Is there a distinctive difference between girders and beams?

Discussion in 'Class Societies' started by ldigas, Jan 2, 2012.

  1. ldigas
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    ldigas Senior Member

    A stupid question, but sometimes I find it hard to differentiate between the two. - expecially on smaller craft where they can sometimes be "pretty much the same". What makes a girder and what makes a beam (both can be long. and transverse) expecially on smaller craft, where there isn't (one big profile) - girder, and (several smaller profiles) - beams, but all are ... well, similar.

    Is it allowable to calculate all your elements of, well in this case - deck, but it can be bottom as well, as beams? Or girders? Do the classification societies allow that?

    Have no idea how to put this more clearly, but I'm sure you all understand what I'm asking here.
     
  2. gonzo
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    gonzo Senior Member

    In a ship, a beam supports the deck atwartships. A girder is a generic name for a type of structural member
     
  3. PAR
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    PAR Yacht Designer/Builder

    Technically speaking a beam resists bending loads, while a girder resists torsional and bending loads.
     
  4. Silver Raven
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    Silver Raven Senior Member

    G'day 'Idigas' Aanswer is; NO - NO Not unless your are nit-picking with a 'thesaurus' - - 'Same-be-same" as they say in the Asian SE countries. Gawd - what would they know - they've only been going for 6000 years - so we all know more than they do - cause we 'is' educated - ha ha what ha ha ha Give me a break, will-yah Please - the shape - for the task - that does the job - is the right ONE regardless of the 'punce' ie legal definition. Ciao, james ? say U others, eh ????????????
     
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  5. murdomack
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    murdomack New Member

    Questions like this can be argued forever. As Par says, a beam resists bending forces. These forces would probably be in one plane, IMO. A girder can be a construction of many parts, bolted or welded together, that resists forces in more than one plane, or torsionally. Of course a girder could simply be a beam with stiffening added.

    Something I came across recently is the difference between a spreader beam and a spreader bar, as used with cranes. Again the beam can have bending forces but the bar must be engineered so that all the forces meet at the one point at the ends and thus has no bending forces. A bar can be many times lighter than a beam, but a beam is more usefull as it allows lifting from different points.
     
  6. troy2000
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    troy2000 Senior Member

    Depends on what the definition of 'is' is....
     
  7. Perm Stress
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    Perm Stress Senior Member

    Stiffeners support the plating, girders support the stiffeners or other girders.
    Both stiffeners and girders are beams in a generic technical sense.
    In old tradition, beams are specifically atwartships structural members, necessary to support deck planks, as opposed to floors on the bottom, and frames on the sides.
    And carlings, stringers, etc., with their respective names.
    If translated to more modern metal shipbuilding terms, all of them will be classified as stiffeners or girders, as explained in the first sentences of this post.
    And all of them will be considered "beams" for strength analysis purposes.
     
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  8. Boston

    Boston Previous Member

    well in the wild world of construction terminology a beam is generally a single member and a girder is a number of members assembled into a truss like structure, generally used to support "beams"

    no clue what the difference is on a boat.
     
  9. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    It seem to me that :

    A beam is a secondary stiffener, witch "support" the plating

    A girder is a primary stiffener, witch "support" the secondary stiffeners
     
  10. ldigas
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    ldigas Senior Member

    That has been somewhat my understanding too ... girders/primary, beams/secondary members which "rest" on primary members. Plating "rests" on beams.

    But, if one has, let's say, a deck ... theoretical example here, just for illustrative purposes. A deck is composed of longitudinals, and deck beams. Both are the same HP profile or L profile ...

    Now, girders are usually considered larger, so in this case, we can say both are beams, right? Beams supporting beams? (Does this make sense?)

    What would be the unsupported length of longitudinals and what of deck beams in such a case? Do longitudinals "rest" on beams, or just on bulkheads far away?



    .... this is generally the main part of my question. When you have identical elements, what is then a beam and what a girder, and what "rests" on what? What is the unsupported length in those cases for longitudinals and what for transversals?
     
  11. ldigas
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    ldigas Senior Member

    When you have a structure like this;

    - two bulkheads 4m apart
    - bottom plating, side plating, deck plating

    - two deck longitudinals 1m from centerline (making up the numbers here ...)
    - deck beams at every frame (0.5m apart, frame spacing)
    - transverse frames
    - bottoms/floors

    Deck longitudinals and beams are the same profile.

    What is the unsupported length of the longitudinals? Is it 4m (distance from bulkheads) or 0.5m (distance of deck beams ... which are connected to frames and so on).
     
  12. Perm Stress
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    Perm Stress Senior Member

    a GRID is number of members :).
     
  13. Perm Stress
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    Perm Stress Senior Member

    In this case you have a homogeneous structure, where what supports what depend on relative stiffness of the elements.
    If your deck area between the sides/bulkheads is longer is one direction, than the shorter elements will be the "spring" supports for the longer ones: because of less unsupported span, they will be stiffer. That is in general. Exact answer is only possible with detailed analysis of particular structure, with careful consideration of interaction between members of similar stiffness -a tedious job, but rewarding for those who like to play with this kind of engineering or/and need to minimise structural mass.
     
  14. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    I think that you can have several "level" of structure on a deck : for example :

    1e level : deck longitudinal (supporting the deck plating) => unsupported length = beam spacing

    2e level : beam => unsupported length = breadth of the hull

    3e level : deck girder => unsupported length : bulkhead spacing
     

  15. Perm Stress
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    Perm Stress Senior Member

    Unsupported length here has very little meaning for practical design, standard solutions for standard cases are largely inapplicable here.
    Here it is necessary to analyse a beam with "spring" supports; those supports represent the action of intersecting beam, and are derived from analysis of that, beam, accounting for that beam interaction with others. :).
    Or it is necessary to use grid analysis methods.
    Hope this will help you.
     
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