Calculating Gross Occupancy Load

[Tim Mailloux]

Are you sure you need to include such “exterior unoccupied” spaces in your gross floor area calculation?

It has always been my interpretation of the BUILDING AREA definition that the unenclosed area under roof or floor projections gets included in the building area for compliance with chapter 5

[BG] AREA, BUILDING. The area included within surrounding exterior walls, or exterior walls and fire walls, exclusive of vent shafts and courts. Areas of the building not provided with surrounding walls shall be included in the building area if such areas are included within the horizontal projection of the roof or floor above.

 

[RHouse]

IMHO
Option 1 is meant for the majority of cases. In my understanding, this is intended to capture a real-world estimate of use. In the example, typically during a normal day, not all offices will be occupied 100% of the time. Some may have 2 or 3 at once. A person from the office down the hall may be talking to someone in the Lobby. Everyone may be in the Conference room (or the Conference room may be used by outside groups, thus calculated separately). But a gross calculation will give you a general case scenario. There is ALWAYS a worse case scenario, but the Code does not provide for that. That would be prohibitive to any new construction.

However, there are times that Option 2 is more valid than Option 1. If this business sole purpose is to have meetings with outside clients in each office all day every day (e.g. financial services, medical billing, etc.), then it might be wise to use the method which provides the higher occupant load. (Ref: IBC 2021 - 1004.5.1 or 1004.8)

I have two relevant experiences, both being restaurants.
1) This restaurant had an open seating plan, but we knew exactly how many seats were being purchased and included the layout of seats and number of people able to be seated plus servers, etc. The AHJ said they don't care. The dining area occupancy is based on the calculation, not person count. So, even though it calculated FEWER people than seats available, the AHJ calculated egress and toilets based on the smaller number. (Which was good for us, as we were on the bubble for an extra toilet.)

2) Another restaurant had a seating area and a long queue for placing orders and paying. Like REALLY long. We could have calculated the entire restaurant based on the Assembly occupancy and been done. But that was a really low number. So we divided the space: queue as "Assembly, standing", dining as "Assembly, unconcentrated", "Commercial Kitchen", "Accessory Storage", and "Business Areas" (for hallways, toilets, and office). The queue had as many people as the dining, but we lost a few in the storage and business areas, so it wasn't unmanageable. We added some egress width and shifted a few things around, but we all felt better about the occupancy load.