A critical decision when designing a sprinkler system is determining the appropriate occupancy classification for a building or portion of a building. This will dictate many of the design parameters for the building, including sprinkler spacing, delivered density, and, possibly, the need for a fire pump. If the wrong occupancy classification is assigned, the sprinkler system could be significantly under designed and result in a system failure during a fire event.
The concept of defining occupancy classification in NFPA 13, Installation of Sprinkler Systems, is much different from that of the occupancy classifications required by NFPA 101®, Life Safety Code®, and model building codes. The classifications in these codes focus on how the space is used, while NFPA 13 occupancy classification looks at the fuel loads and potential heat release rates of those loads.
It is important for the designer to read the various descriptions of the occupancy classifications in Chapter 5 of NFPA 13 to best assign a building, or portion thereof, to the appropriate category. To help designers determine the correct classification, the technical committee on sprinkler system discharge criteria has included the example lists in the annex for each of the hazard classifications. These lists, which are not in the body of the standard and are therefore not enforceable, are provided for guidance to help designers identify the proper category and make the proper decisions.
Unfortunately, many people have taken these lists as the sole source of information on assigning occupancy classification and fail to read the descriptions. It is important to remember that the description is what matters, and not simply the lists in the annex. While the lists in the annex may be appropriate for many cases, they do not always capture the features and fuel loads that may ultimately dictate the appropriate classification.
This is largely the case because not all buildings and spaces are created equally. Take for example the classification of a gymnasium. First, consider a gymnasium that is located in an elementary school. These gymnasiums are often devoid of combustibles and are simply gathering areas for kids to run around and play. Compare this gymnasium to one with foam crash pits and foam mats strewn about the space, and it is easy to see that although both of these spaces are considered gymnasiums, the hazard present in these spaces varies greatly.
For the simple low fuel load, assembly-type gymnasium in an elementary school, the description in Section 5.2 might be appropriate:
“Light hazard occupancies shall be defined as occupancies or portions of other occupancies where the quantity and/or combustibility of contents is low and fires with relatively low rates of heat release are expected.”
This description may not, however, be appropriate for the gymnasium with the higher fuel loads. In order to properly classify a space such as this, the designer may need to look at the descriptions for ordinary and extra hazards.
The designer must not be fooled into thinking about the simple function of the space but, rather, must look closely at the contents of the portion of the building being classified to determine the appropriate category. This process is somewhat subjective, and the decision should be discussed with the authority having jurisdiction early in the sprinkler system design process.