How can I have supports for the longitudinal Girders?

Discussion in 'Class Societies' started by MoeZ, Dec 9, 2017.

  1. MoeZ
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    MoeZ Junior Member

    I am trying to design a boat using ABS HSC rule. There includes a general formula to calculate the Section Moduli of the stiffener/ frame/ girder. This formula involves the square of the span between the supports. I know for the stiffener, the span is between two frames; for the frame between two girders; and for the girder between two bulkheads. But, since the distance between two bulkheads can be sometimes too big that the required section modulus for the girder is so big. So, my question is how can I give supports for the girders instead of bulkheads only? Thank you in advance for your answers.
     
  2. TANSL
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    TANSL Senior Member

    The normal way to shorten the light of the deck girders is to place pillars. You can also place very strong beams and suppose that it is the longitudinal girder that is supported in the beam, instead of the opposite. But I would place pillars that, in addition, is a very good solution to eliminate vibrations.
     
    Last edited: Dec 11, 2017
  3. MoeZ
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    MoeZ Junior Member

    Thank you, that's quite similar with what I had in my mind earlier. But, can you also explain me about the hull girders? (bottom plate girders, side plate girders, main deck girders) because, for a bulkhead spacing of 7 or 8 meters, the deck girders' span (unsupported length) is too big that the profile is too big to be realistic. How should I have additional supports for these elements? I imagine that the pillars might not work in the hull. (for example- for side plate girder, you see). What is the solution for this? Thank you for your answer,
     
  4. TANSL
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    TANSL Senior Member

    Obviously pillars are only a solution for the deck girders. Bottoms and sides usually do not have these problems and CS regulations have specific formulas for these elements. In any case, if the problem persists, the solution is to put more strong intermediate frames. The normal thing is to place a web frame every 4 or 5 normal frames, with which the unsuported length of the girders diminishes.
    I would have to know how you are designing the structure, whether it is longitudinal or transverse structure, in addition, of course, the other conditions that determine the scantlings. Keep in mind that the structure should be treated as a set of elements. The first thing that you must define are the main and secondary elements and how the main elements are supported among themselves.
    Send me a PM if you want to send me information in a more confidential way.
     
  5. vkstratis
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    vkstratis Naval Architect


    You can either reduce spacing by introducing more girders (main longitudinal stiffeners) or/and introduce web-frames that will effectively support the girders. For this to happen web-frames will have spacing the distance between them or the distance to bulkheads and lengths the length (girth) the the webframe which will probably be the girth of the bottom.
     
  6. TANSL
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    TANSL Senior Member

    Yes, the most effective thing is always to reduce the unsupported length of the element, since the bending moment that must be supported is proportional to that length squared. While the width of the "attached plate" to the element only influences proportionally to its width.
     
  7. MoeZ
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    MoeZ Junior Member

    I totally agree with this idea! Thank you for your really helpful answers. It works for the side and bottom girders.
    But, I still have a doubt. For the main deck, I can't place deck web frames here again, because, the length of the span will be again 7 meters, which will make the web frame too big, again. So, are the pillars the only solution for this?


    [QUOTE="TANSL, post: 819901, member: 44745"
    I would have to know how you are designing the structure, whether it is longitudinal or transverse structure, in addition, of course, the other conditions that determine the scantlings. .....
    The first thing that you must define are the main and secondary elements and how the main elements are supported among themselves.[/QUOTE]

    It is the longitudinal structure. I have had it with the longitudinal stiffeners closely spaced, supported by transverse frames with nearly twice spacing as the longitudinal stiffeners. For the transverse frames, the span is between the longitudinal girders again.
     
  8. TANSL
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    TANSL Senior Member

    In longitudinal structures, the so-called web frames are placed every 4 or 5 normal frames separations, that is, every 3000 or 3600 mm. You must bear in mind that web frames must form closed transverse rings. That is to say, the reinforced beam must coincide with a web frame on the side and with a solid floors in the bottom.
    When you talk about very large dimensions of the girders, what are these dimensions really?
    What are the main dimensions of your boat?. Maybe I can find an example that will help you.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    You cannot generalise like that. Every boat is different and depends upon the spans of frames stiffeners and major supports....and what one is willing to compromise in order to achieve the desired result. A frame depth of 500mm may be accept to one dept but not to another as it impedes a/c ducts etc.
     
  10. Barry
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    Barry Senior Member

    Are you saying that if you double the length, the bending moment is squared?
     
  11. Barry
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    Barry Senior Member

    Alternatively, double the girders, so the load is reduced on each one
     
  12. TANSL
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    TANSL Senior Member

    It seems incredible that you ask that. Speaking of uniformly distributed loads, which is more or less what is produced in these cases, for a beam with fixed ends:
    Do you know the formula BMmax = (p * L ^ 2)/12
    where :
    BMmax = maximum bending moment
    p = uniform load applied per linear meter
    L = unsuported length
    That's what I mean.
     
  13. TANSL
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    TANSL Senior Member

    You cannot generalise like that. Most of the air conditioning ducts go through the strong beams that get higher precisely with that idea. On the other hand, in ships other than ferries, there is usually no air conditioning or air ducts in the cargo holds (as you should know). For all that, indeed, many things can not be generalized and I have never intended to do so.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    What Barry means is, if you double the length your bending moment is quadrupled, not doubled.

    It is simple maths.
    A span of 2 squared = 4.
    Double the length span = 4, squared = 16.
    It is your understanding that Barry was questioning, which you failed to recognise....like so much.
     

  15. TANSL
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    TANSL Senior Member

    Ad Hoc, this is what Barry said:
    I think, I have not said that but, anyway, I was wrong or not, what good is that discussion for the OP?
     
    Last edited: Dec 13, 2017
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