ISO 12215-5 - variable height stiffener calculation

Discussion in 'Class Societies' started by Midja, May 5, 2023.

  1. Midja
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    Midja Junior Member

    Hi guys,

    I am in doubt how to tackle this situation.

    I have relatively deep V hull and requirement is to have top of the floor/stiffener in line with machinery room floor.
    So this floor/stiffener is top hat style in cross section, continuous, from portside to starboard, with great change in height.
    Wanted to use H7 table for stiffener analysis, but I am in doubt, what height to take in account? Average between biggest and smallest height of top hat?
    Since I can`t find anything in ISO in case there is stiffener with change in height by its length. upload_2023-5-5_20-42-22.png
     
  2. TANSL
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    TANSL Senior Member

    I know that this is not what you are asking, but couldn't you adopt another solution? Because such a high stiffener can have slenderness problems, local buckling, if the omega legs are not given a very large thickness.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    It all depends upon the application - the purpose of the frame.
    Since you are spanning from side to side, at this depth, it implies something more than 'just support'. Thius clarification of the intended purpsoe would help.

    However, in assessing the modulus requirements, simply by inspection one can concluded the following.
    If we take a section shown in red, the depth of the section - when calculated - is most likely to comply with the minimum depth of section. The section modulus.
    This to be confirmed, and if not, then of course adjust to suit.

    upload_2023-5-6_10-30-57.png

    The section shown in black, is clearly much deeper than that section shown in red.
    Thus one can concluded it is highly likley that this section would easily satisfy the minimum requirement - if this were a conventional top-hat frame.

    So, this brings us to the purpose of this frame.

    You cite:
    So, either the depth of section required or the depth of section for siting the Engine, appears to be the driver for this frame?
    Would you butt the Engine Girders into this frame like so:

    upload_2023-5-6_10-36-3.png

    If not, would you wish to do this, to save weight?
    upload_2023-5-6_10-36-45.png

    or even this:
    upload_2023-5-6_10-39-13.png

    So, you need to identify the objective for the structure and then arrange the structure to satisfy the objective whilst ensuring it passes the minimum section modulus requirements, deflection, stress and buckling checks.
    There is no "rule" as such to follow. The arrangement is down to you and the objective.
    Then it is a simple matter of analysing the arrangement you have created.

    Don't over think this...
     
    jehardiman and bajansailor like this.
  4. TANSL
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    TANSL Senior Member

    The correct solution would go in this direction, adapted to the omega type profile, but it would be necessary to study how the longitudinal girders are to avoid the same local buckling problem.
    upload_2023-5-6_10-40-22.png
     
  5. rxcomposite
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    rxcomposite Senior Member

    Something missing here. You have to consider span (length of longitudinals).
    In calculation, you have to satisfy the stress in the outermost fibers, which in this case is the bottom plating and the floor, the shear on the web of the longitudinal, and the deflection of the span which depends on what type of structure this is. Simplifying, this is a wide flange H beam.
     
  6. Midja
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    Midja Junior Member

    Thank you for detailed answer, this is main structural element in transversal direction.
    Regarding your proposals, my sketch is oversimplified, it has longitudinal elements that finish and start at that transversal structure, but with much lower height:
    upload_2023-5-7_11-14-12.png

    Also, I have one transversal element just like that one above, but further from machinery with laminated plywood bulkhead at top of it, so that whole assembly acts as watertight bulkhead:
    upload_2023-5-7_11-17-8.png

    I`ve already tried to make calculation with lowest height but it failed miserably; thus is my post.
    Also, I did think about "installing" longitudinal elements into frame; making them continuous through frame, but that would create similar problem, in longitudinal direction. That is why I feel like I am in the loop at the moment.
     
  7. Midja
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    Midja Junior Member

    I have lost you little bit in translation, why length of longitudinals? This is transversal structural element (floor/frame); longitudinal structural elements are not continuous through transversal element, thus it should have span of whole its length, am I correct?
     
  8. rxcomposite
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    rxcomposite Senior Member

    Table H2, stiffener with plate require span and spacing input + depth of web. You mentioned table H7?
     
    Last edited: May 7, 2023
  9. rxcomposite
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    rxcomposite Senior Member

    Your drawing appears to be reversed? Is the stiffener supporting the floor but not touching the bottom panel?
     
  10. rxcomposite
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    rxcomposite Senior Member

    I follow the same as AH and Tansl does. Maybe a drawing will clarify things. Transverse.jpg
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Ok...well, let's go back to basics.
    Firstly can you send a sketch, of the overall arrangement. Not frame by frame, or one long.,t and another...as noted in your images above.
    Show exactly how you have arranged the frames, all of them, and the long.,ts, all of them....

    The reason being is to see what is supporting what....and this is where you are perhaps getting confused.

    Thus, the only way to advise further, is by knowing exactly how the structure is being arranged, and then we can establish what is doing what and why.
    Does that make sense?
     
  12. TANSL
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    TANSL Senior Member

    The ISO gives us a value of the design pressure at the midpoint of the stiffener. Therefore, it would not be unreasonable to consider that the height that must be considered to calculate the resistance of the stiffener is the one that it has at its midpoint.

    If you need 2 or 3 long girders in the bottom, it would be logical to make them continuous from bow to stern, with which the longitudinal strength of the hull would be guaranteed, and you take the ribs as secondary elements, something similar (I'm not saying the same) to what which indicates the figure.

    upload_2023-5-8_14-49-59.png
     
  13. rxcomposite
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    rxcomposite Senior Member

    Tansl The drawing is out of proportion. It says 500 mm deep and 80 at the side. That is a deep V hull, around 30-33 degree deadrise.

    I could fit two tall side girder (no center girder), 300 mm depth, and just about maxes out at in the stress and deflection limit at 6 meter.
     
    Last edited: May 8, 2023
  14. TANSL
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    TANSL Senior Member

    Thanks rx, putting the plan in its true proportion, with its real dimensions, will not prevent treating the bottom longitudianls as primary elements and the transversal frames as secondary elements. I'm wrong?. That is what I wanted to say. (The dimensions of the stiffeners in my diagram are not correct either, nor are those of the girders. It may be necessary 2 longitudinal, 3, 4 or who knows!).
    BR

    Edited : The chine that I have drawn is not the same as the one that the OP has drawn. I'm sorry.
     

  15. rxcomposite
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    rxcomposite Senior Member

    I just proportioned it using the 80 mm and 500 mm dimensions. I am treating it as a girder to see how far I can go given the limitation in height. That is a girder supported by two bulkheads (primary) spaced not more than 6 meter apart, or a girder running at least 75% of crafts length(not an ISO rule, just for exercize, warming up).

    Then I will subdivide it for the transverse but I don't know yet the width of the boat.
     
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