small laminated beams

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

  1. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I need to design some small laminated beams. I don't have much headspace, so they need to be designed at the minimums.

    I have done a bit of googling and not much help. Most of the googling results in residential timbers and I don't have that kind of space and a higher degree of complexity.

    The designer of the boat I would like to leave out of the conversation, so please don't. It isn't his decision to place the boat in an area with winter snow loads.

    The beams will support the roof which needs a snow load rating of 50 psf. The dead load rating of the roof is probably around 10 pounds. I could calculate it for a higher degree of precision, when needed.

    There is some curvature to the roof, so the beams need to be curved somehow. The span is about 8 feet and the width is about 13 feet.

    The beam choices are sort of obvious questions.

    1. Which wood has the least deflection, or does fiberglass have a better rating.
    2. Can I build some laminated timbers?

    I am sure there will be more questions.

    Perhaps someone can give me a rudimentary idea about designing cabin roofs. We would have some walking on the roof, but only to service solar.

    The roof itself is going to be made with a 12mm Gurit M80 corecell. I have some scrim for the edges which are curved. I expect to laminate it with something like two layers of 12oz biax both sides.

    I have a full woodshop and can build beams from wood laminations or corecell or just glass like tophats.

    Any help is welcome.
     
  2. BlueBell
    Joined: May 2017
    Posts: 1,112
    Likes: 185, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I made curved beams for my houseboat.
    3/4" spruce, three layers glued and screwed onto a jig.
    Less spring back than expected: 3/4"!
    They were 7" over 9'.
    16" centres
    The roof sits on top of the beams.
    1/4" ply, 2" foam, 3/8 ply, fiberglass, gelcoat black.
    The ply and foam are attached to the beams by 4 1/4" #10 screws.
     
    DogCavalry and fallguy like this.
  3. bajansailor
    Joined: Oct 2007
    Posts: 835
    Likes: 219, Points: 43, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Re maximum strength for minimum depth, would it be feasible to build foam cored glass beams like top hats, but with a strip of carbon tape along the underside of each beam to stiffen it up a bit more?
     
    fallguy likes this.
  4. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    the downside of foam is I can't screw into it,

    but wood with carbon sounds really stiff
     
  5. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I don't have that thickness planned for the roof.

    Pretty sure you don't get a 50psf rating. Not to offend, but do you have snow load? I plan to keep the boat where they get lake effect. I might need to a frame the roof even for winter...

    I was thinking along these lines...perhaps 1.5" x 3 with carbon each side, 12" apart? Any idea how to convert that to psf?
     
  6. Ad Hoc
    Joined: Oct 2008
    Posts: 6,383
    Likes: 507, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Are you suggesting that you wish to sue one beam, over that span and width?
    or
    are you going use several frames to reduce the width (and load per beam) over that span?
     
  7. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Oh I will need many; perhaps on 12" centers to get to 50psf design for snow loads.

    Then the end supports are probably an issue as well. The cabin could literally buckle on the side walls as well. I may have no option, but to a-frame and shrink wrap the roof each year while near Lake Superior

    8x13' is 104sqf@50 is 5200#... the walls made of corecell probably can't handle those loads and neither can windows..
     
  8. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Funny how reality smacks even good intentions. I am probably going to make sure my sidewalls are stiffened at least mid for n aft.
     
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 6,383
    Likes: 507, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Indeed...placing them every 300mm, or 12" is most likely sensible. Then having a central girder running along the roof too. This doesn't reduce the stiffness of the frames, because the span is half, it is merely to aid load paths and to prevent any tripping/buckling.

    You really need to draw or sketch out your arrangement, i.e. 'design it'...so then you can calculate what you need. Not the other way around.
     
  10. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I will provide a sketch later today. I have a wall for the head that can help, but it is not all the way....perhaps we could girder off that wall..
     
  11. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Here is a sketch of the cabin roof. This sketch is proprietary, no copying.

    I will provide a sketch of the interior shortly.
     

    Attached Files:

  12. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I think designing for snow load is a mistake. I will have to shrink wrap an a frame for winter storage.
     
  13. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    This does not diminish the need for a dead load structure for solar panels and even perhaps the shrink wrap guy walking on the roof.

    The inside of the cabin will have a wall for the head that runs for an aft and was designed for the head to be about 32" wide from the port side, but only for about 54" for and aft. This was the only inside wall that would reduce or affect the roof loading at all. Unfortunately, it does not run the entire length of the boat, so there is little to draw.

    Give me another hour or so and I will draw something more for the cabin interior. We really have no options for vertical supports; the space is really too small for it. Although it would be possible for a stripper pole I suppose...not sure I like that idea. Seems lame.
     
  14. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Here is a drawing.

    The cabin is simple. A small head with a shower to port. A galley to star. I will take the stub wall all the way to the ceiling.

    The darker lines will go to the ceiling.

    drawing is to scale...the two lines on the outside show the lower wall and upper wall
     

    Attached Files:


  15. fallguy
    Joined: Dec 2016
    Posts: 2,830
    Likes: 290, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    The span is 2400 mm

    The width of the cabin is 2580. The difference is some angle above the half wall. See pdf
     
    Last edited: Dec 15, 2019
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.