Plywood shear in glulams 2

[fastline12]

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.

 

[Triangled]

assuming, if I understand you correctly, you're creating an Ijoist with glulam for the flanges, and assuming DF material for the glulam, it seems your flange to web connection would be identical and your flanges governed by the appropriate Ft and Fc||. I am not aware of a standard indicating a potential allowable increase in glulam. ...interested in other's knowledge and experience on this....

 

[fastline12]

I found the document. the PDS or plywood design specification, from the APA. I might like another to glance in section 3.8. It does not seem like we can take the full 33% increase due to not being 'glued on all edges" but we are using a T&G in the web and gluing up the webs just for additional shear handling. The flanges can handle it but I just don't like the idea of butt splices. I know there are standards on splice plates but we are working conservatively.


I am still trying to find test data to learn what margins they are using for these "design values".

 

[XR250]

Could you provide a sketch of your beam?

 

[Brad805]

If this is your 100'-0" span building truss, I suggest it is time to hire people that can help.

 

[MotorCity]

What you are describing are two completely different things. A glulam is not similar to a wood I beam with a plywood web. A glulam is a solid, rectangular section composed of several parallel, laminations glued together. The laminations are specifically designed, glued, and oriented in a particular direction to provide the load capacity.

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.

 

[FoxSE14]

Working on a glulam configuration using Plywood, similar to an I beam configuration with flanges being dimensional DF, and plywood web

...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.

 

[HouseBoy]

I think the OP might be describing THREE different things!!!

Can you post a sketch?

 

[Triangled]

yea, i thought he was talking about an I-Joist with glulam flanges....there is or used to be a manufacturer that would glulam (2) 2x4s together creating a net 3.5" wide x 3" thick flange, then notch it like a standard I Joist and glue in the OSB/Plywood sheathing as in a standard IJoist. For this configuraton I cannot imagine a basis for a 33% increase.

 

[HouseBoy]

Maybe APA publication on "Plywood Box Beams" would give some better guidance to the OP.
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.

 

[FoxSE14]

fastline12 - can you post a sketch/photo?

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?

 

[Brad805]

I believe he is talking about the building below (one thread has a sketch). The trusses are to be some sort of a composite glulam/truss concept he has found that others are building. This is something they intend to build themselves. I would not be surprised if this is a hanger for planes. My business partner is a plane guy and it is very odd how frugal they become when it comes to hangers.

http://www.eng-tips.com/viewthread.cfm?qid=402304

 

[HouseBoy]

GOOD SLUETHING Brad805 !
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.

 

[FoxSE14]

Brad - that's great! I had the same hunch based on that very same thread! (notice my question about "hanger/pole barn"? )

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.

 

[Archie264]

The flanges can handle it but I just don't like the idea of butt splices. I know there are standards on splice plates but we are working conservatively.

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.

 

[fastline12]

Sorry if I seem misleading. There are 3 SE/PEs working on a few things for a basic pole barn. No, this is not a hanger, all poles will be in place. We are used to working with other materials but see this as a good opportunity for learning as our own structure.

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.

 

[FoxSE14]

Why not save yourself some the headache, time, and worry and check out the load tables provided by a wood I-Joist manufacturer? I'd be willing to be you could find an I-Joist or other engineered wood that will do whatever it is you need the member to do.

 

[SlideRuleEra]

fastline12 - You mention that you have been using the term "glulam" loosely. Are you doing that also with word "plywood" for the web? I'm asking for a real reason.

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[FoxSE14]

Why not hire a licensed SE to design this for you? Yes there will be a few grand in design fees depending on the scope of work you negotiate, but you can still elect to build everything yourself if you are trying to save money on this structure. You'll also have the benefit of using the structural drawings as an instruction manual for your own erection.

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.

 

[BSVBD]

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.

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.