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Wind exposure B opinion 1

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wikidcool

Structural
Jun 20, 2007
50
US
For ASCE 7-10 26.7.3, Exposure Categories for buildings under 30 ft says B applies when surface roughness B "prevails upwind for a distance greater than 1500 ft".

If you have a site that has a quadrant that's fairly open inside that 1500 ft radius with fields, parking, etc, but if you look at it on larger radius like 5,000 ft and the region is B overall - would you consider that B or C? Obviously, the conservative answer is to use C but what is most accurate for how the wind actually would behave on the building?
 
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I'd say C (at least for that quadrant). No expert here, but I imagine the roughness/shielding/disturbance effect captured by exposure dissipates pretty quickly over 1500ft of open field.

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The name is a long story -- just call me Lo.
 
Wind provisions are very subjective in MHO.

If you want to go conservative, choose C. If you want to give your client a more economical design, choose B.

What's the likelihood of failure if you are not conservative? Do you have redundancy? Are there other areas where additional safety can be built in?
 
There is additional information in the ASCE 7-10 Commentary on determining wind exposure when you have open patches around the building. However, I don't think you'll like what is says. Even though the commentary says that it is estimated that 60%-80% of all buildings are exposure B, all it takes is a decent sized parking lot to make it exposure C.
 
When I doubt, I’ve been going with Exposure C in those situations, which are very common. Almost any school is going to have a large parking lot, football field, baseball field, etc that will ruin an otherwise strong Exposure B setting. It ultimately depends on your subjective interpretation of what they mean by “prevails.”
 
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