Bulkheads in ISO 12215

Discussion in 'Class Societies' started by Mat-C, Aug 7, 2010.

  1. Mat-C
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    Mat-C Senior Member

    I posted this elsewhere, but thought it was possibly worth its own thread....


    I'm having trouble figuring out how one should treat bulkheads in the ISO 12215 (in this case for Aluminium construction, though it may be similar for other materials)
    In the case of the image below, which I would have thought to be typical of a hard chine planing hull, where the frames become CNC cutout bulkheads. How does one asses this? ISO states that it should be treated as a watertight bulkhead, which is fine. But trying to figure out where to go from there has me in a spin!
    My understaning is that it would be treated as a panel - though some of the dimensions ough to be taken as if it were a stiffener (which, of course it is!) If I've understood correctly, the panel is basically be broken down into three sections (shown on my drawing in red), in this case, divided by the natural stiffeners - deck, seat and deck. Though this ignores the chine and keel...
    Am I correct in this basic element?
    Would section 1 be classed as a transverse stiffener (as opposed to a panel) with the length Lu, the distance between chine and deck? And if so, what is the height, as it varies from top to bottom...?
    The other two sections, 2 & 3, would be unstiffened panels with heights of a & b respectively?

    I'm sure this is a lot simpler than it seems, but the more I try to figure it out, the more confused I become...:(
     

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  2. Mat-C
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    Mat-C Senior Member

    Someone...anyone...?
    I'm quite prepared to beg....;)

    BTW.. is there anywhere that provides a basic education in the application of 12215? I mean apart from taking a full blown yacht design course...
    I know what you are going to say... one can't fully undersatand how to apply a standard unless one understands the theory behind it. Which is a perfectly valid point, of course.....
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Mat-C

    You first need to understand what the bulkhead is for, then you can design accordingly.

    Your bulkhead, is it to separate voids into watertight compartments or just for structural support? Your confusion is that you need to understand what this “bit of structure” is doing before you can asses how to design it and what codes it must comply with.

    Assume worse case, ie watertight bulkhead.

    The bottom triangular section, #3
    Is this sealed off, like a tank along the deck btwn #2 and #3, from hull side to hull side?...or just in way of where you have a sole?

    If the ‘tank top’ is hull side to side, then that is your void. If it is not sealed, ie open all the way up to #1 gunwales, or deck as you’re calling it, this becomes your maximum head.

    The pressure to design to is P = rho x g x h

    h = head, ie from the keel to the gunwale.

    Add a factor of safety of 1.5 as a minimum.

    That is your design pressure.

    The rest is treated as just spans and breadth of load area, as the rest of the hull.
     
  4. Mat-C
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    Mat-C Senior Member

    Thanks Ad Hoc,
    I wasn't really thinking in terms of integral tanks or anything... since I don't really have a grasp of a basic shape, I don't want to complicate matters any further!:D
    I was just using that image as an example of a fairly typical CNC cut frame in a smallish aluminium boat - where the frame, or bulkhead, is cut to form supports for things like seats, soles, decks etc.

    11.8.3 of ISO 12215 says that all (metal) bulkheads should be treated as a watertight bulkhead in terms of the design pressure. That bit I get.... it's how to consider the other aspects... I guess in terms of determing the panel sizes / lengths etc that has me a bit bamboozled.

    For instance, is the part near the seat (that I've shown as section 2 of the panel) considered as a separate panel to the other parts (1 & 3), with the length between supports being the seat and the cockpit sole (even though the sole doesn't extend to the sides of the boat...?

    No doubt all very simple when you understand the intent of the rules... but very confusing when you don't....:(
     
  5. Alik
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    Alik Senior Member

    1. Treat any unsupported part of bulkhead as separate panel. Yes it is tricky sometimes for small craft where there are no formal stiffeners and structural furniture is used instead of them.

    2. From my experiecne with bulkheads on aluminum craft, if done according to ISO12215-5 standard they are not stiff enough; same refers to deck. We tend to add additional stiffeners (flat bars, angles) to add rigidity.
     
  6. Mat-C
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    Mat-C Senior Member

    Ok - so, in the case of my diagram, there would indeed be 3 'panels' to consider, basically divided as I've shown in red...?
    And in the case of irregular, or non-square panels (such as the bottom part, #3) one would take the long dimension of the panel as the distance from side to side (the red line) and the shorter dimension as the average of the panel's height (not as I've shown it, as the full height of that section, from keel to cockpit sole)...?
     
  7. Alik
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    Alik Senior Member

    Mat-C, yes, three panels (provided that adjacent bulkheads comply as stiffeners). Dimensions of panels - take average dimensions, there is a scheme in ISO12215-5 of how to measure it.
     
  8. Mat-C
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    Mat-C Senior Member

    Thanks - so, assuming all the frames were either partial bulkheads, like the one drawn, or full bulkheads, then each would be done this way. When you say 'assuming adjacent bulkheads comply as stiffeners', do you just mean each must comply with 12215 when calculated as a panel in the way we have done with this one?
    Interesting to note that even then you've found them not to be stiff enough. Is this in actual strength terms, or more that they don't 'feel' strong enough? Decks flex under foot for example....
     
  9. Mat-C
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    Mat-C Senior Member

    Thinking about this a bit more, if you asses a bulkhead as a panel, then how do you now it's strong enough as a stiffener?
    In the attached image, which is similar to the 1st, only this time there is no seat. Now the frame / bulkhead has what I would assume to be a weak spot where the cockpit sole meets the cockpit 'liner'.
    If all you've done is asses it as two panels, then how have you checked that the frame as a whole is strong enough?
     
  10. Alik
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    Alik Senior Member

    Yes, it has to be checked if it complies as frame, but usually it does. It can be checked as flat bar on edge welded to side/bottom. Important condition that bulkhead should be stiff enough.
     
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  11. Alik
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    Alik Senior Member

    Yes, steel or aluminium deck designed to comply with ISO12215-5 might flex under foot; usually need more stiffeners than required by standard.
     
  12. Mat-C
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    Mat-C Senior Member

    When I've done this though, I often find that because the 'stiffener' height is so much greater than the thickness, then web shear is indicated as a problem...:confused:
    If the panel is welded to the hull and then plug-welded to the sole, or whatever, would you still assess it as a flat bar?
     
  13. Alik
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    Alik Senior Member

    This is why i was talking about stiffness of bulkhead!

    You can assess it as T-bar or angle; depending on details. Usually just flat bar is enough.
     
  14. Mat-C
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    Mat-C Senior Member

    Hmmm... ok, but in order to avoid web shear as a potential issue - particulalry in a deeper section, like #2 in my 1st drawing, thickness goes from say 4mm up to 30 or so... clearly you're not going to have a frame / bulkhead with that kind of thickness...

    Is the height still determined in the same way for an irregularly shaped stiffener... or is the minimum height used...?

    Sorry... so many questions... you've been very kind with your time and patience...
     

  15. Alik
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    Alik Senior Member

    As I said, use additional stiffeners on bulkheads - it will help. For bulkheads, this is just formal check; I would not use actual dimension but would just show dimension that complies (anyway actual dimension is bigger).
     
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