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plywood diaphragm deflection calculation

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In calculating the deflection of a plywood diaphragm, you use the terms “G” (shear modulus) and “t-sub-v” (effective thickness for shear).  I have the 2001 NDS including the Supplements book for Structural Lumber, Structural Glulam Timber, Timber Poles & Piles, Wood Structural Panels and Shear Walls & Diphragms.  On p SP-17 of the Wood Structural Panels section, Table 3.4 has values listed for the “Rigidity through the thickness, “G*t-sub-v”.  Are these the values to use for calculating the diaphragm deflection? 

 

The reason I ask is when I compare with the “Plywood Design Specification” from APA, I find values for G and t-sub-v separately.  I am checking three thicknesses: the first is 15/32” & ½” vs. 32/16 span rating, the second is 19/32” & 5/8” vs. 40/20 span rating and the third is 23/32” & ¾” vs. 48/24 span rating.  I am using “APA Rated Sheathing”; unsanded panels.  I get approximately the same G*t-sub-v for the first and second, but not even close for the third thickness.  Any advice?  

 

I am using “species group 1 for face ply” giving me G=90,000 psi and and effective thickness of 0.298” , 0.315” and 0.445” (corresponding to my 3 checks above).   

 

What am I missing?

 

 

Bruce D. Holcomb, PE

Butler, Rosenbury & Partners

300 S. Jefferson, Suite 505

Springfield, MO 65806

ph. 417-865-6100

fax 417-865-6102

www.brpae.com

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