Occupant load calculations for corridors serving office space and assembly space

[Tim Mailloux]

I am currently in the process of calculating the occupant load for a mixed-use floor plate that includes both Business and Assembly functions. A significant feature of this floor plate is a large central atrium, which is surrounded by common circulation paths on all four sides. To provide more context, the east and west sides of the atrium are are large assembly spaces, while the north and south sides house office areas.

Given that the circulation around the atrium serves both the Business Area gross function and the Assembly area net function, I am seeking clarification on how to treat the area of the atrium circulation. Specifically, should the area of this atrium circulation be omitted from any occupant load calculations because it serves the Net Assembly function? Alternatively, do we prorate the area of the atrium circulation? For instance, if 60% of the floor plate area is office space, should 60% of the circulation area be included in the Business Area gross occupant load calculation?

 

[steveray]

The jerk in me wants to count any corridor that serves the B...but maybe just minimum width....The nice guy in me wants to accept whatever the ethical designer puts forth....

 

[Yankee Chronicler]

It's difficult to respond without seeing the plan. For whatever it's worth, I'll toss out my understanding of why corridors are included in calculating the occupant load for "gross s.f." areas, and see if that helps.

Why would we include corridors in calculating the occupant load for occupancies such as B- Business? After all, the people sitting at desks in the offices and cubicles are same people who will be using those corridors for egress. I haven't seen this in any commentary, but my view is that the occupant load factor is low (B used to be just 1 person per 100 s.f., and now it's 1 person per 150 s.f.), and using the gross floor area accounts for the likelihood that the space may be reconfigured in the future and the code wants to ensure that the overall floor plate/story has sufficient egress capacity and number of exits to allow for future reconfiguration. If we use the gross floor area, as long as the use and occupancy classification don't change, it shouldn't matter how much the plan changes.

Assembly venues are calculated based on much higher occupant load factors, and (particularly for large assembly spaces such as theaters and stadiums) are much less likely to undergo future plan changes that would significantly affect egress numbers or capacity. So (my guess is) it makes less sense to include the corridors and toilet rooms when the occupant load is going to be driven by the number of seats actually available.

All that is kind of a long-winded way of suggesting that whatever portion of the means of egress can logically be "assigned" to the B occupancy should be included in the B occupant load calculation, and the portion attributable to the A occupancy doesn't get counted. In other words -- if the A portion of the building doesn't change but the B area gets completely reconfigured five years from now for a new tenant or tenants, how much of the egress could be affected by the B tenant reconfiguration?

 

[steveray]

It's difficult to respond without seeing the plan. For whatever it's worth, I'll toss out my understanding of why corridors are included in calculating the occupant load for "gross s.f." areas, and see if that helps.

Why would we include corridors in calculating the occupant load for occupancies such as B- Business? After all, the people sitting at desks in the offices and cubicles are same people who will be using those corridors for egress. I haven't seen this in any commentary, but my view is that the occupant load factor is low (B used to be just 1 person per 100 s.f., and now it's 1 person per 150 s.f.), and using the gross floor area accounts for the likelihood that the space may be reconfigured in the future and the code wants to ensure that the overall floor plate/story has sufficient egress capacity and number of exits to allow for future reconfiguration. If we use the gross floor area, as long as the use and occupancy classification don't change, it shouldn't matter how much the plan changes.

Assembly venues are calculated based on much higher occupant load factors, and (particularly for large assembly spaces such as theaters and stadiums) are much less likely to undergo future plan changes that would significantly affect egress numbers or capacity. So (my guess is) it makes less sense to include the corridors and toilet rooms when the occupant load is going to be driven by the number of seats actually available.

All that is kind of a long-winded way of suggesting that whatever portion of the means of egress can logically be "assigned" to the B occupancy should be included in the B occupant load calculation, and the portion attributable to the A occupancy doesn't get counted. In other words -- if the A portion of the building doesn't change but the B area gets completely reconfigured five years from now for a new tenant or tenants, how much of the egress could be affected by the B tenant reconfiguration?

Seems like a very nice assessment...

 

[Tim Mailloux]

It's difficult to respond without seeing the plan. For whatever it's worth, I'll toss out my understanding of why corridors are included in calculating the occupant load for "gross s.f." areas, and see if that helps.

Why would we include corridors in calculating the occupant load for occupancies such as B- Business? After all, the people sitting at desks in the offices and cubicles are same people who will be using those corridors for egress. I haven't seen this in any commentary, but my view is that the occupant load factor is low (B used to be just 1 person per 100 s.f., and now it's 1 person per 150 s.f.), and using the gross floor area accounts for the likelihood that the space may be reconfigured in the future and the code wants to ensure that the overall floor plate/story has sufficient egress capacity and number of exits to allow for future reconfiguration. If we use the gross floor area, as long as the use and occupancy classification don't change, it shouldn't matter how much the plan changes.

Assembly venues are calculated based on much higher occupant load factors, and (particularly for large assembly spaces such as theaters and stadiums) are much less likely to undergo future plan changes that would significantly affect egress numbers or capacity. So (my guess is) it makes less sense to include the corridors and toilet rooms when the occupant load is going to be driven by the number of seats actually available.

All that is kind of a long-winded way of suggesting that whatever portion of the means of egress can logically be "assigned" to the B occupancy should be included in the B occupant load calculation, and the portion attributable to the A occupancy doesn't get counted. In other words -- if the A portion of the building doesn't change but the B area gets completely reconfigured five years from now for a new tenant or tenants, how much of the egress could be affected by the B tenant reconfiguration?

Due to the construction and historical significance of this building, the common circulation areas around the atrium will never change. That being said I agree with your logic and I am leaning towards prorating this common area between the two uses. The business function is currently 74% of the usable area on this floor, as such I will multiple the total area of the common circulation and toilet cores (4,672 sf) by .74 to come up with the prorated portion of the common circulation core and toilet core that will get included in the Business gross calculation (3,457 sf). This only reduces my over all occupant load by 10 people, but that 10 person reduction will allow me to have an additional small conference room without maxing out the stairs

 

[steveray]

Due to the construction and historical significance of this building, the common circulation areas around the atrium will never change. That being said I agree with your logic and I am leaning towards prorating this common area between the two uses. The business function is currently 74% of the usable area on this floor, as such I will multiple the total area of the common circulation and toilet cores (4,672 sf) by .74 to come up with the prorated portion of the common circulation core and toilet core that will get included in the Business gross calculation (3,457 sf). This only reduces my over all occupant load by 10 people, but that 10 person reduction will allow me to have an additional small conference room without maxing out the stairs

Or maybe use the min. required egress width for the B use and see if that has an advantage that is worth pursuing...? Depends on which way you want to push it...

 

[Tim Mailloux]

I was just looking at the 2021 IBC Code Illustrated (application example 1004-1 on page 378) which shows how to calculate egress for a mixed use assembly and business area floor plan with shared common circulation and toilet cores. Per the example given these shared common circulation and toilet areas would not be included in the occupant load calculations

Solution: The occupant load is simply 282, the combination of the assembly and business spaces. It is not necessary to consider the corridor, toilet rooms, and other small accessory spaces that serve the entire building. Note that within the office area itself, such circulation and accessory areas would be included in the calculation.

 

[RLGA]

Being on the west side of the country, I'm usually late to some of these discussions, but I tend to agree with @Yankee Chronicler.

If a portion of the circulation is strictly in support of the assembly occupancy, I would automatically discount that area from the business area occupant load factor. If the circulation areas are separate but not well-defined, showing a logically placed demarcation line on the plans would be sufficient, in my opinion.

If the circulation is used by all persons (e.g., business occupants need to use the circulation adjacent to the assembly occupancy to access restrooms or other business-related functions), then the circulation should be included in the business area occupant load.

However, if none of the above apply, then I could get behind the concept of proportionality based on the ratio of areas.

 

[Tim Mailloux]

Being on the west side of the country, I'm usually late to some of these discussions, but I tend to agree with @Yankee Chronicler.

If a portion of the circulation is strictly in support of the assembly occupancy, I would automatically discount that area from the business area occupant load factor. If the circulation areas are separate but not well-defined, showing a logically placed demarcation line on the plans would be sufficient, in my opinion.

If the circulation is used by all persons (e.g., business occupants need to use the circulation adjacent to the assembly occupancy to access restrooms or other business-related functions), then the circulation should be included in the business area occupant load.

However, if none of the above apply, then I could get behind the concept of proportionality based on the ratio of areas.

RLGA, what are your thoughts on the IBC Code Illustrated example I stated in post #7?

 

[steveray]

Note that within the office area itself, such circulation and accessory areas would be included in the calculation.

That is where it gets funky.....Even with ICC logic...

 

[Yankee Chronicler]

I was just looking at the 2021 IBC Code Illustrated (application example 1004-1 on page 378) which shows how to calculate egress for a mixed use assembly and business area floor plan with shared common circulation and toilet cores. Per the example given these shared common circulation and toilet areas would not be included in the occupant load calculations

Solution: The occupant load is simply 282, the combination of the assembly and business spaces. It is not necessary to consider the corridor, toilet rooms, and other small accessory spaces that serve the entire building. Note that within the office area itself, such circulation and accessory areas would be included in the calculation.

Interesting. The 2021 IBC Commentary seems to say just the opposite! IBC section 1004.3 addresses Multiple Function Occupant Load. The commentary for 1004.3 reads as follows:

In buildings with different functions of space, the
areas are considered separately. In other words,
spaces with net floor area requirements are calculated
and removed from the total area before applying gross
floor area requirements. For example, an office building’s
15-square-foot net floor area for loose tables and
chairs would determine the occupant load in its conference
rooms, and the 100-square-foot gross floor area
for business would determine the occupant load for the
rest of the areas in the building. There can be function
areas within the same room. A fast-food restaurant is a
good example of multiple uses in the same space. To
determine occupant load, a designer could use a 7-
square-foot net area for assembly standing space for
the waiting area in front of the order counters; a combination
30-square-foot net area for unconcentrated
assembly of loose tables and chairs and Section
1004.6 for fixed seating for the dining areas; and a
200-square-foot gross area for the commercial kitchen
areas behind the counter. Not all buildings have occupant
loads for all spaces. The fast-food restaurant
example might not have an assigned occupant load for
the toilet rooms or the corridor leading to the toilet
rooms.

The bottom line, I think, is to be conservative and to have a defensible explanation for how you arrived at your calculated occupant load. I think (and hope) that most building officials will accept anything that looks reasonable.

So what's not reasonable? Last year we had an applicant who had a wide-open space she wanted to rent out as a multi-function even space. The calculated occupant load at 1:15 was something like 200 occupants, and at 1:7 it was significantly over 400. She wanted to arbitrarily limit the occupant load to 150 so she wouldn't have to add any plumbing fixtures. We didn't think that was reasonable, and the Fire Marshal agreed.

From your description, if the circulation around the atrium and the atrium itself will never change, then I could see excluding it from the gross area of the B portion of the building. I think prorating it is going the extra mile and I would hope that the BO in whatever town or city the building is located in will see that as a well-intentioned and responsible solution.

 

[RLGA]

RLGA, what are your thoughts on the IBC Code Illustrated example I stated in post #7?

I don't have a copy of the book, but I'll take your word for it. I know of Doug, and as a consultant for ICC, he's an authority on the subject. However, each situation is unique and would have to be looked at individually to determine how to apply the code and which solution would best serve the occupants in an emergency without being overly burdensome on the owner and their design team.

 

[steveray]

I know of Doug, and as a consultant for ICC, he's an authority on the subject.

I think he's the don't round up people for egress guy, which we also do...

 

[Tim Mailloux]

I think he's the don't round up people for egress guy, which we also do...

he sure is.....didn't he also do an in person seminar last year in CT?

 

[Yankee Chronicler]

he sure is.....didn't he also do an in person seminar last year in CT?

Doug Thornburg. Yes, he was the presenter at an all-day ICC training seminar.