Strength of a wood/foam beam?

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cluttonfred

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Not about but homebuilt airplanes, but still inspired by one.... I am looking at little teardrop trailer and truck camper projects, some of which use largely foam construction. That got me thinking about the Sky Pup's wing spars with wood capstrips and foam webs. Can anyone with more engineering knowledge than me explain what might be the pros and cons of that sort of structure in order to say, cantilever a sleeping area over truck cab? To put it another way, if I take a pair of nice knot-free 8' 1x2 furring strips and bond them to a 8' x 10" x 1.5" foam web, how much could that "spar" support? The easiest thing to do, of course, would be to just build one and test it, but I am traveling for work so that's not an option at the moment. Thanks!

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wsimpso1

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I know that you asked for someone to tell you if your design will be strong enough. Sorry, I am unwilling to do engineering for other folks, but I can send you to the right topics to learn so you can do it for yourself, then iterate the design. I am partial to Excel for placing the formulation, then iteration becomes easy. I will offer to check your work...

That beam you describe and the one pictured are both amenable to analysis. The process of doing so is covered thoroughly in a senior/graduate Mechanical/Aerospace engineering class in mechanics of composites. You might be able to get through this with a mechanics of materials text or instructional video on the details of beam theory and then composite beam theory, which is usually covered from the perspective of steel reinforced concrete beams. For any proposed design, you would need to do a sequence of things, and then iterate the design until it meets your needs. Since iteration is almost always needed, and the design of any one piece feeds back into the larger structure, the analytical approach should be under the control of those designing the rest of the structure. The sequence is generally:

Propose a material set;
  • Define elastic characteristics and strength of each material;
  • Define your Factor of Safety;
  • Propose a design;
  • Calculate centroid of the beam from sum(EAy)/Sum(EA);
  • Calculate sum(EI) of the beam;
  • Apply loading with FOS and calculate tensile/compressive strain and shear strain in each material;
  • Compute stresses from tensile/compressive/shear strains;
  • Check computed stresses against failure criteria for each material;
  • Check interlaminar shear at all joints between materials.
  • Iterate the design until it passes on stresses, deflections, and weight.
Billski
 

Victor Bravo

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Sorry, I do not meet the minimum stated qualifications to make a comment, but I'll offer one small tidbit of "high level engineering" from an old washed up model airplane builder.

I'm guessing you would be much lighter and stronger if you put the furring strip caps on the upper and lower edges of the entire "sidewall", rather than make a 10 inch tall beam.
 

cluttonfred

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Thanks, guys.

Billski, not asking you to design it for me, just wondering if the approach is at all realistic and hoping for some rough guidance. If a back-of-an-envelope calculation says a beam like that supported at the ends would snap with just 100 lb applied to the middle then the approach is pretty flawed. If it would take 1000 lb then it might be worth pursuing. I just don’t have the math or engineering to do that quick estimate and am unlikely to gain those skills on this trip.

VB, I think you are talking about using the full height of the walls as spar webs. My thought was that the wood and foam girders would form a load-bearing frame and the rest would be just foam to fill in the holes with some wood inserts to support windows, doors, vents, etc.
 

Victor Bravo

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Make the walls your spar beam, yes that was what I proposed. If you also put in two or three of your furring strips vertically between them, and allowing the foam to do the shear and keep everything aligned, you have a pretty good chance of being able to sleep in it. Worst case scenario, if it flexes too much or cracks, you have to stop and buy a couple of pieces of thin door skin plywood and contact cement it to the outside of your walls as a shear web.
 

mcrae0104

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To put it another way, if I take a pair of nice knot-free 8' 1x2 furring strips and bond them to a 8' x 10" x 1.5" foam web, how much could that "spar" support?
...not asking you to design it for me...
Well, in fairness, that is a design question, or more precisely, an analysis question. Analysis requires a full set of known facts (also known as a design). Without all of the information required to perform even a rough analysis, the best--really, the only possible--answer is, "it depends." It's like asking how strong a 10" deep beam is, and then saying, "well I didn't ask you to design it for me!"
 

Hephaestus

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Grab a free truck camper - strip it for parts - to resell/recycle into the new trailer - eyeball the structure. Even back into the 60's - the overhang didn't have much support, 1x2's, that 1/4" wood paneling - all held together with staples - that was it, if I recall some of the better ones got a little 1/8" bent steel "T" plate there for 'extra' support and to tie in the corner jacks through the structure.

The new ones are even worse.
 

vhhjr

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I've done some testing of beams made from foam with fiberglass and/or wood facings. A comon failure mode is in the foam just inside the glue line. Once that happens the beam fails quickly. One can greatly increase the strength of such a beam by using a slightly higher density foam. I also think that running a roller with short spikes in it over the foam surface before bonding the face sheets on will increase the foam/glue line shear strength with only a slight increase in beam weight.

Vince Homer
 

TFF

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Like above. The density of the foam is not close to the wood. It moves when it’s not supposed to. The web job is to keep the cap from moving. How do you keep the cap from moving when it’s so soft.

As for the SkyPup, it’s got 5-6” of web for the caps. At some point you can add enough foam but it takes a lot more than the wood.
 

Victor Bravo

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Matthew if you want to go the minimalist route for a one- or two-trip camper shell, you can do the same "foam and tape" method from the R/C models. They figured out a couple of clever things.

Spray the foam with 3M 77 spray glue, or some sort of non-solvent thin contact cement or glue rolled on. Let it dry. Then come back and cover it with colored packing tape. It becomes a lot more attractive than you might think.

The glue soaks in and fills the grain/roughness of the foam. It makes the packing tape far more permanent and a much better bond.

But before you apply the colored packing tape, you can put in tensile strength by applying filament "strapping tape" where it is needed. If you have two furring strip "caps", and two or three furring strip "uprights" between them, you can create the equivalent of a wire braced truss with the tensile strength of the strapping tape. I'm guessing you can easily add enough strength to the truss with this stuff to support the cantilevered sleeping platform above the truck cab.

You can make a reasonably attractive, expensive, sturdy-enough, and rain-proof camper shell this way.

OR, just use a few more feet of furring strip, with foam in between, and you don't have to worry about the filament tape:

Camper.jpg
 
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cluttonfred

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Thanks, all, for the suggestions.

My thought process was that my large self and a guest in the cantilever sleeping area was the critical load scenario.

I was thinking wood/foam beams the full length of the camper (above side hatch and below scout logo in image) resting on a T-shaped bulkhead behind the cab and two rectangular bulkheads at the tailgate (because of the door. The rest would be glued and taped foam with light wood inserts or plywood frames to strengthen door and window window openings.

I have seen the spray glue and packing tape approach with models, though I was thinking more of open weave polyester cloth filled with latex paint. Maybe I could find adhesive vinyl sheet in rolls and still use the tape approach?
 
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