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Wind Exposure Category

Wind Exposure Category

Wind Exposure Category

(OP)
I am currently working on analyzing a 150 foot monopole that is technically in an Exposure B area. The problem I have is that the surrounding structures are no more than two stories in height. Therefore, it seems to me, that the upper reaches of the monopole should see a higher wind load based on either Exposure C or D, but there is no guidance in the code that I can find for this condition as to where the break line should be, let alone the application.

I remember looking at Department of the Navy Hydraulics tests in school and the turbidity height based on the surface roughness, It seems that this factor is not dealt with in the codes, ASCE7, TIA-222, or otherwise, yet.

It also seems that neglecting this phenomenon could lead to an under designed structure if Exposure B is assumed, leading to the use of Exposure C which would be conservative, which is what I am forced to do here.

Any thoughts.

Mike McCann, PE, SE (WA)


RE: Wind Exposure Category

I don't think it's overly conservative to use C. It's prudent and you have some technical reasons.
I don't think that exposure B is meant for a very tall structure in the midst of a bunch of midgets, even though the definition implies that you could do that.

RE: Wind Exposure Category

I would use C. Don't forget that pesky vortex shedding thing

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