FASTENING MULTI PLY WOOD BEAMS 1

[CJJS]

When a multi ply wood beam is loaded on its top flange, how does one go about determining the fastening of the plies. I think that when it is top loaded you can assume that all three plies receive equal load and thus do not have to worry about transferring shear forces across the vertical plane of the plies. Do you look at horizontal shear as the controlling factor? Thanks in advance.

 

[Lion06]

You can do that, but then you are not taking advantage of the composite properties of the section. That is equivalent to having them side-by-side.

 

[CJJS]

OK. So, supposing I want to design the multiply beam to act compositely (even though it is top loaded). What process do you use to determine the fastening?

 

[Lion06]

just check the shear flow. VQ/(It). That will give you a shear flow in K/in. Then just design your fasteners for that load.
V=max shear
I=moment of Inertia of the composite section
t=thickness at location of joint
Q=(y')*(A') where A' is the area of the segment being connected to the beam and y' is the distance from the neutral axis of the composite section to the centroid of the connected part. This is probably explained a little better in a mechanics of materials textbook (look up shear flow for a more precise definition of Q)

 

[CJJS]

I am referring to a multiply beam with the long dimension oriented vertically and loaded on the top narrow face. I am not sure if I am missing something here, but why would horizontal shear be of concern here?

 

[Lion06]

your plies are oriented vertically so that they are side-by-side? That makes a big difference then. My mistake on mis-reading the OP.
I would just provide a nominal fastening (maybe 1/2" diameter bolts at 16"-24" o.c.) to help ensure they deflect together.

 

[CJJS]

No, problem. My original post was a bit confusing. I was looking to see if there were any calculations one might do to determine a minimum fastener spacing for this condition. Thanks for your replies.

 

[Lion06]

I think this is more important in flitch beams where the load actually has to be transferred from the wood to the steel through the bolts. You will have the load transmitted directly to all plies through bearing because they are all the same dimensions so there really is no load transfer. I would probably still provide some connection.