Section Modulus for Calculating Concentrated Load on Plywood ???

[JennyNakamura]

Hi everyone,

I have never had to do this before, but I am trying to calculate the capacity of a plywood panel to support a concentrated load. For checking flexural stresses, I am using the typical f=M/S. This is consistent with what I have seen in the NDS 2001 and 2005 Manuals. However, upon an internet search, I have come across older documents that us an "effective" section modulus "KS" in lieu of S. My old Breyer textbook mentions this also, but doesn't go into any detail. Based on some tables I found on line, "KS" is much less than just plain old "S".

Should I be using "S" or "KS"???

Thanks in advance.

 

[hokie66]

For bending, you can only use the plies that are in the direction of the span.

 

[Lion06]

I have a paper from the APA that deals with this. I'll look for it.

 

[chrislaope]

I can see the reason why "KS" is used instead of "S". Because the bonding between glued layers of the plywood may not be able to guarrent an integrity feature of the plate, i.e. the layers may have relative movement during bending (the glue may not strong enough to resist the shear between glued layers during bending). Take a 3 layer plywood as an example with each layer thickness equal 0.5", if you can guarrent the glue provide a strong enough bonding between layers, then, you can take it as a 1.5" thickness plate to calculate the "S". On the other hand, if the 3 layer is only superposed together without any bond between each other, then the sectional modulus will become a minimum value which equals 3 times the sectional modulus of a single 0.5" thickness plate.

 

[Ron]

Take a look at the attached APA publication (Lion06...might be the one you were referencing). In particular, look at Tables 1,2 and Page 20.

Please note that APA pdf publications are generally free (and this one is), so no propriety breached.

 

[Lion06]

That's exactly the one, Ron. I downloaded all of them. I stumbled upon them a while back and thought they were a great resource. I think there were quite a few of the plywood design specs and supplements for a lot of different applications.

 

[dhengr]

A slightly different slant on Hokie’s and Crislaope’s posts....
--The orientation of the grain in the ply must be aligned with the normal bending stresses for that ply to be effective. And then, the ply’s distance from the neutral axis comes into play too.
--The glue bond doesn’t actually fail. It is that the grain perpendicular to the normal bending stresses starts to fail due to cross grain horiz. shear, as if it were a pile of soda straws rolling w.r.t. each other under loading.

 

[RFreund]

Just an FYI I believe there is a 2008 version available now of that APA attachment.

EIT

 

[JennyNakamura]

Ron-Lion06-RFreund,

The APA documents were exactly what I needed! Ron and Lion 06's 1997 version use the "KS" and RFreund's 2008 version uses just the "S". The 2008 stress tables have the panels broken down into the sheathing and single floors grades (24/0, 48/24, 24 oc, 48 oc, etc.)for use with "S" and the 1997 version just has plywood broken down into species and thickness for use with "KS". I was working with an existing structure with 3/4 plywood (assumed DF-Larch) over a 4 foot span. Both methods give about the same answer.

 

[bgatto]

I myself would use S. In addition to what dhengr said I would like to show you this: