Exposure Category in Mountain Regions

[SeanTee]

I have been trying to understand how ASCE 7 wind exposure categories work in inland mountain regions.

I guess my main question is do upwind obstructions refer to obstructions along the lay of the land or looking out horizontally from the building.
If there is a building high on a wooded mountain that has been cleared just around the building. Looking out from the building there are no mountains or any obstructions in font of the building for miles. Would that be category B? Or would this be an "unobstructed area" and count for D? Though it seams D is meant for coastal regions. So maybe C?

I am, of course, taking into account escarpment effects.

Thanks

 

[Aesur]

Take a look at the commentary in ASCE7-16. It gives a great explanation of how to calculate and determine exposure categories including finally explaining the real meaning of open patches. In your case it would be either B or C. As you said D is for open water/marsh/swamps, etc.. where the water surface is so smooth that it can allow the wind forces to greatly increase.

 

[SeanTee]

Thanks Aesur, I read though that commentary and it does clarify some things, however it didn't seam to address my confusion directly.

Suppose you have a building that is on a steep slope and has a clearing for 500' in front of it. If you look downhill you see the trees 500' away but if you look out to the horizon there is no obstruction for many miles. Is the windward direction considered along the slope or along a level line?

Considering winward as along the slope you would get exposure B. If you considered it along a level line it would be C.

My reading of ASCE 7 seams to suggest it is in the downhill direction, though it doesn't explicitly state that, so I'm not confident about it.

 

[Aesur]

My understanding is that the wind acts parallel to the slope. If you have a clearing that is 500' in front of the building, you would need to look at open patches in each quadrant to determine if B or C. The fact that the building is located on the side of the mountain is considered in the topographic factor.

 

[BridgeSmith]

If the ASCE categories are similar to the AASHTO wind categories (which it sounds like they are), I would consider the situation you describe as Category C. It doesn't seem like there is enough of substantial height above the surrounding terrain to lessen the exposure to wind for the structure, which I understand to be the basis of the reduction in wind load allowed in Category B. You might be able to justify using Category B based on the wording in the spec, but good engineering judgment about whether the conditions warrant such a reduction still falls on the shoulders of the engineer.

Edit: Anything that you would consider as being effective to lessen the wind exposure must also be permanent.

Rod Smith, P.E., The artist formerly known as HotRod10