Plywood Allowable Stresses 4

[David Deck Guy]

After a recent career change, it looks like I have a lot of existing truss repair ahead of me. Alexander Newman mentions sandwiching light frame trusses with plywood in Structural Renovation of Buildings as a repair option.

I'm looking for allowable stresses for plywood to use with the NDS. I'm not talking allowable shears applied to diaphragms and shear walls for given heights, nailing conditions, etc.. I'm looking for allowable stresses I can check against when doing repair of wood trusses. How Can I check the forces in my gussets? The compressive forces in the plywood when scabbing a rotten top chord? Any help here would be appreciated.

 

[ProgrammingPE]

Check out Chapter M9: Wood Structural Panels of AWC's Manual for Engineered Wood Construction.

Structural Engineering Software:

http://www.structuralcentral.com

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http://www.youtube.com/structuralcentral

 

[Lomarandil]

also the APA Plywood Design Specification

 

[David Deck Guy]

Sometimes I forget the AWC manual exists. This is exactly what I was looking for. Thanks!

 

[Eng16080]

For designing connections with the plywood gussets and nails, you'll also want to follow the requirements of NDS Chapters 11 and 12. The lateral (shear) capacity of each nail used to connect the plywood gusset plate to the truss is a function of the dowel bearing strength, which for plywood is provided in Table 12.3.3B (of NDS 2018, p. 88). Also helpful are Tables 12Q and 12R, which list nail shear values for connecting wood members to plywood. Based on Table 12.3.3B, Structural 1 and Marine grade plywoods as well as OSB have higher dowel bearing strengths than lesser grades of plywood. In your case, it may be beneficial to use the higher grades to potentially minimize the number of nails needed.

 

[David Deck Guy]

I'm accustomed to reading of 5/16" - 3/8" thick panels, sheathing, and plywood. The manual references 3-ply - 5-ply. An internet search tells me 5-ply is about 4mm thick, or about half of the smallest thickness I expect to see these panels, 5/16" (8mm). Obviously there is something I'm still missing here.

 

[Eng16080]

David Deck Guy, I just took a quick look, and you're right, it is confusing. I would have thought that the actual thickness would be reported as well.

Table C9.2.3 (NDS code commentary) shows a relationship between span rating and nominal thickness. Hopefully that helps.

 

[David Deck Guy]

FYI for anyone referencing this thread in the future, if I could do this all over again, I would have jumped straight to full depth gussets that transform the member into a box beam. The below article gives good advice for when to ditch scabbing of individual members and jump straight to full depth ply on the sides. In my case, there just wasn't enough room to get in my connectors.

https://www.structuremag.org/?p=13781

As for designing as a box beam, there is a lot of good advice in APA supplement 2-23 Design and Fabrication of Glued Plywood-Lumber Beams. However, the engineer will still need to adapt the advice in the publication to design with screws instead of glue.

https://www.apawood.org/publication-search?q=s812&tid=1