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# plywood diaphragm deflection calculation

• To: <seaint(--nospam--at)seaint.org>
• Subject: plywood diaphragm deflection calculation
• From: "Bruce Holcomb" <bholcomb(--nospam--at)brpae.com>
• Date: Fri, 9 Apr 2004 08:57:57 -0500

```I submitted this yesterday, but got a message that it was undeliverable, so I am trying again.  I am basically asking what values to use for "G*t-sub-v" in calculating wood diaphragm deflection.

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" & 1/2" vs. 32/16 span rating, the second is 19/32" & 5/8" vs. 40/20 span rating and the third is 23/32" & 3/4" 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
Architecture, Engineering, Interior Design, Planning & Development

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