small laminated beams

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

  1. bajansailor
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    bajansailor Marine Surveyor

    Re how you cannot screw into foam, would it be feasible to use a female threaded nut set into and bonded to the beam, as per these from Bighead Fasteners?
    Extended range | bigHead Bonding Fasteners https://www.bighead.co.uk/bighead-solutions/extended-range/

    Or maybe a female bushing like these from Enfasco in the USA?
    https://www.enfasco.com/clickbond/PDF/cn555.pdf

    Although I realise that you could then have difficulty in locating the nuts under the panels when you put them up.
     
  2. fallguy
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    fallguy Senior Member

    well, a glued in bushing is a good option

    no nuts are needed beneath; I am going into wood beams, so the enfasco would work; thank you, the screw head load would be balanced and not crush the foam if the right thickness (12mm)

    the bighead pin type might work, but I couldn't find a detail

    I could also fasten the foam with just velcro in the middle and screw onto the trim boards only and this reduce the span needed for the trims to something nicer, say 2'
     
  3. fallguy
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    fallguy Senior Member

    If I built a small beam, say from walnut or hickory, and applied carbon to it; does anyone have an idea on sizing it? Like, say 1" high by 2.25" wide glue up with carbon each every 24" would deflect x amount?

    The dead loads would be from the foam and 24 oz glass and resin each side and perhaps a scrim filler; the beams themselves, and solar panels..cables from the solar and some other lighting; the headliners; easy to calculate..

    then it should support a man walking at the middle as it will happen for the a frames for the shrink wrap snow roof which is much larger load than the dead load

    It also has a wee bit of arch to it; which I discounted for the headliner
     
  4. BlueBell
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    BlueBell Ahhhhh...

    We get snow and my roof was designed for it.
    I've had over a foot of wet snow on it.
    I can walk on it at 200 pounds.
    I can stand on the 3' unsupported overhang.
    I've bounced on top of a single beam without failure, cracking sounds but no failure.
    With 9' ceilings I've got lots of head room but you don't.
    By burying the beams inside your roof you will gain headroom of course, and strength as you'll create an I-beam.
     
  5. fallguy
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    fallguy Senior Member

    No I-beam.

    Just girders under like a deck. And technically, I am losing headroom as I have a cabin roof height limit driven by the helm which overlooks the cabin. That said, I could go up higher, but I already have a 21" helm riser for a modest cabin height. The riser will get gaudy if I keep raising it. However, 3" might be possible...
     
    Last edited: Dec 16, 2019
  6. BlueBell
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    BlueBell Ahhhhh...

    What do you mean by that?
     
  7. bajansailor
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    bajansailor Marine Surveyor

    In an earlier post you mentioned that the 'standard' roof has a 12 mm thick core.
    If you were then going to add roof beams that are say 1" deep on the underside, would it be feasible to fill in the gaps between the beams with roof core foam, effectively making the roof thickness a uniform 36 mm all over plus the laminate thicknesses? This would have the advantage of giving you extra insulation as well.
    Could you do the same for the cabin sides as well?
    You could then even have a layer of glass between the layers of foam, creating a sort of 'triple I beam' effect.
    If the beams are fibreglass tophats then you could just add strips of carbon going across the roof on the underside.
    If they are timber, with carbon strips on the underside, then you could maybe use varnished battens over the carbon strips.
    Re calculating the strength of all this in an arch, I suppose the easiest would be to assume that it is straight rather than arched (hence already incorporating a factor of safety re strength).
    And then calculate the second moment of area of the cross section - you could do this for the beam itself (and include a section of the roof laminate above the beam to effectively form a 'T' section), and also for a section (same width as the 'T' section with the beam) of the roof layup comprising the 3 layers of foam with glass 'flanges' in between them. And see how they compare?
    Ad Hoc is the expert to consult re then calculating the loads - how would you calculate these if you have two different materials in the beam 'flanges', ie fibreglass and carbon, with significantly different moduluses (moduli?) for each?
     
    Last edited: Dec 16, 2019
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  8. fallguy
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    fallguy Senior Member

    Clerical error; deleted.
     
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  9. fallguy
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    fallguy Senior Member

    So, we spent the day doing some calcs.

    We have an ample supply of redwood. Not too stiff, but pretty light.

    We tested the beam only and backed into Youngs at 1,050,000 psi. Wood database used 1,250,000.

    We then did a simple load test with the planned core and glass atop the beam and calculated a new Youngs of the augmented beam (not glassed even) and got 2,010,558.

    We then used this number to establish a deflection calculation.

    We decided to use 100 pounds as a point load estimate which sort of assumes the person standing would not stand in one spot or that the glassed core balances the load a bit.

    We arrived at a beam height of 1.5" by width of 2.75" (stock is 2.75"x5.5"). And this gave us a deflection of 1.2". The 1.2" is what we arbitrarily decided an acceptable limit.

    We were unable to really determine the span dimension and are assuming 18".

    Any replies are welcome.
     
  10. fallguy
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    fallguy Senior Member

    It should be noted notching and glassing the beams to the walls is planned and glassing the beams to the roof is planned and laminating the beams 3x is planned to develop the arch.
     
  11. DogCavalry
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    DogCavalry Senior Member

    Hi Fallguy. It's worth reminding you that stiffness of a component is proportional to the 3rd power of thickness. Also that your ribbons of carbon must go as far apart from each other as possible. So your beam should be an integral part of the roof, not something the roof rests on. Fine to insert solid structure between the upper and lower skin of the roof, then bond beam to that. That gives you a beam depth equal to the entire structure. Your intuition is correct. Sides of the structure are then the most likely point of failure.

    But snow load is easily managed if you're attentive. You sweep or shovel it off. In the collapsed roofs I've dealt with (I'm a residential carpenter/ex combat engineer in Canada) it wasn't snow alone. It was heavy rain that saturated the snow. That'll bust some stuff. So you plan for that. Plan on clearing the snow.
    Shrink rap is dandy, but surprisingly expensive. If you're planning on fall alterations, then a well attached tarp, on an A frame should have enough positive slope that anything will slide off.
     
  12. DogCavalry
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    DogCavalry Senior Member

    Ah, Bajansailor beat me to it. What he said.
     
  13. fallguy
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    fallguy Senior Member

    I will be 4 hours from the boat by car. It will get shrink wrapped. The cabin is only 16' long by 9' wide by 9' high or so. $2-300 a season most likely

    We will laminate a section of core, see drawing, up the sides of the structure and tape it both sides to the walls. To port is a head wall.. To star, we will put in a short stub wall about 12" wide all the way to the ceiling and bond it to the walls and ceiling and floor, like a big giant mid-cabin (for and aft) T beam on its side to keep the walls from buckling if the shrink wrap ever failed.
     
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  14. Barry
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    Barry Senior Member

    Bajan has offered the easiest solution. If you build beams to support the roof without ensuring that the beam to roof interface will carry any shear flow stresses, you are kind of over designing the
    system. If you add a 1 inch x 3 inch beam down into the cabin you will lose headroom so you may as well just increase the thickness of the roof and make it one big "beam"
    In Kitimat BC several years ago they had 6 feet of snow in 36 hours, and at least 3 boats turned turtle and sunk. If you can even increase the thickness by an inch and take this dimension up, I understand your helm issue or split the difference 1/2 up and 1/2 down you will end up with a single panel roof able to handle the load and not lose much headroom.
    Not sure if foam is the best core material. I would not include a center layer of fibreglass between two layers of foam as it will not really add to the strength of the roof panels as it would be
    pretty much at the neutral axis.
    Ie make the roof thicker leave the beams out of the equation.
     

  15. bajansailor
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    bajansailor Marine Surveyor

    Here is a rather extreme case of using foam to create a very deep 'beam' thickness - I was privileged to work for a while at the RNLI HQ 30 years ago, and while I was there I was involved in the design of their FAB (fast afloat boat) 3 and 4, which became the Severn and Trent classes.
    I seem to remember that the Severn has foam cored topsides, but is single skin below the chine; the Trent is foam cored below the chine as well.
    And the hull bottom thickness on the Trent is massive - 4" / 100 mm of foam, plus layers of epoxy with glass and kevlar.
    Trent Class Lifeboat - The RNLI’s Lifeboat Fleet - RNLI https://rnli.org/what-we-do/lifeboats-and-stations/our-lifeboat-fleet/trent-class-lifeboat
    Edit - the website mentions that the topsides on both have 4" thick cores as well.

    I am guessing that the 'new' Tamar and Shannon classes are built in similar fashion (while allowing for technology developments over the last 30 years).
    Tamar Class Lifeboat - The RNLI’s Lifeboat Fleet - RNLI https://rnli.org/what-we-do/lifeboats-and-stations/our-lifeboat-fleet/tamar-class-lifeboat

    Shannon Class Lifeboat - the Latest All Weather Lifeboat https://rnli.org/what-we-do/lifeboats-and-stations/our-lifeboat-fleet/shannon-class-lifeboat
     
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