number of occupants in any given area will be much lower than in an assembly, educational, or mercantile occupancy. On the other hand, occupants in assembly, educational, mercantile, and business occupancies are more likely to self-evacuate or make their way to safety with minimal assistance from firefighters.
Pre-fire planning for specific buildings in their response areas, coupled with occupancy factors, provides an IC with an indication of the resources needed to evacuate those buildings. In some cases, pre-fire plan information may indicate that an evacuation is not the best course of action.
This column is adapted from the author's book Structural Fire Fighting, available at www.nfpa.org or (800) 344-3555.