Plywood shear in glulams
Plywood shear in glulams
(OP)
Working on a glulam configuration using Plywood, similar to an I beam configuration with flanges being dimensional DF, and plywood web. I seem to remember a standard that detailed a potential allowable increase in glulam but I cannot find it. Right now we are designing right out of the books for structural 1 plywood shear through the thickness that seems to be governing our design but I need to make sure we are referencing the right data.
The APA references other standards in glulams but most of these documents are rather vague.
The APA references other standards in glulams but most of these documents are rather vague.






RE: Plywood shear in glulams
RE: Plywood shear in glulams
I am still trying to find test data to learn what margins they are using for these "design values".
RE: Plywood shear in glulams
RE: Plywood shear in glulams
RE: Plywood shear in glulams
An I beam is not considered a glulam simply because its pieces are "glued" together. So any allowable increase that you find pertaining to glulams would not apply to your wood I beam.
RE: Plywood shear in glulams
...please note this does not constitute a glulam. Per 2015 NDS 5.1.2: "grain of all laminations is approximately parallel longitudinally."
This is a glulam:
This is NOT a glulam:
Perhaps you should contact any various engineered wood suppliers (Red-Built, TrusJoist, etc), they pre-fabricate something very similar to what you're proposing.
RE: Plywood shear in glulams
Can you post a sketch?
RE: Plywood shear in glulams
RE: Plywood shear in glulams
I agree, I don't see any way to use a 33% increase for anything here.
Shear flow seems like a major challenge. I imagine I joist makers get from testing.
Still, a sketch or visual aids will help.
RE: Plywood shear in glulams
What exactly is the goal of the (custom?) built-up wood member you are working on? Is this a roof beam? Frame for hanger/pole barn? Something else?
RE: Plywood shear in glulams
http://www.eng-tips.com/viewthread.cfm?qid=402304
RE: Plywood shear in glulams
I wonder if this is for the header and wonder when someone will mention how the lateral stability is affected by the removal of some of those poles!
Seems like he will need to take out a few of them on one side at least.
RE: Plywood shear in glulams
I'll cheer all day long for 'do it yourself' on most things, but would hate to see something get missed and someone get hurt/plane/big kid toys get damaged. At very least, hopefully he and his team will get a local SE to at least provide them some peer review comments over dinner and a beer before it's all said and done.
RE: Plywood shear in glulams
If you're referring to splicing the flange of an I-joist for continuity then let me just say that I would not trust such a thing. All the more so if there are also splices in the web, whether at the same location or elsewhere.
If this is something you're assembling out of dimensioned lumber and plywood then I think you should reconsider, first for safety and secondly for cost: you'll be unlikely to be able to build such a thing cheaper than you could buy it from an engineered lumber or truss manufacturer.
RE: Plywood shear in glulams
I assure you all, there is a big pile of excel calcs and FEA done. However, we will of course discuss our design locally with someone more familiar to ensure a safe structure.
I apologize for the 'glulam' confusion. I commonly use that word simply to reference glued members. The joist in question is a basic I beam configuration. Please see attached pic but disregard any dimensions as I just grabbed that from the Inet. Would basically attach DF 2x4s to top and bottom as configured.
We are learning that manufactures have to be performing tests to prove out their designs beyond referenced standards. We are seeing commercial products that seem to induce around 400psi in shear through the plywood web. Obviously we cannot do that but trying to understand a few things. I mentioned the 33% increase but does not look like we can use it.
RE: Plywood shear in glulams
RE: Plywood shear in glulams
Actual plywood, as it has been known for the 100 years, or so, is terrible for "rolling shear" loading - exactly the loading applied to plywood used as the web of beam. The alternating layers of wood that are glued together to form a sheet of plywood guarantee that some of those layers will be positioned in the worst possible way. In the layers that are perpendicular to the horizontal shear the fibers tend to "roll" over each other.
For a high quality sheet of plywood, allowable shear through the plys (say, used as a floor) may be 190+ psi.
For the same sheet of plywood, allowable rolling shear (used as the web of a beam) is 75 psi, or less.
Depending on exactly how the plywood is positioned in the web, a 1/3 increase in allowable rolling shear is possible. Perhaps that is what you have come across.
Note that Wood I-Beams, as shown in the photo above, use OSB for the web... because OSB is structurally superior to plywood for this application.
IMHO... forget about using plywood altogether for this application - it is a very poor choice. Buy a beam.
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RE: Plywood shear in glulams
With sincere respect to your goal of going at it yourself: Any licensed SE that you hire will not need to generate, as you say, a 'pile of excel calcs and FEA' among 3 SE/PEs for a pole barn.(!!) Sure, the SE will have some number of calculations to run through, but something tells me that an SE will spend less time generating a construction ready set of plans for you than you and your crew have already spent to this point (and will spend from here to the finish line if you elect to continue going at it on your own). Many SEs are familiar with this type of construction on a daily basis.
RE: Plywood shear in glulams
Fastline12, from your pic, it appears as though you have nothing more than a 10 mil plywood beam.
Without the top chord(s) bearing directly over the top of the web, as in a true I-Joist, and the web bearing directly within and above at least half of the bottom chord, in the case that you show, there is no credible continuity of overall structure of this member other than the glue alone. It appears as though you have (5) separate members depending on shear of the glue alone.
It is true that the glue plays a significant role in the web of the I-joist as well as the plys of an LVL microlam and even a PSL beam. BUT, in all of these members, there is a credible continuity of the "wood" or fibrous structure from top to bottom.
RE: Plywood shear in glulams
FEA is not used in wood design except at the University level.
RE: Plywood shear in glulams
1. Is the pole building you have asked about elsewhere, the building you have in mind for these site fabricated "beams"?
2. If so, what building component are you thinking of to use them for?
RE: Plywood shear in glulams
Like others have mentioned above, I can certainly appreciate the enthusiasm to tackle a project yourself and be a DIY'er, but this is a pretty sizeable project for those not familiar with pole barn design. To me, it is a red flag that "there is a big pile of excel calcs and FEA done". My advice would be to stick with "off the shelf" type of members whose structural performance is known and tested instead of trying to fabricate your own members. Next, I suggest hiring a professional experienced in wood design and maybe show him or her everything you have done up to this point and let them take it from there.