Means of egress width

[bill1952]

I thought that it was not permitted to reduce means of egress in the direction of egress travel. Is that so or have I misread or perhaps applied Life Safety Code, where I'm pretty sure that is the case.

I'm looking at 6' wide exit door at second floor from an assembly occupancy to a landing, and from landing to grade is a 5' wide stair. Is that allowed?

I'm looking at 1003.6 (2015 IBC) in particular last sentence: "The minimum width or required capacity of a means of egress system shall not be diminished along the path of egress travel."

Maybe my strict interpretation is wrong, but when I look at the classic large loss of life fires and similar crowd incidents, people jammed up at some restriction is a common theme. The 2003 E2 nightclub in Chicago comes to mind. (Yes, many other contributing factors.)

So set me straight if it's ok for 6' doors to lead to a 5' stair. Yes, I think with the 5' stairs there is still sufficient means of egress capacity for the occupant load, barely.

 

[ICE]

The minimum width or required capacity

That's the key to this.

 

[bill1952]

All components meet the minimum widths and the aggregate width - based on the minimum of any component in a path - seems to be sufficient. It's being wider at one point and narrower at a later point that concerns me. Doors with a capacity of near 360 leading to stairs with a capacity of 200. Seems risky.

And no issue with aisles, etc., within the assembly occupancy. Colleagues are amused that I know all that by heart - IBC, LSC, and Chicago Building Code.

 

[Yankee Chronicler]

So set me straight if it's ok for 6' doors to lead to a 5' stair. Yes, I think with the 5' stairs there is still sufficient means of egress capacity for the occupant load, barely.

Consider that the required with of vertical means of egress (stairs) has to be greater than the required width of horizontal means of egress (corridors and doors) by a factor of 1.5 (0.3" per person as opposed to 0.2" per person). So if a stair has a required width of 60 inches, it has a capacity of 200 people. The minimum width required for horizontal egress components to accommodate 200 people is 40 inches.

The section you quoted is found in 1005.4 of the 2021 IBC. That section has a fairly lengthy explanation in the Commentary, which might be helpful:

The requirement that both the minimum width and
required capacity from any floor are to be provided
along the entire exit to the termination, typically down
the stairway to the exterior exit door at the level of exit
discharge, results in an egress width that is adequate
for the exit discharge.
The total capacity of the exits that serve a floor is not
to be less than the occupant load of the floor as determined
by Section 1004.1. If an exit, such as a stairway,
also serves a second floor, and the required
capacity of the exit serving the occupants of the second
floor is greater than the first floor, the greater
capacity would govern the egress components that the
occupants of the floors share. For example, suppose
an exit stairway serves two floors, with occupant loads
of 300 on the lower floor and 500 on the upper floor.
Assuming that two stairways serve each floor, and
using the upper-floor occupant load of 500 as the
basis of determination, the two stairways would be
designed for a capacity of 250 people each. Note that
the doors to the stairways on the lower floor would be
designed for a capacity of 150 and the doors to the
stairways on the upper floor would be designed for a
capacity of 250. Reversing these two floors would
result in the portion of the stairways that serves the
upper floor to be designed for a capacity of 150 and
the stairways that serve the lower floor to be designed
for 250. Requiring the egress component to be
designed for the largest tributary occupant load
accommodates the worst-case situation.
Also note that the capacity of the exits is based on
the occupant load of one floor. The occupant loads are
not combined with other floors for the exit design. It is
assumed that the peak demand or flow of occupants
from more than one floor level will not occur simultaneously
at a common point in the means of egress,
except as provided for in Sections 1004.2.2 and
1005.6.

 

[bill1952]

As I have tried to say, each component is wide enough. It's the downstream reduction I thought was not allowed.

Say for instance a 5' corridor is adequate. Is it ok to begin with a 10' corridor and the narrow it to 5'?

 

[Yankee Chronicler]

As I have tried to say, each component is wide enough. It's the downstream reduction I thought was not allowed.

Say for instance a 5' corridor is adequate. Is it ok to begin with a 10' corridor and the narrow it to 5'?

What's the minimum required corridor width?

If a 5-foot corridor is the minimum required width, it doesn't matter if it's 20 feet wide upstream.

This provision really applies to stairs. Take a 3-story building. The third floor requires a minimum 5-foot wide stair. The second floor only requires a 4-foot wide stair. The stair has to be 5-feet from the third floor all the way to the exit discharge -- it can't reduce to 4 feet at the second floor.

 

[bill1952]

But it's ok if it only needs to be 4', is 6' ft it top floors, and the necks down yo 4' for last floor? Seems like crowd crush is certain in an emergency egress at full capacity.

 

[Yankee Chronicler]

But it's ok if it only needs to be 4', is 6' ft it top floors, and the necks down yo 4' for last floor? Seems like crowd crush is certain in an emergency egress at full capacity.

If it only needs to be 4 feet, then 4 feet is okay.

Did you read the commentary? Do you understand that when exiting from multiple stories, the occupant loads of the upper stories to NOT get added to the occupant loads of lower stories?

The code section addresses required width of the means of egress. "Just fly the mission."

 

[bill1952]

Do you understand that when exiting from multiple stories, the occupant loads of the upper stories to NOT get added to the occupant loads of lower stories?

Yes, I've learned that in the late 80's. The this stair is exterior and only serves one room on one floor.

 

[walker.t]

But it's ok if it only needs to be 4', is 6' ft it top floors, and the necks down yo 4' for last floor? Seems like crowd crush is certain in an emergency egress at full capacity.

If it only needs to be 4 feet, then 4 feet is okay.

Yankee Chronicler is correct, the 2018 IBC Illustrated Handbook has a commentary that says, “The actual width of the means of egress may be reduced throughout the travel path as long as the required width is provided.” Then there’s a diagram showing a 10’ wide corridor necking down to 6’. But I’m with you in thinking that such a reduction in actual width seems like it could cause problems but the code allows it.

The Illustrated Handbook also has a helpful graphic of a section cut through a building showing how a required capacity is maintained as multiple floors dump into a stairwell, this scenario is also explained in the commentary Yankee Chronicler shared.

 

[bill1952]

OK. I was confused by requirements for assembly seating - what I've done for 40+ years - and there are requirements for some aisles to be uniform width. The commentary even uses the term bottlenecks as the reason they can't get narrower even if still it still exceeds the minimum width or capacity.

I do think there are problems with a 4 foot aisle leading to a 3' door and the potential for crowd crush, but it's allowed.

 

[walker.t]

I do think there are problems with a 4 foot aisle leading to a 3' door and the potential for crowd crush, but it's allowed.

Regarding crowd rush and reducing the width of a path of travel, it’s interesting to note that the width of stair landings cannot be less than the width of the stairway served (1011.6) - therefore no bottlenecking allowed on stairs. The 2018 IBC Illustrated Handbook makes this comment: “Where the stairway reaches capacity across its width during egress, a landing of reduced size will create an obstruction to the flow pattern that has been established.” Evidently the established “flow pattern” in horizontal travel is forgiving enough to allow reductions in actual width as long as the required width is maintained.

 

[bill1952]

Evidently the established “flow pattern” in horizontal travel is forgiving enough to allow reductions in actual width as long as the required width is maintained.

I guess that is based on an assumption the code has it right?

Assembly seating - most of what I do (or did) - seems to bring out some odd situations, and not surprising since the same code applies to a 100 seat theatre applied to a 1000 seat recital hall that applies to a 100,000 seat stadium. In the small performing arts spaces, aisle widths are almost always governed by minimum width, not capacity, nearly opposite the stadiums. So 2 - 48" stepped aisles serving 80 occupants each converge into a level cross aisle - how wide? 36" base on capacity and minimum width or 48" based on not reducing or 96" as the illustration in the commentary would have you believe. And all the time I'm fighting for every inch to get the audience closer to the performer and for acoustics.

Rant over.

 

[walker.t]

I guess that is based on an assumption the code has it right?

And all the time I'm fighting for every inch to get the audience closer to the performer and for acoustics.

It can be a challenge to balance the minimum requirements of the code, the program and priorities of the project owner (including pressure to only do the minimum,) and the design professional’s judgement that the design should exceed the minimum.

My only experience with “large” assembly was for a renovation involving a 670-occupant church, I did the drafting for the architect, he oversized the total exit door width to carry an additional 470 occupants - I’d say he wasn’t confident that the code had it right.