Structural question - Epoxy Fiberglass plate

Discussion in 'Boatbuilding' started by mvoltin, Oct 29, 2018.

  1. mvoltin
    Joined: Nov 2017
    Posts: 9
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    Location: ALABAMA

    mvoltin Junior Member

    Designing 13' diameter round "plate" that will be suspended by three anchors around the perimeter (it will serve as area cover for rain/sun - like a contemporary gazebo). Using epoxy + fiberglass over Extruded Polystyrene (XPS) Rigid Foam and would like to have thin profile while still being able to withstand moderate winds. Otherwise, there is no additional load on the "plate".

    It is a big project but I do not mind and not afraid of failing. just wanted to get more input about the engineering. Currently, considering two options:
    1. 2" thick XPS edge glued and cut to form 13' diameter circle and then simply covered with two layers for glass from each side.
    2. 1/2" XPS edge glued and cut to form 13' diameter circle and then using 2"x 1" ribs about 2' apart.
    Is there clear advantage of one over the other? Building with ribs is little more complicated but is it much stronger to make it worthwhile? Design #1 will be slightly heavier (more foam) but the difference is negligible. On the other hand, the design #2 will be 2.5" thick (vs. 2" thick) but, again, the difference is negligible for me. Primary concern is structural strength.

    thank you in advance for any input and hearing about any other potential approach will be great.
  2. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    You would do better if the roof had some shape, as in if you made it from triangular pieces and that way it would have some pitch. If it gets some snow on it a flat top isn't going to cut it. Also, this will get rain to run off like it should. If you have sufficient thickness to the core you don't need ribs. Think seriously how you're going to support it, not so much from holding up, but more importantly to hold it down if you get some strong wind.. You can imbed pieces of wood into the body to give you something to screw into, but unless you do something on that order it's going to get ripped off when a strong wind comes along. That much epoxy is going to cost a lot, I would think that a foam compatible with polyester and going with a less expensive laminate would be a lot less expensive than going with epoxy.
  3. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    It will be tons of work making the ribbed version look good.

    Add some camber for rain n snow loading.

    YJ-good advise on the need for structural inserts.

  4. mvoltin
    Joined: Nov 2017
    Posts: 9
    Likes: 0, Points: 1
    Location: ALABAMA

    mvoltin Junior Member

    Thank you both. The main takeaway for me is that, with sufficiently thick core I do not need ribs. Initially, I was not sure if, with the same overall thickness, structure with ribs would be stronger but, based on YJ feedback, I will go with just the 2" foam - much easier and straightforward.

    Also, totally agree and appreciate your other recommendations and here are additional clarifications I did not mention in the original post to keep it short:
    1) There will be three 2" metal I-beams from the center out (and welded at the center) inserted in between the foam to provide solid anchor points (it is anchored on three metal posts)
    2) There will be plywood inserts between the foam every 2'. It will be used to anchor 1/4" or thinner wood on the underbelly (cosmetic only - example file attached)
    3) The patio is in a corner and, by design, it is going to be installed at 20 degrees pitch and rain and snow (we do not have much here) should not be problem.

    Also, higher cost of epoxy is not an issue since I already have epoxy and fiber-cloth left over from another project. Plus, in my limited experience, working with epoxy is so much easier and it does not shrink anywhere as much as fiberglass resin (I had huge problems with shrinking and warping when glassing large flat surfaces). In addition, I do not buy brand-name epoxy (I probably would if building an actual boat) and epoxy cost is only 20%-30% higher which makes it totally worth it in many ways. In addition to Epoxy being easier to work with (less fumes, less shrinkage, etc.), finding acceptable foam core locally and cheaply is very difficult for fiberglass resin (I have never been able to find polyurethane anywhere).

    Attached Files:

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