ASCE7-10 Wind

[spats]

I am using a spreadsheet called ASCE710W that I found on the AISC Steel Tools site. The notes in the spreadsheet say that Exposure D should only be used outside of hurricane-prone regions,and that Exposure C applies to water surfaces in hurricane-prone regions. I can't find this anywhere in ASCE 7-10, including the commentary. Can anybody shed any light on this?

 

[UcfSE]

ASCE 7-05 had the phrase "...outside hurricane prone regions." in the definition of surface roughness D. ASCE 7-10 has removed that phrase. Exposure D can apply in hurricane prone regions.

 

[spats]

Thanks for the reply, but I'm sorry to hear that. Why is it that architects can't give you proper shear walls... Nothing but windows!

 

[Wallache]

The commentary of 7-10 has some back background on the research behind the change. Section C26.7

 

[TehMightyEngineer]

As ASCE 7-10 hasn't been adopted where I work this is very interesting and annoying. Plus, wasn't the whole reason that hurricanes produced so much turbulence that they could never take advantage of the exposure in a category D area? Made sense to me if that's the case.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[spats]

I found the Section C26.7 information Wallache referred to. However, it talks about the roughness of the ocean. In my situation, located on Anastasia Island in St. Augustine, Florida, there is well over 600' of Exposure B/C between the structure and the ocean (see Section 26.7.3). However, there is over 5000' of open grasslands/swamps and an inland waterway to the west without the 600' buffer. When the structures'location is on the back side of a hurricane, the wind direction would be blowing offshore, across the inland waterway/swamp, but is that the same as an open ocean? I'm still confused.

 

[UcfSE]

What are you confused about?

The surface roughness and exposure definitions do not use the phrase "open ocean".

 

[spats]

UcfSE,

Technically you are correct, but the entire logic of the discussion in C26.7 has to do with the surface roughness of the ocean. Under ASCE 7-05 I would use Exposure C because it applied to "ALL water surfaces in hurricane prone regions". Now, because of the surface characteristics of the open ocean, which has nothing to do with my situation, I have to use Exposure D. Makes a lot of sense, right?

 

[UcfSE]

It's telling you that a little bit of Exp B or C between your site and Exp D doesn't overrule the classification of Exp D in that upwind direction. If, in the direction of the inland waterway, you have less than 600ft of B or C and then D, you use D, otherwise you fit to a different category. Remember that D is associated with a certain fetch distance as well, not simply a surface roughness.