Enclosed vs Partially Enclosed...another thread

[bsh117]

I understand the conditions for a partially enclosed building per ASCE 7. It states:

1. 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%, and
2. the total area of openings in a wall that receives positive external pressure exceeds 4 ft2 (0.37 m2) 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%.

Both conditions must be met.

However, I have a questions about buildings in a hurricane prone region. Per the ASCE Guide to the use of wind load provisions, it states that if the windows are not wind-borne debris rated, then you should be designing as a partially enclosed building.

My question is: why doesn't condition #1 need to be met? I have a hard time getting the numbers to work out to meet condition #1. For example, lets use a square office building with the same window pattern on all four sides. If we say that all the windows fail, how do the total openings on the positive face exceed the sum of the other three sides by 10%?

I would love to hear your input!

Thanks in advance.

Brian

 

[ron9876]

Because only the windward walls will fail due to debris impact with the balance of the windows still in place.

 

[Ron]

Only openings receiving POSITIVE pressure are considered. Openings receiving negative pressure (as on the opposite side of the building) don't count as openings in the calculation.

 

[bsh117]

Ron & Ron,

Thanks for the input. I understand what you guys are saying.

But let's put in another way. It appears that the wind design is being penalized because there are windows in the structure to begin with. Let's say that the initial design didn't have windows in the structure at all. If we say that the structure is rectangular and we load the broad face with wind. The openings in this broad face would not exceed the sum of the openings at the side face and leeward face. Therefore, it wouldn't meet condition #1 and the building would be designed as an enclosed building. Is this correct?

Thanks!

Brian

 

[Ron]

No. If you have no windows, you have no openings...therefore enclosed. By definition, a partially enclosed structure must have openings.

 

[bsh117]

Ron,

Sorry, I meant to say that I have openings, but there are no windows in the openings.

If the initial design didn't have any physical windows, but just punched openings on all four sides. Or, if it's easier to imagine that the structure is a parking garage. Let's say there are spandrel openings on all four sides. If we place the wind load on the front face, it would receive positive pressure. The side faces and the leeward face receive negative pressure. Therefore, the area of openings on the front face do not exceed the sum of the openings on the side and leeward faces. Therefore, the parking garage would be considered an enclosed structure. Is this correct? It appears counter-intuitive.

Thanks!

Brian

 

[Ron]

The criteria includes an area of openings. The area must be 4 sq ft or greater to be considered. That's not much. I've found on PEMB hangars that the gap at the doors can exceed 4 square feet....and the engineer designed the structure as "enclosed".

If you have punched openings, then you have to look at what is in those openings. If louvers with air flow, then yes, they would be the same as windows. If protected with something equivalent to small missile and large missile impact and no air flow, then they would be protected openings.....and they don't count.

 

[ron9876]

bsh117 yes that is correct. But it makes sense. You can't build up pressure from wind on one side if there is a larger area of openings in the balance of the walls to relieve the pressure.

 

[bsh117]

Ron9876,

What you are saying makes sense, but to me, there can't be an internal pressure component if there is a larger area of openings in the balance of the walls to relieve the pressure. I guess, to me, it sounds like the building should be an open building, not an enclosed building.

 

[a2mfk]

OP- I think I understand what you are saying. But a building with doors and windows in a wind event may only have one or two windows get blown in, and not all of them- so it COULD be partially enclosed at some point during a wind event, so you must design for this being the worse case. Garage door failures during wind events are a common cause of overall structural failures, the wind enters and has no place to go, so the internal pressure builds until a wall or roof member fails...

SIDEBAR ON PARTIALLY-ENCLOSED BUILDINGS IN WINDBORNE DEBRIS REGIONS
I don't personally believe that they are doing any property owners or anyone for that matter any favors by allowing you to design partially enclosed structures in wind borne debris regions by assuming the windows will fail. I believe the openings should be protected by shudders,etc. or be impact resistant.

Being cheap on the windows and doors is just a short-sighted savings. I had this argument with an architect and client on a project before, on a retail strip box type store with tons of storefront glass! Sure the structure will be standing since we designed it as partially enclosed, but the property will need to be gutted and will be useless for a year.

I am sure Ron can attest to this- during hurricanes the majority of property damage is due to water damage from roof leaks, window and door leaks and failures, and of course flooding. Improving the envelope of the building will reduce the economic impact of a hurricane by a larger factor than improved structural codes at this point in time, IMO. I am not saying by any stretch that the structure is not important, or that structural damage cannot and does not occur, or that the science of wind engineering should not keep advancing. BUT, I would have to look up the numbers, outside of Hurricane Andrew, I would be surprised if wind-related damage came anywhere close to the amount of water-related damage in economic terms.

Sorry- OFF SOAP BOX.

 

[Ron]

a2mfk...I agree. Even if you design a building for partially enclosed conditions, protecting the openings protects property loss and occupant loss. If a window blows in during a code-level storm event, the wind movement inside can be catastrophic to the occupants. Furniture gets blown around, pressure changes pop eardrums, etc. Yes, the building will stand, but the damage to occupants and other property can be devasting.

 

[bsh117]

a2mfk/Ron/Ron,

Thanks so much for your input. I'd like to ask another question that is related to my initial question but has nothing to do with wind borne debris or hurricane prone areas.

In your interpretation of ASCE...should a parking garage be considered an open, partially enclosed, or an enclosed building. The garage has spandrel panels on all four sides. There are no windows, grills or louvers between the spandrel panels....it is just open. The only difference between the four faces of the parking garage is that there is an opening on two sides (not opposite) for the drive aisle.

Thanks again!

Brian

 

[Ron]

Assuming you have exterior barrier walls, not guardrails, then you have to check the open area against the solid wall area. Might or might not be open, but likely not, since you have to have 80% open as compared to the gross area.

If you have guardrails instead of barrier walls, it would most likely be open.

Then check the partially enclosed condition. It might not meet both conditions, checking flat and oblique.

Then, by default, it is enclosed (makes no sense, but that's the way ASCE 7 defines it).

So, depending on the exposed face configurations, it could be either of the three.

 

[ron9876]

a2mfk at least where I do most of my work protection is required in the debris zone. But water is certainly the cause of most of the financial loss.

The answer to the garage question, in my opinion, is like most everything with ASCE7. It depends. Most garages have ramps that, when combined with spandrels and columns, will result in the structure acting like an enclosed building for MWFRS. But if I was checking the wind loads on an individual element I would use the worst of the possible conditions for that element.