Definition of unsupported span

Discussion in 'Class Societies' started by DUCRUY Jacques, Feb 26, 2011.

  1. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    Hello,

    I have a little problem for the definition of "unsupported span" for a transverse frame.

    On a sailboat (ABS ORY and ISO are agree !), this is the chord length between the support points (i.e. between end of floor and gunwale).

    But it seems that for the english and the german Lloyds, the definition is not the same : the span of the transverse frame is slitted in two parts : bottom frame between centerline and turn of bilge, and side frame between turn of bilge and gunwale ; if this definition seems logical for a motor boat (V hull) ; what do you think for a sailboat ?

    Thank you inadvance

    Best Regards


    Jacques
     
  2. Petros
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    Petros Senior Member

    I am an engineer that has worked with the GL requirments. It appears purpose of the GL definition is to recognize the "arch" effect of curved rib. In an arch you have the ends in compression, and bending loads in the center. On a pure simple supported beam you get bending in the center and shear at the ends. The older English standard is an over simplification of a complex loading condition. I would speculate than the more modern GL breaks the rib length down differently to take advantage of the extra strength that can be achieved with a more detailed look at the load conditions, saving weight and material cost.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Jacques

    There is no confusion.

    You need to understand what the structural member is doing and what it is supporting. For example, if you have a member that is straight and is supported at each end and this span is say 2.0, clearly the span is 2.0m between the supports.

    If this very same beam is not longer straight what occurs, how do we account for this? Well as a general rule, if the knuckle (location of joint or change of direction) creates an angle of more than 150 degrees, or no more than 30 degree total (depending upon your frame of reference), from support to support, this is considered to be "straight", thus the span remains 2.0m.

    If however, the knuckle is say 130 degrees, this is less than 150 degrees. This is then no longer considered “straight” and the span must be adjusted accordingly. If this knuckle joint occurred say in the centre of the 2.0m span, then the span is between the support to the location of the knuckle i.e. 1.0m. If the knuckle occurred at say 500mm from one end, the max span is then 1.50m.

    So, to go from keelson to gunwale is impossible unless you have a very odd looking hull. The shape must turn through an angle of at least greater than 30 degrees to form a bilge or chine, from the keelson, and then up to the gunwales.

    Thus, your span is from one of a beam to the other, but the other end, if joined/butts into another beam, has an angle of greater than 30 degrees from its original “straight” extended line.
     
  4. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    Thank you very much for the explanations.

    So, can I suppose that, in the ISO definition of the unsupported span (chord length)for a sailing boat, the curvature factor is a "pseudo" substitute at the division of span in two parts ?

    Best Regards


    Jacques
     
  5. Alik
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    Alik Senior Member

    I agree about differencies between GL and ISO - this span issue is a bit tricky.

    What is also interesing about GL that design bottom load on high speed craft is much less than one in ISO. So their methods of measurements are matching their design loads.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Jacques
    When i look at the ISO rules, again the definitions are clear. The assumptions are just as i described above. In 12215-5:2008 (the version i have) in 9.1.4.3 there are 4 figures.

    If you look at a "typical" hull shape, figure c), they are noting the location of S1 (gunwale) and S2 (keelson) as the only supports. There then is a b1 - c1, factor to account for curvature.

    However this is misleading. Since if you look in 9.1.5, figure 10, the hull shape (hard chine) the angle , aplha, between each knuckle, (or chine) if greater 130 but less than 150 degree falls into their "its ok" type of category.

    But this is a complicated way of say, that which I described above. If the subtended angle is greater than 30 degree (at the knuckle), it cannot be considered to be a single span from end to end, it is from end to the “knuckle”. Which means at that location (knuckle/chine turn of bilge) this is the start/end location of the span.

    This is clearly shown in fig's.11 a to d. The side frame is not part of the span for the bottom frame. Because the angle from the side to the bottom part of the hull is greater than 30 degrees. Fig.11 e is the classic definition of spans.

    ISO just simply complicates a very simple structural definition, because ISO is meant for a wider audience. Class rules and structural theory are generally used by professional naval architects, whom all have formal training and education in structural theory, thus they know what is or is not the correct definition of "a span" for a structural member. ISO, in their attempt to define a span, they have just simply made it worse, it is actually very simple.
     
  7. rxcomposite
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    rxcomposite Senior Member

    Perhaps an illustration would better serve the explanation.

    The 1st page is from ABS illustrating a chine craft and the second is from LR with a bilge radius. Both class rules treat chine an bilge radius the same. The illustrations are not complete so I have to cut and paste.

    As you can see as the frame transition from midship towards the bow, the knuckle angle changes and at angles greater than 150 degree,the distinction from a "bottom frame" to a "side frame" vanishes and the frame is simply treated as one.

    The panel curvature (3rd page) is treated as "additional strength gained", hence a reduction in scantling is allowed under the rules.
     

    Attached Files:


  8. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    Hello,

    Thank you very much again.

    Rxcomposite, your drawings are very explicit, and it is clear for me now that a rounded bilge craft is "assimilated" to a hard chine craft.

    ad-Hoc, I have a 2006 version of ISO 12215-05, but (Thanks God), the figures are the same !

    I had made a little calculation for a famous steel racing sailboat, the Tina (D. Carter 1966). The minimum pressure bottom (ISO) is 17.91 N/mm² and I take a admissible stress of 192 N/mm. The frame space is 0.27 m.

    1) if I take as unsupported length the chord length between centerline and gunwale => l = 2.25 m, I find a SM of 5.33 cm^3 with a curvature factor of 0.5.

    2) If I take as unsupported length the chord length between the centerline and a point situated at level of waterline (choose as "turn of bilge") => l = 1.58 m, I find a SM of 5.26 cm^3 ... with a curvature factor of 1 !

    The two results are very similar : is it a coincidence or not ?

    Best regards



    Jacques
     
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