Applying ASCE 7 Wind Loads

[Splitrings]

The attached sketch shows two buildings that by ASCE 7 definition are considered enclosed buildings. When designing the MWFRS, the wind loads are applied to the "projecting area" according to figure 6-9 of ASCE 7-05. Even though the pole building has open area, I have been applying the loads to full area as shown. I have some coworkers that believe you only need to apply the loads to the solid areas. I don't see anywhere in ASCE 7-05 where this is stated.

I don't have my new copy of ASCE 7-10 yet, so I apologize if it is covered in the new standard. Thank you for your comments in advance.

 

[larsacious]

You need to apply the wind load to the full projected area. If your pole buiding qualifies as being open per ASCE 7-05, section 6.2 then the provisions of section 6.5.13 may qualify as an approach which allows you to ignore the open areas.

 

[Lion06]

I've never dealt with a partially enclosed building, but reading the definition I don't see how a building could be classified as a partially enclosed building unless it's for a specific wind direction. I just don't see how the definition given could possibly apply to a building for a wind, independent of direction.

That being said, let's take the pole building example to more of an extreme. If it's 30' tall and only has 3' of wind surface at the top I don't think I would look at it with a full wind surface. That doesn't make sense.

 

[larsacious]

Parking structures can often be classified as partially enclosed.

 

[Splitrings]

If you had an opening on a side receiving positive pressure that is at least 10% greater in size than the sum of the remaining openings, in the remainder of the envelope, it would be partially enclosed. An example would be an open garage door on one side and an open person door on one of the three remaining walls.

The pole building as drawn is neither partially enclosed or open. In order for it to be open all sides must be at least 80% open. Even the gable end, with just the gable sheathed, is less than 80% open.

Intuitively it does not make sense to think of the pole building as anything but open. I am guessing ASCE could not cover every possible "openness" case. So, if the structure is not completely open, it is conservatively lumped into the enclosed case. I am guessing this is the reason. Of course you could do wind tunnel testing.

 

[Lion06]

When I read the definition of partially enclosed, I read it as though every wall that could receive positive pressure needed to meet that criteria, but that's not physically possible. It makes more sense that if ANY wall that receives positive pressure meets that criteria it would be classified as partially enclosed.

 

[UnneutralAxis]

Splitrings - Wind tunnel... haha. I think, in reality, I've only dealt with a handful of cases that by ASCE 7 definition didn't need a wind tunnel analysis. Can you imagine telling the architect that he needs to ask the client for however many thousand dollars for a wind tunnel test of their "cut-up" office building?

I have and will always say that our wind loading procedures are a bit of black magic for most buildings out there. But it seems to work.

Back to the post - yes, I would conservatively apply it to the entire surface.

 

[a2mfk]

I have always looked at these definitions as important for one thing- internal pressures. Partially enclosed is obviously always the worse case, which has been proven in hurricane forensics and I have seen it first hand. An enclosed structure without opening protection has windborne debris that strikes the windows or a garage door is blown in, making it a partially enclosed building as the wind rushes into the structure and internal pressure builds...

Other than that, I look at the way the wind acts against each wall like I would any other structure. If there is no surface for the wind to hit, then why would we pretend it is there?

Maybe I am missing something here....

 

[Lion06]

I'm with you a2.

 

[ron9876]

I would treat the pole barn as an open building with restricted flow for the roof pressures. The walls would require some guess work. I would consider sign loading for the walls and know it is conservative.

 

[firai]

The forces only need to be applied to the solid areas of the wall. I cite ASCE 7 C6.2 "Building, Enclosed, Open, Partially Enclosed" as a reference, where it is stated that "the internal pressures would act on the solid areas of the walls and roof" in an "enclosed" (as per ASCE 7 definitions) parking garage with large openings. This passage discusses the internal pressure, but the intention is equally applicable to external pressures. In addition, analytically, it is impossible to have a force on the openings as there would be no load path to carry it.

 

[firai]

I forgot to address the "projected area" you referred to in the initial post. That is only talking about surfaces that may not be perpendicular to the principal structural axes; it does not have anything to do with openings.

EIT, New York, NY

 

[csteve]

The comment by firai seems intuitively correct, but I am not fully convinced. Section C6.2 doesn't mention external pressures which are normally applied to gross projected areas, and although it may appear that buildings with large openings in two or more walls would have little or no internal pressure, the +/-.18 internal coefficients clearly apply. Also, even though nothing is in the opening, a load path could be assumed as though the opening were solid.

In addition, a Minimum Pressure of 10 psf applied on projected areas is required. I assume this is intended to apply to the gross projected area, regardless of openings.

IMHO, ASCE 7 is not clear enough on the issue. Without clarification from ASCE, it may be prudent, even if overly conservative, to use the full projected areas.

 

[Teguci]

Consider this. The wind blows under the end truss and into the interior of the barn. It would not be conservative to assume that this wind flows through the building without any resistance. Instead, this wind will be resisited by the interior trusses and walls. Any interior surface may resist some of the wind velocity. Now, how do you account for that?

On the other hand, you could have a smooth interior surface and the wind cleanly blows right through the barn, but it is noted that even our air duct systems (sheet metal) has calculated resistance to air speed.