All plywood beam, no flanges

[hayeska]

I am looking to see if anyone has experience designing an all plywood beam without a top & bottom flange.

Beams (rafters) are for an eyebrow dormer.

I have downloaded "Design and Fabrication of All-Plywood Beams" from APA, as a design reference.

Beams will be shop fabricated, cut on a CNC machine.

I am just getting started reviewing the material and was hoping to ask some intelligent questions soon.

Thanks in advance.

 

[hokie66]

Why not use LVL (laminated veneer lumber)? It is manufactured for the purpose of using as beams, while plywood, because the plies go in alternate directions, is not as appropriate. So LVL, which is just a specific type of plywood, is probably what you need.

 

[hayeska]

It is an eyebrow dormer.

The rafters will be similar to an arch like shape.

 

[hokie66]

I see, in which case rolled steel RHS would be my choice.

 

[hayeska]

It is for a residential wood framed roof; max span 13 feet.

I am designing it for someone who makes these dormers in the shop and then ships them to the site for installation.

 

[jdgengineer]

I've done something like this before. I wouldn't say the contractor was happy about it but in the end we cut out the shape out of a 22" LVL. The loading was relatively light.

Given this was an expensive hoise, so in the grand scheme of things this wasn't very expensive.

 

[jayrod12]

I envision this more like the plywood glu-lam arches I see in barns. Strips of 3/4" x 2" plywood bent to the desired profile and glued together.

 

[hayeska]

The fabricator will create the beam by gluing/stapling layers of plywood vertically, not like a glulam horizontally.
It will then be cut on a CNC machine to the desired shape (arc, ellipse).
They have done small spans before. Now they are increasing the size (13 ft) and want to verify that the design works.
Working through the formulas and specifications now.
Thanks to all for the replies.

 

[Canuck67]

I have done these type beams a couple of times in the past, typically for eyebrow dormers but some other applications as well.

The first thing I tend to do is to check what horizontal (non-arc'd) dimensional lumber would span the opening (i.e. 2x10 @ 16"o/c). This gives me a common sense perspective on where the design is going.

Material quantity and type is usually not a significant cost issue, so I tend to go on the heavy side. Location and staggering of joints is important to consider. I make the beams constant cross-sections, i.e. without the top and bottom flanges being thicker and with the plies being vertical. It's minimal extra plywood and easier lamination/fabrication to do it this way and in the end you will have a more robust solution. All the plies are glued and screwed.

Based upon feedback from the site and my own site visits, the beams I have designed have been quite robust for 40-50psf snow load area.

 

[manstrom]

I've seen the detail before for dog leg stair stringers. You can probably pull values out of the NDS for plywood or OSB. The tricky part will be the joint since you can't readily find a 13' piece of plywood.

I would imagine that a few plies of plywood with a good lap splice will work.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[RHTPE]

While I do not know what the profile of your final piece will look like, but I can tell you that I have done a number of creative structural members out of 3/4" Struct I Class I plywood. I would NOT recommend the "typical" plywood used for residential sheathing as the inner plies are crap.

Having had a number of custom plywood pieces cut by a CNC router, I would inquire about the maximum depth of cut (thickness of plywood) that the router can do. 1-1/4" LVL members can be found in various widths - perhaps the stock can be found that will serve your purpose (both in width and length) so you can avoid the limitation of 4' x 8' sheets of plywood.



Ralph
Structures Consulting
Northeast USA

 

[XR250]

Look at the APA plywood design specification. I usually end up with something about twice as thick as it would be with dimensional lumber.

 

[Hughatstruartapp]

Having just designed a deep plywood box beam with flanges, my main issue would be the depth to span ratio - i.e. is it working more as a shear beam than flexural action. In which case the absence of flanges may not cause an issue.

Structural Engineer
Hugh Morrison Associates