Exit Width of Corridor connecting Two Stairs

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5 comments

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    David Kaplan

    Josh,

    My personal view on this, and I welcome others', is that the corridor width is based on the total number of occupants - period.  So in the case of 324 people, it's 324 X 0.2" per occupant = 64.8" width.  If you think about it, looking at that floor plan above, when the fire alarm goes off, EVERYONE goes into that corridor at once.  It needs to be able to handle all those people.

    I think what might be confusing the issue for you is the 50% occupancy rule for stairways, meaning that if one stair is blocked, the other must be designed to handle at least 50% of the total occupant load.  I agree that the proper allocation for the 324 people scenario for the stairs is 162 people stair.  That meets the code.  But I don't agree that you would design the corridor the same way.  Think about what that would really mean.  In the drawing above, half of the occupants would be nicely walking towards the left stair, the other half to the right.  But in reality, in an emergency/panic situation, that would not occur.  People on the west side of the building might panic and run all the way to the east for whatever reason.  And, vice versa. 

    The other thing I’d offer is, looking at your second scenario of 600 people, think about what the math would work out to if you did the 600 / 3 rule.  That would be 200 people in the corridor.  200 X 0.2” per occupant = 40” corridor width using that math.  Hold up your hands 40” apart.  Does that look wide enough to handle 200 people running up and down the corridor to the fire stairs? 

    Any other thoughts here?  Hope this helps. 

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    Theodore Diamond

    I am going to disagree with David here, because a means of egress cannot be sized for smaller groups as it goes along.  If you size the corridor for 384 people, you hit a bottleneck at the stairs and this is a dangerous condition.  I think that you have to treat this as a corridor for 162 people heading in each direction.

    Regarding the second part of the question, we know that in the pictured version, the minimum width allowed is 52", which is a 260 person capacity.

    Similarly, with the 600 person capacity, you don't need to hold your hands apart to determine whether 40" is wide enough to handle 200 people, you need to again reference IBC and see that a corridor with 50 people or more must be at least 44" wide.

  • Avatar
    David Kaplan

    Theodore/Josh,

    I did some looking into of the 2012 IBC Code Handbook which provides commentary.  The book identifies that it is a bit unclear in the code, but that Theodore's approach is the commonly-used controlling concept.  And it makes sense too.  The book agrees that the best approach is to equate the required exit width and the corridor width, provided that you at least the 44" minimum (btw Theodore, the only reason I did the "hold your hands up and look at 40" was because in the initial post it said "please ignore the minimum corridor width for this."  But yes you're absolutely right.)

    So this being said, you CAN in fact use 162 people in your first example.  162 x 0.2" = 32.4".  But again, don't forget that 44" minimum must be met for an occupant load > 49.  But for the sake of understanding the principle, this is correct.

    Thanks!

  • Avatar
    Tsai Tse Josh Lo

    David and Theodore,

     

    Thanks guys for this constructive dialogue. Agree to use Theodore's approach which is consistent with the concept of the code, "means of egress capacity may not diminish in the direction of egress" or the watercourse analogy.

     

    That said, I share David's initial view point because it is logical in reality consider one stairway could fail, and I have seen it proposed by others (see example below). But on the other hand, the code often appears to relax its requirements for the second/backup means of egress, ie. travel distance limit only governs the most remote corner of a floor to the "closest" exit stairway.

     

    Josh

     

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    Rogelio Marzan Hernandez (Edited )

    This was a tricky one! And I was wrong on my first thought, thinking same as David that we need to consider the total occupant load for the corridor width calculation. 

    But after reading the discussion between David and Theodore, I reviewed this topic with the Architect Studio Companion for a better understanding than the one in the Code. On page 302 is an example on how to determine the widths of egress components. Theodore explanation is correct. Once we have determined the OL, we have to divide it with the numbers of exits to calculate the width of the corridor, but remembering that the min. is 44 inches (with few exceptions).

    Definitely this discussions helps to understand better the codes requirement.

    Thanks guys

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