Stress Panel Construction

[Rocosil]

Among the many posts related to framing and light-weight, insulated construction I have yet to find any reference to building TDs with stress panels. Maybe there is a thread on this subject somewhere in this marvelous forum and I simply haven't found it yet. The search feature gives me lots of returns for "stress" and for "panel", but not for the two words linked as one concept. If it is somewhere, please point me to it and don't bother to read the rest this voluminous blather.

I'm thinking of stress panels as used in home construction, i.e., a sandwich of two layers of plywood bonded to a core of styrofoam, extruded poly foam, or what have you, with epoxy, polyurethane glue, or perhaps contact cement. For the sides of a TD, these panels could be made up of two layers of 1/4" (perhaps 3/16" or even 1/8") ply and 1" of foam. Where more structure is required, like to attach doors and windows, the foam would be gouged or routed out and solid wood would be glued in. At the anterior, top, and posterior edges of the sides, the inner layer of plywood and core foam would be smaller than the outside by the height of the spars plus the thickness of the interior, ceiling skin. The rabbet, or shelf, thus created would support the ceiling skin. The spars would go on top. The ceiling skin would be held in place with clamped blocks and/or tie-down straps while a 3/4" fillet of putty made from epoxy thickened with wood flour (sanding dust) and coloidal silicate is applied to the joint from below and allowed to set up. Fiberglass tape may need to be applied with epoxy to add strength. The spaces between the spars would be filled in with solid wood before the outer roof skin is installed. The foam in the bottom edge of the side panels would be routed out to allow them to fit over, and be glued to, rails standing proud on the edges of the floor. Panels that need to be larger than the standard 4'x8' would be scarved to size. Possibly, this type of construction could be used for the floor as well.

Wiring, or conduits for wiring, would be incorporated into the walls before the inner skin is bonded to the foam -- at least, in the vertical sense. Horizontal wiring could be located in the same cavity that fits over the rails standing on the floor.

I believe that building a TD this way would give more strength and rigidity for less weight, not to mention better insulation, than conventional framing plus insulation. It would almost certainly be lighter than solid 3/4" plywood with finish paneling inside. For additional strength and adhesion, epoxy putty fillets covered with fiberglass tape could be added where panels meet at 90° angles. Cabinets could be pre-fabricated and installed as a unit and attached with epoxy putty fillets and fiberglass . Alternatively, cleats could be attached to the inside plywood layer with glue and epoxy putty fillets plus glass tape to support cabinets built in place.

Since this is all pretty theoretical and comes from a guy who has never even towed a camper, let alone built one, I would appreciate comments from people with experience in this sort of thing before I commit myself to building a rolling disaster. So, (1) does this sound feasible, or am I completely nuts and (2) do you think that the benefits, if any, would be outweighed by the added complexity compared with conventional insulated construction?
Cheers,
Bob

[toypusher]

Bob,

Do you have any sketches? Please do some if you can and post them.

Kerry

P.S. There is a commerical teardrop manufacturer that does this. I can't think of the name and don't have a link, but I'm sure someone else on this board knows it.

[IraRat]

Hey, Bob.

I THINK you're talking about the stuff that's used in a lot of prefab home construction and the like, right? Like, just purchasing the composite panels and modifying them?

I'm not an engineer, but if so, I don't see why not, except for added expense. However, you're still talking about a lot of work modifying them, so if the reason you want to do this is less work, I don't think it will work out that way.

If you're talking about making the panels yourself, I would say it's overkill.

Is ultra-light your ultimate goal? Ultra-strength? Ultra-easy? Ultra-cheap?

Like I said, I'm no engineer, but you gotta balance in those 4 factors to see if it's a plan worth pursuing.

[Chris C]

Rocosil,

The only difference between what you are suggesting and what most are doing is the foam is cut and installed rather than formed between the sheets.

I used to own a plastic manufacturing company and have "shot" a lot of polyurethane foams. I've even been involved in the process you describe. But let me tell you, for a novice to accomplish that without expensive equipment would be a total disaster. Expanding foam has a hydraulic pressure most people don't understand or respect. I've seen people try to insulate their homes with foam and actually blow the bricks right of the side. Talk about a disaster!

You mention the use of "plywood only" sides. I don't think too many people are building that way.............although I'm sure it is perfectly acceptable. Plywood is an extremely resiliant material. But a sandwich building method with insulation inbetween the sides is actually stronger than a single sheet.

I've not built a tear yet, but I've years of experience building with all types of materials. There may, someday, be a commercial teardrop manufacturer who will take advantage of the method you describe. Until then, I'll be satisfied with using inner and outer skins and an insulated void inbetween.

[Steve Frederick]

Chris, Check out my build journals, as well as those of Asian Flava. We're using stress skin panel methods. I'm on my third Tear using a little "tweaked" system that I developed that places the headliner and bulkheads in recesses, built in the panels as I go. I rough in for wiring in conduits, as I build the panels also.
Build Journals

[IraRat]

Hope he comes back, but I don't think that's what he was talking about. I still think he means the prefab pieces used for prefab/moduular home construction, where you buy a 4 by 8 sheet of material that's ply which already sandwiches and is glued to the foam in the middle.

[Rocosil]

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[Rocosil]

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[Rocosil]

Thanks for the responses, folks.
IraRat,
Not quite. I'm thinking of making the panels myself from scratch. The stuff used in home construction is too thick and heavy and usually made with OSB on one side and sheetrock on the other. I should have been clearer about that.
What I'm after is the following:
1) Gain strength by using monocoque construction in which two structural elements become stronger than the sum of their individual strengths as they are bound together at a distance by a weaker material, like those manufactured joists and rafters, you see a lot these days, that consist of two pieces of dimensional lumber with a web of OSB in between, like an I-beam.
2) Take advantage of that gain in strength by using lighter and, maybe, cheaper materials.
3) Maximize the efficiency of the insulating foam board by reducing to a minimum the structural frame, which acts as a temperature conductor between the inner and outer skin.
But you're probably right -- the gain may not be worh the effort.

Chris,
I'm not talking about injecting foam between plywood panels, but rather gluing plywood to either side of a preformed foam panel.

Kerry,
I'll take a shot at sketches and posting them when I've given this idea some more thought and, of course, when I've figured out how to do that on this board.
Cheers,
Bob

[TRAIL-OF-TEARS]

I think they are also called SIP's (Structural Insulated Panels). If I remember correctly, There is a tear drop maker out in West Texas that is using them the only problem with them is the price. if You do as Asian and Steve (as well as others) you are basically making the same thing. Good luck

[denverd0n]

I think most of the people on this forum refer to that as "sandwich" construction, because you are sandwiching the foam between plywood panels. My impression is that this method of construction is actually MORE common than single-thickness, plywood walls. I think that if you a search for "sandwich" you'll find a lot of discussion.

[An Ol Timer]

I made up a complete set of plans with my CAD program using a lot of these methods. My only change from some of the other forum members is that I built up frames and then placed them onto the outer skin. I then used 1" thick foam insulation to fill all the spaces between the frames. My only change from most is that all my foam was glue in place. When the inner skin was glued on it was glued to both the frames and the foam. this makes a solid sandwich. I also cut my inner skin and the frames from the point of the galley hinge around to the front lower corner smaller by the thickness of the inner ceiling panel and the top frames. This allows you a shelf to place your inner ceiling on and then the top frames. I glued insulation in between the top frames next and then glued the top skin to the sides, frames and the insulation. After spending a lot of time drafting all this out, I was on the forum saw where Steve Fredericks uses the same or similar process so it can't be all bad. I did it because I can't do a lot of work on the inside. Too old and arthritic with knee replacements. It's a GOLDEN YEARS thing!!!!

[asianflava]

All my panels and shelving are built this way. My floor and walls are bonded with epoxy thickened with microfibers and the shelves use Titebond II.

Here are some good threads on the topic:

http://www.tnttt.com/viewtopic.php?t=725
http://www.tnttt.com/viewtopic.php?t=1154
http://www.tnttt.com/viewtopic.php?t=1459

Here is a pic where I am getting ready to apply the epoxy on the skin and the core. I have it on plastic so it won't get glued to the floor. After I put them together, I put a bunch of my "clamps" (weights) on it. I considered vacuum bagging it but dropped the idea due to the complexity.

[Dee Bee]

Interesting Idea. It might work...

It think careful consideration would need to be given to how the corners are fastened. side to deck, roof to sides doors to sides tear curve to sides, etc.... These corners need to ne strong and secure or the whole TD falls like a house of cards. Solve this, find a way to do it inexpensively, and you might have a new assembly technique.

Keep thinking and post some more...

It seems to me that in a modified way, the "sandwich construction" creates a wall that is close to this concept...

JMHO

DEE

[Chris C]

Steve, I think I must have been unclear. I think the method you use to build your sides is the best for teardrops. I call it a sandwich type construction. I used to own a plastic manufacturing company and we were part of the pioneer group of people who worked in stressed panel construction. We had a machine into which we placed two 4' x 8' sheets of plywood. We'd inject polyurethane foam between the panels. As they traveled throught the machine they expanded until the machine confined them to a total of 4" thick. When they came out the other side, we had a stressed panel ready for the building industry.

I plan to build my tear sides exactly the same way you do..........except I'll be using a different insulating material. When I converted my 18' Dodge panel van to a camper, I installed 3 layers of a bubble-pack/foil covered material which has an R-value of 4 for each layer......and it's 1/4" thick. http://www.insulation4less.com/highr_FBF.asp My van is super warm in the winter, and super cool in the summer.