plywood questions

[ajk1]

Can anyone help with any of the following questions?

1. Where plywood comprises an even number of plies, is it standard practice that the plywood is made with the middle pair of plies in the sheet with their grain running in the same direction? This is what I see on site in a piece of 3/4" thick plywood with 6 plies and no tongue and groove edges. I suspect that this is done so the face plies on both the top and bottom faces of the sheet are in the same direction i.e. parallel to the span of the plywood.

2. Where do I find the moment of inertia of the plywood so that I can calculate the deflection of the sheet when laid flat as floor deck to span between joists?

In Table 7.3A of the 2010 Canadian Wood Design Manual it gives "Bending Stiffness" EI, but it lists it in two columns labeled "Orientation of applied force relative to face grain" and the columns are labelled 0 degrees and 90 degrees.

There is another pair of columns labelled "Axial Stiffness" EA and that also has two columns labeled "Orientation of applied force relative to face grain" and these columns are also labelled 0 degrees and 90 degrees. I understand this "axial stiffness" as being that the load is being applied to an edge, but I do not understand how the "bending stiffness" can be in terms of orientation of applied force relative to face grain.

Can anyone explain this to me?

 

[jayrod12]

for the bending I've understood that to mean which way is it spanning. as the tensile and compressive stresses induced in bending happen along the span.

 

[ajk1]

Yes, but then how can there be load applied at 0 degrees the face grain? Do you have a copy of the Canadian Wood Design Manual handy?

 

[EngineeringEric]

The EI that you mention is relative to which bending... The plywood as strong and weak directions. Typically a 4'x8' sheet is meant to bend in the 8' direction. This means the bending stresses are acting with the grain. The other direction the stresses are acting cross-grain.

To calculate the deflection of floor sheathing, when in a multi span configuration (such as joists spaced 16" o.c. and board is 8ft long), you should use the stiffer of the two values when installed correctly/strong-direction.

I do not have the US equivalence (AFPA/AWC) to provide more info but it is there... Web Searching the AWC website should even show some useful graphics to help illustrate.

 

[EngineeringEric]

A useful links i just found for your assistance, with use of the 90/0 angle table:

http://www.tecotested.com/techtips/pdf/tt_plywooddesigncapacities

 

[woodman88]

Attached pdf (an USA APA info) on page 15 shows an three layer four ply plywood.
This link is to Canada info

http://www.canply.org/pdf/main/plywood_designfund.pdf

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ajk1]

Engineering Eric- thank you for what you sent. The table headings and footnote definition in the document that you referenced are 100% clearer than that in the Canadian Wood Design Manual! I think the heading in the Canadian Wood Design Manual is nonsensical and wrong. I will write to them if I have time. You have solved my problem.

Woodman88: thank you for the tables and reference. Much appreciated.