Consultation regarding ISO 12215-5

Discussion in 'Class Societies' started by TANSL, Nov 25, 2020.

  1. TANSL
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    TANSL Senior Member

    When calculating a panel, from the bottom of a ship for example, the standard indicates a certain value for the design pressure. The question comes when when calculating the design pressure of the reinforcements that delimit that panel, the pressure of these is appreciably greater. Any reasonable explanation why that is so? Thanks a lot.
     
  2. rxcomposite
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    rxcomposite Senior Member

    Delimit? You mean the constraint?
     
  3. TANSL
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    TANSL Senior Member

    The reinforcements that form the perimeter of the panel, in which the panel edges are supposed to be fixed. Stiffeners supporting panel boudaries.
     
    Last edited: Nov 26, 2020
  4. rxcomposite
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    rxcomposite Senior Member

    Glad you clarified. Design pressure is a different set of calculations than the reinforcements of the tophat stiffeners. If you look at ISO figure H3 “top hat” illustration, the defining parameters is there. In table H4 top hat worked example, the bonding flange web is also there, just before the attached plating. This is typically a “plate with stiffener analysis”

    If you look back to table H2 “Laminate stack analysis” you will see the illustration of the region of compression and tension. Although the illustration shows the stiffeners with bonding flange table H2 is just that, a single skin plate analysis. No frills or details on differing thickness occurring.

    To answer your question “Does the extra thickness at the base of the laminate “substantial”?”. It is both a Yes and No. Look at the illustration. At the outside “wet” skin where the pressure is, it says it is in compression but in the opposite end, at the base of the stiffener, it is in tension. The stiffener is fixed and the twisting reaction is against the rotating motion, thus the reversal.

    Table H2 if done in Excel will only analyze a uniformly distributed load at the central portion with the outer surface in compression and the inner surface in tension. The only way you can do a complete analysis is when you program it in such a way that will allow a reversal of the laminate (or forces) plus the extra thickness at the bonding flange (I did). And oh, you have to modify your bending moment formula to “test at” a distance away from the center of the stiffener. Lloyds has this but not ISO. It allows me to test for reversal and test at “a distance from”.

    So back to the question. Is it “substantial”?. NO (it depends). Composites are weaker in compression than tension. To compensate, it is made thicker. That is why the outer skin (generally in compression) is thicker than the inner skin. Near the base, at the reversal of the forces, the inner skin is in compression and the added thickness of the bonding flange compensates. How much? It depends on how much and type of material you put in. It is about the thickness of the stiffener web so it is more than substantial.

    Where it matters most is in table H3 where the stack analysis is “rationalized” into a simple Web flange attached to a wide plate (Illustrated in LR) and analyzed as a beam. Here the layer by layer stress analysis will show where the individual layers away from the neutral axis fails. With 50mm minimum each side for the bonding flange, never had a failure.
     
    Last edited: Nov 26, 2020
  5. TANSL
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    TANSL Senior Member

    Dear Rx, thank you very much for your explanations. They are really valuable and I appreciate the time spent with me and I think that many people will find it useful, everything you have explained to me.
    That said, I have to admit that I have explained myself very badly because my question, the doubt I have, arises long before I could check whether a reinforcement works under tension, compression or in any other way. My question is general, it does not depend on the material of the reinforcement. It is limited only to the fact that, when applying the formulas to calculate the design pressures (chapter 8 of the standards), the pressure corresponding to a panel is less than that corresponding to a reinforcement that is in exactly the same position. I do not understand why they are different, I wonder if they should be the same and, since they are not, what is the reason or the explanation that they are not.
    I hope I was clearer this time.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Perhaps a sample calculation would be clearer. Freehand on a piece of paper will do.
     
  7. TANSL
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    TANSL Senior Member

    See attached pictures and thank you very much for your time.
     

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  8. baeckmo
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    baeckmo Hydrodynamics

    What happens when you go to the longitudinals calculation; is there a third value applied then?
     
  9. TANSL
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    TANSL Senior Member

    Yes, there is a new value.
     

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    Last edited: Nov 26, 2020
  10. baeckmo
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    baeckmo Hydrodynamics

    Ok, please bear with me, I am speculating here. It does matter if the design pressure is strength related or deflection related. There is also a longitudinal pressure distribution to observe. If the panel and the transverse stiffener refer to different longitudinal positions, the design pressures differ.
     
  11. TANSL
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    TANSL Senior Member

    You are right, the design pressure varies throughout the length of the boat but, as you can see in the screenshots I have shown, the distances from the centers of each element to the after end of the WL are exactly the same.
    (I do not rule out some type of error on my part but, for the moment, I cannot detect it and I think that perhaps there is some technical reason that explains these results.)
     
  12. baeckmo
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    baeckmo Hydrodynamics

    I notice that the mid-panel position is not the same as mid-position for the long or transverse, which might cause a difference in design pressure. That should be checked versus design rules. As for the stiffeners, there is a difference in the position of the neutral layer, probably linked to the influence of the wood attached to the longitudinal stiffener. A difference in the position of the bulb profile's neutral layer indicates a difference in the stress levels. But if it is the full explanation of the pressure variations, I can't tell. If the calculus sheet is using curve fitting algorithms for design pressure depending on longitudinal position it could result in a bug at specific positions, f.i.

    If you move all components forwards to about midships, I'd expect similar pressures if stress related, but if there is a deflection limit (set as a design constraint), the pressures must differ for the various load carrying components.
     
    Last edited: Nov 26, 2020
  13. TANSL
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    TANSL Senior Member

    Thanks for your comments, @baeckmo. Indeed, the distances to the aft end are not the same, which was my mistake. However, once this has been corrected, the design pressures remain different for each element, with differences greater than 30%.
    Once the formulas have been analyzed, it is observed that there is a factor, in the pressure formulas, called "kAR" which in turn depends on the design area "AD". This area is AD = lu * s but it will not be taken greater than 0.33 * lu ^ 2

    lu = unsuported span of the stiffener
    s = stifferners separation.

    These limitation of the value of AD is what makes the kAR factor can be very different, which leads us to those strange values of the design pressure. Frankly, I do not know if the parents of the rule have taken into account the effects of the limitation. In any case, can someone explain the why of it?

    Take for example a panel of 550 * 450, of area AD = 550 * 450 (247500). In the case of transverse reinforcement, for example, it should not be taken greater than 0.33 * 550 ^ 2 (99825) but in the case of longitudinal reinforcement, its value should not be taken greater than 0.33 * 450 ^ 2 (66825). In the first reinforcement AD = 99825 and in the second reinforcement AD = 66825
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed it is.
    All rules have these variations, depending upon whether the structural member under investigation is a frame, stiffer, or plating.. and each has limitations such as you note.

    The scantling determination in 11.4.1 notes that P = design pressure as per section 8.
    There are 2 design pressure formula presented with the greater value being the value to use.

    One pressure calculation has values of kAR and kL in it.
    And as you note kAR has limitations placed upon it and is different whether the member under investigation is plating or stiffening - which thus yields different 'fudge' factors, hence the difference in pressure values you highlight.
    Additionally, as Baeckmo notes, another factor that influences the pressure value is the kL...the long.t distribution.

    Thus depending upon where in the boat you're investigation..aft...midships..fwd...etc and depending upon the panels sizes selected, the design pressure used to calculate the SM in 11.4.1 will oscillate between the 2 possibles, giving the max value to use, as per section 8 requirement.

    Why this is - is another topic.
    But most Class rules also present the design pressures and application of said pressure to structural members in the same way.
     

  15. TANSL
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    TANSL Senior Member

    Of course, everything you say is obvious and is well explained in any regulation. The only thing I have done is to check once more that I had not made a mistake, since the values obtained are strange, but the initial question remains unanswered. Is there any explanation for the fact that a reinforcement, depending on whether it is transverse or longitudinal, must withstand a different design pressure or that these are different from that acting on its attached panel?
    Thank you for your time @Ad Hoc .
     
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