Wind Design, Width of pressure coefficient zone, "a"

[Aesur]

For years I have been using the full building dimensions of complex, multi-diaphragm buildings to calculate the distance "a" for the wind pressure coefficient zones. Recently while discussing wind design with a few engineers we came to the conclusion that this is a grey area. Some are basing the "a" calculation on the least dimension of each separate diaphragm, whereas others are basing the "a" calculation on the whole building. Using the whole building would result in a larger edge zone for MWFRS lateral design, increasing forces. I am curious what others are using and if anyone has any justification on which method is correct.

 

[JAE]

The wind will behave in its manner based on the larger overall building.
In other words, the wind doesn't have a clue where the diaphragms are, only how it is being interrupted by the overall building on its path.

So the "a" dimension we use is the overall building and not based on smaller portions or segments.

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[Tomfh]

I’ll use reduced value if in my judgment the wind can slip around that part as easily as it could were that part an isolated building.

 

[structSU10]

I think the concept of the 'a' dimension is to account for the effect of a mass in the way of the wind path, and as a building gets larger, and larger volume of wind is trying to find its way around the ends, increasing the pressure in those zones. If there is no other area to relieve the wind pressure, you have to consider the whole size of the building, regardless of how the building frame is broken up.