What is an exit capacity factor?
In our last segment we talked about the number of exits and the sizing of exits.
How do we size an exit?
We said it was related to the number of occupants that are assigned to that exit, [03B] but how is that done? That’s what we’re covering today – and the answer lies in an exit capacity factor.
This video is strictly in the context of the International Building Code. We’ll cover the IBC here, and then in our next video, talk about NFPA 101 and the differences between the two.
So, what is an exit capacity factor? How was it developed? What are the exit capacity factors for various means of egress components? How specifically do I use it on my projects?
In 1927, NFPA determined that the average person required 22 inches of standing space on a stair, meaning that if a stair was 44 inches wide, it could accommodate two people standing side by side. This led to the NFPA requirement that new buildings must be provided with stairs at least 44 inches in width and that existing buildings must have stairs at least 36 inches. The building codes generally adopted these requirements and they can still be seen today.
Confusingly, the 22 inches became known as a “unit of exit width” and early codes required occupant loads to be compared to available units of exit width. Meaning every exit was divided by 22 inches, resulting in fractions of exit width units.
Thankfully for all of us, this has been simplified.
Instead of dividing by 22 to get “units of exit width,” we divide the width of the exit (in inches) by the exit capacity factor.
The exit capacity factor has units of inches per occupant, so by dividing an exit width, say a 44” stair by a factor of 0.3” per occupant, results in 146 occupants (rounding down to the nearest whole number).
This is then compared to how many occupants there are on the floor. Much easier to apply and understand than 22-inch units of exit width.
CALCULATING EXITING CAPACITY FOR STAIRS
I mentioned 0.3” per occupant. Where does that come from?
In the IBC, under Chapter 10 for Means of Egress, then 1005 for Means of Egress Sizing, then 1005.3 for Required Capacity Based on Occupant Load, we work our wall all the way to Section 1005.3.1. Here, it says that the capacity of stairs is the occupant load served multiplied by a factor of 0.3 inches per occupant. I find that wording to be particularly confusing.
Let’s explain it a different way. The exit capacity factor for stairs is 0.3 inches per occupant. To determine the maximum capacity of stairs in terms of number of occupants it can handle, take the width of the stair and divide it by 0.3. That’s why I previously said that a 44 inch stair can accommodate 146 occupants.
Now if we have two stairs, then the total occupant load these two stairs can accommodate is 292 total.
If the calculated occupant load of the floor is less than 292, we’re good because we can accommodate up to 292 people.
If our actual occupant load exceeds this, this is not compliant and either (a) the number of occupants needs to be decreased, (b) the stairs need to be made wider, or (c) an additional interior exit stair is required.
REQUIRED VERSUS AVAILABLE
The terminology I like to use is required versus available.
The required exit capacity is what is required based on the calculated occupant load. This can be presented in terms of number of occupants or inches. If we calculated 250 occupants to be on the floor, this means there needs to be a total of 75 inches of stair width on the floor. That’s 250 occupants, times 0.3 inches per occupant, to get 75 inches of required exit capacity.
The available exit capacity is what is being provided. If we provide two 44-inch wide stairs, then we are providing a total of 88 inches of stair width. That’s the available exit capacity in terms of inches. In terms of occupants, divide 88 inches by 0.3 and you get 292 occupants.
So in this scenario, we have a required exit capacity of 250 occupants, and we have an available exit capacity of 292 occupants. That’s good.
Or, in terms of inches, we have a required exit capacity of 75 inches, and an available exit capacity of 88 inches.
Since there is more exit capacity available than required, we’re good. If we have less available than what is required, that would be noncompliant.
FACTOR FOR DOORS AND OTHER EGRESS COMPONENTS
So we introduced an exit capacity factor for stairs of 0.3 inches per occupant. What about for doors, and other egress components? It’s actually different.
The factor for doors and all other egress components is 0.2 inches per occupant.
Why is this different?
Traveling up or down stairs [09B] generally takes longer than traveling through a door. Because it is easier to move through a door, these have a higher occupant load factor, meaning they can accommodate more occupants by width.
Similar to above, we take the clear width of the door and divide it by 0.2”. Clear width means the actual size of the opening.
When a door is open to 90 degrees, the door leaf itself restricts the size of the opening, since it projects into the door frame. The door stop also restricts the size of the opening. [10b]
For a typical 36” door, the leaf and the stop contribute about 2” of total projection into the clear width of the door, so typically a 34” clear width is available for a 36” door. The 34-inches is the clear width here, and that’s the measurement we must use in our calculation.
Taking 34” clear width and dividing by 0.2 inches per occupant results in an available exit capacity of 170 occupants. So a 36-inch door here has an available exit capacity of 170 occupants.
Similar to before, we need to compare the available exit capacity in terms of occupants for all exit doors to the calculated occupant load in order to determine whether the means of egress system is compliant.
STAIR AND DOOR EXAMPLE
Now stairs usually have a door to enter the stair. In this case, both the door clear width (at a factor of 0.2” per occupant) and the stair tread width (at a factor of 0.3” per occupant) must be evaluated. If we use the example we introduced just now, our 36” door permits 170 occupants but the stair only permits 146.
We must use the more restrictive component.
146 occupants is less than 170 occupants, so the stair width is the more-restrictive component.
If the stair were increased to, say 60”, then it would have a capacity of 200. That’s 60 inches divided by 0.3 inches per occupant, to get 200 occupants. But since the door to the stair is only 36” (with a 34” clear width), it only permits 170 occupants. So if our stair is increased to 60-inches, then our door is now the more restrictive component of the two. Considering both the door and the stair combined, a 60-inch stair and 36-inch door can only accommodate 170 occupants from this floor, due to the smaller exit capacity of the door.
There is a code benefit here to having a fully-sprinklered building alongside a voice-fire alarm system.
The IBC permits an exit capacity factor increase when a building is protected throughout with an automatic sprinkler system and an emergency voice communication fire alarm system. If the building has both, then the factor for stairs changes from 0.3 to 0.2.
This is an increase, meaning more occupants can use the stair.
For our 44” stair, the capacity using a factor of 0.2 is now 220 occupants (44 inches divided by 0.2 inches/occupant to get 220 occupants). That’s up from a capacity of 146.
Similarly, the occupant load factor for a sprinkler and voice evacuation system for doors [13B] is an increase from 0.2 to 0.15. For our door with a 34” clear width, the capacity goes up to 226 from 170 occupants.
What is exit capacity factor?
In order to calculate the size of an exit component, we multiply the number of occupants, by an exit capacity factor, to get the size of the exit component.
The exit capacity factor is the constant that is used to translate an occupant load to an exit capacity.
We can determine an available size of an exit by taking the width, like a stair width, and dividing it by the exit capacity factor to get an available exit capacity for that stair.
We can also do the opposite, determine a required size for an exit by multiplying an actual occupant load with the factor to get a required exit size.
In either case, we’re looking to make sure that the available capacity is more than the required minimum capacity.
These exit capacity factors are different between stairs and other exit components, and under the IBC we have increases for fully sprinklered buildings that have an emergency voice evacuation fire alarm system.
Now NFPA 101 handles exit capacity factors a little differently, which is the topic in our next video.
I’m Steven Barrett, this is MeyerFire University.
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