stitch and glue over a steel tube framework

There, that got your attention!
Greetings from a former boat builder. My father and I built several Glen-L designs, as well as some of dad's own creations.
I came here seeking answers. I'm not building a boat with these materials, it is a travel trailer roof structure. My choice in materials is due to my love of wood and boats and bright finishes, and lightweight strong construction.
I have this design in my head that is a retractable roof structure. I *think I need steel for ease of the lifting hardware structure and to add strength to the frame.
If I build this welded tube framework, paint it properly, and glue and stitch 4 MM okoume ply over it, epoxy and glass in and out, linear poly finish, will it perform OK?
Or will I be plagued by differential contraction/expansion problems?
It will be used much like a trailerable boat, it will not sit in the damp and cold of winter out in the weather. It will get covered and heated to a degree.
My concern is how the two (three, four?) materials will interact thermally.

 

Steel and light weight aren't generally used in the same sentence. You could develop a fully taped steam structure that is lighter then the steel and taped seam method you envision. Without a much better idea of what you're attempting structural issues are difficult to address, but mating steel and wood is possible though often fraught with difficulties at the contact points (one reason as you've mentioned, but there are others). If the whole structure is bonded with epoxy and tape, you'll eventually have issues, as one or the other tries to "break out" of it's bindings. You can handle this with a flexible bedding and incorporate anticipated movement into the joints and contact areas.

 

Thank you for the reply.
I apologize in advance for a lack of drawings. It is all in my head so far with only a few brief sketches.
I'm using steel tubing for structural requirements that can't easily be met with wood. Imagine an upper roof structure that is 2" thin, that slides over a lower box for 2'. In one corner of that upper structure a door frame will extend even farther down.
It has a slight crown or radius to the entire 8' wide roof, and the corners are 12" radii.

In addition, the longitudinal box tubing will act as a top support, using machined dowels.
When the roof gets lowered, it will pin to the wall sections of the lower unit and bridge gaps in a fixed top rail.
These gaps would be for the door and a slideout on each side.
None of that seems plausible or economically feasible with a wood-only structure.
what I had planned was a 3/4" tube bow on 16" centers, welded to the longitudinal box at the top of the straight side section.
I would cut the top plywood to go across the top, and either pre-mold the 12" radii sections with a couple of 2mm bending plys with a form and vacuum bag, or strip it in to approximate a radius. I would put an adhesive on the tube and wire down the plys until set, then remove wire and eventually cover with a 4~6 oz cloth and epoxy.

So you'd find an adhesive that will have a little give at that metal/plywood glue joint?

 

A 1/4" 1088 plywood sheet, epoxied to a 1.5" thick, closed cell foam sheet, pre-bent to the roof crown, with a matching plywood sheet, epoxied to the underside will create a rigid, beam free, very light roof, that's self supporting and will maintain the roof camber. A perimeter off soft wood to accept fasteners for the retract mechanism would be "let" into the edges of the foam cored roof panel. This eliminates the need for a steel/wood interface and their potential issues, not to mention will have some insulation properties that could be enjoyed in your neck of the woods. Lastly you'd want to sheath the whole shooting match (walls, roof, everything) to improve abrasion resistance and waterproofing. The retracting mechanism should be considered a separate item and handled accordingly. It would fasten to the roof on a bedding such as polyurethane or polysulfide. At this point some hand sketches to better describe what you are trying would be helpful.

 

I do plan on insulating the structure, so no worries there. Maybe somebody else has some insight on the wood/metal interface and expansion/contraction issues.

 

PAR, you have given me much food for thought. I am currently working overtime on equipment that lets me daydream about things.
I have already abandoned the steel bows.
I have redesigned the door.
I will get some sketches going, possibly tonight.
Thanks again.

 

Pars comments are spot on, Steel and epoxy expand at different rates, so that metal fastenings will work themselves out of an epoxy and glass structure, for example.

If you are planning insulation, you can use it to your advantage. You will get a great result if you sandwich plywood ( with suitable glues) between rigid foam panels.

You can do a search on ply foam sandwich on this site and come up with lots of useful hints.

 

Here are some sketches.
This is a profile of the trailer itself, without the roof breakout detail. The idea is an aerodynamic, low profile, low drag trailer that can pop up 2' when you get where you are going. The whole roof does not elevate, the last 4' or so is storage and equipment space.



Here is a bit of roof detail. It has forward and aft bulkhead, but no other bulkheads. It slides over the bottom section, so no crosswise reinforcements are possible.



I would make the roof crown a large radius instead of the peak as drawn.
I don't know how I'd make the 12" radius sections with closed cell panels.
If that is not practical, I could make plywood ribs, and attach the outer skin to those and glass.
Wire any lighting, then spray in a foam insulation and attach an inner skin.

 

Plywood ribs are wasting 40% of the cross grain stock in the veneers (read heavy and not very strong). Foam can be very easily bent over just about anything, then thin plywood glued to it, at which point it'll hold it's shape.

This technique is well documented and proven. Buildings are now being constructed with SIP (Structural Insulated Panels) which are just as I described, foam with plywood glued to both sides.

 

so I'd need a construction form to bend the foam over? It is new ground for me. Does it take heat? what is the cost per sq foot and what brand/type of material would you suggest?

 

Hi Skyking, reminds me on Lucille Ball in "the long, long trailer"http://www.youtube.com/watch?v=UaES0zGBIlg&feature=related, after watching that one rainy afternoon I drew up & built an 11M(37') trailer(flat top) to store some tooling on & work as the basis for a fiberglass laminating table.
Quick query, on the trailer geometry as you've drawn it- the overhang looks a bit excessive & clearance at the rear pretty low, your vehicle rules may be different in US- in AUS the axle cluster is usually no greter thn 3.6M(12') from the trailers end.
Awesome project, I can see that cruising down the highway, also I think 3M has double sided tape trat is used to attach the panels & windows to the sides of buses.
Regards from Jeff.

 

Some good guides and techniques all over the web

eg

http://www.amateurboatbuilding.com/articles/howto/foam_sandwich/planking.html

and

http://homepages.paradise.net.nz/garyd/quikama.html


you wont need expensive boatbuilding foam like Diab etc

 

Thanks Jeff.
This is about 13' overhang, within the limits here.
I am showing the trailer in the high speed configuration. It has an adaptive hybrid suspension, comprised of torsion axles on swing arms with airbags.
It could lift up, both at the axles and the 5th wheel, an additional 6".
As shown it would get high centered on driveways, railroad crossings, etc.
I will design it with an automatic and a manual mode.
I can get speed information from the VSS feed on my PCM, and automatically raise it up below 20 MPH. No need for the low drag configuration at the low speeds.

Thank you for the guides. I had looked up costs on 1.5" corecell A500 at Jamestown distributors, not knowing any better. At ~$500 per sheet, it was not going to happen!
The Dow polystyrene should be considerably less expensive.

 

Thank you all for the inputs. I am working on the interface with the plywood/foam sandwich sides to the steel frame of the trailer.
I will have vertical posts in a couple of places inside the trailer, braced at angles for rigidity.
I can construct the ply foam sandwich flat, then mount to the posts. I am breaking up the sides into a few pieces no longer than 13' to minimize the expansion problems. I came up with an idea to use spring steel interfaces that will allow some bending without any damage or loosening fasteners. It would still be sufficiently rigid.
Those vertical posts would also be the lifting mechanism points. I plan on using a recirculating ball nut on a jack screw, with a common drive shaft down each side. I'd joint them with a chain drive in the equipment spaces. The 4 jacks will move in sync, driven by an optional electric motor. The design will allow for hand cranking. A clutch will prevent over driving, along with limit switches.

EDIT: I found this high compressive strength foam from Dow Corning.
Link to PDF:
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0813/0901b80380813717.pdf?filepath=styrofoam/pdfs/noreg/179-02548.pdf&fromPage=GetDoc

What do you think of that? It seems impressive at first glance. The 40 and 60 PSI is 2.2LB/ CF, the 100 is 3.0LB
I plan on using no more than 4MM plywood in the main, and 2 layers of 2MM for the tighter radii. This would shore up the impact resistance.

 

You'll pay a fortune for that foam. Just use the blue or pink polyurethane from Lowe's/Depot or the XPS that's available locally and cheap.

You will want to glue the plywood to the foam (both sides) with the foam pre-curved. If you do it while they're flat, then they'll stay flat. If you try to bend a flat sandwich panel, then you'll find you can't easily, which kind of defeats the whole purpose of using the foam/plywood sandwich thing. You could also do a 'glass/foam sandwich, where a laminate of fabric replaces the plywood, though there's a lot more sanding and goo work involved with this method.

Once your curved roof panel is made, it can be bedded in polyurethane and hard fastened to a frame (steel).