Ch. 3 - Occupancy Classifications and Loads

Delia Tacheny
INT 324 - Building Codes
Reading: The Codes Guidebook for Interiors

Keywords:

• occupancy class
• mixed occupancy
• incidental uses/ hazardous areas 
• accessory occupancy
• occupant load 

Every day we enter a variety of buildings... home, school, work, the grocery store, the doctor's office, etc. We go there for a particular purpose and activity, and then we leave. Have you ever given much thought to the safety of the buildings you enter? Have you ever considered what might happen in an XYZ situation? Have you ever considered why the exits are placed where they are, or the significance of that one sign that says the max. occupancy of a room? 

If I were to guess, I'd say the answer is probably not. Which is okay! Because someone else does. As we've made clear so far in our study of building codes, codes exist to protect occupants and ensure a smooth relationship between us and the built environment that we encounter. However, there are SO many different types of buildings, how are we supposed to apply the building codes to both a hospital and a grocery store when they serve such different purposes? That is where Occupancy Classifications and Loads come in handy. 

Occupancy Classifications

When working on any project, determining the occupancy classification and design load should be first on the to-do list as all other codes will be based on these factors. In general, an occupancy class is a way to label buildings based on their use. The three primary factors to consider are 1) the type of activity occurring, 2) the expected number of occupants, and 3) whether any unusual hazards or risk factors are present. The ICC and NFPA breakdown occupancy classes into very similar categories. Both, the I-Codes and NFPA codes also subdivide certain classifications into smaller, more specific categories which also may differ: For example, the IBC divides its Assembly occupancy into five subclassifications by types of use; the NFPA codes -- by number of occupants. Below is a chart to compare the general categories of both sets of codes. 

For the ICC, many of these categories are then broken up into subcategories by number; for example, under Residential there is R-1, R-2, R-3, and R-4. These subcategories are defined by additional factors such as how familiar occupants are with a space, the physical capabilities of occupants, or the level of "hazards" within different spaces (Ex. a hotel is R-1 as a "transient" unit where people stay temporarily, while a single-family home is R-3). 

It is also important to be aware of the sometimes-blurry exceptions amongst classifications. For example, when following the IBC, a day care center that serves fewer than five children may be considered Residential, but if it has more than five, it may be considered Institutional or Educational. So, if the number of occupants increase or decrease, the classification may have to be adjusted accordingly. 

Some other considerations for projects are whether or not the classification is considered "new" vs "existing" which will affect the codes that must be applied, especially when dealing with the NFPA Life Safety Code. Occupancy for a building may change if any of the factors mentioned above are changed. 

If a building contains multiple occupancies of fairly equal dominance, the building is considered mixed occupancy. 

"When the different occupancies are divided by the required rated assemblies (e.g., walls, floor, and/or ceiling assemblies), these occupancies are considered by the IBC to be separated mixed occupancies... Once separated, each occupancy only needs to meet the requirements of its own occupancy classification...

When there is no rated separation between the occupancies, it is considered by the IBC as a nonseparated mixed occupancy. (This term is not used by NFPA.) When the occupancies are considered nonseparated, the occupancies must both meet the requirements of the most stringent occupancy classification, including construction type, allowable area, finishes, fire protection, and exiting requirements."

Incidental uses (IBC) or hazardous areas (NFPA) are areas or uses in a building that are considered more hazardous, but small enough to not be labeled as a separate occupancy class. Example, a laundry room in a preschool facility. Some buildings contain two occupancies where one is much smaller than the other and is therefore defined as an accessory occupancy. According to the IBC, this area must be less than 10% of the total floor area. 

Occupant Loads

A brother to occupancy classifications, design loads are an important part of occupancy and the applicable codes. Occupant loads should not be confused with other design loads such as live and dead loads which are structural applications. 

The occupant load is the safe and comfortable number of occupants allowed in a space. The occupant load is needed determine egress components, and well as the number of plumbing fixtures, and other mechanical calculations. 

In order to calculate a reasonable number of people for typical uses within a building, the codes provide an occupant load factor, which is basically a determined amount of square footage assigned per person. In simple form, the occupant load is determined by dividing the total square footage by the determined load factor. 

These calculations can get a bit confusing though when it comes to gross load factors vs. net load factors. I recommend watching the video below to understand this concept better:

Source(s):

Kennon, Katherine E., and Sharon Koomen Harmon. “Chapter 3: Occupancy Classifications and Loads.” The Codes Guidebook for Interiors, John Wiley & Sons, Inc., Hoboken, NJ, 2022.