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wind load on 2 sided structure

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I have always thought you cannot go wrong with a partially enclosed model. If it is fuzzy and doesn't quite meet the definitions, I believe this will always give you the highest values. Now I don't have ASCE in front of me, but 2000 IBC has a little formula in 1609.2 for partially enclosed. Open means each wall is 80% open, like someone else said.
 
 I think about it this way. Any amount of wall you put up will block the wind, and send it upwards to your ceiling or U/S of the roof, and around the wall. One entire solid wall still will direct a lot of wind up and against the roof. But if you have two adjacent solid walls and that gets hit by wind at a 45 degree angle, and the walls go all the way up to the roof, there is not much open wall area for the pressure to be relieved, it just keeps building up and some escapes around the edges. 3 walls and one open end is like holding a paper grocery bag in front of a large fan, the pressure is going to be high on all the surfaces...  I have seen plenty of pics and heard plenty of stories of wind blowing out a garage door then taking the roof out.
 
Now, I am mainly thinking about C + C loads. I suppose if you were designing things for shear, or uplift at the foundations, you may tweak your model a bit. But I like to be conservative with C + C loads, as well as with structures that don't fit code models well. But why bother splitting hairs, a small increase in wind pressure does not seem to have a huge impact on the design and cost of most low rise buildings (in my experience). Not for the peace of mind at least. You still need roof connections, tie downs, bars at 48" o.c., screw down the deck, etc.
 
HTH,
Andrew Kester, PE
Lake Mary, FL