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

    Ugh ... after reading most of the above I'm more confused then when I asked the question. This unambiguity of terms is one of the reasons why structure is the part of design I like the least. It also shows that there is a need for more clearly written rules.

    If we disregard most of the above, could someone help me with just the few following questions?
    GL rules for yachts under 24m, have formulas for deck beams and deck girders. Beams can be both longitudinal & transversal, girders also it seems.

    What is the unsupported length when considering longitudinal girders and what is the unsupported length when considering longitudinal deck beams?

    Can a stronger element (judging by section modulus) be supported by a weaker element connected to a stronger element, or must an element always be supported by a stronger element? I.e. if we take a longitudinal, is its unsupported length from bulkhead to bulkhead, or is it from one deck beam to another?
     
  2. Petros
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    Petros Senior Member

    Generally the term "beam" is a straight supporting member (as in "beam of light") and the girder is not necessarily straight. Beams are usually straight members that support floors (which are presumably flat). In building construction beams support both joists and roof rafters. Beams are always horizontal members.

    Girders support beams. Girders are the "collector" members that support other structures. Girders could be built-up members or trusses, or one piece.

    These terms are used in both boat building and land based construction. I suspect the origin of the term date to antiquity and are likely from different root languages.

    These terms are often used interchangeably, which is improper and only leads to the confusion. Most dictionaries track usage, so if the term is misused long enough it takes on a new meaning. which is why we have so much trouble communicating.
     
  3. Perm Stress
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    Perm Stress Senior Member

    Welcome to the world of structural engineering :D.

    GL Rules, by the way, are very thorough, clear, and well structured.

    What you seem to lack, is versatility with translation of real structures to "models for analysis". That take time, practice, and head scratching in first stages. After a few designs, executed in full cycle, fluency will come.

    __________________

    For such ambiguity it is only necessary to make one set of structural members MUCH stiffer than the others. All the rest (type of cross section, span, end clamping condition) being equal, biggest factor by far is the height of profile .
    Say, your patch of deck is almost square, beams (transversal ! :) ) are ~1.5 times the height of longitudinals.
    Than span of beams is from side to side (or to longitudinal bulkhead), span of longitudinals is bulkhead to beam, than beam to beam and so on.
     
  4. Boston

    Boston Previous Member

    um
    I've built everything from high rises to bungalow's

    a beam is a single member

    a girder is a heavy truss like object made up of multiple members typically used to hold up beams, joists, whatever.

    [​IMG]

    if you notice the girder is the fat one holding up the trusses that dive into it
     
  5. Perm Stress
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    Perm Stress Senior Member

    Apologies,
    I missed you were talking about construction, not shipbuilding.
    From posts already here, it is obvious that "girder" in building not all that similar to "girder" in ship.

    In shipbuilding, the "girder" is member A which do not support the plating directly, but support the other members B, C, ..., which support the plating.
     
  6. Perm Stress
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    Perm Stress Senior Member

    Than in broad sense, "girder" mean the same in building and in ship:
    it is something that do not take the loads directly, but receive them via other parts of structure.
     
  7. ldigas
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    ldigas Senior Member

    Okey, to bring this to an end ... a real example (this is the actual project I'm working on, and which is causing me headaches, although not only from a structural point of view).

    This is the boat. What you see on the top view (as in view from the top) in dashed lines, represents the side part of the deck (around the cabins).
    Below that plating is a longitudinal L50x30x5 that goes all the way from R11 to R32 (R are frames). R11 is the bulkhead, R22 is the bulkhead, R26 is the bulkhead and R32 is the bulkhead.

    The longitudinal is connected via FB 5mm thick, to side frames (transverse frames, also L50x30x5) on every other frame (0.8m).

    When calculating that longitudinal according to GL rules, I can calculate it either as a longitudinal beam, or a longitudinal girder. In both I need to determine the spacing (half the width of the side passage, let's say 0.3m) and the unsupported length. The big question! Is the unsupported length 4.4m (length between R11 and R22) or is it 0.8m (length between stiffeners which connect the longitudinal to transverse side frames)?

    The thing is, if I calculate it with the unsupported length of 4.4m, it is grossly underdimensioned (this is an already built ship which is undergoing reconstruction), and it I calculate it with 0.8m for unsupported length, it is grossly overdimensioned, in terms of required section moduli vs. actual section moduli of the longitudinal.

    There my reason for insecurity in my calculations.
     
  8. Boston

    Boston Previous Member

    exactly

    cheers
    B
     
  9. Perm Stress
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    Perm Stress Senior Member

    If understand you correctly,
    this dashed line represents BOTH de deckhouse side above deck and 50mm L profile below deck.
    If this is correct, than this longitudinal carry no load at all, because ~0.5m high deckhouse side will take all the vertical loads first! This longitudinal only sit there for more convenient connection between transversal beams and deckhouse side. It could be replaced by small brakets in plane of DH side without any change in strength.
    Funnily, but quite often everything that is above deck, is only considered as load and not as part of structure. ;).
    Quite experienced engineers, close to retirement age, sometimes fall in this trap.

    The L profile is only really necessary below doors of wheelhouse, where the wall is discontinuous.
     
  10. Perm Stress
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    Perm Stress Senior Member

    :idea:
     
  11. ldigas
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    ldigas Senior Member

    The dashed line actually represents the boundaries of the plate which forms the main deck, starting from the bow and then going along the side.

    Along the middle of the plate, at the side of the boat, goes the longitudinal (below the plate, of course).
    Below the deckhouse walls (where you assumed was the longitudinal) is a, I don't know, FB40x5 profile, also, along the whole length of the deckhouse wall.

    The stiffeners are also below that plate at every other frame, connecting the deckhouse walls (the FB profile), and the longitudinal to the side frames.

     
  12. PAR
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    PAR Yacht Designer/Builder

    The structural elements of a boat are a girder, while some elements of this structure can be considered beams. Beams usually tie to girders, not the other way around.

    The way you handle them strongly depends on the framing system employed. A stressed skin structure, such as a taped seam build, would employ a box beam type of arrangement, while a longitudinal ring frame setup (common on larger ships) would be a trussed girder, yet again a traditional build with keel, floors, ribs and deck structures, would be a true girder, with the deck planking and deck beams serving as the upper flange, the ribs and bulkheads as the webs and the keel, floors, stringers and bottom planking serving the lower flange role.
     
  13. hoytedow
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    hoytedow Carbon Based Life Form

  14. ldigas
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    ldigas Senior Member


    They are not clear if ten people are having a two page long discussion over the difference in terms. (They are well structured though).

    Ugh, maybe I'm just tired ... but I cannot make head or tails of it ... even after re-reading all this.
     

  15. PAR
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    PAR Yacht Designer/Builder

    By initial post was from a structural engineering stand point and is how it's described in text books. You can buy into the hoopla of "we called it this or that" but in strict engineering terms a beam carries a bending load, typically along a single axis and a girder bending and torsional loads often along multiple axises. Anything else is discussion fodder, but not "footnote" qualified or quantified.
     
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