Tophead Floor Calculation Question

Discussion in 'Sailboats' started by Repelsteeltje, Jul 13, 2010.

  1. Repelsteeltje
    Joined: Jul 2003
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    Repelsteeltje Junior Member

    For my project i need to calculate the scantlings of the floors above keel end below the mast. Calculating the loads is not problem, ABS, ISO 12215-5 and Principles of Yacht design are quite clear on that.

    Problem is as i have some height and weight restrictions i would like to go for stiffeners with a ud (may be even carbon) top head. Can anybody explain how this can be calculated? Both Section modulus and moment of inertia? If i remember correctly for moment of inertia IE can be used.

    Some yaers ago i have also read a very interesting article on this subject in Professional boat builder magazine. Unfortunately i can not find my copy anymore. I would be very grateful if somebody could provide me with this article.

    I truly hop you guys can help me out. your help is much appreciated.
  2. Perm Stress
    Joined: Sep 2009
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    Perm Stress Senior Member

    You have to recalculate I assuming same STIFFNESS E of materials.
    Say (to make numbers easy) UD has 2 times the stiffness of woven fiberglass. say you add 1cm2 of UD on top of your floor. then, for calculation, you assume 2cm2, spread HORIZONTALLY, and woven material calculated as it is. So you have both I and W of composite section.
    there is a further problem: while UD in itself is much stronger than woven, the stress in place where UD ends (just below the edge of UD tape) has to be well below permissible for weaker i.e. woven material. So unless some special design measures are taken, STRENGTH of UD will be not used. While it will still significantly increase STIFFNESS of composite section.

    One fundamental to be kept in mind:
    when we have certain bending moment to be taken by section, height of could be reduced only to some critical value. Reduce the height too much, and you quickly move to situation when no exotic materials with super strength are able to hold it. Or, in less extreme case, 10% reduction in height will mean 100% increase in weight + questionable stiffness.
    For fiberglass structures stiffness is often more difficult engineering problem as strength.

    After all, a couple of structural floors, protruding few centimeters above floorboards are not so big nuisance. Even fancy production boats have them sometimes.
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  3. Repelsteeltje
    Joined: Jul 2003
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    Repelsteeltje Junior Member

    Perm Stress: thanks for your reply. I understand stiffness this will be the summation of I*E.

    Strength however is another issue here I should consider first ply failure. This makes things much more complicated. And I still do not understand how to calculate. Can you or somebody else on this forum maybe provide a sample calculation for my refference?

    Furthermore I think web shear and buckling stability could become an issue. I would appreciate some guidance. I think professional boatbuilder had an article explaining all this unfortunately I have not been able to find this copy.

    As I expect the (carbon) UD will also shift the neutral axis away from the bottom shell the reduction of height will not reduce SM and I as much as might be expected.

    Well any further help remains much appreciated

  4. terhohalme
    Joined: Jun 2003
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    terhohalme BEng Boat Technology

    There is an example and explanation in ISO 12215-5, annex H, Laminate stack analysis, H.3 Method for stiffeners.

    ISO 12215-9, which is the standard for keel and rigging attacments is not ready yet. Actually 12215-9 is for keels and 12215-10 for rigging if I remember correctly.
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