Exposure Categories

[drewtheengineer]

So I'm doing what I normally do on Saturday night, when I come to Section 6.5.6 of ASCE 7-98 (Exposure Categories). It seems that Exposure D is described as Flat, unobstructed areas exposed to wind flowing over open water. Well one site that I have in mind is just that, however it sits up on a hill (therefore not flat).

Later on that night I came to Section 6.5.7.1 item 5 which reads "...H is greater than or equal to 15 ft for Exposures C and D and ...". That H is referring to the height of a hill but I thought Exposure D was supposed to be flat.

If anyone can set me straight here I'd appreciate it.

Thanks,
Drew

 

[dougantholz]

If your building has a direct exposure to a lake of at least 1 mile in windward length is exposure D. If the building is on a hill, it is still exposure D, but you need to modify according to the escarpment provisions.
That's my reading.
Any others?

 

[drewtheengineer]

Thanks Dougantholz, that's how I'm reading it as well. But the fact that Exposure D is defined as "Flat.." is what's throwing me. If it's not flat, but meets all the other criteria, is it Exposure D?

 

[dougantholz]

Yes I believe it is. I think that D is used where the wind is un-obstructed and allowed to pick up speed. This isn't always a lake. I have spent a lot of time in Nebraska on I-80 where the land is flat for several hundred miles and usually picks up a good bit of speed.
If my building were out in the middle of nowhere with no trees or anything else to break up the wind, I would use D.

 

[drewtheengineer]

I disagree with you there. Exposure C includes "flat open country, grasslands and shorelines in hurricane prone regions". Exposure D "shall only apply to those buildings and other structures exposed to the wind coming from over the water".