Shear Forces Within Ply Memeber Screwed Together

[Ozy001]

Hi everyone,

I’m trying to work out the shear force between two ply boards screwed together and I’m not too sure how to do it. Basically we require 2 sheets of 24mm ply to be screwed together to act as one. How would I work out the shear force between the two ply members if I know the applied moment? If anyone could help that would be excellent. I can also upload a sketch to help visualise the problem if need be. Ideally I would like to note on the drawing what type of screws and what centres are required to ensure no shear slip occurs. From my first impression it seems very similar to designing shear studs on composite beams. Again I really appreciate the help.

Thanks!

 

[rb1957]

i think a key part of your sketch, i think you'll need to post one, is are both sheets loaded together ? if both are loaded and reacted together then i don't see load transferring from one to the other. if the load is applied to one sheet and reacted on the other, then yes there is load transfer, and it should be easy enough to calc.

 

[dhengr]

Ozy:

Provide a sketch; dimensions, loads, reaction points, plywd. orientation, etc.

If the two plywd. sheets are standing on edge and supported and braced at their ends, as a simple beam, and the load is truly applied to both sheets at their edges; they both take half the load and just have to be held together to prevent out of plane buckling.

If the two sheets of plywd. are laying flat and supported at their ends and the loads are applied perpendicular to the plane of the sheets; then they each have the same independent section properties Sx, Ix, etc. and take half the load, but slip w.r.t. each other. Alternatively, if you fix them together then you have a beam member of double the depth and improved section properties. The faying surface btwn. the sheets must be treated like the horizontal shear problem in any wooden beam, and your fasteners must take the shear flow btwn. the two sheets at that faying surface. This will be discreet shear forces at x inches on center, for a total shear flow per foot.

Screws are generally not considered a good fastener for this type of connection, they have threads in the faying surface, might be hardened and brittle, etc., clinched nails may be better. Also, a glue which you can trust in the given environment improves this joint by distributing the shear stress over a much larger area.

 

[dhengr]

Keep in mind that grade of plywd., orientation of the plys and the number of plys, and orientation of the outer ply grain w.r.t. the orientation of normal stress, in bending, are all critical in your problem. And, that plywd. standing on edge and loaded as a simple beam is almost useless because only the plys which are oriented with the grain horiz. come into play in this bending.

 

[hokie66]

I thought your sheets were flat until dhengr raised the possibility that they were vertical. If flat, you are correct that the calculation for horizontal shear is similar to that for composite beams, but you won't like the number for doing it with screws or nails. Takes a lot of them.

I would talk to your plywood supplier about providing a suitable adhesive. Or perhaps they can make 48 mm thick sheets for you. This is done all the time for timber bridge decks, and LVL sections are just thick plywood.