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- To: <seaint(--nospam--at)seaint.org>
- Subject: plywood diaphragm deflection calculation
- From: "Bruce Holcomb" <bholcomb(--nospam--at)brpae.com>
- Date: Thu, 8 Apr 2004 12:47:42 -0500
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
Architecture, Engineering, Interior Design, Planning & Development
Your Vision. Our Focus.
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