When calculating transverse frames what would be ...

Discussion in 'Class Societies' started by ldigas, Jan 3, 2012.

  1. ldigas
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    ldigas Senior Member

    When calculating transverse frames what would be the unsupported length (GL rules) when one has a strong deck beam inside from an accomodation deck?

    [​IMG]
     
  2. tobbe
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    tobbe Junior Member

    A

    "A".
    Regards
     
  3. Paul Kotzebue

    Paul Kotzebue Previous Member

    In the example you have shown it is "B". The transverse frame supports the lower deck beam.
     
  4. ldigas
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    ldigas Senior Member

    Ah, good ... as long as we have a concensus :)

    Seriously now ... wouldn't a beam as strong as a frame count as adequate support? If it doesn't that means the inner deck structure doesn't participate in the stiffening of the structure at all?
     
  5. Petros
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    Petros Senior Member

    I can tell you as an experienced professional engineer, that if the floor is structurally tied to the frame, I would instantly say "A" is the unsupported length.

    Tobbe is correct, any other answer is incorrect.

    Also note that the frame has continuous lateral support, so laterally the frame has "zero" unsupported length in lateral direction if the skin or planking is continuously attached to the frame. this affects the size limits of the frame (it can be lighter since it is restrained from buckling laterally).
     
  6. rxcomposite
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    rxcomposite Senior Member

    Not familiar with GL rules but here are from LR and ABS which I posted sometime ago.

    Seems your frame angle is greater than 150 degrees.
     

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  7. Paul Kotzebue

    Paul Kotzebue Previous Member

    I don't think most classification societies would consider a lower deck beam "floating" between port and starboard transverse frames as adequate support for the same frame that is supporting the beam. However, if the lower deck beam were effectively tied to the floor (the structural transverse floor at the keel) with pillars and/or brackets, as in double bottom construction, then it may provide adequate support for the transverse frame and the span may be reduced to "A".
     
  8. rxcomposite
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    rxcomposite Senior Member

    I checked the GL rules as downloaded from the link provided by MikeJohns. You may search it if here if you don't have it.

    "Chapter 1 section 3 C. Unsupported Span
    1. Stiffeners, frames
    The unsupported span is the true length of the stiffeners
    between two supporting girders or else their
    length including end attachments (brackets)."

    And "I - Part 1
    GL 2011
    Section 9 Framing System" which shows there is a distinction between bottom transverse and side transverse.

    However, your illustration shows a section which seems to be a (forward/aft) end of the vessel. Is this a discontinous floor or a double bottom? I interpret it as one is a raised floor (or horizontal bulkhead). This being the case, the unsupported span will be "A" for the aft part and "B" for the forward part assuming both floor (or 'tween deck) is bracketed as Paul and Petros has correctly interpreted.

    Can you post a profile of the floor?
     
  9. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    Hello,

    A little question about the sketch given by RXComposite : il seems to me that the L.R. and the ABS rules have different interpretation on the "middle point" for a curved traverse frame for calculating the local pressure.

    Is it true that :
    for LR, the midpoint take in account the curvature on the frame ;
    for ABS, the midpoint is simply the mid-length of the chord between the gunwale and the end of the floor.

    In the first case, the pressure is generaly bottom pressure, et in the second, this is the side pressure ?

    Thank you in advance


    Jacques
     
  10. rxcomposite
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    rxcomposite Senior Member

    Jaques,

    In the original post of the illustration, it was to show that the distinction between the bottom transverse and the side transverse vanishes when the angle is equal to or greater than 150 degrees. It simply becomes one single transverse. It was also to emphasize that curved or chined, the rule is the same.

    So for angles less than 150 degrees, two calculations are needed. One for bottom transverse, and one for side transverse.

    As for the curvature, a factor is added to the calculation to account for the increased stiffness of the structure brought about by the curve.

    The determination of midpoint as shown by the rules is important because;

    1. It determines where the additional stiffeners will be located.
    2. It determines the point where the pressure will be applied for calculations where midpoint has the greatest moment.
    3. It determines the point from the y axis the amount of pressure that will be exerted. That is pressure increasing as the point gets deeper. GL measures this from the baseline, LR measures this from the waterline.

    No. 3 is simplifying it a bit. Dynamic pressure is used (that is when the boat is moving).

    When the boat is moving, a bow wave (and stern wave) is created, raising the static waterline and increases the height of pressure. which explains why the "V" in the fore end or aft end is treated as one member.

    Using the pressure formula, try plotting the pressure along the length of the boat and at different depths. you will find that the distribution is greater on the fore end and aft end of the boat (ship) and diminishes towards the midship.
     
  11. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    Thank you for your answer.

    In fact, I thinked to a sailboat : as the curvature is "soft", it is difficult to slit the frame in two part : bottom et side.

    The ABS ORY rules do not take account of the curvature factor, but it is the case for ISO rules ... And it seems to me that the ISO use the same method of L.R. ...
     
  12. rxcomposite
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    rxcomposite Senior Member

    You're welcome.

    For the curvature, it is taken into account. See the rule on "c", the correction factor for curved plating. For ABS ORY, it is not to be less than 0.70 and a table is given, but for steel vessel it is simply stated c=0.915. See attached rules on plating.

    My rulebook is old so there might be some changes.
     

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  13. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    Thank you very much for your answer.

    But it seems to me that ABS ORY take in account the curvature factor only for the plating, but not for the transverse frame ... But maybe my interpretation is wrong ?
     
  14. Perm Stress
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    Perm Stress Senior Member

    This cross tie is not fully effective for support for the frame as soon as loads are asymmetric (ex, wave slap from one side) , becouse cross tie itself is supported by identical frame the other side. So, span is B, but, if you are SURE cross-ties will be actually built as in the drawing, you may skimp a bit on frame scantlings.
    if loads are symmetrical, like in submerged submarine, span would be A, provided that this cross tie is safe for buckling.

    ____________________

    To play safe, I would assume span B.
     

  15. ldigas
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    ldigas Senior Member

    I'm not sure whether the same assumptions apply, but in this case I was referring to GL Rules for yachts under 24m (I-3-3 part of the Rulebook). It is an error on my part for not explicitly mentioning that - I thought it would probably be assumed by looking at the L50x30x5 profile, which is rarely found on larger ships.
    The mentioned rules (from what I can see) make no distinction between side transverse and bottom transverse, unless you're referring to transverse frames and bottom floors.

    Yes, it is in the fore part - approximatelly 1/3L from the forward perpendicular. I'm not sure about the term "discontinous floor". The floors on the bottom are bottom floors, upon which one diesel tank is fixed, and then above that are the beams, and a wooden deck sole on them, where the cabins are. So, it could be "a raised floor".
     
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