small laminated beams

Discussion in 'Materials' started by fallguy, Dec 15, 2019.

  1. DogCavalry
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    DogCavalry Senior Member

    That's some sound engineering Barry.
     
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  2. Barry
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    Barry Senior Member

    Another aspect as you are building your own roof panel. I would add say 3 to 4 "ears" per side at the outside perimeter of the cabin roof. While you could just insert some solid wood to
    act as a place to attach any item that you might want to add in the future, ie search lights etc, any penetration of the roof membrane can be a place for water ingress.

    If you form some ears say 2 inches high with a 1/2 in hole through them, solid glass of course, they would be an attachment for your newly designed snow shedding tarp frame or
    even places to lash docking poles, halibut spears, long handled deck wash down brushes, crab traps and the list goes on. Easy to add on at this stage
     
  3. fallguy
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    fallguy Senior Member

    The downside to this solution is there is no wire chase for ceiling lights and for the solar panels. I could try to plan some pvc chases I suppose if I two paneled it with a 12mm stringer or some such.

    It should be noted there is a 5" curve at the ends of the roof. I have already purchased 12mm scrim foam for this roof. However, I do have two sheets of 1" plascore on hand as well that gets very stiff with a couple layers of 12 oz biax. Perhaps that core with a one inch beam would be sufficient? It won't be as easy to work as the scrim, but I will have to get fancy and glass the outside of the roof first and then take it off and glass the inside upside down, so lighter would be preferred as well...

    An extra complexity is we did plan eyebrows, but I suppose those could just be worked in same as rest.

    It would get to be a lot more laminating and a lot more room for errors to try and get two laminations done to allow for a wire chase.. not sure other strategies for wiring..
     
  4. fallguy
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    fallguy Senior Member

    I might have enough plascore for the roof; not sure. It is super flexible stuff, not as rigid as corecell..

    If I were going to try to build the entire thing from plywood, perhaps I could build up the plywood underneath and glass only the outside and down onto the sides. Then the inside would not require glassing and the job might go tons faster without needing to remove the top at all. We could just cut away the support system from underneath?

    Would I be able to build a plywood box beam? Say 4-6mm ply top over some 1" thick redwood beams and all that atop another piece of 4-6mm ply? And could I not glass the inside then? The downside is the roof could rot, of course, but we have some plywood in the build already.

    Thoughts? Any guidance on sizing the plywood? I have some okume here I can go check the thickness; pretty sure it'll make the bends. Fast build no glass inside? Or do I need to?

    Thanks.
     
  5. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    I feel like I'm missing something here and am hesitant to comment...

    Yes glass inside.

    Ply-foam-ply all epoxied together with appropriate curve/camber.
    Fiber-epoxy the interior and exterior accordingly.
    Paint.

    4-6mm ply, 50mm foam, 4-6mm ply.

    WRCedar deck beams built in between the ply, sectioning the foam
    would add rigidity and strength and something to screw the ply to!
     
  6. fallguy
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    fallguy Senior Member

    How about this?

    I build the laminated beams, say 2 layers of 1/2" each, placed 18" apart. I notch the walls and set them in on a temp jig to keep shapes.

    I place 6mm ply on top of them and bond the ply to the beams with epoxy thickened with cabosil.

    I place scrim foam on top of the ply and bond with epoxy thickened with cabosil.

    I fill the scrim with poly filler and epoxy and glass the top of the cabin to the sidewalls.

    I remove the jig from underneath and bond the sidewalls to the plywood and the beams. The plywood is okume and requires no internal glasswork.

    The roof, then also requires no removal in the build process.

    I am also putting hooks on the roof, so the cabin can be craned up and onto the beams. So, the plywood can also be the backer for the crane hooks and I could actually even put some G10 or metal plates in under the hooks...

    pushback?
     
  7. fallguy
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    fallguy Senior Member

    Any takers on my last post?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Lots of interesting ideas...but..from my point of view...you still need to establish the 'design' and then stick to it until proven otherwise....
    There have been many options thrown about..but you need to decide which 'design'...by that I mean, the structural arrangement, that you wish to use.
    Then comments, at least from my side, can be more focused...

    However:-
    Need to make sure the hooks have solid foundations and adequate load paths.
     
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  9. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I really like/need to pursue the idea of building a jig and then building up to avoid glassing the roof bottom off the cabin.

    To avoid fiberglassing the bottom, if possible.

    This would be ...

    1. Build a jig to build on.
    2. Notch the walls every 18" for a laminated beam 1-1.5" thick, 0.5" boards. I would have a test modulus to determine..we want less than 1" of deflection from a 100 pound load?
    3. Epoxy 6mm plywood onto the beams with thixotropic mix. The roof width will be 8' exactly.
    4. Laminate scrim and 12mm core onto the plywood.
    5. Glass two layers of 12 oz biax on top of the scrim.
    6. If needed, I could also glass on top of the ply which is effectively the scrim bottom, but this layer is probably not going to add much.

    load paths for the hooks will be glass tapes off the hook points down to the cabin sole.. I need to hook away from the windows...

    Is that enough or should I develop the testing now?

    I just don't know how wide to put my beams apart...just going on humans typical size and foot distances walking...

    Thanks for any. I do have a request into the designer, but he is at sea last I checked.
     
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  10. fallguy
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    fallguy Senior Member

    I can also tape the beams to the ply if the thixo bonds are considered a potential weak point.. probably double the depth of the beam, so a 1" beam would get a 2" tape
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    If you have drawn up the arrangement you are happy with - and suits your internal accommodation issues as well - then all you have to do is calculate if what you have proposed, is feasible. No need for testing just now..that comes afterwards.
    You can either use sales figures for strength or Class rules for strength..or make small coupons to test of the proposed layup. This is all that is required to ascertain of the arrangement satisfies the load and hence strength requirements.
     
  12. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Terrific progress here.

    Fallguy,
    I'm thinking a curved roof that has the rigidity and strength of an I-beam.
    How you arrive at that I-beam structure is up to you but epoxy-fiber
    on the outside (top) and inside (bottom) would provide maximum
    compression and tension resistance at the optimum moment arm of the I-beam.
    4 - 6 mm ply would add huge structural integrity and reduce the fiber-epoxy weight needed.
    Then you need something to provide the vertical separation like foam,
    western red cedar, or both.
    The thickness is the key.
     
  13. fallguy
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    fallguy Senior Member

    I have no plans for an I beam.

    The plan is for beams on the bottom, ply on top and scrim above that, 24 oz biax above that..

    The beams can be epoxied and glass taped to the ply. The scrim and foam can be bonded to the ply and the top can be glassed.

    The ply on the bottom is a skin for convenience and strength and the skin on top is for strength and to get us thickness without weight, if you will.
     
  14. gonzo
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    gonzo Senior Member


  15. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

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