Wind Corner Zones on Walls Theory 1

[TysonH]

ASCE is clear that the corner zone width 'a' is "10 percent of the least horizontal dimension or 0.4h, whichever is smaller, but not less than either 4% of least horizontal dimension or 3 ft." Working for a curtainwall company, I have to use this rule on a regular basis. But, I was asked WHY the least building width influences ALL of the corner zone dimensions, and I can't come up with a legitimate reason other than "the code says so." Does anyone know how ASCE arrived at this rule or have an intelligent theory on it? Thanks for the help.

 

[JAE]

Most of the ASCE 7 wind coefficients and rules came out of several wind tunnel tests and I would think that the "a" dimension was a result of those tests, not some theoretical analysis done on paper.

Read the commentary for Chapter 30 to get a feel for where all these came from. That's my guess anyway.



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

 

[msquared48]

So, are you insinuating that this is a situation of the tail wagging the dog?

Mike McCann, PE, SE (WA)

 

[klaus.]

We have similar situation with the height of the building
The wind load depends on the total height of the building
SO if you a high building the wind load is higher also for the lower bottom parts of the building

for example...designing the entrance facade at ground floor
two buildings... different wind load for the entrance facade because one of the buildings is much higher than the other one

somehow not logical.... or ?








best regards
Klaus

 

[ajh1]

Actually, at least for windward pressures, you are allowed to use qz, which is the velocity pressure based on the height above ground that the component you are designing is located. There are cutouts on that in that for z <= 15' you have to use 15' minimum. For roof pressures obviously, and for leeward suction forces you use the mid-height of the roof above ground. For most structures there probably isn't enough difference between using z and h so people don't bother, but for something like an entrance canopy on a high-rise it could be significant.

 

[TysonH]

JAE, I'm not saying it's not in there, but I just looked at the ASCE Commentary and saw nothing. msquared48, you might be on to something here.

Here is an example to better illustrate my confusion: Let's say you have a 100'x30' building and a 100'x90' building. Assuming the 'h' is large enough and the same for both buildings, the corner zones will be 3' and 9' respectively, correct? It is hard to imagine WHY there would be a different corner zone width for the 100' faces of the two buildings when they are the exact same face. In my line of work, the corner zone may require additional reinforcing, fasteners, weld, etc. so this dimension is pretty important.

 

[JAE]

The commentary, in many instances, refers to the wind design data within the code as coming from experimental wind tunnel tests (i.e. research projects).

These typically would be modeled building shapes of various sizes and configurations and from those tests I speculated that the size of the building may have revealed a relationship between the higher peak corner pressures, their extent away from the corner, and the dimensions of the modeled building.

The fact that you can't "picture" this between a 30 ft. wide and 90 ft. wide building is reasonable - but my "guess" is that wind pressure behavior at corners is quite complex and not as intuitive as may be supposed.

You might try contacting Texas Tech university, or perhaps Dr. Kishor Mehta or his firm (here:

http://www.depts.ttu.edu/ceweb/faculty/faculty.php?name=Kishor C. Mehta)

as they/he does a lot of wind research.



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

 

[SAIL3]

when the wind hits the windward face of the building the only 3 routes for the air to escape are around both sides and over the top....the larger the width the more air volume is involved and thus the increased turbulance at the corners......anyway, that's my take on it..
Klaus...the leeward side of the building experiences the affects of wind blockage mentioned above...this affect(suction), I would assume is approximated by using the full height of the building...if for example, the first floor was totally open, then,IMO, one may be able to use the varying press according to hz for the leeward side....

 

[Ron]

One reason for the difference in height values is that a higher building needs to be strengthened (i.e., stiffened) in the lower stories so that drift is reduced.