ASCE 7 Wind Enclosure Classification Question

[Ron247]

Is the Enclosure Classification (Open, Enclosed, Partially Open, Partially Enclosed) a single designation for the entire structure or can it be a separate designation for each of the 4 directions? I had a similar question weeks back about the topographic factor (Kzt ) and everyone pretty much agreed it can be a different value for each direction. I was curious if the Enclosure is similar or not.

 

[BridgeSmith]

That makes sense, Ron, but when would the .18 produce a critical loading case?

 

[Ron247]

For enclosed or partially open it has very little affect on the overall lateral forces. You have 10 pounds inside pushing the right with 10 pounds inside pushing to the left. But if you had 1 side completely open and due to that you were classified as Partially Enclosed (remember this also means "Mostly Open") you have .55 pushing on 1 end of the building but not pushing on the other end since there is no surface to apply it to. In that case, you get a .55 increase in total lateral force but at the same time you have no exterior wind on that missing wall surface. While it may not create a controlling load combination, it can change the load path for your bracing. The .55 internal pressure adds with the leeward wind coefficient and the total lateral force is pulling on the bracing and in other cases it is a combination of pushing and pulling on the lateral bracing.

 

[BridgeSmith]

So the effects due to loading subject to the .55 factor is additive with the effects due to loading with the .18 factor? Man, ASCE really does hate you guys!

 

[Ron247]

No, they are not additive. They are applied based on what wind direction you are analyzing once you determine them. For Wind-X positive .18 may be correct while for Wind-Z negative .55 may be correct. Also consider they have to be applied as internal suction and later as internal pressure (+/-). That makes it even more aggravating.

For most buildings, using one Enclosure for the whole thing is a lot easier but at times you hit something like some bracing system is good for 265 plf but you have 290 plf. If you do a little more work, your load may get below 265 and you can use the system. I rarely have had a partially enclosed but if you did hotels in Hawaii you may have one. I have seen several buildings with a front entrance that does not have solid walls or a door. The airport is similar in some areas. In the formulas, the windward wall holes let air in but any hole (non-windward wall, floor or roof) can let air out.

This is an internal pressure calculation that is additive to your external pressures. But since it is interior suction or interior pressure and applied separately from each other, it gets real aggravating.

 

[BridgeSmith]

Definitely sounds aggravating. I'm still not sure I grasp when the different factors would be applicable. I'll take another stab at it anyway, and you can let me know if I'm way off base. If one side of the building has a large opening, like a garage door, you would factor the wind load on the opposite wall (and the side walls) with the .55, but you could potentially factor the wind load on the remainder of the wall with the opening using the .18?

 

[Ron247]

No, the .55 and .18 never get mixed in the same analysis. If for Wind in the +X direction, I am classified as partially enclosed, the .55 gets applied to all actual surfaces (horizontal or vertical) as a suction load and in a separate analysis, as a pressure load. Later, for wind in the -X direction, if I was classified as Partially Open, the .18 gets applied to all actual surfaces (horizontal or vertical) as a suction load and in a separate analysis as a pressure load.

Yes, a lot of derivations. The question I am trying to confirm is whether the worst case gets used in all directions of wind or can we separate them as described above.

The openings calculation is only to determine the magnitude of the internal wind coefficient (.55 or .18). The application of these coefficients is like all other wind application except the application is to the interior surface of the wall and roof. If you are have an open wall, there would be no application of any wind (exterior or interior) to that open wall area but all other existing walls would get a force on them.

As I said, mos of the time it is sufficient to be conservative but at times it is good to be able to reduce the load. I deal with a lot of existing buildings for modifications. New construction is not the issue as much as existing buildings. For new buildigns, no harm in being conservative. Especially if they were designed before the wind learned it could create an internal suction or pressure.

 

[GC_Hopi]

I think the intent of the code was to classify the building then use that classification for all directions. They use the terminology of "A building that complies with..." or "A building that does not comply with.." If they expected each wall to be classified different I think the terminology would be different such as "A wall of the building that complies with..

 

[Ron247]

GC, I have not read those definitions that close but your comment makes sense. That makes sense for sure on Enclosed and Open. Enclosed says "Total area of openings in each wall" and Open says "each wall at least 80% open".

For Partially Enclosed it says 2 conditions must be met but from the wording it does sound like only 1 wall is needed to have this classification for the entire building tot hen have it. From the 1st rule, windward wall alone has 10% more openings than the rest of the roof and walls combined. For the 2nd rule, lesser of 4 sqft or 1% of the windward wall while the openings in the rest of the building do not exceed 20%. My only question to that is does Rule 1 and 2 have to be the same wall or can they be separate walls where one meets one criteria and one meets the other criteria. I think they must be the same wall. This is the .55 one.

Partially Open is the default if the other 3 do not apply. It is the other .18.

There is also a statement on Multiple Classifications but it only addresses Open and Partially Enclosed. Defaults to Open. I would have though they would default to Partially Enclosed but I guess not.

 

[JAE]

I think you also have to keep in mind that the enclosure classification affects BOTH MWFRS winds and C&C winds.

I agree with what Ron247 has been saying and here is my attempt to summarize it all:
(Based on ASCE 7-10)
Enclosure Classification
1. Work through the Partial Enclosure calculations found in section 26.2 (Definitions) to determine if your building is Partially Enclosed for each direction.
2. Section 26.10.4 states that

a) if a building has both Open and Partial conditions, the whole building should be considered Open.​

b) If a building is NOT either Open or Partial then it is ALL Enclosed​

c) Otherwise it is Partially Enclosed​

C&C Wind
1. Since C&C wind doesn't depend on direction of wind (i.e. building orientation) then the directional aspects of Partial Enclosure doesn't apply.
2. That means that for Open, Partial, or Closed, you have ONE Cpi factor for all elements of a building regardless of wind direction.
3. That Cpi factor is the largest one you derive from the classification calculations.

MWFRS Wind
1. If the building is OPEN or CLOSED then you have one Cpi factor for the entire building.
2. If the building is PARTIALLY ENCLOSED then you have a different Cpi factor depending on wind direction.
3. For a unified building (i.e. a building without expansion joints) the directional aspect of Cpi doesn't really matter as the GCpi pressure cancels out in each direction.
4. For a building with one or more expansion joints through it, the GCpi pressure does make a difference and you would possibly have different analyses for the "open" direction and the other three directions. This is because the expansion joint separates the wind forces on the windward and leeward sides of the joint and GCpi results in different lateral forces on each building portion.

Let me know if the above looks correct. Any corrections/alterations?



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Ron247]

JAE, based on GC_hopi post, I am still not sure about Partially Enclosed (PE). Open and Enclosed definitions clearly state all walls must meet the criteria to be classified as them. So if just one wall does not meet their criteria, it is only left with the 2 partial options. Partially Enclosed is the only one left with any definition. Partially Open (PO) is what you are left with if it does not define as PE. In the PE definition below, is sounds like if you get one wall that meets the 2 criteria, you have a PE in all directions. It is not clear if the 2 criteria have to be the same wall or any two walls. While I see it has the classification could be unique for any direction, their wording sounds like it says "one for all and all because of one". I wish they would just outright state it. If the codes states it is to be that way, it does not matter what we view it as.

It is like "foundations shall be a minimum 12" into the ground". Since they have that requirement outright stated, I do not have to rationalize if I can put them any shallower.

Another thing on the Partially Open issue. I never see it stated in any of the wind analysis options that they allow for it although I view it as being so close to Enclosed it would be somewhat the same design. I think even the authors have confused themselves. Partially Enclosed on the surface sounds like a kissing cousin to Enclosed when in fact Partially Open is closer. That is why I mentally refer to Partially Enclosed as "Mostly Open".

 

[JAE]

I look at the phrase above - "a wall that receives positive pressure" as a windward wall as leeward and side walls are always negative pressure.

For wind in +X direction - the windward wall may have a large opening (as you stated above) and thus for that +X direction you would have a Partially Enclosed condition.
For wind in the other three directions - not a Partially Enclosed condition for those wind directions.

As I stated in my outline above - for all three (Open, Partially Enclosed, or Enclosed) this directionality doesn't usually matter unless you have building expansion joints that separate the effects of the resulting internal pressure.

I don't think there is such a thing as Partially Open in ASCE 7.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[GC_Hopi]

JAE: Never say never...they added Partially Open to ASCE 7-16. haha

 

[Ron247]

yeah, it was an add to the 2016 version. Problem is, they did not modify all the wind analysis options to say you could use them for Partially Open. That is why I said, they have confused themselves to some degree. Per the code, if you get a partially open building, you are not allowed to design it even though the internal pressure coefficient is the exact same as Enclosed. Fortunately for us, I do not think the wind is smart enough to follow the code properly.

 

[Ron247]

That wording is what we are now trying to confirm. It sounds like if just one wall that receives pressure meets the opening criteria for both the rules, then the entire building gets classified as PE. I think it should be per direction, but their wording sounds like the presence of 1 (or 2) walls that meet their criteria causes all directions to be PE.