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ASCE 7-10 Service Wind

ASCE 7-10 Service Wind

ASCE 7-10 Service Wind

(OP)
IBC 2012 Table 1604.3 Footnote f permits 0.42 times the components and cladding wind pressure for checking deflection. I know this is relating the ultimate pressure to a 10-year service pressure and the derivation of this pressure includes the occupancy category based wind speed.

However, when checking drift (or service wind in general) by ASCE 7-10 Commentary Appendix C, the equation is D + 0.5L + Wa where Wa is taken from Figures CC-1 (for 10 year wind service wind for example), CC-2, CC-3. This comes directly from the maps no matter the category of building.

ASCE 7-05 Appendix C used D + 0.5L + 0.7W where W was developed based on the old importance factors.

So it appears that IBC 2012 "permits" us to check C&C service deflections using the a factor on the wind developed based on the occupancy category. And ASCE 7-10 Appendix C does not account for the building category for its service wind which should apply to either drift or C&C deformation checks. And this is a change from ASCE 7-05 and is not coinciding with what IBC 2012 is "permitting".

Thoughts?

RE: ASCE 7-10 Service Wind

I think what is going on is that the IBC is just saying use 0.7 times 0.6 which reduces the MRI and the wind to service level. However, the 0.7 isn't applicable in higher wind areas. It's actually 0.74^2 which comes out to 0.55. This means that the IBC is conservative in high wind areas. I'm guessing (I don't use Appendix C) that the ASCE 7 maps don't take the risk category into affect because it's a serviceability check. Cladding doesn't distinguish between risk categories but in an ultimate level event, the risk category certainly would matter.

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