ASCE 7-10 Wind Design Loads 1

[SteelPE]

I am in the process of designing a large single story warehouse (340'x176'). The building is to be designed utilizing ASCE 7-10. To make the building architecturally pleasing, at 2 locations along the front of the building the architect is placing a 6'-0" tall parapet (these parapets also help define the entrances to the building). In order to give the entrances some depth, the architect is returning the parapet into the building 30' or so. I am trying to calculate the wind loads on these parapet returns.

The closest these return walls are to the windward edge of the building is 42'. I can't really seem to justify designing these systems as a parapet in accordance with ASCE 7-10 as they are no located at the edge of of the building (being located 42' from the windward edge). I am leaning towards designing these return walls in accordance with section 30.11 (rooftop structures). Is this the correct approach?

 

[JAE]

That makes sense to me.

The question is whether the wall parapet pressure immediately drops to an equipment pressure once it is off the exterior perimeter.
I might be inclined to keep the parapet pressure going some distance inward...perhaps a distance equal to the wall height?

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[KootK]

Interesting question. In what follows, put my level of confidence around 65%. I'll not be throwing myself upon this particular knife.

1) An argument for using parapet wind load. As I see it, the mechanism involved in parapet wind loading is that you've got wind forced to slow down in front of the wall (high pressure) and then quickly swirled into fast moving eddies (low pressure) over the top/back of the wall. The net effect, obviously, is a significant net pressure. It seems to me that this same mechanism will apply to your interior parapet walls. I don't think that the wind cares that it's last fifty feet of travel have taken place over a parking lot or a flat roof.

2) An argument for not using parapet wind loads. Perhaps wind slow down in front of something 6' tall is not as significant as it would be in front of something of full building height proportions? Meh.

For me, #1 outweighs #2 and I'd be inclined to go with the parapet wind baring new and convincing evidence to the contrary.

 

[SAIL3]

I also vote for parapet wind pressure...

 

[SteelPE]

FYI, the building height is 27'-0" +/-.

I left the design utilizing the rooftop structure wind pressure. This was based upon a parapet being placed at the perimeter of a building (which is how the dictionary defines a parapet). If the parapet was withing distance "a" from the edge of the building I might be more inclined to use the parapet pressure.

The project is in the very early stages at the moment and might not even go forward. I ended up calculating qh = 26 psf with a GCr=1.9 for a total wind pressure of about 50 psf (not exactly small).

 

[TehMightyEngineer]

What about the criteria in ASCE 7-10 Fig. 29.4-1 for solid freestanding walls?

I've used these provisions before and it appears most applicable to your situation.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[charliealphabravo]

I'd be thinking about freestanding walls as well.

 

[JAE]

I don’t think the ASCE wall provisions apply to things on a roof. The wind dynamics are totally different when the wall is on a roof.

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[SteelPE]

The reason why I didn't use 29.4-1 is the word freestanding. This is a wall on top of a building so I would not clarify this as freestanding.

By the non-consensus on this thread, from some of the best forum members, it appears as if there is some confusion with how the code applies to this item. I don't think my interpretation is incorrect, just open for debate.

 

[Ron]

I would treat the wing walls in the same way as you would a screen wall for hiding rooftop equipment. Similar to your approach.

 

[KootK]

The advice that I gave previously in this thread was wrongheaded. I dug into Ron's suggestion a bit deeper and I now agree with the approach suggested by him and, by extension, SteelPE. I thought it prudent to report back, acknowledge my error, and share what I've found with the gang. The clips below are taken from ASCE 7-16.

I would treat the wing walls in the same way as you would a screen wall for hiding rooftop equipment. Similar to your approach.

 

[JAE]

Ya, that 1.9 factor is a killer.

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[TehMightyEngineer]

Nice find, KootK. I think I know what your first quote is from but can you confirm the source?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[KootK]

I think I know what your first quote is from but can you confirm the source?

Both quotes are taken from ASCE 7-16

 

[TehMightyEngineer]

Oh really? I was going to guess Guide to the Wind Load Provisions of ASCE 7; but it appears there's no version of that text for ASCE 7-16 yet.

I still have yet to get a copy of ASCE 7-16.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/