small laminated beams

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

  1. gonzo
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    gonzo Senior Member

    If you built a beam for testing, you can use the calculator backwards to get E. Enter the deflection value on the formula. How critical is the weight on the beams? You could use a species with higher density like pine to get a stiffer beam.
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    is there anything horrible about building up and using plywood for a lower skin ??

    the stack would be 1" redwood (have tons of old growth on hand). I am aware of the properties of redwood as less than aircraft spruce or most pines, but I have lots

    plywood, 6mm okume

    12mm scrim filled with polyester stuff

    2 lyers of 12 oz biax

    all the layers are bonded with epoxy thixo with cabosil/aerosil
     
  3. gonzo
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    gonzo Senior Member

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  4. fallguy
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    fallguy Senior Member

    I suppose the scrim and glass above simply add to that section.

    If I place the beams on 18" centers, would you consider the top of the t-sections to be 9"?

    I can do some crude testing of the stacks.

    The designer seems to be satisfied with just a 24 oz glass each side, but glassing the bottom is so damned difficult; I prefer the build up method. Thank you for contributing. Dan
     
  5. missinginaction
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    missinginaction Senior Member

    As I read through this thread it got me thinking about my own cabin roof. No beams were used. I'll put some links to some 10 year old threads that have photos of the work I did. There have been no problems with the cabin roof/sides or windshield/windows since she was built in 2009. I remember cutting what I call roof formers out of some inexpensive plywood boards. To get the arc I wanted I had to determine the proper size circle that would create an arc over the width of my cabin roof. In other words say I wanted a 5" crown over an 8 foot wide cabin top. I'd need to determine the size of the circle necessary to describe that arc. Once that was calculated I carefully made up a string with a pencil at one end and a nail at the other. The length of the string was the radius of the circle I had already figured, 1/2 the diameter of the circle. It was a long string but with the help of a friend we traced the proper arc out on the driveway. I made up 4 or 5 formers and used them to get the proper arc for my cabin top.

    My cabin top is about 10 feet fore and aft and slightly less than 8 feet port to starboard. Not as big as yours but the width is similar. It also has seating and additional structure as it is the second helm station on my boat so there is additional fixed weight up there. If you're not putting a car up there (no snow loads) this might be an option that would work.

    I used two layers of 1/2 Okoume marine ply laminated together with epoxy. 1/2 inch ply isn't to difficult to bend. Two layers laminated together are very difficult to bend. To make things easy I finished and painted the sides of the panels that would be the "ceiling" inside the boat. I hate painting upside down. Then I carefully installed the formers and laid the bottom layer on, getting everything lined up. Once that was done I weighted the bottom layer so it laid tight to the formers, tacked it to the beams that are part of the cabin sides and mixed up some thickened epoxy to tack everything into place.

    Once the resin cured I filled in the gap that existed between the convex roof panel and the cabin sides with resin. If you were inside the boat this is the joint between the top of the cabin sides and the ceiling. Next step was laminating the top layer to the bottom. Got everything set up, mixed up quite a bit of slow cure resin with silica to give it a little thickness. Plopped the first top panel on tacked the center into position and went at it with a pneumatic stapler. Let the roof panels run long port to starboard and the cut them flush with the cabin sides once everything was cured. I added 5/4 x 6" Philippine Mahogany stock to the front and aft portions of the roof to give it some added stiffness over the windshield and where it overhangs that rear cabin wall. Did the finishing work and that was it.

    You probably won't want to read the old threads I'll put below but there quite a few photos that might give you some ideas. No beams at all and no movement.

    Old Boat, new life...saga of my 1973 Silverton continues.... https://www.boatdesign.net/threads/old-boat-new-life-saga-of-my-1973-silverton-continues.33850/
    MIA's 73 Silverton finally looks like a boat again https://www.boatdesign.net/threads/mias-73-silverton-finally-looks-like-a-boat-again.35186/
    This flybridge came out of my head. https://www.boatdesign.net/threads/this-flybridge-came-out-of-my-head.44559/

    IMG_20180803_161025057_HDR.jpg Finished product....well, it's really never finished, is it?

    Best Regards, MIA
     
    Last edited: Dec 29, 2019
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  6. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    SO T BEAMS ARE OKAY BUT ANYTHING THAT RESEMBLES AN I BEAM IS NOT OKAY.
    Oh, sorry I didn't realize I was yelling...
     
  7. fallguy
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    fallguy Senior Member

    oh an I would be fine, but is more work is all

    if you want to install wiring; a simple headliner below the bottom of the beams is just easier than fishing wiring and planning conduits
     
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  8. fallguy
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    fallguy Senior Member

    thanks mia

    I like the idea more to install beams over a jig and ply over that and foam and glass above that..

    Mine won't be as beefy as yours; nor as thin, but I can run wires through.

    The thing I like about your work is it, too, did not require removing the roof to glass the bottom of the ceiling..

    I don't really want a circle...more of an ellipse?
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Re more of an ellipse shape, you can calculate the standard camber of your roof from the formula

    a = C x (d*d/B*B)

    where a is the camber 'drop' at a distance d off the centreline,
    C is the total camber (eg 8" over say 12') and
    B is the half width of the roof.

    I have a slide rule that I used at school 40+ years ago; I was still using it 25 years ago for doing this calculation (but that was it's only useful purpose really).
    And it is quicker than using an electric calculator....... :)
     
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  10. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    When you consider them a T section, the spacing between the beams is the flange width. An L and a T section will have the same deflection.
     

  11. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Okay, so I am having some trouble with the engineering calcs.

    I want to test the following stack.

    1/2" x 2.75" redwood
    1/2" x 2.75" redwood
    6mm okume
    12mm scrim foam
    17 oz biax glass

    I know the deflection ratings of the redwood and a single piece bends 31" under 100# loading (fails).

    two pieces bend 6"

    When I try to add the flat panel components, I get results that are nonsense because the I of the section increases so much I can't calculate deflection accurately.

    I have wracked my noggin trying to understand the calculation, but can't.

    It seems like the flat panel should have a deflection rating and then the beams would reduce it further.

    Has anyone got an idea how to do this?

    I basically made a stack of okume, and core over redwood and am trying to determine how much redwood to use. Intuition tells me one inch to get to the 6" deflection ratinng and then the roof reduces that significantly. I basically am hoping for an assumed max deflection of an inch, but that may cause delam.

    Any pointers on how to design without lots of testing?

    For example, can I just build the beam and add the flat panel thesame width as the beam and calculate a modulus for the 'system'? That seems plausible, but a bit off because when I think of young's, it seems material specific

    So, how then to combine the various layers in the stack?

    If I just build the beam and stack at 2.75" and test the deflection, isn't the panel outside the beam going to be missed in the calc?

    Thanks for any reply.
     
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