How many people can fit in this space? What do occupancy signs mean?
If you have ever walked into a restaurant or any other business, you may have noticed a sign indicating a maximum occupancy for the building. What is an “occupant” and where does that maximum occupancy number come from?
As architects start working on a building design, one of the first things we do is determine the number of people that can be in the space. The technical term for this is the “occupant load.” The occupant load (OL) is a significant number because it affects many other decisions about the building or space. Such as the number of toilets and sinks needed, whether or not the building will need a fire protection system, and how many exits will be required. These are just a few items that are affected by the OL.
So what voodoo formula is used in determining an OL? Actually, it’s pretty straight forward. As I mentioned, architects start by determining how the designed space will be used, such as for a restaurant, a theater, or maybe an office building or warehouse. Each type of building use has an alphanumeric (something that contains letters and numbers) occupancy classification (OC) within the International Building Code (IBC).
For example, a restaurant might be classified as A-2, a theater would be an A-1, the office building would be a “B,” and the warehouse would be an “S.” The letters are related to the use of the space: “A” = Assembly (groups of people), “B” = Business, “S” = Storage, and so forth. There are many types of occupancy classifications and sub-classifications. Click here to see classifications
After the OC is determined, table 1004.5 (shown below) in the IBC provides the “occupant load factor” (OLF). I’m not quite sure how these numbers were determined (I’m sure it involved many late nights of research and numerous cups of coffee); these numbers are a strong factor in determining the OL. For example, according to table 1004.5, the OLF of an A-2 restaurant is 15. To determine the maximum OL, architects have to do the math: take the square footage of the building and divide it by the OLF to determine the OL.
Have I lost you yet? Maybe an example will help.
Let’s assume we are building a 2000 square foot restaurant (A-2) with an OLF
of 15. 2000/15 = 133. Thus, we can have a maximum of 133 occupants in the space at one time. That is the occupancy load for our imagined restaurant. Now the designers can determine the exits, the sinks, etc., as well as the correct type of fire protection system that would be needed to safely manage a fire if one broke out when the restaurant was at capacity.
The OLF for an “S” occupancy is 300. If we have a 40,000/sf warehouse, the OC is calculated this way:
40,000sf/300 = 133.
As you can see, we have the same OL even though the square footage is significantly greater. This is because of the higher OLF in this type of building. It stands to reason when you think about it: a restaurant is going to have many more people (staff, patrons, etc.), but a warehouse typically holds a lot of “stuff” and not many people.
The next time you’re in a restaurant or office building, and you see the occupant load sign, you’ll know how the architect arrived at the OL. Do a quick headcount and see if the occupants in the space are consistent with the occupancy limits, and know that the architect thoughtfully planned for fire protection, egress, and trips to the restroom.
All Information was obtained from 2018 International Building code and 2015 International Building code.