ASCE 7-05 Sec 6.2 Wind Loads :Enclosure classification 2

[inju]

I have some dilemma on ASCE 7-05 Sec 6.2. Definition of Building enclosure classification.

According to ASCE 7-05 Sec 6.2.,
Building OPEN: a building building having each wall atlest 80% open.
Building PARTIALLY ENCLOSED: a building which complies with both
1-total area of opening in the wall receiving positive external pressure exceeds total area of opening in the remaining building envelope by more than 10% ( area of windward opening shall be 10% more than total area of opening in the remaining sides including roof)
2-total area of opening in the wall recieving possitive external pressure exceeds the lesser of 4ft2 or 1% of area of that wall (minimum area of windward opening shall be lesser of 4ft2 or 1% of area of windward wall )
Building ENCLOSED: a builing that does not comply with the requirements of open or partially enclosed building.

Consider a one-story, 10ft tall box-shaped building having 10ftx10ft footprint. no opening in the roof. Assume each side of the building has 70% opening in walls. How would you classify this building? I have a feeling that it should be classified as "PARTIALLY ENCLOSED". But, according to ASCE 7-05 Sec 6.2 it appears to fall in "ENCLOSED" catergory.

I requested a code opinion from ICC staff and I am waiting on their response.What do you folks think?

 

[Ron]

In general, though there are exceptions....

If the openings are subjected to negative pressure, then the structure is partially enclosed.

The code now considers that the openings subjected to negative pressure "do not count". So the equation in ASCE 7 is almost always satisfied to "partially enclosed".

 

[ajh1]

Could you clarify where in the code the statement is that "openings subject to negative pressure do not count"? I couldn't find that sort of a statement in ASCE7-05, its commentary, or IBC 09.

 

[inju]

Ron and ajh1, thanks for keeping this discussion going.

I don't expect an opening to be subject to whether negative or possitive pressure. In simple terms, For North-to-South wind, windward (north) walls will be subject to positive pressure and leeward (south)walls to negetive pressure.

I was just trying to understand the rationale behind the definition of BUILDING ENCLOSURE. ASCE 7-05 and its Commentary do not elaborate on this.

If anyone cares to classify 'a simple box-shaped building' mentioned in the begining of the thread and discuss their reasoning it's greatly appreciated.

 

[vincentpa]

ASCE 7 is tying to confuse you. They make the definitions of partially enclosed and open so you can calculate them. They give three categories. If your structure is not partially enclosed or open by calculation (not by what you see), then it is enclosed. The structure is considered enclosed even if the building is partially open. This doesn't make sense but if you read the commentary and start making phone calls and emails, you will get this explanation. I did this to try to get the answer to the same question you asked. The definition of enclosed is counterintuitive it seems when you are looking at a building that is partially open. ASCE 7 has never adequately clarified except for a pathetic example in the commentary. What ASCE 7 should have is four definitions of enclosure, the last being "Other." The "Other" enclosure would have the same GCpi as Enclosed (+/- 0.18). This would avoid the confusion you are experiencing. But why would they do that? What fun would it be if everything were clearly written and easily understood?

 

[kslee1000]

For partially enclosed, my interpretation is:

1. A (pos) = 70%*(10x10) = 70 sf
A (rem) = 3*70%*(10x10) = 210 sf
A (pos) = 70 sf > 10%*A(rem) = 10%*(210) = 21 sf

2. A(1%) = 1%*(10x10) = 1 sf < 4 sf
A (pos) = 70 sf > 1 sf

----> Partially Enclosed

The language of provosion (1) is not well writen, I guess.

 

[inju]

vincentpa:
thanks for your insight. I am glad you have many calls/email correspondence with ASCE about same question. I will wait and see what ICC staff will have to say on this. but, i couldn't find much about the "other" enclosure classificaiton after reading thru the commentary.

kslee1000:
i guess on the 1st part of your calcs,
A(pos)=70 sf < A(rem)=1.1*210 sf =231 sf that will make it non-compliant to Partially enclosed classificaiton. (According to ASCE 7-05 sec 6.2 Ao>1.1Aoi )

so, this building is ENCLOSED!?! eventhough 70% of the entire building(excluding the roof) is open. really strange, right?

 

[kslee1000]

inju:

That's the reason I "guess" the provison wasn't languaged right. If as stated, the intention does not make sense to me. How often we have the case that opening on one wall more that all others combined + 10% over? Jail house, maybe

But a guess is a guess, wait to see the clarification.

 

[Ron]

To clarify my earlier comment...

There is a body of thought, and it has been incorporated in some codes, that if an opening has negative pressure and the remainder of the openings have positive pressure but no impact resistance, the area of the negative pressure openings is assumed to be zero....in this consideration, the equation will point to "partially enclosed"

 

[kslee1000]

Ron:

Do you know where it is written (which code)? Just curious.

 

[Ron]

kslee1000...Florida Building Code..applying to windborne debris areas.

 

[kslee1000]

Thanks, Ron.

The originator of this thread may want to look into it. There must be a explanation on why it deviates from other sources, such as ASCE 7-05.

 

[Ron]

In this case, the code is used to define the specific element and how the computation is considered in ASCE 7.

To my knowledge, the IBC (2006) does not contain the same consideration. It is relatively silent on opening protection for windborne debris.

The consideration makes some sense in that windborne debris would only affect the opening when something blows into it (as in positive pressure); whereas debris would be blowing away from the opening for negative pressure considerations.

 

[CFSEng]

The way I understand it, enclosure clasification only controls the GCpi. Enclosed is +/-0.18, Partially is +/-0.55 and Open is 0.

If you have 70% openings on all sides (non-compliance with partially enclosed), that just means the CGpi coefficient is +/-0.18 to be used when determing wall pressures. So exterior and 'interior' walls with exterior exposure would still be designed for wind using the +/-0.18 GCpi.

I would imagine there could be some exceptions for this possibly based on geometry, but that is where engineering judgement comes into play.

 

[chichuck]

kslee1000,

Consider an aircraft hanger with the big doors open.

regards,

chichuck

 

[inju]

Let's assume that the structure is NOT located in wind-borne debris region. I just wanted to rationalize the defination for enclosure classification as applied to general conditions.

As I have described earlier, I found the definition of "partially enclosed and enclosed" to be ambigious and incomplete.

 

[CFSEng]

call it ambiguous or incomplete if you want but it is clearly defined. Your openings determine your building classification. Your classification determines the GCpi you use.