Attached plate to a stiffener.

Discussion in 'Class Societies' started by TANSL, Mar 20, 2016.

  1. TANSL
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    TANSL Senior Member

    The various regulations provide several formulas to calculate the width of the plate associated with a given stiffener. My question is: Should we add the width of the base of the reinforcement to the width that formulas give us ?. In the case of "top hat" profiles it is important to know the right way to proceed.
    What is the correct practice for you?.Thank you.
     
  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

    It's not clear what you mean by the term 'width of the base of the reinforcement'.

    Most stiffener-plate combinations have two elements, like a piece of plate and a stiffener, such as an angle iron or tee-section.

    Are you looking at a section which contains three or more elements in it?
     
  3. TANSL
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    TANSL Senior Member

    This is what I mean (see figure) : When calculating, for example, the second moment of area, must be considered "be" or "be + w"
     

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  4. NavalSArtichoke
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    NavalSArtichoke Senior Member

    I think you would use the width be + w, unless the spacing of the hat sections makes a shorter width. That's what the ABS Rules for Yachts specifies.

    See ABS Guide for Building and Classing Yachts, 2016 edition, 3-1-2/7.1

    This Guide can be downloaded free of charge from www.eagle.org

    You can check with other class societies to find out their practice in such situations, but it will probably be the same as ABS'.
     
  5. TANSL
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    TANSL Senior Member

    I consulted the DNV-GL, Lloyd's SSC and ISO 12215-5 and had no clear answer. That is why I am asking.
    I have gone a little further and I think that indeed in top hats must have "be + w" but I still doubt for solid wood stiffeners, which are not as wide at its base as a top hat.
     
  6. rxcomposite
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    rxcomposite Senior Member

    You must consult Lloyd's or Marine Composites.

    When analyzing plate, use only the width of the plate from the base of the stiffener.

    When analyzing a stiffener with attached plate, the effective width is used as shown in the drawing. The section must be transformed (idealized) into a simple H beam with a wide flange at the bottom. The web is the total thickness of both sides. Height is measured from the top of the attached plate/tabbing and the bottom of the top flange or cap. For accuracy, the stiffener tabbing is also included as shown in the LR diagram.

    Plate with stiffener is the most difficult to program in Excel and has been a challenge to me.
     

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  7. NavalSArtichoke
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    NavalSArtichoke Senior Member

    DNV-GL uses a more complex stiffener model than ABS does. I don't know about Lloyd's or the ISO.

    Wood is tricky to analyze unless you are certain that there is enough shear connection between the sheathing and the stiffener, so that both components bend as a unit.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Wood can be analyzed also as a reinforced beam. See illustration as it appears in Strength of Materials by Pytel and Singer.

    Clue. Use the ISO method of tabulation for sandwich construction and substitute wood properties for the core.
     

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  9. TANSL
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    TANSL Senior Member

    Frankly I think the model for reinforcements used in any CS is the same.
    In wood and fiber reinforcements we must analyze the shear stress in the reinforcement/plate interface. But that has little to do with the width of the plate that can be used to calculate the modulus of a reinforcement working in bending.
    I just read in ISO 12215-5, 10.6 that "when the reinforcement has a significant width, it can be added to the width given by the formulas". I have, therefore, the answer to my question. I will assume that "significant width" means a width equal or greater than the thickness of the plate. In this also can pose other opinions.
    Thanks to both of you.
     
  10. rxcomposite
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    rxcomposite Senior Member

    I wonder why you missed it TANSL.

    In table H4 of ISO 12215-5 (stiffener with attached plate) the effective width is defined as 20 tp + 70.

    In LR rule, it is 0,5bw + 10tap

    In USCG it is bw + 18t

    Class rules may look deceptively similar but each has their own approach and when not followed to the letter may produce erroneous result.
     

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  11. Ad Hoc
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    Ad Hoc Naval Architect

    Quite right and Tansl is incorrect.

    Each Classification Society has different methodologies and thus different formulae for effective breadth whether it be aluminium or composite. It is based upon their in-house R&D experience and feedback from vessels in service.

    Effective breadth is simply that, how effective is the attached panel and how much it contributes to the stiffness of the section. In metals it is relatively straight forward. In composite less so, owing to the fact it is ostensibly anisotropic. Hence the differing formulae in each CS.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    That wholey depends if the tabbing is merely to "fill" up the section/radius, or whether it is genuinely part of the structure that is carrying and transmitting the load path from one member to another. That requires an understanding of the build practices and hence the QA of the yard.
     
  13. TANSL
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    TANSL Senior Member

    Yeah, me too I wonder, now. I was plugged the brain for a few hours. Even the figure that I have attached in post #3 answers my question. Thanks.

    How can I be incorrect if all I've done is raise a question?

    Thanks for such a detailed reply, although does not answer my question.

    Thanks also to NavalSArtichoke, you were correct : be + w .
     
  14. vkstratis
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    vkstratis Marine Designer

    For composite top-hat-like stiffeners you will take be+w for second moment of area calcs. For wooden beams you will only consider the stiffener's width and not the plate's. This was stated in ABS Rules for Classing FRP Vessels (1978). New rules that consider only FRP stiffeners, require be+w (considering all other specs for the stiffener in question are met).
     

  15. TANSL
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    TANSL Senior Member

    vkstratis, although there are people who never have doubts, there is no doubt that, in this respect at least, are possible different opinions. Not everyone is clear about the concept of plate associated
    With regard to small boats governed by ISO 12215-5 (required to obtain the "CE"), wooden stiffeners, and standard timber construction, requires special studies but, as I've seen in the ISO standard, the plate associated must be taken into account for the first and second moments of the stiffener. What happens is that you should check the shear force in the intereface stiffener/plate and there, of course, you should only take into account the width of the reinforcement. In the case of top-hat stiffeners, there must take into account the tabing, and calculate its width so that the shear is within the allowable limits.
    All this is my opinion but it would not surprise me to hear opinions against.
    Thanks vkstartis.
     
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