Calculating Gross Occupancy Load

[benjamin_flight]

I've been wanting to post this question to the group for a long time. I finally took time to draw up the two options to (hopefully) clearly describe my thoughts.
I understand that "Floor Area, Gross" and "Floor Area, Net" are defined in Chapter 2, and that Table 1004.5 Maximum Floor Area Allowances Per Occupant exists to help classify uses of spaces. My question is how to calculate gross areas, as it seems that there could be two ways of doing it.

In Opt 1, I used the term "gross" quite liberally and just made all Business spaces one area. You'll see that the result tallies to 31 occupants total for the tenant space.

In Opt 2, I used the more conservative method of calculating each occupied space individually and the typically-unoccupied spaces as one area. Note that the occupant count jumps to 34 using this method.

I feel that the calculation method of Opt 1 is what the code intends. However, the case could be made that large offices (such as Offices 4, 5, and 6) can easily fit 2+ people (and often do). The tenant space in this example is small, but imagine a floor with dozens of large offices; the occupancy count could jump quite a bit if using Option 2, and might be warranted. However, I don't see that the code intends for gross areas to be partitioned like Option 2.

Am I overthinking this? Is the calculation method of Option 1 the standard and I should just rely on that? Or is the more granular method of Option 2 warranted?

(Side note: please ignore the restroom situation. I'm sure a code-savvy person would catch that I'm not counting the shared restrooms. Still working through that...)

Thanks!
Benjamin

 

[fatboy]

I always used Option 1.

 

[Sifu]

Option 1.

 

[steveray]

Option 1 but...

[A]107.2.3​

The construction documents shall show in sufficient detail the location, construction, size and character of all portions of the means of egress including the path of the exit discharge to the public way in compliance with the provisions of this code. In other than occupancies in Groups R-2, R-3, and I-1, the construction documents shall designate the number of occupants to be accommodated on every floor, and in all rooms and spaces.

Sometimes you need to know who is in each room when you are flirting with thresholds like 50 OL and things like that...

 

[Blazer]

Option 1 is the code intentions in most circumstances. The code and commentary do not allow any reductions for gross takeoff except for exterior walls, vent shafts and interior courts; that's it.

I will say, that even when I have had a net area takeoff occupant load factor (for assembly unconcentrated) the AHJ's wanted an occupant load applied to the staging or "pre-function" space (corridor basically) outside of the large conference center rooms. We argued it is non-simultaneous use, either occupants are in the conference room or they are mingling in the pre-function space. This cost the project tremendously ($$$$) by adding extra stair width and extra plumbing fixtures beyond reason. Everyone interrupts the code differently.

 

[arwat23]

Option 1 is ideal, but some jurisdictions have forced me to use option 2. Their argument for option 2 makes sense from a logical standpoint to me, but it also makes things more complicated.

Let's say you have a bunch of 100sf offices. After enough offices, the OL will likely be less then the number of offices (since business areas have a 150sf gross load factor), meaning that, according to code, some offices wouldn't be occupied.

I run into this situation often on the projects I work on. It's gotten to the point where, up until recently, I defaulted to option 2.

 

[Yankee Chronicler]

Option 1.

The code is clear what the term "gross" means in this regard. The problem you face was created when the ICC changed the occupant load ratio for offices from 1 person per 100 s.f. to one person per 150 s.f. It had been 100 s.f. for decades, even long before the merger of the ICBO, SSBCC, and BOCA into the ICC. I'm sure someone made a good case for changing it to 150 s.f. per person, but i the real world it doesn't work.

Keep in mind, however, that the basic rule is the occupant load as calculated using Table 1004.5 or the actual occupant load, whichever is greater. From the 2021 IBC Commentary for Table 1004.5:

Table 1004.5 establishes minimum occupant densities
based on the function or actual use of the space (not
group classification). The table presents the maximum
floor area allowance per occupant (i.e., occupant load
factor) based on studies and counts of the number of
occupants in typical buildings. The use of this table,
then, results in the minimum occupant load for which
rooms, spaces and the building must be designed.
While an assumed normal occupancy may be viewed
as somewhat less than that determined by the use of
the table factors, such a normal occupant load is not
necessarily an appropriate design criterion. The greatest
hazard to the occupants occurs where an unusually
large crowd is present. The code does not limit the
occupant load density of an area, except as provided
for in Section 1004.5.1, but once the occupant load is
established, the means of egress must be designed for
at least that capacity. If it is intended that the occupant
load will exceed that calculated in accordance with the
table, then the occupant load is to be based on the estimated
actual number of people, but not to exceed the
maximum allowance in accordance with Section
1004.5.1.

 

[JPohling]

Option 1

 

[Yankee Chronicler]

Option 1 is ideal, but some jurisdictions have forced me to use option 2. Their argument for option 2 makes sense from a logical standpoint to me, but it also makes things more complicated.

Let's say you have a bunch of 100sf offices. After enough offices, the OL will likely be less then the number of offices (since business areas have a 150sf gross load factor), meaning that, according to code, some offices wouldn't be occupied.

I run into this situation often on the projects I work on. It's gotten to the point where, up until recently, I defaulted to option 2.

Nope. This relates back to a discussion here a few weeks ago about whether occupant calculations get rounded up or down. I was always told to round up, because there are no partial people. Even in ordinary math, you generally round up if the fraction is one-half of greater. So a 100 s.f. office calculates out to .66666666667 occupants. That's one occupant, whether by the "There are no fractions of people" rule or by the general rule of 1/2 or more gets rounded up.

Where the general mathematical rule fails is for very small offices. Say there's a bunch of offices that are 9' x 8' -- 72 s.f. each. 72/150 = 0.48. The general math rule would say to round down, but it makes no sense to assign a zero occupant load to an office that will almost certainly have an occupant. By the "There are no fractions of people" rule, every office gets at least one occupant.

The code says to use the number as calculated by Table 1004.5 or the actual number, whichever is greater. So I start by taking the overall, gross area of the office floor plate or suite and divide that by 150. That's the base occupant load. Then I look at each office and whether it's going to have one, two, three or 'X' occupants, and add up the occupant loads of the individual rooms and spaces. Whichever number is higher is the number I need to provide egress capacity for.

 

[walker.t]

This relates back to a discussion here a few weeks ago about whether occupant calculations get rounded up or down

Maybe this thread?

https://www.thebuildingcodeforum.co...ant-load-calculations-round-up-or-down.36668/

I checked a life safety plan I have on file, four-story building, approximately 24,000 s.f./floor, Business with accessory Assembly and Storage, they used Option 1.

 

[Yankee Chronicler]

Maybe this thread?

https://www.thebuildingcodeforum.co...ant-load-calculations-round-up-or-down.36668/

I checked a life safety plan I have on file, four-story building, approximately 24,000 s.f./floor, Business with accessory Assembly and Storage, they used Option 1.

I believe that's the one.

 

[arwat23]

Nope. This relates back to a discussion here a few weeks ago about whether occupant calculations get rounded up or down. I was always told to round up, because there are no partial people. Even in ordinary math, you generally round up if the fraction is one-half of greater. So a 100 s.f. office calculates out to .66666666667 occupants. That's one occupant, whether by the "There are no fractions of people" rule or by the general rule of 1/2 or more gets rounded up.

That's if you go with option 2, in which case I agree. Round up each room to determine the OL. Option 1 says you don't calculate each room individually, so rounding up each room isn't necessary. I believe IBC commentary supports this (I don't have access to that to double check though).

Which section of code says that you have to use the actual occupant load if it's greater then what's calculated with 1004.5? Genuine question, because I don't see that (even though it makes logical sense to me).

 

[Paul Sweet]

1004.5.1 allows you to increase the occupant load, but doesn't require it.

If I was forced to count each space separately (option 2) I wouldn't count corridors, restrooms, mechanical spaces, and other spaces that aren't listed in Table 1004.5, I would assume that the fractional occupants from rounding up are in these spaces and use the greater of the room by room count or option 1.

 

[Yankee Chronicler]

That's if you go with option 2, in which case I agree. Round up each room to determine the OL. Option 1 says you don't calculate each room individually, so rounding up each room isn't necessary. I believe IBC commentary supports this (I don't have access to that to double check though).

Under section 107.2.3, the 2021 IBC requires that In other than occupancies in Groups R-2, R-3, and I-1, the construction documents shall designate the number of occupants to be accommodated on every floor, and in all rooms and spaces.

Which section of code says that you have to use the actual occupant load if it's greater then what's calculated with 1004.5? Genuine question, because I don't see that (even though it makes logical sense to me).

The 2021 Commentary for section 1004.1 says:

The design occupant load is the number of people intended to occupy a building or portion thereof at any one time; essentially, the number for which the means of egress is to be designed. It is the largest number derived by the application of Sections 1004.1 through 1004.8. Occupant density is limited to ensure a reasonable amount of freedom of movement (see Section 1004.5.1). The design occupant load is also utilized to determine the required plumbing fixture count (see commentary, Chapter 29) and other building requirements, such as automatic sprinkler systems and fire alarm and detection systems (see Chapter 9).

The 2021 IBC Commentary for Table 1004.5 says::

Table 1004.5 establishes minimum occupant densities based on the function or actual use of the space (not group classification). The table presents the maximum floor area allowance per occupant (i.e., occupant load factor) based on studies and counts of the number of occupants in typical buildings. The use of this table, then, results in the minimum occupant load for which rooms, spaces and the building must be designed. While an assumed normal occupancy may be viewed as somewhat less than that determined by the use of the table factors, such a normal occupant load is not necessarily an appropriate design criterion. The greatest hazard to the occupants occurs where an unusually large crowd is present. The code does not limit the occupant load density of an area, except as provided for in Section 1004.5.1, but once the occupant load is
established, the means of egress must be designed for at least that capacity. If it is intended that the occupant load will exceed that calculated in accordance with the table, then the occupant load is to be based on the estimated actual number of people, but not to exceed the maximum allowance in accordance with Section 1004.5.1. Therefore, the occupant load of the office or business areas in a storage warehouse or nightclub is to be determined using the occupant load factor most appropriate to that space—one person for each 150 square feet (13.9 m2) of gross floor area.
. . .
Table 1004.5 presents a method of determining the absolute base minimum occupant load of a space that the means of egress is to accommodate.

 

[walker.t]

My question is how to calculate gross areas

If I was forced to count each space separately (option 2) I wouldn't count corridors, restrooms, mechanical spaces, and other spaces that aren't listed in Table 1004.5

I think you have to include corridors, restrooms, and mechanical spaces when calculating gross areas:

2018 IBC Definition of “Floor Area, Gross” (partial quote, emphasis added)

The floor area within the inside perimeter of the exterior walls of the building under consideration…without deduction for corridors, stairs, ramps, closets, the thickness of interior walls, columns or other features.

When calculating net floor area those areas may be excluded:

2018 IBC Definition of “Floor Area, Net” (emphasis added)

The actual occupied area not including unoccupied accessory areas such as corridors, stairways, ramps, toilet rooms, mechanical rooms and closets.

 

[Yankee Chronicler]

I think you have to include corridors, restrooms, and mechanical spaces when calculating gross areas:


When calculating net floor area those areas may be excluded:

Correct.

 

[brokenkeys]

Option 1 is ideal, but some jurisdictions have forced me to use option 2. Their argument for option 2 makes sense from a logical standpoint to me, but it also makes things more complicated.

Let's say you have a bunch of 100sf offices. After enough offices, the OL will likely be less then the number of offices (since business areas have a 150sf gross load factor), meaning that, according to code, some offices wouldn't be occupied.

I run into this situation often on the projects I work on. It's gotten to the point where, up until recently, I defaulted to option 2.

Option 1 is correct in my opinion, but so many municipalities I work in require it by room that I also default to option 2.

Also, chapter 3 lets you classify your conference room as business. So, do you keep that at 1:15, or drop it down to 1:150?

This is a great topic, especially when plumbing calcs are part of the calculus.

 

[arwat23]

Option 1 is correct in my opinion, but so many municipalities I work in require it by room that I also default to option 2.

Also, chapter 3 lets you classify your conference room as business. So, do you keep that at 1:15, or drop it down to 1:150?

This is a great topic, especially when plumbing calcs are part of the calculus.

Ch 3 has nothing to do with Table 1004.5. The conference room may be classified as 'B', but the "function of space" is still unconcentrated (15 net).

What matters is how the space is used, not what it's occupancy classification is. For offices, I always make the break room, conference room, and the waiting room 15 net since "Unconcentrated (tables and chairs)" is the closest function of these spaces.

Here's a link to a very short article (referencing an older version of IBC) discussing this in far clearer language than I can write: https://coderedconsultants.com/insights/cracking-the-code-occupant-loads-part-i/

 

[Tim Mailloux]

For offices, I always make the break room, conference room, and the waiting room 15 net since "Unconcentrated (tables and chairs)" is the closest function of these spaces.

if a break room has tables and chairs I will use the 1 per 15 net Unconcentrated tables and chairs factor, but if its just a small kitchenette where people grab a cup of coffee and stash their lunch (no tables and chairs), then I include that space into the gross business area 1 per 150 calculation.

 

[arwat23]

if a break room has tables and chairs I will use the 1 per 15 net Unconcentrated tables and chairs factor, but if its just a small kitchenette where people grab a cup of coffee and stash their lunch (no tables and chairs), then I include that space into the gross business area 1 per 150 calculation.

Depends on the layout imo. For example, if it's an entirely separate room from the rest of the space (rather than just a counter / sink that's just open the the rest of the office), I'd still use 15 Net (or worse, "standing space"). That said, I can see an argument for Business Area if the space wasn't a separate room.

Only problem, in my opinion, is that a kitchenette doesn't really have the "function" of a business area. Business is not conducted in a kitchenette typically. Maybe I'm splitting hairs and being too anal about it...

 

[Tim Mailloux]

Depends on the layout imo. For example, if it's an entirely separate room from the rest of the space (rather than just a counter / sink that's just open the the rest of the office), I'd still use 15 Net (or worse, "standing space"). That said, I can see an argument for Business Area if the space wasn't a separate room.

Only problem, in my opinion, is that a kitchenette doesn't really have the "function" of a business area. Business is not conducted in a kitchenette typically. Maybe I'm splitting hairs and being too anal about it...

Toilet rooms, corridors, stairs & mechanical chases don't really have a business function either, but they all get included in the Business gross area.

 

[arwat23]

Toilet rooms, corridors, stairs & mechanical chases don't really have a business function either, but they all get included in the Business gross area.

Those all serve a "business area" and there's no other function that would be applicable to spaces like those. "Business area" (or any other "gross" load factor functions) is the most accurate function for those spaces due to the definition of "floor area, gross" in IBC.

In contrast, there's another function that could apply to kitchenettes. If a different function applies to break rooms, why wouldn't a different function apply to kitchenettes without seating as well? I would argue that one of the assembly without fixed seating functions is a more accurate function to a separated kitchenette than a business function. Especially since nothing is stopping anyone from just adding furniture later, which would trigger an assembly function as you described previously.

Now, again, if the kitchenette is something like the image below, then sure, I see an argument for the kitchenette being a Business Area function. It shares the same space as what is clearly a business area, there aren't well defined boundaries for the kitchenette, no complaints here. If it's in it's own room, what makes it any different than a break room in terms of function? In my experience on the projects I work on, few people end up using the break room as a space to take a break anyways. Most of the time, they just take their food back to their office or desk. I've yet met a BO that would accept that argument as a way to make it a "business area" function instead. There's a function for standing space.

 

[PatrickGSR94]

My firm always does it by the total Gross area, except for smaller net areas as needed. Years ago we used to do it room by room, but that wasn't couting wall widths and things that are part of the Gross area, so we changed our methods. What I do is typically calculate any net areas separately (classrooms, assembly spaces and so forth), then add in the Gross areas that cover the rest of the building including wall widths, corridors, restrooms etc. and run that calculation. So an office with a conference room and a lunch room would have the net areas at 1/15 for each of those two ancillary spaces, and then 1/150 for the rest of it, measure to the inside surface of the exterior walls. I typically also include any canopies or outdoor coverings attached to the building, and list them as "exterior unoccupied" spaces. That way they're included in the overall Gross Floor Area calculation, but do not contribute to the occupant load calcs.

 

[walker.t]

I typically also include any canopies or outdoor coverings attached to the building, and list them as "exterior unoccupied" spaces. That way they're included in the overall Gross Floor Area calculation, but do not contribute to the occupant load calcs.

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

2018 IBC Definition of “Floor Area, Gross” (partial quote, emphasis added)
The floor area within the inside perimeter of the exterior walls…

“Canopies and outdoor coverings” are not inside the inside perimeter of the exterior walls.

2018 IBC Definition of “Floor Area, Gross” (partial quote, emphasis added)
The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above.

An area that can be used will have an occupant load. Your “exterior unoccupied” spaces are unoccupied and therefore not usable and not included in the gross floor area.