Catamaran composite beam design

Discussion in 'Multihulls' started by groper, Apr 29, 2012.

  1. MikeJohns
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    MikeJohns Senior Member

    Hi redruben

    The load is the applied force and remains constant, I presume you mean the stress in the outer laminate skins ?
    The thicker the panel cross section (like the web of an I beam) the lower the extreme fibre stress for a given amount of material and the stiffer it will be.
     
  2. redreuben
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    redreuben redreuben

    "I presume you mean the stress in the outer laminate skins ? "
    Yes, thanks for your reply, yep understand, talked to quick, and forgot the I beam analogy, thinner core = thicker skins, it's all coming back to me now !
     
  3. cavalier mk2
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    cavalier mk2 Senior Member

    I think what you are getting at catbuilder is a second set of cycle fatigue occurring in the panel with flex movement. The movement will cause a cycle failure not experienced by the panel with no movement because it has added motion in an additional direction. Both will be affected by the original cycle. Sort of like ripples on the waves for a visualization if that makes sense.
     
  4. rxcomposite
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    rxcomposite Senior Member

    This thread went cold before I could ask the question.

    In calculating the stresses in a crossbeam, shall I use the simply supported beam theory as in illustration "A" (attached) or,

    Should I use the firmly fixed beam as in illustration "B"? In illustration "B", the beam is firmly fixed to the side of the demi hull. Demi hull strength is calculated in accordance with class rules and the beam is calculated with the ends firmly fixed.
     

    Attached Files:

  5. Ad Hoc
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    Ad Hoc Naval Architect

    RX

    You need to define the load case first.

    Is this a transverse bending moment calculation...or twin hull torsional moment (long.t or transversely), a simple, on dry docking one hull is left "hanging" type.

    Lots to think of :)
     
  6. Alik
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    Alik Senior Member

  7. MikeJohns
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    MikeJohns Senior Member

    Also don't presume you have sufficient global stiffness to assume complete end fixity. You can count on partial fixity but the amount depends on the stiffness of the whole stucture. From a conservative call I'd analyze it as a pinned beam and use the full bending moment( the bending moment reduces as the end fixity increases).
     
  8. Richard Woods
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    Richard Woods Woods Designs

    Alik

    What does подмачтовой mean?

    I haven't studied any Russian for 40 years

    Richard Woods
     
  9. rxcomposite
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    rxcomposite Senior Member

    AH,

    I am not yet on torsional moment or cantilever calculation and even the concentrated load imposed by a structure on the bridgedeck. For now, the load is simply placed in the middle.

    LR has a rule on cross deck structure strength but I am not sure on how to define the moments. "A" or "B". For the initial load, I will assume it is the (displacement/number of crossbeam) x Vertical acceleration. That would give me a local zone that I sketched on the lower left hand portion of the paper.

    LR calls for an FEA analysis on varying conditions of crossdeck structure. I am not ready for that until I can define my moments.

    Alik,

    Thanks but could not open the link. Will try later in the evening when net traffic is much less severe.

    If I can define the moments, then I intend to create a spreadsheet based on several conditions, I.E., simply supported, cantilever, torsional, then align the composite fibers along the direction of the stress.

    LR software has a limitation as it does not account for a double sided skin on composite beams, or box beams. Only the lower skin (wet deck) is included in the tophat stiffeners. So for a simply supported beam, the lower skin is in tension but on a cantelever design, it is the upper skin on tension which does not exist on the composite profile build up.
     
  10. Alik
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    Alik Senior Member

    под мачтовый - under mast; here they mean the beam supporting the mast, under-mast beam.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    RX

    If you're doing this to LR...firstly look at their load cases and bending moments, then see if any of your own superseded theirs.

    FEA is generally only required for approval of vessels over 50m.

    What you're doing is a global calculation; principal loading. Top hat stiffeners etc are for secondary loads. This if you wanted to establish the total modulus for bending moments, you'll have to do that by hand, unless you have written a small spreadsheet for it ;)
     
  12. rxcomposite
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    rxcomposite Senior Member

    Thanks Alik. I was able to open it. It is Russian!:confused: It will take sometime for me to digest it. I can see, they are using the outermost side of the hull as the maximum span.

    Mike, Thanks. I have thought of the pinned beam design but the hull moves when the beam flexes. But you have a point there that I will have to think about. The secondary fixed point at a distance from the edge. A gusset, haunch, or fillet, whatever they call it. A secondary support on a beam.
     
  13. Alik
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    Alik Senior Member

    Once one knows theory behind and can understand formulas... can even read engineering books on Chinese :cool:
     
  14. rxcomposite
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    rxcomposite Senior Member

    Thanks AH. Thats good to hear. I have written a spreadsheet on box beam based on cantilever design, evenly distributed load, divided into zones which gives me the upper and lower cap thickness and web shear. I want to modify it so it will fit the load model in discussion.

    I will find it quite usefull as most crossbeams discussed are assumed to be heavily constructed but sparsely populated. I have seen designs on small cats/tri with monocoque constructions where there is no box beams, just laminated foam with unidirectional caps on upper and lower face.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Don't make it any harder for me.:D Russian, maybe. Chinese, maybe no.:)
     
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