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Exit stairs required to be the same width?

N

nealderidder

Sawhorse
Joined
Dec 7, 2010
Messages
420
Location
Sacramento, CA
Anyone of the opinion that exit stairs (scenario: three story office building with 2 interior exit stairways) are required to be the same width? For example, if one is 7' wide, the other cannot be 5' wide even if the egress load is accommodated.

CBC 1005.5 notes that the loss of one stair must not result in the loss of more than 50% capacity...

What flexibility do I have in "assigning" occupants to a stair? In my 3-story rectangular office building, if I have more intense uses on the west vs. the east, can I send more occupants to the west and provided a larger stair?

Any insight would be appreciated!

Neal
 
So long as you meet the minimum width capacity and can meet the 50% capacity requirement with the loss of one stair, then there is no issue with one stair being wider than the other. Note that the 50% capacity issue is of the required egress capacity, so you can still be larger that that but your minimum width for either stair is going to be 1/2 of the total egress capacity width (if there are only two stairs).
 
To add to lifesafetyinfo's post, let's assume you have an occupant load of 330 and there is no sprinkler system. The required egress capacity for the stairs must be 0.3 inches per occupant times the 340 occupants, which equals 102 inches. Each stair must provide at least 51 inches of egress capacity (1/2 of the required capacity). You could not provide one 44-inch-wide stair and one 58-inch-wide stair (102 inches total) since the loss of the latter stair would reduce the capacity by more than 1/2 (the remaining stair could only support less than half).

Now if one stair is 58 inches wide and the other is 66 inches wide, then the loss of either stair would not reduce the capacity by more than 1/2 since each stair has the capability of supporting more than 1/2 of the occupant load.
 
RLGA said:
To add to lifesafetyinfo's post, let's assume you have an occupant load of 330 and there is no sprinkler system. The required egress capacity for the stairs must be 0.3 inches per occupant times the 340 occupants, which equals 102 inches. Each stair must provide at least 51 inches of egress capacity (1/2 of the required capacity). You could not provide one 44-inch-wide stair and one 58-inch-wide stair (102 inches total) since the loss of the latter stair would reduce the capacity by more than 1/2 (the remaining stair could only support less than half).

Now if one stair is 58 inches wide and the other is 66 inches wide, then the loss of either stair would not reduce the capacity by more than 1/2 since each stair has the capability of supporting more than 1/2 of the occupant load.
Click to expand...
Got it. What about the idea of "assigning" more load to one stair than another? I've got a western exterior deck which needs two exits. One exit will be an exterior stair. The other exit will be accessed by entering the building in a common corridor and going to the nearby western stairway. I've got to pass the western stairway to get to the eastern stairway via common corridor. Seems like the western stairway will get the bulk of those exiting the deck. But if that backed up people could continue to the east stairway... Do I always evenly divide the load regardless of proximity of loads to one stair or the other?
 
nealderidder said:
Got it. What about the idea of "assigning" more load to one stair than another? I've got a western exterior deck which needs two exits. One exit will be an exterior stair. The other exit will be accessed by entering the building in a common corridor and going to the nearby western stairway. I've got to pass the western stairway to get to the eastern stairway via common corridor. Seems like the western stairway will get the bulk of those exiting the deck. But if that backed up people could continue to the east stairway... Do I always evenly divide the load regardless of proximity of loads to one stair or the other?
Click to expand...
Your description sounds as if you may have converging exits and do not comply with the separation requirements. A plan would be advantageous.
 
e hilton said:
If i was in a building when the fire alarm sounded, assuming there isnt a crowd at the door ... I’m going to take the nearest exit regardless of what i am assigned to.
Click to expand...
Me too. But I'm wondering, should we make that call and make the stair likely to get more use wider or simply divide the floor occupant load and provide half the width in one stair and half in the other ignoring that one is likely to get more use.
 
nealderidder said:
Me too. But I'm wondering, should we make that call and make the stair likely to get more use wider or simply divide the floor occupant load and provide half the width in one stair and half in the other ignoring that one is likely to get more use.
Click to expand...
How big are the occupant loads? On many projects, the minimum stair width can still have a significantly higher egress capacity than 1/2 of the occupant load so how the occupants split up doesn't really impact things much or really "overload" one exit or another.

Generally, the building code assumes that egress stairs will be separated enough that their use will be roughly evenly split. The big exception would be assembly spaces where there are additional requirements that take into consideration people's tendency to try to exit back out the way they entered, etc.
 
e hilton said:
If i was in a building when the fire alarm sounded, assuming there isnt a crowd at the door ... I’m going to take the nearest exit regardless of what i am assigned to.
Click to expand...
well then based upon your description you have converging exits and you have much bigger problems than stair width.
 
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