Wind exposure B opinion 1

[wikidcool]

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?

 

[Lomarandil]

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.

 

[blakeschatz]

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?

 

[spats]

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.

 

[abusementpark]

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.”