ASCE 7-10 Partially Enclosed Structures 1

[dianium500]

I have recently been asked to design the foundation for a tent structure. Upon reviewing the tent structure engineering, I noticed they have designed the structure as enclosed. The ends of the structure are partially enclosed w/ equal openings. Based on how I am interpreting ASCE 7, by definition this structure is partially enclosed, however the engineers who manufacture the tent disagree stating that because the openings are equal at each end, it doesn't fit the definition of partially enclosed. I feel they are sum what correct, in that it doesn't fit the definition, but in my opinion it doesn't fit the enclosed definition either. It's this is really a cross between an open structure and a partially enclosed structure. I have attached the structure so you guys can take a look and give me your opinion. Thanks for the help.

 

[mike20793]

If you think it is really a cross between open and partially enclosed by definition, then 26.10.4 would apply and it would be classified as an open building. However, if there are no openings at all along the longitudinal walls, then I tend to agree with the tent engineer and this is an enclosed structure since those walls can receive positive wind pressure and they don't meet the opening requirements. That being said, if it were my design, I would treat the same way you do and treat it as partially enclosed.

 

[BAretired]

You didn't include drawings of the end walls (L1 - L3) but you state that end walls have equal openings. In considering wind pressures, openings can either be open or closed so the worst combination must be considered in design. Why should it matter whether the building is classified partially enclosed?

BA

 

[dianium500]

I have uploaded the end wall. It has a header that comes down leaving a 16 ft high opening on each end.

 

[boo1]

internal pressure?

 

[dianium500]

They have the internal pressure listed at 0.18, and I think it should be the 0.55.

 

[KootK]

I don't like the enclosed classification for this. In my mind, the defining characteristic of enclosed is localized air leakage at a slow enough rate that one could reasonably expect uniform interior pressure on all perimeter faces of the building, generally cancelling out for lateral design. This scenario does not possess that characteristic in my opinion.

On a logical basis, I find it irrational to go with a less punitive classification simply because correlation with a more punitive classification is imperfect. That's not what we do.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SAIL3]

I usually try and understand why there is a difference in the wind press coeff between open, partially enclosed and enclosed and then check the bldg for the possibility of any of these wind conditions occurring.
One might state that if the wind is perpendicular to the opening that eventually air flow would stabilize within the bldg and the air in-flow would equal to the air out-flow without any buildup of press within the bldg. But what happens when the wind is @ 45deg(or 30deg) to the openings.
What are the contents of the bldg that might result in air blockage within the bldg.
Even with wind perpendicular to the long axis of the bldg I would expect unpredictable airflow and turbulence at the openings at both ends that may affect the structure locally at the ends of the bldg.
Without wind tunnel tests I would try to get an upper-bound solution by checking out the various wind load conditions.

 

[wannabeSE]

See the definitions in chapter 26

To be open, each wall needs to be 80% open. The structure doesn't have significant openings on all sides, so it is not an open building.

To be partial enclosed, the structure needs to meet two criteria. The first requires 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 envelope (walls and roof) by more than 10%. Since the structure has equal sized openings on two ends, it is not partially enclosed.

To be an enclosed building the definition is "A building that does not comply with the requirements for open or partially enclosed buildings." So, enclosed appears to be the proper classification.

The commentary mentions an enclosed building may have large openings on two or more sides.

When the wind is blowing toward one of the large opening, the opening in the other end will prevent the internal pressure from building-up inside the structure. If the structure has an internal division that restricts air movement, a partially enclosed classification may be justified.

 

[KootK]

wannabe's convinced me. I retract my previous answer and support his in its place.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RobertHale]

I concur with wannabe's interpretation as well. A while ago, after delving into this issue for a parking garage, I reasoned that a "partially enclosed" building has to behave like a plastic grocery bag in the wind. You need a big hole in one side with no potential outlets to get that increased internal pressure. Just my two cents.

Robert Hale, PE

 

[dik]

Would you not want to try for a flat roof? rather than an airfoil? Why so high?

Dik

 

[dianium500]

I agree w/ wannabe's interpretation, but that's looking at only two walls. There are four walls here and if the wind blows at a 45 degree angle, you can still get a build up of internal pressure.

I feel ASCE got this wrong. To consider this a fully enclosed structure is not an accurate classification and they should have a provision in the code for calculating the internal pressure coefficient based on the number of openings per wall and area.

 

[BAretired]

Is there or could there be in the future provision to close the openings? If one opening is closed and the other is open, large internal pressures are possible. If neither opening can be closed, the building is not secure.

BA

 

[SAIL3]

I also agree with Wannabe's recap of the code. However, the code can not cover every different bldg config. To me, this is a non-typical bldg case because of the large full-width openings in both ends. I would, at a minimum, investigate the affects of oblique wind on the end bays and the end frame in particular.
Say the wind is @ 70deg(or 60deg) to the longitudinal axis....I would anticipate a zone(16 to 18ft high) where air seperation could occur behind the windward wall and also the leeward wall...how far that zone extends into the end bays would be a judgement call....it may or may not control design in that area but, at least, I have attempted to address that possibility.

 

[KootK]

I agree w/ wannabe's interpretation, but that's looking at only two walls. There are four walls here and if the wind blows at a 45 degree angle, you can still get a build up of internal pressure.

While I appreciated wannabe's able code interpretation, it was really his succinct explanation of the theory that did if for me. And, in a similar vein, Robert Hale's take.

When the wind is blowing toward one of the large opening, the opening in the other end will prevent the internal pressure from building-up inside the structure.

a "partially enclosed" building has to behave like a plastic grocery bag in the wind. You need a big hole in one side with no potential outlets to get that increased internal pressure.

I (now) see the partially enclosed situation as requiring both of the following in order to generate the associated wind pressures:

1) A significant opening through the windward wall through which air can be pumped into the space faster than it leaves.

2) The absence of a comparably significant opening in any other surface of the building through which the air from #1 can escape at a similar rate to which it enters.

This jives with the code provisions nicely. And I contend that it's irrelevant which of the non-windward surfaces serves as the pressure release valve. It could be the roof for all the difference that would make theoretically. Heck, it could be some kind of intelligent HVAC system.

and they should have a provision in the code for calculating the internal pressure coefficient based on the number of openings per wall and area.

Be careful what you wish for. I, for one, have no desire to see the ASCE wind provisions grow in complexity.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.