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.

 

[JAE]

After setting up a spreadsheet for this it appears to me to allow different conditions for different directions.

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[Ron247]

JAE, that is how I am treating it right now. I also wanted to know because of setting up my own spreadsheet. I am hoping that will be the consensus opinion.

 

[ajh1]

With the exception of Partially Enclosed, I would say that the other conditions all would need to be the same for all directions. Open is defined as all walls more than 80% open. Enclosed is now something like no more than 4 sq. ft. of opening. Not sure how either of those could be construed differently based on direction. Partially open is simply the default case when all other cases are shown to not be valid. Partially enclosed is working off of a ratio of open area on the windward wall vs. open areas on the remaining walls, so I suppose that one could be viewed differently by direction, although I'm not sure I would consider it worth the trouble. Some one could argue that if you ran different conditions for different directions, what do you have when the wind is projected diagonally in from a corner, where one wall translated to partially enclosed and the other did not.

 

[Ron247]

From what I can tell of the formulas, you have to treat it from four directions at a minimum. It appears to calculate and compare wind entering the structure to wind exiting the structure. If your balloon has way more holes that allow air out than what you are trying to blow into the balloon, you have a low internal pressure. After you check four different directions, you can have from 1 to 4 different conditions. Based on how they calculate it, it appears you could use the worst internal pressure for all direction for simplicity or you could choose to treat each direction differently.

 

[Lomarandil]

I've done it (most recently for a shade structure with one side plus half end walls enclosed, and the opposite half open.

i.e. [ ] where one bracket had brick, and the other bracket was empty.

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The name is a long story -- just call me Lo.

 

[dianium500]

It certainly would make your design more economic to treat each direction differently, but is it really worth it? I mean how much extra concrete and connectors are you saving by doing this? If it's a repeat structure that a builder will build a few hundred of, then yes it's absolutely worth the trouble, but not on the custom ones. I also agree with ajh1, what happens when wind is at a 45 to the structure, how do you deal with that in this scenario?

 

[Ron247]

dianium, This is a general question, not a project specific one. As far as savings, going from .55 to .18 is fairly significant especially in a high wind speed area. The other reason for sometimes wanting a reduction is when you have some demarcation line in a code you can avoid passing by using the more exact values.

The 45 degree scenario is probably covered by the fact you assess 4 directions anyway.

 

[BridgeSmith]

I'm not familiar with the ASCE, but when all factors are considered and the positive or negative wind pressure is applied to all the walls and the roof (I'm assuming we are talking about the pressure differential between inside and outside, regardless of which side has the opening), do the wind speeds really change that much from one angle of attack to another?

 

[Ron247]

If the wind is 100 mph but on a 45 degree angle, I would change the speed to 70.7 mph in the 2 directions if I were going to work it that way. I would not resolve a 100 mph pressure, I would resolve the wind speed. 100 squared = 10,000 while 70.7 squared is 5,000, so although I am checking 2 directions, it is at a much lower pressure. Internal pressure has no effect on the overall loads on an enclosed structure, but can have a significant effect on a partially enclosed one. If one end is completely open, you can have a net added wind due to internal pressure since one wall is not present to counteract the opposite wall.

As I stated, this is not a project specific question, but more of one regarding how that portion of a code is intended to be applied.

 

[Lomarandil]

Ron, that would be an incorrect method...

You could multiply the resultant pressures by 0.707 and combine the orthogonal directions, but not wind speeds. We're looking for a vector sum of forces here, where forces and pressures are linearly related.

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The name is a long story -- just call me Lo.

 

[BridgeSmith]

What I was trying to get at is that my understanding would be that the pressure used for design of a partially enclosed structure would be based on the wind pressure on the side with the opening, but would be applied to the entire building envelope as a pressure differential between the inside and outside. So, unless the wind pressure was significantly different on the open side (such as another building that substantially blocks the wind from that direction), the design wind pressure is the same regardless of which side is open, right?

 

[Ron247]

Since you calculate the 4 main directions, you can get 4 different answers. Lets say 1 of them is partially enclosed (same as Mostly Open). Then you have a .55 internal pressure coefficient that gets applied to all interior surfaces. If one side is completely open, then there is no .55 being applied to it. Hence, you have a net increase in lateral force whereas in a closed structure, there is no net change in total lateral force. Once the internal pressure is determined for a given wind direction, it gets applied to all interior surfaces at the same magnitude (.55 or .18).

 

[GC_Hopi]

I thought that a partially enclosed building and enclosed building are mutually exclusive by definition.

 

[Ron247]

Lomarandil, While this is not the reason for my original question, it is an important point to analyze. If a wind at 100 mph strikes at a 45 degree angle, at what speed is it striking perpendicular to the X-face and what speed is it striking perpendicular to the Y-face? Speed is still a vector isn't it? The pressure we calculate created by the wind is based on the speed of the wind.

Where is a good wind tunnel when you need one?

 

[Ron247]

Partially enclosed is more like "Mostly Open". Partially Open (more like Mostly Enclosed) and Enclosed have the same internal pressure. Partially Enclosed has the higher internal pressure. Their choice of wording makes this hard to follow in the code. Look at the definition of Low-rise, it does not include Partially Open although they have the same internal pressure. Enclosure only has to do with internal pressure and which parts of the code you are allowed to use.

 

[BridgeSmith]

If there are openings on 2 sides, at any given time, one may be open and the other closed. With the largest one open and the other closed, you'd get the largest pressure differential, which you would apply to everything except the area of the opening. Theoretically, with the smaller opening open and the larger one closed, you could apply a lower pressure, but that would only affect the design of the door covering the larger opening, because everything else has to be designed for the higher pressure produced by the larger door being open. Am I missing the point?

 

[Ron247]

First, I think we have 2 different but related discussions kind of overlapping here.

Discussion 1: The original discussion only has to do with determining the internal pressure coefficient based on the openings present in the various walls of a structure. That is a known procedure in ASCE7. For each "positive wind direction or windward wall" you do a set of calcs and arrive at an Enclosure. Assuming 4 directions, you have 4 possible outcomes. From that point, you either have 1 determination that applies to all directions, 1 determination for each direction (2 total) (X or Y), 1 determination for each pair of walls that touch (this would address 45 degree) or 1 determination for each of the 4 directions. The formulas used in these determinations take into account holes in the windward wall differently than it does holes in the non-windward walls. Basically, windard is wind into the building and non-windward is wind escaping the building. It just wants to analyze is wind gets out faster or slower than it gets in. It is not just the fact one opening is bigger than another, it is how much bigger in this determination. Even if I apply the 45 degree logic to one corner (say wall 1 and 2) and it was more severe, what about the wind blowing in the reverse direction on wall 3 and 4? It might not be more severe. That still leads to the same question. Would I use .55 for all wind or .55 for walls 1 and 2 but .18 for walls 3 & 4.

Discussion 2: This more has to do with applying the wind into your structural model. In this case, we are not only applying internal pressure, but also the application of external pressures. This is what lead to 45 degree wind being resolved based on pressure or speed.

 

[BridgeSmith]

With regard to Discussion 1, not knowing how the spec reads, as I said, I'm just trying think about the mechanics of it. From that perspective, I would have to say the .55 would have to be applied to all walls because the pressure differential would create forces on all the walls. The loading will vary widely for each wall and locally in areas of each wall, but it will be impossible to predict the pattern of that loading for even one particular wind direction, much less the infinite number of possible wind directions that can occur.

With regard to discussion 2, the AASHTO spec we use for bridges, according to the commentary, combines the windward pressure and leeward suction into a single coefficient. Unless the ASCE really likes punishing its users, I suspect it does the same. If you're going to resolve it, it would have to be based on the pressure, being that the relationship between wind speed and force is not linear. As a slightly conservative simplification of the actual resolution of forces for wind at any given angle, we use 100% loading in one direction and 20% loading in the orthogonal direction. (Wind at 10 degrees from perpendicular to the surface is only marginally less than these values).

 

[Ron247]

The .55 (or .18) is applied to all interior surfaces when it is applied for a given wind direction. When you look at wind in the orthogonal direction it may be classed as .18. When you apply the .18, it is to all interior surfaces (vertical and horizontal).