Barge structural details: pillars

Discussion in 'Boat Design' started by Vinassman, Jul 23, 2012.

  1. Vinassman
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    Vinassman Junior Member

    Hi all,

    I have a stupid question regarding the usual barge structures. On the below midship section, ones can see that there are 3 pillars connected to the main deck at the same location and then spreading on the bottom plate. What is the purpose of those pillars?

    The fact that they are connected below main deck is a kind of mystery for me: obviously the span of deck beams will be reduced but why using 3 pillars instead of 1 stronger?

    I believe the fact that the 3 pillars are connected at different location on the bottom is to spread the load, isn't it?

    In such a case, why not to use 3 vertical pillars which will divide the deck beam in 4 sections (thus reducing dramatically the unsupported length) and connected to the bottom plate at 3 different locations as well?

    One more comment/question: for this kind of barge, the deck load can be around 5 to 7.5 ton/m2 which is not so high. Then, why do we bother to fit pillar when a deck beam strong enough will do the job as well?

    [​IMG]

    Thanks in advance and sorry if my questions are stupid.

    VM
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Your question is not so stupid. You have answered it yourself really :)

    The purpose of those pillars is either

    1) The designer didn't know what s/he was doing..and thought...ooohh..maybe i need some pillars
    or
    2) To help transfer large shear loads, which i doubt very much on this type of boat.:eek:

    One thicker pillar is better for buckling as well, so those 3, hmm...they look long and slender! :eek:

    And if the deck beams and bottom frames are stiff enough on their own...yup..why bother with the pillars! The long.t BHD does that too!

    Just looks a mess.
     
  3. Vinassman
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    Vinassman Junior Member

    @Ad Hoc: nice to see I was not going in the wrong way :)

    However, I found in ABS rules for barges (cf. below) that all the examples they give show those pillars in the midship section... Is there any other possible reason to fit those stuffs?

    [​IMG]

    P.S: I attached again the previous snapshot since it doesn't work anymore...

    [​IMG]
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Strange!

    All you need to do, is a simple check list

    1) Is the stress ok, for the given load
    2) Is the deflection ok, for the given load
    3) Is there sufficient shear area to transfer the shear loads.

    Once you have answered those, you can make the arrangement however you like. The use of pillars, is simply to reduce the spans, as you've noted. I can only assume that ABS have "assumed" most will do this to reduce the beams scantlings (possibly because the deck loads are high???) from being too onerous/heavy. Other than that...no idea why they are being shown.
     
  5. Vinassman
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    Vinassman Junior Member

    Indeed, there should have nothing magic behind this: if stress/deflection are ok, then there should have no issue.

    I am still puzzled by ABS rules showing those 3 pillars arrangements though...
     
  6. TANSL
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    TANSL Senior Member

    Perhaps to avoid racking?
     

    Attached Files:

  7. mydauphin
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    mydauphin Senior Member

    Triangulation is used to prevent putting too much stress in one area one you move stuff around. People forget that on barges the loads are moving, like if you have a crane on tracks. The loads are not static.
     
  8. TANSL
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    TANSL Senior Member

    I disagree with your reasoning, my friend. First, you can not "put stress" in any structure. You only can apply loads on a structure that will cause stress on the elements of it. Second, however much the charges are not static, the distribution of loads is achieved in many ways more effective than using triangulation.
    There are not only moving loads on a barge. Think of the ro-ro ships, in which garages you can not find anything that looks like triangulation.
    The modern bridges, by circulating many loads, have no triangulation in their transverse sections.
    On the other hand (this does not have to do with your response, mydauphin), assimilate those straps to pillars is an absolute error. A pillar, by definition, should only work in compression. A profile, inclined, is completely ineffective as a pillar.
    The raking can be avoided with large consoles in the corners but it's much cheaper to use "triangulation". That's why the Classification Societies recommend this type of reinforcement.
    That is my opinion.
    B.R.
     
  9. Petros
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    Petros Senior Member

    TANSL got it right I think, but it is more complex than that. by using diagonal members the whole assembly acts like a truss, this makes the hull stronger and less susceptible to deformation in all directions. the truss can act structurally like a bulkhead, but by using discrete members, some in diagonal, it saves material and construction costs.

    if there is no reason to separate the areas a head or behind an internal bulkhead, than using truss like structures are less costly, easier to inspect/repair and saves weight.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    As noted previously:

    The are many ways in which to do this, using pillars, is just one way. It is also SOR dependent too.
     
  11. TANSL
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    TANSL Senior Member

    OK Ad Hoc, I think we both agree on the diagnosis of the problems that arise in this type of structures. It is possible that the relationship breadth / depth, something great in these cases, increase the distortions created by the shear. What do you think?
     
  12. latestarter
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    latestarter Senior Member

    The above seem to be inconsistent

    If that were true trusses would not work.
     
  13. TANSL
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    TANSL Senior Member

    A pillar is a reinforcement that works only in compression. That's what I mean. A beam is a transverse and a girder is a longitudinal element. Everything has a name and the name has been given aims, in some way, identify their function in a structure.
    We can not say that a longitudinal beam works well or badly because a beam is never longitudinal. So I say that an inclined pillar makes no sense, we would need, to withstand the compressive forces, a lot of material adcional to make it as effective as it would be a "vertical pillar".
    "If that were true trusses would not work". Trusses never will work as pillars, trusse will only work, wery well, as trusses.
    I know that probably everyone is talking about the same thing, but it should assign the correct word to properly explain a concept. And here I have to apologize because my english sometimes not allow me to express myself in complete correction.
    I would like to introduce here a reflection that, of course, is obvious: Most Important in the calculation of structures, and any other aspect of life is to make a good diagnosis of the situation. Only then will we be able to detect the real problem to be solved and therefore be able to find the best solution for it. Otherwise, we can build a structure full of surplus or redundant items.
    Another comment (my opinion): the distribution of shear can be varied primarily by varying the shear areas of the cross members. The addition of strusses can change the distribution of shear stresses, but not determinative. What it does do very well, in my opinion, if we don't like to increase the shear areas, is to avoid the distortions produced by these efforts.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Not necessarily. Since for any wracking or indeed shear to occur, the structure that is withstanding the load must be stiff.

    In your little pictures above, where you have labeled "deformed section" only occurs if there is insufficient shear area and/or the stiffness of the joint is insufficient to transmit the load as a single unit, ie the whole frame work.

    For example.

    If, again in your frame work, if you applied a load mid-span on the outer vertical member, and if that member was made from rubber...what would occur? Obviously the member would simply bend locally between the upper and lower supports.

    If that same member was now infinitely stiff, the load that is applied is directly transferred as an in-plane load to the horizontal members on the upper and lower joints. Since the infinitely stiff vertical member wont move.

    If the beams, had insufficient shear area and low buckling strength, then that horizontal member would fail. So if these were also infinitely stiff, the load would be taken as a direct in-plane load.

    BUT, how does the load go from the mid-span vertical member to the horizontal members, if both are infinitely stiff?

    If the web of each member have sufficient shear area to transfer the load but, more importantly, able to transfer the load from the vertical member to the horizontal, without a stress concentration, then we can say that under the given loading the frame work is satisfactory.

    If the webs, calculated end up being huge and adding weight, then one (of many) simple solution is to introduce pillars, in well positioned locations, to take the direct in-plane load in compression and to transfer this load to another member. (Large brackets also as you suggested).

    So, if the whole frame work is sufficiently stiff that globally the "box" does not wrack (deform section in your picture), then it comes down to how the shear loads are addressed and transferred from one member to another within their given allowable design limits..
     

  15. TANSL
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    TANSL Senior Member

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