Room occupant load calculations: round up or down?

[dbartley]

When calculating the net occupant load of a room the math almost never results in a whole number. In those cases do you round your math up or down?

As an example, I do a lot of school work, and we typically try and keep classrooms under 1,000sf as to not require two exits and exit doors that have to swing in the direction of egress travel. But say you have a 985sf classroom with an occupant load factor of 1 person per 20sf. This works out to a calculated value of 49.25 people in that classroom. There is no such thing as ¼ of a person, and logic would say you can round down to 49 occupants. But it seems far more common for people to blanketly round up all calculations related to occupant loads.

I attended a chapter 10 egress seminar this past spring, the guest instructor was an ICC Staff member. I asked the instructor this question off-line during one of the breaks and he was in favor of rounding down.

When calculating number of exits, the tables (in 2021 IBC and later) now state "MAXIMUM OCCUPANT LOAD OF SPACE" OR "MAXIMUM OCCUPANT LOAD PER STORY"
So, I read that to mean if it's any decimal over 49, or 29, or whatever the maximum is for that occupancy, it needs a second exit.
For a third exit it states "from 501 to 1000," and four if any "greater than 1000." I would read those literally.

 

[mtlogcabin]

And what happens when you have classrooms on one side of a corridor (20net) and offices (150 gross) on the other? How much of the corridor goes with the gross?

1004.4 Multiple occupancies.
Where a building contains two or more occupancies, the means of egress requirements shall apply to each portion of the building based on the occupancy of that space. Where two or more occupancies utilize portions of the same means of egress system, those egress components shall meet the more stringent requirements of all occupancies that are served.

 

[Mr. Inspector]

I don't assign occupant loads to individual offices as they are part of a larger gross occupant load calculation encompassing the whole business area which includes corridors, toilet rooms, stairs & mechanical shafts. This approach is consistent with both the IBC code commentary and IBC Code Illustrated books.

Our state replaced Chapter 1 of all the I codes with the PA UCC. This requires an egress plan with the occupancy of each room. But the total occupants can still by the above.

 

[steveray]

1004.4 Multiple occupancies.
Where a building contains two or more occupancies, the means of egress requirements shall apply to each portion of the building based on the occupancy of that space. Where two or more occupancies utilize portions of the same means of egress system, those egress components shall meet the more stringent requirements of all occupancies that are served.

I get it, but the classrooms don't need to add the corridor, but the offices do....Do they add the whole corridor? What if it is 8' wide? Does it really matter?

 

[Mech]

Our state replaced Chapter 1 of all the I codes with the PA UCC. This requires an egress plan with the occupancy of each room. But the total occupants can still by the above.

How long has the egress plan been in effect? I may have only had to make one or two.

 

[mtlogcabin]

And what happens when you have classrooms on one side of a corridor (20net) and offices (150 gross) on the other? How much of the corridor goes with the gross?

Corridors do not have an occupant load based on their sq footage per person. The OL is determined by the width of the corridor and therefore needs to be large enough to handle all the occupants that will be exiting through that corridor.
An "E" occupancy is required to have a minimum 72" wide corridor (360 people) if more than 100 people using the corridor which is about 3 classrooms.
Don't over think this rounding up or down it will likely not make a difference for egress purposes but could drastically make a difference with fixture counts and when a fire suppression system and/or alarm system is required.

 

[arwat23]

Corridors do not have an occupant load based on their sq footage per person. The OL is determined by the width of the corridor and therefore needs to be large enough to handle all the occupants that will be exiting through that corridor.
An "E" occupancy is required to have a minimum 72" wide corridor (360 people) if more than 100 people using the corridor which is about 3 classrooms.
Don't over think this rounding up or down it will likely not make a difference for egress purposes but could drastically make a difference with fixture counts and when a fire suppression system and/or alarm system is required.

I think the question relates to if you include the corridor area in the occupancy calculations. In a office building, you include corridors in the area since the function of space has a gross load factor. When you have two different functions on opposite sides of the same corridor, one with a net load factor and one with a gross load factor, would you include the corridor area when determining the OL or not?

 

[mtlogcabin]

would you include the corridor area when determining the OL or not?

Per the definitions corridors are included with the gross floor calculations. see post #68

 

[arwat23]

Per the definitions corridors are included with the gross floor calculations. see post #68

Yes, that's what the definition says.

"Net" wouldn't include the corridor in floor area while "gross" would. So, when a corridor serves both a net and gross load factor space, the entire corridor gets included in the gross load factor floor area. Is that correct?

 

[mtlogcabin]

That's correct IMHO

 

[Paul Sweet]

The interpretation I've usually seen in classroom buildings is that corridors serving mainly classrooms or other net load factor spaces don't contribute to the occupant load, but corridors within office suites off the main corridors do contribute to the occupant load.

In a building with offices on one side of a corridor and classrooms or labs on the other side it would be reasonable to not assign an occupant load to the corridor if you are listing occupant loads for every room and rounding up. An example would be where the office might have 0.75 occupants and the corridor would have the remaining 0.25 occupants.

 

[steveray]

Even I didn't know there were this many ways to skin this cat.....Probably why it hasn't been "fixed" yet...

 

[Tim Mailloux]

Yes, that's what the definition says.

"Net" wouldn't include the corridor in floor area while "gross" would. So, when a corridor serves both a net and gross load factor space, the entire corridor gets included in the gross load factor floor area. Is that correct?

I would say it depends.......

 

[Tim Mailloux]

The interpretation I've usually seen in classroom buildings is that corridors serving mainly classrooms or other net load factor spaces don't contribute to the occupant load, but corridors within office suites off the main corridors do contribute to the occupant load.

this is the approach I have always taken.

 

[Penguin87]

My plans have been rejected on multiple occasions when I've tried that method. At least in the jurisitication I work, the definition of "space" is interoperated as a definable space, such as a room (California added a definition for the word "space" in their codes, but I don't see how their definition conflicts with IBC). A room is a space by definition, therefore each room, gross or net, should be calculated independently, rounded up, then added. For gross load factors, measure to the center of the wall so that the thickness is accounted for. That's how a few Building Officials in my area have explained it to me at least.

I've can often get plans approved using the method you show in that diagram when there's a 3rd party plan review, but when it's in-house at the jurisdiction, they rarely get approved.

I have had the same experience, in multiple jurisdictions - ie, Portland & Seattle, North Carolina etc.