Exit stair sizing

[rosegamble]

Hello, everyone. I understand conceptually that if my occupant load for a single-story building is large enough to require two exits, I can divide the occupant load by 2 to generate the egress sizing. Let's say we have 90 occupants and two stairs are required. I understand that 45 occupants can be considered for each stair (and that this sizing also needs to be weighed against 1005.3.1 and 1005.3.2).

However, where does the code say I can split the occupant load if multiple exits are required? I'm not seeing it anywhere. I am familiar with 1005.5 but this isn't explicitly stating that the occupant load can be divided by the number of exits to generate the sizing of each exit.

I'd just like to cite the actual code allowance for this, but cannot find it.

Thanks!

 

[RLGA]

Technically, all stories require at least two exits regardless of occupant load size (Refer to IBC Table 1006.3.3). However, there are conditions that allow single exits. If you conform to those conditions, then you can have one exit; otherwise, at least two exits are required.

Now to your specific question. The code does not dictate how the egress width is distributed among the provided exits beyond what Section 1005.5 states. As long as the egress capacity of all exits meets or exceeds the required minimum capacity and the loss of one exit does not diminish the required capacity by more than 50%, then you can distribute the occupant load any way that you want.

 

[rosegamble]

Technically, all stories require at least two exits regardless of occupant load size (Refer to IBC Table 1006.3.3). However, there are conditions that allow single exits. If you conform to those conditions, then you can have one exit; otherwise, at least two exits are required.

Now to your specific question. The code does not dictate how the egress width is distributed among the provided exits beyond what Section 1005.5 states. As long as the egress capacity of all exits meets or exceeds the required minimum capacity and the loss of one exit does not diminish the required capacity by more than 50%, then you can distribute the occupant load any way that you want.

Thank you! So it’s just “understood” that a inches calculated for egress width can be split into multiple exits? I just don’t see that in the code anywhere.

 

[RLGA]

As an example, assume a story has 600 occupants with no sprinkler system or emergency voice alarm communication system (EVACS). Three exits will be required since the occupant load is greater than 500.

The total stair capacity must be 0.3"/occ x 600 occupants = 180 inches

You can distribute the capacity equally among all three required stairways so that each will have a required width of 60 inches.

Or, let us say you want one stair at the minimum allowable width of 44 inches, which leaves 136 inches that must be provided by the remaining two stairs. The remaining 136 inches can be divided equally (68 inches each), or you can make the second stair 46 inches and the third stair 90 inches. However, you could not make the second stair also 44 inches, because that would require the third stair to be 92 inches, which is greater than 50% of the required capacity.

However, you can provide a 92-inch capacity stair as long as the remaining two stairs provide at least 50% of the required capacity (i.e., 90 inches).

Finally, you can always provide more capacity than is required (e.g., three 72-inch-wide stairs with a total 216-inch capacity).

 

[rosegamble]

As an example, assume a story has 900 occupants with no sprinkler system or emergency voice alarm communication system (EVACS). Three exits will be required since the occupant load is greater than 500.

The total stair capacity must be 0.3"/occ x 600 occupants = 180 inches

You can distribute the capacity equally among all three required stairways so that each will have a required width of 60 inches.

Or, let us say you want one stair at the minimum allowable width of 44 inches, which leaves 136 inches that must be provided by the remaining two stairs. The remaining 136 inches can be divided equally (68 inches each), or you can make the second stair 46 inches and the third stair 90 inches. However, you could not make the second stair also 44 inches, because that would require the third stair to be 92 inches, which is greater than 50% of the required capacity.

However, you can provide a 92-inch capacity stair as long as the remaining two stairs provide at least 50% of the required capacity (i.e., 90 inches).

Finally, you can always provide more capacity than is required (e.g., three 72-inch-wide stairs with a total 216-inch capacity).

Thanks for your thorough explanation! I'm on board with all of this and in agreement. However, I've been trying to find where in the code it says I can distribute the egress capacity (say 92" in your example) between multiple exits? Or is it simply assumed?

 

[RLGA]

Thanks for your thorough explanation! I'm on board with all of this and in agreement. However, I've been trying to find where in the code it says I can distribute the egress capacity (say 92" in your example) between multiple exits? Or is it simply assumed?

If the code does not regulate it, then silence on the subject means it is permitted.

 

[Timothy C. Ryan]

As an example, assume a story has 900 occupants with no sprinkler system or emergency voice alarm communication system (EVACS). Three exits will be required since the occupant load is greater than 500.

The total stair capacity must be 0.3"/occ x 600 occupants = 180 inches

You can distribute the capacity equally among all three required stairways so that each will have a required width of 60 inches.

Or, let us say you want one stair at the minimum allowable width of 44 inches, which leaves 136 inches that must be provided by the remaining two stairs. The remaining 136 inches can be divided equally (68 inches each), or you can make the second stair 46 inches and the third stair 90 inches. However, you could not make the second stair also 44 inches, because that would require the third stair to be 92 inches, which is greater than 50% of the required capacity.

However, you can provide a 92-inch capacity stair as long as the remaining two stairs provide at least 50% of the required capacity (i.e., 90 inches).

Finally, you can always provide more capacity than is required (e.g., three 72-inch-wide stairs with a total 216-inch capacity).

Why is that you used 600 occupants in you example and not 900 ?

 

[RLGA]

Why is that you used 600 occupants in you example and not 900 ?

Ah, you caught that. I mistyped it. I'll edit my post so it doesn't create any future confusion.