All plywood beam, no flanges
All plywood beam, no flanges
(OP)
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.
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.






RE: All plywood beam, no flanges
RE: All plywood beam, no flanges
The rafters will be similar to an arch like shape.
RE: All plywood beam, no flanges
RE: All plywood beam, no flanges
I am designing it for someone who makes these dormers in the shop and then ships them to the site for installation.
RE: All plywood beam, no flanges
Given this was an expensive hoise, so in the grand scheme of things this wasn't very expensive.
RE: All plywood beam, no flanges
RE: All plywood beam, no flanges
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.
RE: All plywood beam, no flanges
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.
RE: All plywood beam, no flanges
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
RE: All plywood beam, no flanges
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
RE: All plywood beam, no flanges
RE: All plywood beam, no flanges
Structural Engineer
Hugh Morrison Associates