Consider the following: You have a large warehouse (tilt up, steel bar joist, metal deck roof). The warehouse was designed as an "Enclosed" building. You experience an EF1 tornado. Uplift pressures cause a portion of the roof deck to blow off. Assume the deck that blew off was 18' x 3' = 54 SF. The warehouse is 280,000 SF. Assume all dock doors remained closed and the only puncture in the building envelope is this 54 SF opening in the roof. Does the building now have an internal pressure per a "Partially Enclosed" design?
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Roof Opening & Enclosure Classification per ASCE 7-10
- Thread starter Ben29
- Start date
No. ASCE 7 determines enclosure by wall openings, not roof openings. That's not to say it should be ignored, but I think whatever consideration you give it will come from judgement/accepted literature and not a more prescriptive approach from ASCE 7 or another code.
Yes, there is internal pressure, evident by houses been blown apart by wind due to a broken window/door during a storm event. I think this problem is supposed to be avoided by provide better connection/fastening between building frame-foundation, frame-frame, and frame-roof panels. The former two are in the hands of the structural designers, and the latter is up to the roof contractor.
phamENG,
Per the definition of "BUILDING, PARTIALLY ENCLOSED" in ASCE 7-10, (Section 26.2 Definitions):
One of the criteria for a partially enclosed building is:
[blue]The total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building enveloped (walls [red]and roof[/red]) by more than 10 percent.[/blue].
So the roof openings do enter into the issue a bit based on that.
Per the definition of "BUILDING, PARTIALLY ENCLOSED" in ASCE 7-10, (Section 26.2 Definitions):
One of the criteria for a partially enclosed building is:
[blue]The total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building enveloped (walls [red]and roof[/red]) by more than 10 percent.[/blue].
So the roof openings do enter into the issue a bit based on that.
- Thread starter
- #6
Ao = total area of openings in a wall that received positive external pressure.
If they wanted this to mean "wall and roof" or "building envelope" then I think they would have stated it that way. (or at least I hope they would have)
Wind blowing towards (and over) a building would lead to uplift pressures (negative pressures) on the roof. So if there is an opening on the roof, you wouldn't expect wind to come in through that opening and cause additional negative pressures the way you would expect it to come in through an opening in a wall.
I understand that there is a possibility for additional internal pressures if there was an opening in the roof but I'm not convinced that they are of the same magnitude for an opening in the roof vs the wall.
If they wanted this to mean "wall and roof" or "building envelope" then I think they would have stated it that way. (or at least I hope they would have)
Wind blowing towards (and over) a building would lead to uplift pressures (negative pressures) on the roof. So if there is an opening on the roof, you wouldn't expect wind to come in through that opening and cause additional negative pressures the way you would expect it to come in through an opening in a wall.
I understand that there is a possibility for additional internal pressures if there was an opening in the roof but I'm not convinced that they are of the same magnitude for an opening in the roof vs the wall.
The opening in the roof would increase your Aoi, and could therefore swap the inequality for condition 1 and move you from a partially enclosed building to an enclosed building. Interesting - hadn't thought of that before. It would also shift condition 2 favorably (if at all). That said, I would certainly never consider it in any design.
So while I agree that the roof opening plays into it, having openings or expanding openings in a roof will not - by itself - push from enclosed to partially enclosed. Going back to my original post, though, there could be other ramifications. If it's possible for the wind blowing over the now breached roof to draw a negative pressure within the building (seems reasonable for an otherwise enclosed space), then you could end up with an effective internal pressure coefficient somewhere between -0.18 and -0.55. To find out just what would have to happen (size of opening relative to roof size/internal volume, location of opening, etc.) for that to happen would require a dive into the background research for wind loading. Something I'd love to do, but don't have time right now.
So while I agree that the roof opening plays into it, having openings or expanding openings in a roof will not - by itself - push from enclosed to partially enclosed. Going back to my original post, though, there could be other ramifications. If it's possible for the wind blowing over the now breached roof to draw a negative pressure within the building (seems reasonable for an otherwise enclosed space), then you could end up with an effective internal pressure coefficient somewhere between -0.18 and -0.55. To find out just what would have to happen (size of opening relative to roof size/internal volume, location of opening, etc.) for that to happen would require a dive into the background research for wind loading. Something I'd love to do, but don't have time right now.
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