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Gross floor area for stairway - which floor?

Discussion in 'Commercial Building Codes' started by Glenn, Jul 9, 2019.

  1. Glenn

    Glenn Corporate Supporter
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    Two stories connected by a single stairway with two stairs and an intermediate landing. All functions of spaces require gross floor area calculation. Which floor does the area of the stairway apply to?

    1) one stair and half the landing to each story?
    2) All to the bottom story?
    3) All to the top story?
    4) Whatever works best to limit occupant load where the designer wants?

    What is your interpretation? Thanks.
     
  2. cda

    cda Sawhorse

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    A?

    All areas shall have code compliant exiting.

    A simple floor plan would help.
     
  3. mtlogcabin

    mtlogcabin Sawhorse

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  4. e hilton

    e hilton Bronze Member

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    If there was no second floor, then there would not need to be stairs, so charge the stairs to the second floor. Wait ... if there was no second floor, the space would be part of the first floor.
     
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  5. mtlogcabin

    mtlogcabin Sawhorse

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    Technically you do not get a deduction for stairs on any floor so you include each area the stairs takes up to each floor so the number is the same on all floors. You look at each and assume no stair exist

    FLOOR AREA, GROSS. The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, closets, the thickness of interior walls, columns or other features
     
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  6. Glenn

    Glenn Corporate Supporter
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    I see your rational here using the definition, and that is the answer that leans toward safety. However, I don't see the rational of counting the area of the stairs twice. Trying to explain that in an educational environment reduces faith in the sensibility of the code. If the function of space dictates an occupancy density, it would be hard to convince someone that twice that density would be hanging out on the stairs.
    My thought was they would be best counted toward the second floor because the question is of occupant load and thus number of exit access. The occupants on the stairs would only be on the stairs for the sake of going to or coming down from the second floor. First floor occupants would not need the stairs. In a particular example, if the stairway was counted toward the second floor, it would tip the scales to needing two exit accesses from the second floor. Since the stairs are not providing exiting means for the occupants on the first floor, it would make the most sense to apply them to the second floor.
     
  7. Glenn

    Glenn Corporate Supporter
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    Just imagine a two story building with a stairway in the middle. For a gross floor area you calculate the floor area inside the exterior walls. However, at the stairway, there is only one "floor area" (surface for people to stand) that covers both stories. If counted twice, it would be assuming twice the density on the stairway over any other place in the building. That is hard to justify.
     
  8. Inspector Gift

    Inspector Gift Sawhorse - Made in USA

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    Glenn,
    With residential plans we usually include the stairs are part of the floor they serve, but also include any usable space under the stairs with the floor on which they are located.

    For instance, entry hallways with stairs often times have a powder room located under a portion of the stairs. In that case, the usable space under the stairs is calculated. The same goes for a closets under the stairs.
     
  9. ADAguy

    ADAguy Sawhorse

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    OK, so what if the exterior walls of a building are not vertical, they lean inward ( "A" frame) or outward (reverse "A" frame), then where do you measure the gross sq. ftg. to?
     
  10. RLGA

    RLGA Sawhorse

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    I use the entire area of the stair enclosure at each story measuring to the inside of exterior walls. For stairways in large floor openings, such as atriums, I use the entire horizontal projected area of the stairway at each story and the entire atrium/opening area on the story in which the atrium/opening floor is located.
     
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  11. cda

    cda Sawhorse

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    I believe the code addresses this already.

    As you say multi floor building, idea first floor is mostly gone before the second floor gets to the exit/ stair.
     
  12. cda

    cda Sawhorse

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  13. mtlogcabin

    mtlogcabin Sawhorse

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    OL is used for plumbing fixture counts and other things in the code. That is why I chose #4 in my original post. If it aids the designer and does not effect egress width and number of exits then I will probably agree with his OL numbers


    1004.1.2 Areas without fixed seating.

    Exception: Where approved by the building official, the actual number of occupants for whom each occupied space, floor or building is designed, although less than those determined by calculation, shall be permitted to be used in the determination of the design occupant load.
     
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  14. Tim Mailloux

    Tim Mailloux Registered User

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    That's not the answer that leans towards safety, that IS the answer.
     
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  15. mtlogcabin

    mtlogcabin Sawhorse

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    A 48" wide stair with 10 ft between floors will use about 880 sq ft per floor so in a 2 story building that can be 18 people in a B occupancy under the 2012 IBC or 36 people under the 2018 concentrated business use area. Naturally the numbers will increase per floor that is where as a BO or plans examiner you needs to be very cautious and not give up to much in accepting a design occupant load that is less than the tables. If you are required two stairs in a "B" occupancy you would be giving away a required WC per floor under the 2012 codes or 2 WC per floor under the 2018 concentrated business use area.

    Glenn
    I don't think you should be looking at this as OL on the stairs for an educational environment. I think you would be better to explain "net floor area" and "gross floor area" and why historically everything with the exterior walls except shafts are included in the OL calculation. As pointed out in previous post the area under the stairs is a usable space and depending on the height between floors it could be a receptionist desk, waiting area, or any number of things.
     
  16. steveray

    steveray Sawhorse

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    I'm on team both....That being said, if fixture count was the issue and not egress, I am sure we could find a way around it...
     
  17. Tim Mailloux

    Tim Mailloux Registered User

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    Mechanical shafts should be included in gross floor area for occupant load calculations. Per a technical opinion I received from the ICC any part of the building with a roof over it shall to be included in the gross floor area, this includes stairs shafts, elevator shafts and mechanical shafts. The 'vent shaft' exception in the definition below is not the same as a mechanical shaft. These vent shafts are open air vent shafts (no roof) like you might find in an old NY City apartment building

    FLOOR AREA, GROSS. The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, closets, the thickness of interior walls, columns or other features
     
  18. mtlogcabin

    mtlogcabin Sawhorse

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    Personally I have never deducted out any shafts when calculating gross floor area. Although I not familiar with the open air shaft you reference I do believe IMHO the technical opinion is wrong since it is one mans opinion. I would be curious to see if a Committee Interpretations would be the same as the technical opinion of one staff member

    The "Gross Floor Area" definition in the IBC and the old UBC "Floor Area" are exactly the same "exclusive of vent shafts and courts" so the language has been in the code a long time

    Coming from an HVAC background the term vent shaft to me means the shaft is used by an HVAC system so my thought process might be slightly biased.
    But enough on this rabbit trail
     
  19. RLGA

    RLGA Sawhorse

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    You need to take a look at the very last sentence: "The gross floor area shall not include shafts with no openings or interior courts."
     
  20. Tim Mailloux

    Tim Mailloux Registered User

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    Take a look at the code commentary for this definition, the commentary is consistent with the ICC technical opinion I received.....

    Gross floor area is that area measured within the perimeter formed by the inside surface of the exterior walls. The area of all occupiable and non occupiable spaces, including mechanical and elevator shafts, toilet rooms, closets and mechanical equipment rooms, are included in the gross floor area. This area could also include any covered porches, carports or other exterior space intended to be used as part of the building’s occupiable space. This gross and net floor area is primarily used for the determination of occupant
    load in accordance with Table 1004.1.2.
     

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