Stress values for GLB (in wood layers near neutral axis)

[cenee]

I am trying to find out stress values (fb, fv) for those layers between top and bot. layers of a GLB. The reason is because I have this situation where the top layer (the strong one) of this cantilever GLB has to be cut (it hits the roof). This cantilever (5ft only) beam suports a point load (another beam)and now I have to check if it fails in shear and bending (because the top layer will be cut).
If I could have and idea as to how much the 2400psi and 195psi values reduce when the top layer is missing.
Beam is 8.75x48
Thanks.
M

 

[cjd97]

I personally do not have a lot of experience with Glue Lam. However, I have used a company called Unit Structures for specification writing and sizing of Glue Lam members.

http://www.unitstructures.com/

 

[Dinosaur]

My concern would be for the layer immediately below the cut remaining able to resist the bending stress and possibly the shear near the neutral axis. You said the beam is a cantilever, about 5 feet long. You did not say where the cut portion would be. Is it at the support, or nearer the free end of the beam? You also need to be concerned that the glulam purchased was intended for use in negative bending. There are two different layups used for glulams depending on whether they experience only positive bending or also reverse bending. If a positive only layup was used and you have a cantilever bending situation, you could have a problem because tension laps are higher quality than compression laps. Beams in negative bending often get tension laps in both faces. There are ASTM specs for computing section properties of glulams. You should be able to get the layup spec from the supplier of the glulam to determine what grades were supplied in each lap. Good Luck.

 

[RARWOOD]

If you know the glulam combination you can look up the beam layup. For example 24F-V4 Douglas Fir is layed up using L1, L2 & L-3 laminations. When laminations have been removed you can look at the remaining laminations comparing the layup to the lay up for lower stress combinations.

On that basis you can estimate an allowable stress. When taking this approach I try to be conservative to account for notch effects and other factors.

 

[cenee]

Thanks to all. And yes, the point load at the cantilever part is at 4 ft from the support. I have the shear and bending at that point and I just need the stress values; for now, I am assuming the middle layers are DFL-1 fb=1000, fv=95. Bending should be ok. the beam was choosen to get the bending at midpoint of the beam (at 35ft), but I thing shear could be critical. The problem was that the glb is with a slop and the support is actually the ridge, so the cantilever part needs to be cut (top layers) 2.5 in to have the shape of the other side of the roof and slope down. I will look into your info. and see what I can find out.
Thanks a lot.
M