Corridor Width vs. Stair Width

[Phil B]

Just making sure, but if I have a 2nd level facility with a total 2nd level occupant load of 450 being served by 2 stairways, one at each end of the building:
The corridor width needs to be 7'-6" wide minimum (.2 x 450)
The stair needs to be 5'-7 1/2" minimum (450/2) x 0.3)

The stair width isn't based on 0.3 x the total occupancy, correct? My concern is that the stair width is less than the corridor width, and you can't reduce the required exit path along the path of travel.

 

[walker.t]

The corridor width needs to be 7'-6" wide minimum (.2 x 450)

Incorrect per the 2018 IBC Illustrated Handbook:

2018 IBC Illustrated Handbook commentary on 1020.2 (partial quote)
The code is not totally clear on how the required corridor width is determined when the occupant load is large enough to require corridor widths in excess of the minimums. A common approach to determining the required minimum widths of corridors is that the width should be related to the required capacity of the exit to which the corridor leads.

They show a diagram with a long straight corridor with a stair at each end, total occupant load of all spaces opening to that corridor is 480 occupants, they use the following calculation to determine the width:

0.2” x 480 occupants = 96” required width.
96”/2 exits = 48” clear width at doors minimum.
96”/2 directions of travel = 48” clear corridor width minimum.

The stair width isn't based on 0.3 x the total occupancy, correct?

Correct, per 2021 IBC 1005.3.1:

2021 IBC 1005.3.1 Stairways (partial quote, emphasis added)
The capacity, in inches, of means of egress stairways shall be calculated by multiplying the occupant load served by such stairways by a means of egress capacity factor of 0.3” per occupant.

With an occupant load of 450 occupants you need two means of egress from the story per Table 1006.3.3. The occupant load will need to be divided evenly between the two stairs so that loss of one of the exits will not reduce the required capacity to less than 50% per 1005.5. Therefore you divide the total occupant load of 450 by 2 to get 225 occupants x 0.30”/occupant = 67.5” = 5’-7 1/2” clear.

My concern is that the stair width is less than the corridor width, and you can't reduce the required exit path along the path of travel.

As long as the stairway has a required capacity of at least 225 occupants to receive the 225 occupants traveling down the corridor to that stairway you are good.

 

[Phil B]

Thank you both for your replies. It's comforting that the verbiage from the 2018 IBC Commentary example in walker.t's reply above states that the code is unclear. At least I'm not alone.
The FBC section 1020.3 refers to 1005.3.2 which in the FBC states that the component (corridor) can be calculated at either 0'2" or 0.15" per person depending on sprinklering with voice evac or not. It does not state that you divide the amount in 2 which results in a width larger than the stairways. That wouldn't fly with the code section that you shall not reduce exit width along the exit path.

 

[walker.t]

Thank you both for your replies.

You’re welcome!

It does not state that you divide the amount in 2 which results in a width larger than the stairways. That wouldn't fly with the code section that you shall not reduce exit width along the exit path.

For reference, that provision is…

IBC 2021 1005.4 Continuity
The minimum width or required capacity of the means of egress required from any story of a building shall not be reduced along the path of travel until arrival at the public way.

2018 IBC Illustrated Handbook commentary on 1005.4 Continuity (partial quote)
It is important to remember that because different factors are used to determine the requirements for stairways than for all other components, it is really the required capacity, not the minimum width, of the egress system that is not permitted to be reduced.

We would see this at work in a building with a relatively low occupant load when we have a door with a 32” clear opening at the bottom of a stair that has a minimum 44” width.

 

[Sifu]

From the commentary to 2018 IBC 1005.3.2, which is the way I have always done it.

When determining the required capacity for the
corridor, where the corridor runs in two directions to
two different exits, similar to the room example, the
number of occupants moving into the corridor to exit
are divided by two. That is the number of occupants
used to determine the minimum width.

 

[Paul Sweet]

I agree. The corridor would be sized for 225 occupants because half could go each way.

 

[steveray]

I agree. The corridor would be sized for 225 occupants because half could go each way.

But at some point there are 450 people in it.....So the corridor SERVES 450...

 

[Phil B]

But at some point there are 450 people in it.....

And therein lines my quandary. I want and need the wider corridor but don't want to run afoul of 1005.4, although I never have.

 

[steveray]

1005.3.2​

The capacity, in inches, of means of egress components other than stairways shall be calculated by multiplying the occupant load served by such component by a means of egress capacity factor of 0.2 inch (5.1 mm) per occupant.

Exceptions:

1. 1.Facilities with smoke-protected assembly seating shall be permitted to use the capacity factors in Table 1030.6.2 indicated for level or ramped aisles for means of egress components other than stairways where the entire path for means of egress from the seating to the exit discharge is provided with a smoke control system complying with Section 909.
2. 2.Facilities with open-air assembly seating shall be permitted to use the capacity factors in Section 1030.6.3 indicated for level or ramped aisles for means of egress components other than stairways where the entire path for means of egress from the seating to the exit discharge is open to the outdoors.

1005.4​

The minimum width or required capacity of the means of egress required from any story of a building shall not be reduced along the path of egress travel until arrival at the public way.

1005.5​

Where more than one exit, or access to more than one exit, is required, the means of egress shall be configured such that the loss of any one exit, or access to one exit, shall not reduce the available capacity or width to less than 50 percent of the required capacity or width.

 

[steveray]

And therein lines my quandary. I want and need the wider corridor but don't want to run afoul of 1005.4, although I never have.

It always narrows at every doorway.....What they are trying to avoid is an abrupt "squeeze" in an MOE full of people....And stairs are narrower than landings...and...

 

[Phil B]

It always narrows at every doorway.....What they are trying to avoid is an abrupt "squeeze" in an MOE full of people....And stairs are narrower than landings...and...

Understood. I just wish 1005.5 had added something like "however, the corridor width needs to be the full width as determined by .02" x occupant load", then I could rest easy.

 

[walker.t]

I just wish 1005.5 had added something like "however, the corridor width needs to be the full width as determined by .02" x occupant load", then I could rest easy.

Closest I can think of your wish being granted is 1005.2 followed by 1005.3:

1. 1005.2 says that the component has to meet the minimum width for that component as stated elsewhere in the code. So for corridors we look to Table 1020.3 Minimum Corridor Width. In your example, a minimum 44” width would be the minimum width by component. This lets you carry 44”/0.20” = 220 occupants.
2. 1005.3 then says we have to provide the “required capacity” based on the factors in 1005.3.1 (stairways) and 1005.3.2 (other egress components.” When you get above 220 occupants 1005.3 is more restrictive and you have to provide 225 occupants x 0.20”/occupant = 45” for your corridor.

For up to 220 occupants you can forget about 1005.3 because it doesn’t apply yet, once you get to 221 occupants you can forget about 1005.2 because it no longer applies.