Figuring safety in mixed-use structures

NFPA Journal, May/Jun 2002 by Carson, Chip

NFPA 101(R) includes provisions to determine if an occupancy is mixed-use or incidental.

Most larger buildings have mixed-use occupancies. Hotels, for example, contain residential, offices, assembly, and storage areas, even parking garages. Office buildings have meeting rooms, cafeterias, and storage areas. And factories have cafeterias, offices, and storerooms.

The question facing designers and authorities having jurisdiction is whether such multiple uses represent mixeduse occupancies or incidental occupancies. The answer to this question will determine how the codes are applied.

NFPA 101(R), Life Safety Code(R), defines mixed-use buildings as those with two or more occupancy classifications so intermingled that separate safeguards can't be provided. Incidental occupancies, on the other hand, are areas that are either considered incidental to the buildings' primary occupancy classification or areas that have a nonresidential use that allows for an occupant load under the occupancy threshold established in Chapter 6 of the Life Safety Code. For example, a meeting room that holds no more than 50 people isn't an assembly occupancy and so can be considered incidental to the building's primary use.

Where incidental occupancies occur, the building must comply with the requirements that apply to its primary occupancy. Residential uses can't be considered incidental. Any space containing sleeping accommodations must be classified as a residential occupancy.

Other than sleeping accommodations, the Life Safety Code doesn't establish any qualifications for determining when an occupancy is incidental, since one can always play "games" with such things as size limits, staying just one square foot under the limit or breaking the space up into several smaller spaces.

However, the code considers a building to contain mixed occupancies if those occupancies share portions of the required means of egress.

Separate safeguard

When a building is considered mixed-use, the most stringent requirements of the occupancies involved apply to those particular areas. For example, office areas must comply with business occupancy requirements. However, requirements that affect the entire building, such as sprinklers or fire alarm systems, must apply to the entire building.

Portions of the means of egress shared by the various uses must also meet the most stringent requirements of the occupancies involved. Separate safeguards for egress have generally been interpreted to mean that people in the different occupancies have their own separate means of escape from the occupancy.

Although the Life Safety Code doesn't require that occupancies have a specific fire-rated separation between them to be considered separate occupancies, the building codes used in a particular jurisdiction may.

Determining the question of mixed or incidental occupancies can present designers and authorities having jurisdiction with quite a challenge, but an egress analysis can help.

CHIP CARSON, P.E.

WAYNE "CHIP" CARSON,

P.E., is president of Carson

Associates Inc., a fire

protection engineering

and code consulting firm

in Warrenton, Virginia.