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0.7 Reduction in C & C Pressures for Deflection Calcs

0.7 Reduction in C & C Pressures for Deflection Calcs

(OP)
I work for a company that manufactures and installs aluminum composite cladding panel systems. We've recently ran into a buzz-saw of questions regarding why we are taking a 0.7 reduction in the wind pressure for our deflection calculations. I have referenced all who question this reduction to foot note f of table 1604.3 in Chapter 16 of the 2009 IBC.

I feel that this note adequately addresses the reviewers concerns and is pertinent to our cladding application.

My question is if I am making the correct assumption on taking a 0.7 reduction in wind pressure for deflection serviceability purposes only and if not could someone please explain why this note does not support my calculations.

RE: 0.7 Reduction in C & C Pressures for Deflection Calcs

I have used it in the past with C&C loading as you described.

RE: 0.7 Reduction in C & C Pressures for Deflection Calcs

(OP)
Do you know if this is addressed in ASCE 7-05 or 7-10.

I have also heard from other engineers that say they use the 10 year Mean Recurrence Interval Wind Speed maps for deflection calculations only. I haven't found anywhere in the code books where it permits using the 10 year MRI, any help would be greatly appreciated.

RE: 0.7 Reduction in C & C Pressures for Deflection Calcs

The commentary chapter for serviceability (ASCE 7-05) addresses the wind load factor of 0.7 for deflections. The 0.7 factor essentially converts a 50 year MRI wind (strength design) to a 10 year MRI wind (deflection calcs).

We are Virginia Tech
Go HOKIES

RE: 0.7 Reduction in C & C Pressures for Deflection Calcs

This is straight out of the commentary from ASCE 7-05.

"Use of the factored wind load in checking
serviceability is excessively conservative. The
load combination with an annual probability of
0.05 of being exceeded, which can be used for
checking short-term effects, is
D + 0.5L + 0.7W"

The first printing of the code did not have the commentary for serviceability and was issued in an errata.
 

We are Virginia Tech
Go HOKIES

RE: 0.7 Reduction in C & C Pressures for Deflection Calcs

We use 10 year winds for serviceability checks all the time.

I don't remember the exact wording of the footnote in IBC, but I don't think there is any ambiguity.

The only thing I would say is that the 0.7 factor is most appropriate for 90 mph winds.  The factor is a little different for higher winds - per a paper I have (I can post later this morning if I have it electronically).  

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