ASCE 7 Partially Enclosed vs. Enclosed 1

[EngineerRam]

Per ASCE 7, a partially enclosed structure is defined as such:

1. The total openings in a wall exceeds the sum of the areas of openings in the balance of the building
envelope by >10%.
2. The total area of openings in a wall that receives positive external pressure exceeds 4ft2 or 1% of
area of that wall, and the percentage of openings in the balance of the building envelope ≤ 20%.

I am in a situation where I can go from partially enclosed to enclosed by increasing my openings in the building shell.

If my total openings in any wall is greater than 10% of the openings in the balance of the building, I add openings to the roof, or walls with less openings so that condition 1 is not satisfied. Because 1 is not satisfied and both need to be satisfied for it to be partially enclosed, I can assume the building is enclosed(as all walls are not 80% open).

This seems illogical and counterintuitive, however I don't believe I am violating any provisions in the ASCE by doing this. Can anyone provide any reasoning as to why this is?

Thanks

 

[ajh1]

For ASCE7 up through the 2010 you are correct.
New to ASCE7-16 is a "Partially Open" condition that covers situations like this. It is all semantics as the internal pressure coefficients for "Partially Open" are the same as for "Enclosed". "Partially Open" becomes the default case if none of the other conditions apply. "Enclosed" is limited to each wall having openings totaling no more than 0.01Ag or 4 sq ft whichever is SMALLER.

 

[briancpotter]

Basically, when one wall has more openings than the rest of the building, the building can become pressurized, as more air is entering the building than is capable of leaving it. Reducing a walls openings OR adding more openings to the other walls will both eliminate this condition. "Partially enclosed" doesn't really mean "how many openings the building has", it means situations where a building might become pressurized.

FEMA 543 has a good explanation of this (skip to page 3-11). [URL unfurl="true"]https://www.fema.gov/media-library-data/20130726-1557-20490-6102/fema543_chapter3.pdf[/url]

 

[MrHershey]

For me, it's easier to think of what the wind is actually doing rather than getting hung up in the nomenclature. Call them condition 1, 2, 3.

Condition 1: Wind free to travel through structure in all directions with little to no hindrances or blocks.

Condition 2: Large opening on one side allows wind to build up pressure inside without a release. Like blowing up a balloon.
Example:

Condition 3: No large build up of wind pressure inside the building. Could be either (A) because there aren't sufficient openings on any face for wind to come inside, OR (B) because there's sufficient openings to relieve internal pressure. The Condition 2 building from the image above would become Condition 3 building if we either (A) remove the large opening, OR (B) include openings on other faces to relieve internal pressure.

Condition 1 is open
Condition 2 is partially enclosed
Condition 3 is enclosed

This is an instance where I think nomenclature gets in the way. Saw a webinar recently that had a great example of a building that was completely open on three sides with a solid wall on the fourth. Using the condition descriptions above it's pretty clear it's condition 3. Wind can't travel freely in all directions because of the solid wall on the fourth side so it's not condition 1, but it can't build up pressure either so it's not condition 2. That leaves condition 3. But certainly odd to look at picture of a building that is wide open save for a single long wall on one side and call it 'enclosed'. Perhaps that's why they're adding 'partially open' in 7-16, though as ahj1 points out it's the same numerically as 'enclosed'.