ASCE 7-02 Wind Loads - definition for enclosed structure 1

[chichuck]

All,

I recently finished an online self-learning course on basics of ASCE 7-02 wind loads, for continuing education hours needed for license renewal. I won't reveal the source here.

The course outlined the use of the simplified method for analyzing wind loads. The definition specified in the course for enclosed buildings was slightly different from the one in ASCE 7-02. The instructor said that you must have less than 4 ft^2 openings or you must cover them with debris-resistant covers (or use debris-resistant glazing.)

At the end of the course, there was a quiz. One question was as follows:

If a building is "Enclosed", what does that imply?

The building windows must be covered or impact resistant
There are minimal openings (4 sf)
You can use the Simplified Procedure
All of the above

The correct answer was given as "all of the above."

I say their definition for enclosed buildings was wrong, I say you can have an enclosed building with more than 4 sf openings on windward wall, no coverings on openings. And, I say, you might also not be able to use the simplified method for analysis.

I've defined a building that is enclosed, has > 4ft^2 openings in windward wall & cannot use simplified procedure. Its an actual building I'm working on at the moment. I have a 3-page write up, a spreadsheet for calcs and some sketches of the building. Too much to describe here, but can anyone help - I cant post my email, where can I post those documents for use on the forum?

I'd like to figure out if my interpretation of the definition of enclosed buildings is correct. I also want to check here before going off and telling the intstuctor he's wrong. (I may be the one who's wrong here.)


Thanks in advance for any insight.


chichuck

 

[ajh1]

I would agree with you. "Enclosed" is a catch-all anytime that "open" and "partially enclosed" are not met. The best example is of a building where both sidewalls are completely sheeted and both endwalls are completely open. Checking the conditions:
"Open" - each wall at least 80% open - Fails
"partial" - rule 1: total area of opening on one wall exceeds the sum of the areas of openings in the balance of the building envelope by more than 10%. - Fails, area on front endwall matches area on rear endwall.
therefore building is "enclosed" even though it has two completely open walls.
Note that in this case, care should be taken in determining the wind loads to apply to the bracing system in the longitudinal direction as there may be more wind on the system than if the building was completely sheeted. The key here is that "partially enclosed" will generate a ballooning condition due to internal pressure that neither enclosed or open will have.

That is the general rule for "enclosed", however the wind-borne debris rules requirement for the simplified approach could mean that the building could be "enclosed", but not qualify for the simplified approach. The instructor's limits are probably appropriate for the simplified approach, but not "enclosed" in general.

 

[chichuck]

ajh1,

I agree with your comments, especially the note about "ballooning", that is exactly what I think is the most important part of the three requirements for "partially enclosed" structures.

Here is the instructor's definition for Enclosed Buildings:

Building, Enclosed- A building that compiies with both of the following conditions:

1) The total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of the openings in the balance of the building envelope (walls and roof) by less than 10%, and...

2) The total area of openings in a wall that receives external pressure does not exceed 4ft^2 or 1% of the area of that wall, whichever is smaller, and the percentage of openings in the balance of the building envelope does not exceed 20%.


This is NOT correct, it does NOT match the definition in ASCE 7-02. This is merely the converse of the requirements for Partially Enclosed Buildings. (At least, converse of two out of three requirements.) That's not what the code definition says.

Briefly, for partially enclosed buildings, per the code, there are three tests:



1) Ao > 1.1 * Aio

2a) Ao > 4.0 ft^2

2b) Aio < 0.20

Your example fails 1), meets 2a) and meets 2b) and is enclosed, so it fails on partially enclosed and therefore is enclosed. (My example is not open on two sides but works the same way as yours.)

Whether or not you can use Simplified Method for load calculation is completely unrelated to the question of whether or not it is enclosed. There are a whole different set of requirements for that.

So, his question/answer looks wrong to me for that reason as well.


I note that if one accepts the instructor's definition for enclosed structures, your example and mine do not meet the requirements for open, do not meet the requirements for partially enclosed, and also do not meet the requirements for enclosed. So, then, what would these two examples be? I believe I need to pose that question with the instructor.

Thanks for the insight.


Regards,

chichuck