Table 20 in ISO 12215-5 for plywood structure

Discussion in 'Class Societies' started by roryact, Jan 27, 2018.

  1. roryact
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    roryact New Member

    I'm designing a plywood internal structure for a small (30ft) sailing yacht, and would like to use the furniture as structural members, eg. bunkfronts and berth tops as stringers.

    According to table 20 and 21, my plywood flat-bar stringers can have a maximum height 10 times their thickness.
    • Does this mean for a 400mm height bunk front I need to use 40mm ply just to pass this minimum? surely not?
    • Or if i'm using 12mm ply, can I only consider the first 120mm of the bunkfronts as structure? In which case my SM is too small.
    I also have a similar question about plywood bulkheads. If I follow the rule in 11.8, they must be 7 times the hull depth (roughly 10mm), but there is no provision for how much can be cut out of them. If I take it as a transverse stiffener, again, I run up against max height=10 x thickness leaving me with a very small SM or very thick frame.

    Any guidance on how to interpret this rule, and how to get reasonable structural design out of plywood would be greatly appreciated!
     
  2. TANSL
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    TANSL Senior Member

    As for the stringers, the interpretation you make is correct. It is about preserving the structural stability of the reinforcement itself.
    As for the transverse bulkheads, it is not correct to calculate them as if they were a transverse stringer because the load assumptions of these two elements, bulkhead and stringer, are totally different (see attached picture).
    Regarding the cut outs, you can do what you want as long as you conveniently reinforce the opening.

    Snap57.jpg
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Correct, the FB has a max 10:1 ratio of length to thickness.

    Correct. So if your hull has a depth of 1.0m the thickness is 1.0 x 7 = 7mm.
     
  4. roryact
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    roryact New Member

    Thanks TANSL and Ad Hoc for the help. I suppose I'm asking too much of my tall stringers and will need to add more to reduce their spacing.
    If you can give some guidance on my solution it'd be also appreciated!

    I think my other option is to call the berth top the "flange" of my bunk front stiffener, providing they're suitably bonded, In which case I can increase the height to 40 x the thickness (12mm ply=480mm, plenty for ergonomics). Can I still use the berth top as a separate side stiffener?
    Im guessing I can't also use it as a L stiffener, as I'd be counting the same element twice, but If I have a 600mm berth top, can I call the inboard 100mm a flange, and the outboard 100mm a flat bar?
    Is this the best optimisation for lightweight plywood structure?


    Re: the bulkheads, I suspected this as this is the approach taken in Principles of Yacht Design. Though it appears to me under the 12215-5 rules you could then cut out 99% of the middle of them and have it still meet that minimum?
     
  5. TANSL
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    TANSL Senior Member

    I would like to help you but for this I would need more details of your boat. The structure must be considered as a "whole" and it is risky to try to fix an element without knowing, at least, those adjacent to that element. Could you send me a PM with your e-mail, to be able to exchange more information, drawings, dimensions, ...?
     
  6. gonzo
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    gonzo Senior Member

    One solution is to add stiffeners to the bulkheads. It is the usual method, which is lighter and cheaper.
     
  7. TANSL
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    TANSL Senior Member

    I do not understand you. Solution ... what for?. Lighter and cheaper .... than what?
     
  8. roryact
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    roryact New Member

    TANSL, I'll take you up on your offer to have a look at it. Right now my structure is mostly spreadsheets, so i'll punch out some drawings and send you a PM

    I've seen bulkheads with welded structure for larger metal yachts if that's what you mean? I'm not sure how I can use it to strengthen longitudinals?
     
  9. gonzo
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    gonzo Senior Member

    The maximum height or depth is based on the force a thin column can take before buckling. If you add stiffeners, it is not necessary to use thicker plywood. For example, you can glue perpendicular plywood webs.
     

  10. TANSL
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    TANSL Senior Member

    Buckling of a column, thin or thick, occurs when subjected to compression. I do not know what that has to do with what is discussed in this case. Well, yes, I know, it has nothing to do with it.
    Could you explain better what this solution consists of and, above all, what does it intend to solve?
     
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